Technical Guideline for Design of Headworks

Transcription

1 Japan International Cooperation Agency (JICA) Oromia Irrigation Development Authority (OIDA) Technical Guideline for Design of Headworks May, 2014

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3 Foreword Oromia Irrigation Development Authority (OIDA) is established on June, 2013, as a responsible body for all irrigation development activities in the Region, according to Oromia National Regional Government proclamation No. 180/2005. The major purposes of the establishment are to accelerate irrigation development in the Region, utilize limited resources efficiently, coordinate all irrigation development activities under one institution with more efficiency and effectiveness. To improve irrigation development activities in the Region, the previous Oromia Water Mineral and Energy Bureau entered into an agreement with Japan International Cooperation Agency (JICA) for The Project for Capacity Building in Irrigation Development (CBID) since June, 2009 until May, CBID put much effort to capacitate Irrigation experts in Oromia Region through several activities and finally made fruitful results for irrigation development. Accordingly, irrigation projects are constructed and rehabilitated based on that several Guidelines & Manuals and texts produced which can result in a radical change when implemented properly. Herewith this massage, I emphasize that from Now on, OIDA to make efforts to utilize all outputs of the project for all irrigation activities as a minimum standard, especially for the enhancement of irrigation technical capacity. I believe that all OIDA irrigation experts work very hard with their respective disciplines using CBID outputs to improve the life standard of all people. In addition, I encourage that all other Ethiopian regions to benefit from the outputs. Finally, I would like to thank the Japanese Government, JICA Ethiopia Office, and all Japanese and Ethiopian experts who made great effort to produce these outputs. Feyisa Asefa Adugna Addis Ababa, Ethiopia May, 2014 General Manager Oromia Irrigation Development Authority

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5 Introductory Remarks Growth and Transformation Plan (GTP) from 2011 to 2015 intensifies use of the country s water and other natural resources to promote multiple cropping, better adaptation to climate variability and ensure food security. Expansion of small scale irrigation schemes is given a priority, while attention is also given to medium and large scale irrigation. In Oromia Region, it is estimated that there exists more than 1.7 million ha of land suitable for irrigation development. However, only 800,000 ha is under irrigation through Traditional and Modern irrigation technology. To accelerate speed of Irrigation Development, the Oromia National Regional State requested Japan International Cooperation Agency (JICA) for support on capacity building of Irrigation Experts under Irrigation Sector. In response to the requests, JICA had conducted "Study on Meki Irrigation and Rural Development" (from September 2000 to January 2002) and Project for Irrigation Farming Improvement (IFI project) (from September 2005 to August 2008). After implementation of them there are needs to improve situation on irrigation sector in Oromia Region. JICA and the Government of Ethiopia agreed to implement a new project, named The project for Capacity Building in Irrigation Development (CBID). The period of CBID is five years since June, 2009 to May, 2014 and main purpose is to enhance capacity of Irrigation Experts in Oromia Region focusing on the following three areas, 1) Water resources planning, 2) Study/Design/Construction management, 3) Scheme management through Training, On the Job Training at site level, Workshops, Field Visit and so on and to produce standard guidelines and manuals for Irrigaiton Development. These guidelines and manuals (Total: fourteen (14) guidelines and manuals) are one of the most important outputs of CBID. They are produced as standards of Irrigation Development in Oromia Region through collecting different experiences and implementation of activities by CBID together with Oromia Irrigation Experts and Japanese Experts. These guidelines and manuals are very useful to improve the Capacity of OIDA Experts to work more effectively and efficiently and also can accelerate Irrigation Development specially in Oromia Region and generally in the country. Finally, I strongly demand all Irrigaiton Experts in the region to follow the guidelines and manuals for all steps of Irrigation Development for sustainable development of irrigation. Adugna Jabessa Shuba Addis Ababa, Ethiopia May, 2014 D/General Manager & Head, Study, Design, Contract Administration & Construction Supervision Oromia Irrigation Development Authority

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7 Table of Contents 1. GENERAL DESCRIPTION Introduction Definitions Aim of the Guideline Scope of the Guideline Components of Headworks Basic Considerations of Headworks BASIC DESIGN INPUT DATA Data for River Conditions River Flow Regime... 9 (1) Discharge Condition (2) Water Level and Discharge (3) Sediment Load (if data are available) Condition of Riverbed (1) Condition of thalweg (2) Riverbed slope (3) Riverbed Materials Studies on the Influence of Flood Control and Water Use Flood Control Plans Situation of Upstream and Downstream Drainage Dikes, Bridges and Other Structures Present Condition of River Water Utilization Geotechnical and Geological Investigations Drilling Test Pitting Bearing Capacity Tests (1) Standard Penetration Test (SPT) i

8 (2) Loading Test Investigation on Riverbed Deposit Groundwater Investigation Assessment & Planning for Construction Works Meteorology, Surface Water, Groundwater and Riverbed Conditions Construction Equipment and Materials Transportation of Equipment and Materials Power Source for Construction Availability of Enough Labor for Construction Topographical Survey Topographic Survey Longitudinal and Cross-Section Survey Surveys for Other Temporary Works and Compensations Collection of Topographic and Other Related Maps Data for Temporary Works Annual Maximum Daily Rainfall and Annual Maximum Hourly Rainfall Annual Daily Rainfall Environmental Impact Assessment DESIGN OF HEADWORKS Basic Design Design Conditions (1) Design water intake discharge (2) Design intake water level (3) Design flood discharge (4) Design flood level (5) Study of riverbed evolution ii

9 3.1.2 Position of Headworks (1) Points of selecting Headworks position (2) Process of selecting Headworks position Method, Location and Type of Water Intake (1) Method of water intake (2) Location of intake (3) Type of weir and direction of weir axis Design Dimensions (1) Design water intake level (2) Elevation of crest height of weir (3) Ensuring creep length (4) Study of possible effect on the river control of upstream Detail Design Movable Weir (1) Sill elevation of movable weir (2) Spillway by movable weir (3) Scouring sluice (4) Pier Fixed Weir (1) Section shape (2) Type of fixed weir (3) External forces (4) Determination of section (Stability analysis) (5) Correction of trapezoidal section (6) Apron Riprap (1) Basic concept for engineering of riprap work (2) Conditions required for riprap (3) The shape of riprap (4) The length of riprap of upstream side iii

10 (5) The length of riprap A of downstream side (6) Riprap B of downstream side (7) Use of Bligh's formula (Reference) (8) Structural engineering of riprap Foundation Work (1) Functions of foundation work (2) Types of foundation work Upstream and Downstream Cut-off Walls (1) Upstream cut-off wall (2) Downstream cut-off wall Inlet (1) Function of Inlet (2) Location of inlet (3) Features of Inlet Design (4) Flow Discharge at Inlet (5) Water Level Calculation for Inlet Gate (1) Selection of type of gate (2) Lifting Height (3) Material (4) Dimension of gate for Slide gate and Stop-log Related Structures (1) Settling Basin (2) Protection of bank and major bed Control Facilities (1) Operation equipment (2) Power receiving and distributing facilities (3) Operation and maintenance bridge (4) Other operation facilities (5) Operation iv

11 4. DATA SHEET, CHECK LIST AND OTHERS Data Sheet Check List Coefficients of Roughness EXAMPLE OF DESIGN FOR HEADWORKS Basic Design Input Data Discharge through Float Measurement Method Riverbed Slope Basic Design Design Water Intake Discharge (1) In case of getting discharge data in or near river basin of project site (2) In case of getting discharge data by actual measurement Design Intake Water Level (1) Water level of the field at the highest elevation of the irrigation area (2) Water level at the starting point of the main canal (3) The hydraulic loss between the intake and the starting point of the main canal (4) Other structural losses at the intake (hydraulic loss of entrance) (5) Calculation result of design water intake level Design Flood Discharge (1) In case of getting past flood discharge data in or near river basin of project site (2) In case of using the maximum flood in the past based on flood mark or discharge capacity of the river by slope area method Design Flood Level v

12 5.2.5 Elevation of Crest Height and Length of Weir Possible Effect on the River Control of Upstream (1) Water depth of the river where the place of headworks before construction as design flood discharge (Tail water depth) (2) Water depth on the crest as design flood discharge Detail Design Fixed Weir (1) Section shape (2) Determination of section (Stability analysis) (3) Apron Riprap (1) Calculation of water depth at the weir toe as design flood discharge (2) Calculation of water depth at the beginning point of hydraulic jump (3) Comparison with h 1a and h 1b (4) Calculation of supercritical flow length (5) The length of hydraulic jump (6) Necessary Length of riprap A (7) Length of riprap B (8) Length of upstream riprap Foundation Work Upstream and Downstream Cut-off Walls Inlet Gate Settling Basin (1) Width and depth of sedimentation ditch (2) Length of sedimentation ditch Protection of Bank and Major Bed vi

13 5.3.9 Scouring Sluice (1) Diameter of riverbed materials (2) Design of scouring sluice intake (3) Engineering of upstream portion of scouring sluice (4) Cannel width of scouring sluice (5) Design of upstream slope of cannel (6) Design of downstream cannel How to use Goal seek References List of Authors/Experts/Editors/Coordinators vii

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15 1. GENERAL DESCRIPTION 1.1 Introduction Irrigation development, irrespective of scale i.e. small, medium or large, needs detail study and design. The study and design needs to be conductuted rigorously with minimum standard quality. Unless it results loss of money, wrong construction that inturn can result negative environmental impact (water loss, salinity, gully, conflict,etc.) and in general unsustainable development. The study and design phase plays decisive role in irrigation development. To enhance the quality of the study and design of irrigation projects, it is necessasry to attain minimum quality standard. For this reasson, it become necessary to prepare standard terms of refrence, guidelines and manuals. 1.2 Definitions In this guideline the following words and phrases are defined as follows: Headworks is a structure constructed across the river to effect local storage and rise water level locally to divert part or all the supply in to a channel. The height, quantity and period for which the supply is stored make it different from dam. In this guideline, headworks is defined as the facilities which divert water from a river (lake and marsh areas, excluding reservoir) into a canal for irrigation use. [Reference] In the Multilingual Technical Dictionary of Irrigation and Drainage issued by the International Commission on Irrigation and Drainage (ICID), headworks is defined as "A collective term for all works (weirs- or diversion dams, head regulators, upstream and downstream river training works and their related structures) required at intakes of main or principal canals to divert and control river flows and to regulate water supplies into the main canal (s). " 1.3 Aim of the Guideline The aim of this guideline is to show the basic and important way of design concepts and approaches of headworks for small and medium 1

16 scale irrigation. Irrigation engineers are expected to design technically efficient, socially acceptable, economically viable and environmentale friendly headworks through this guideline for implementation. The guideline assists the engineer to follow a specific design procedure country wise to save time and materials. 1.4 Scope of the Guideline This guideline discusses the general procedures to be considered in the design and construction of small and medium scale irrigation diversion headworks. In Ethiopian, irrigation schemes are classified in to three types based on the area. Small scale irrigation scheme is less than 200ha,medium scale irrigation scheme is 200-3,000 ha and large scale irrigation scheme is greater than 3000ha (In Oromia ha micro irrigation, ha small and the rest has the same classification with country level). Some of the concepts in this guideline need specific data. Material and equipment that are difficult to get at present situation of the country requires time, qualified and experienced human resource for accomplishement. In such cases, supplementary description is given in box on what to do at least in current condtion, how to estimate and design with available data without significant impact on the quality of project. The guideline takes into account all internal and external forces acting on a weir, how they affect weir and make sure the structure functions normal throughout its life time the purpose intended to serve by providing appropriate dimension to all elements. Technically, the guideline deals with all components of headworks including intake facilities, diversion weir for maintaining water level at the intake (except weirs with storage function), their related structures and operation and maintenance (O & M) facilities. The manual is also applicable for simple intakes without detail weir body. 2

17 Fig.1.1 Layout of headworks 1.5 Components of Headworks Components of headworks are shown in Fig Standard figures of headworks plans and cross-sections used in the figure are shown in Fig. 1.3 and Fig Fig.1.2 Components of headworks 3

18 Wing wall Devide wall (Guide wall) Canal Settling basin Scouring sluice gate Inlet/ Intake River Protection (Gabion) Riprap(D/S) Apron (D/S) Hirna River River Flow Wing wall Weir Riprap (U/S) Apron (U/S) Fig.1.3 General Plan of headworks Settling basin Scouring sluice gate Inlet / Intake Wing wall Riprap (D/S) Devide wall (Guide wall) Apron (U/S) Riprap (U/S) Apron (D/S) Weir Fig.1.4 (a) Standardized layout of headworks 4

19 Cross section Wing wall Scouring sluice gate Intake gate Weir Wing wall Settling basin Devide wall (Guide wall) Apron (D/S) Fig.1.4 (b) Standardized cross section view of headworks 5

20 Longitudinal section Apron (U/S) Weir Apron (U/S) Riprap (U/S) Riprap (U/S) Cut-off wall Apron (U/S) Riprap (U/S) Scouring sluice gate Weir Devide Wall Apron (U/S) Riprap (U/S) Cut-off wall Fig.1.4 (c) Standardized longitudinal cross section view of headworks 6

21 1.6 Basic Considerations of Headworks In principle, headworks should have the necessary water intake facilities, maintain safety from any external forces, harmonize with flood control and water use of the river and be basically economical structure. The structures of headworks should be designed to secure the intake with enough irrigation water at all times, to release flood flow smoothly, to be safe from any external forces during flood and to maintain the original river functions identical to previous functions. The design of headworks must comply with relevant laws and regulations of the country.the design of headworks should be carried out by appropriate procedures with special attention to interrelated aspects as mentioned previously. At the basic design stage (Section 3.1), a general layout of each component of headworks is determined. Then a detailed design of each facility follows as Section 3.2. However through out the whole design procedures, feedback of intermediate results should be made if necessary to make each component harmonized as a whole. General design procedures of headworks are shown in Fig

22 Fig. 1.5 Flowchart for designing headworks 8

23 2. BASIC DESIGN INPUT DATA Design of headworks needs input data that have good quality in different stages of the study. The design of headworks has to identify kinds of data, availability of data, quality of data in each stage. Headwork design not only identifies input data but also assesses impacts that have to be tested like effects of flood, upstream and downstream effects and others issues. In this section basic design input data and the reasons for the requirement of these data are briefly explained. This section helps the engineer to understand what they are designing, what to observe including impacts and other issues. The necessary basic primary and secondary data have to be identified and then to be collected. The collected data have to be examined. Planning is the primary work in designing i.e. what data to collect, how to collect, when to collect and where to collect. Data collection and examination has to be considered not only for design and construction phase but also for operation and maintenance aspects after construction. This is very helpful in securing the headworks to function properly, to economize the project during planning, design, construction and operation stages. Keeping this in mind, it is important to carry out efficient & effective planning considering the relationship between the basic design input data & design, construction and O & M. When basic design input data is not possible to be accessed with the same method as explained in guideline, it is necessary to get the information and data in another method. It is not recommendable to design without ungrounded information and data. 2.1 Data for River Conditions River conditions are the major factors to select design approach and parameters. These data shows the target river morphology before the design is done. This data includes river flow regime, river bed conditions, and saltwater intrusions of tidal rivers River Flow Regime River flow regime is concerned with the river geometry. This includes: 1. Discharge condition, 9

24 2. Water level and discharge, 3. Sedimentations (1) Discharge Condition Discharge condition is to understand and evaluate the river water flow situation. It is important to design intake discharge, design intake level and design flood discharge in Design Conditions explained in section A flow duration table (or curve) shall be prepared based on the observation records of river discharge: 1 95-day discharge (probable discharge occurring more than 95 days in a year), 2 Ordinary discharge (probable discharge occurring more than 185 days in a year), 3 Low discharge (probable discharge occurring more than 275 days in a year), 4 Base flow (probable discharge occurring more than 355 days in a year), 5 Yearly average discharge(average of yearly daily mean discharge), 6 Average discharge of irrigation period(average of daily mean discharge during irrigation period), 7 Seasonable discharge variation(dry season discharge, wet season and rainy season discharge) When there are no records at the proposed site, a flow duration table (curve) should be prepared using records obtained from the stations close to the site on the same river or by transferring data from close to similar gauged catchments. < Supplementary explanation > Data is required for planning. For example, Irrigation engineers have to address the intended benefit of the area in consideration of the proposed river. In our country current condition, most of the rivers don t have recorded data for such condition. This will lead to collect information from local community (elders). This approach has a limitation because of the memory of the person with whom we discuss. This has to be corrected after identification, pre-feasibility and detail study made after measuring the flow at least weekly and monthly if possible daily by district and 10

25 kebele experts. This data has to be supported by rainfall - run-off analysis (when there is rainfall data of the area is available). Please refer to Manual for Runoff Analysis in detail. (2) Water Level and Discharge Rating curve is prepared by using long-term records of water level and discharge obtained from the data collected or analyzed from the previous section (1). In principle, discharge measurement should be applied by the currentmeter. When the current-meter is not available, float measurement method can be applied. 1) Current meter method The current meter method is a method of calculating discharge by multiplying the flow area and the mean flow velocity observed by current meter. The follows are noted on measuring the flow velocity. 1 To increase the accuracy of measurement, both the water depth and the mean flow velocity should be measured more than twice at the same cross section if possible. If the results of measurements are quite different, another measurement is required. 2 In principle, the measuring points should be set at equal intervals on a measuring line crossing the river. However, the intervals should be reduced in a complex sectional feature or uneven velocity distribution (Table 2.1). 3 In case of shallow (less than 75 cm) water depth, the one point method, measuring at 60% of water depth, should be applied. When the water depth is deeper than above, the two point method, measuring at 20% and 80% water depth, shall be applied. 4 The water depth measuring line should be set at each boundary of dead water zone as shown in Fig The discharge is the total of the numerical value obtaining by multiplying the mean flow velocity in the measuring point by the small cross sectional area that the mean velocity represents. 11

26 Table 2.1 Interval of measuring points for water depth and water velocity Width of water surface (B)m Interval of measuring points for water depth (M)m Interval of measuring Points for water velocity (N)m Less than 10 10~15% width of water Surface N=M 10~20 1m 2m 20~ ~ ~ ~ ~ ~ More than Fig. 2.1 Velocity measuring point in the current meter method Photo 2.1 Measuring by current meter method 2) Float measurement method The float measurement method is a method of calculating discharge by multiplying the flow area of planning point and the mean flow velocity measured by the float. The measurement of flow velocity is calculated by dividing the distance among two transversal lines by the time which a float flow the section. The follows are noted on measuring the mean flow velocity: 12

27 1 The float measurement method is used when the current meter method cannot be used, 2 The transversal line is set more than two, and the interval is not less than 50m basically. But the case of small and medium river, the interval can be changed to not less than 10m, 3 There is the planning point between the transversal line of upstream and the transversal line of downstream, 4 The measuring line of velocity of float is set in the direction of the flow of the river from the transversal line of upstream, 5 The relation between width of surface water of river and the number of measuring line of velocity of float is expressed in Table 2.4, 6 The point which the float starts is about 30 m upper from the transversal line of upstream basically. But the case of small and medium river, it can be changed for small rivers 5-10m, for medium rivers 10-20m, 7 The mean flow velocity at the planning point is calculated by multiplying velocity coefficient (Refer to Table 2.3) and average flow velocity calculated based on the results of measurement in each measuring line of velocity of float with time record. Measuring line of velocity of float Start line Approaching section about 5 30m Transversal line of upstream Planning Transversal Measurement section not less than 10 50m line of downstream Fig. 2.2 Layout of float measurement method 13

28 Fig. 2.3 Float type and stick type for float method Table 2.2 Selection criteria of float number and submerged depth of flow Water depth ~0.7m Number 1 0.7m~1.3m 2 0.5m 1.3m~2.6m 3 1.0m 2.6m~5.2m 4 2.0m 5.2m~ 5 4.0m Submerged depth of float Table 2.3 Velocity coefficient for float method Number Water depth (m) ~ ~ ~ ~ ~ submerged depth of float (m) Float type Stick type velocity coefficient

29 Table 2.4 Width of river and number of measuring line of velocity of float Width of river ~20m 20m~100m Number of measuring line of velocity of float 100m~ 200m 200m~ < Supplementary explanation > The flow data is used for planning and design of headworks. Discharge is the basic data for planning irrigation capacity. When Dam and reservoir is planned not only minimum discharge but also annual data is required for design. Design discharge is needed for the design of stability and safety of headworks. Data about the level of water throughout the year is required to decide the necessary design and construct weir that will not to bring unacceptable water level raise. When there are no measured data discussion from local community has to be done through checking the flood mark, by taking survey of cross section and river slope. The discharge can be calculated by Manning formula: Q = A V 1 2 V R n I Where:, Q: Discharge (m 3 /s) A: Cross-section area (m 2 ) V: Mean velocity (m/s) I: Hydraulic gradient (River bed slope ) R: Hydraulic radius (m) n: Coefficient of roughness (refer to reference) This method doesn t help us to know the probability year of the discharge because it is only one time data. If rainfall data can be found, then it is possible to support the result by rainfall run-off analysis. Please refer to Manual for Runoff Analysis in detail. (3) Sediment Load (if data are available) River sediment must be measured in order to fix the sill elevation of the intake, to decide the necessity of a settling basin, to design the scale and 15

30 frequency of sand removal from the settling basin. Sediment load can be divided into two categories: 1 Suspended load 2 Tractable load (bed load) Suspended load is part of the sediment transport suspending in the river water without touching the riverbed for a certain period. Tractable load is part of the sediment transport that is jumping on the riverbed or moving along the riverbed. When the sill of an intake is designed it has to be high enough to avoid the suspended sand entry to a canal. Fig. 2.4 Uniform velocity In general, the distribution of suspended sand within a flow section is not even, so the following sampling method should be applied to obtain reliable data: 1 During small to medium floods, sampling should be done along several measuring lines (at right angles to river flow) near the proposed intake site, 2 At each measuring line, the distribution of flow velocity is checked in the vertical direction. The vertical section is then divided into several sections with similar flow velocity. Samples should be taken at the center of each section, 3 Furthermore, each measuring line should be divided horizontally into several sections by flow velocity and samples should be taken at the center of each section, 4 The quantity and grain size of the samples taken by the above method should be recorded and analyzed. In addition, the tractable sand should be estimated at several sections at 50 to 200 m intervals up to about 1 km upstream from the proposed site of the headworks. < Supplementary explanation > The sediment load is crucial for design of Scouring sluice gate and settlement basin. When there are no data and no time for measuring, observation of the river and information have to be gathered from local community (elders). In addition catchment data & experience, research, 16

31 model results and literature can be consulted to make the information collected from the community more viable Condition of Riverbed Data has to be collected regarding river bed conditions on 1condition of thalweg, 2 river bed slope and 3 river bed materials. (1) Condition of thalweg The river flood never flow uniformly covering all width of the river. The water collection of flood flows fluctuates. The river thalweg, or stream centerline, is formed after the floods. Therefore, the location of the headworks should be selected at a site where the thalweg is stable and located near the river bank where the intake is installed in the scheme. The scouring sluices are set in the thalweg. Study has to be carried out on the magnitude and frequency of floods, which may cause the movement of bed materials of average grain size. The maximum flow capacity should be estimated based on survey data on width, depth and gradient of the existing thalweg. Rivers that don t have stable thalweg needs investigation of the moving condition of the thalweg. When headworks is planned without weir type or when the river channel at the proposed location of headworks is wide and meandering, the following studies should be carried out: (a) The past changes of stream centerline based on old topographic maps, river trail maps and so on, (b) The characteristics of meandering river upstream and downstream of the proposed site of headworks, (e) The condition of rock foundation on the river bank and effects of scouring on the riverbed and (d) The influence of river structures such as piers of bridges. In order to understand these characteristics, a river survey of reaches upstream and downstream of the proposed site of headworks is required. < Supplementary explanation > Condition of thalweg is required for design of location. Particularly, it is important for design without weir type. When there are no data and no time for measuring information from local community has to be gathered. According to river situation, if the discharge is not including flood and is small and thalweg change is big, then it is better to make weir or change weir location. If the discharge is big and thalweg change is not a problem, 17

32 then this modification is not desirable. (2) Riverbed slope The assumption of aggradations and degradation of the riverbed are vitally necessary in order to determine the sill elevation of intake and to design foundation of headworks and related structures. For this purpose, stability and tendency of aggradations or degradation of riverbed should be studied carefully. Riverbed slope can be calculated by following formula. I avg = H avg / L.(F. 2.1 ) Where, Iavg: Average Riverbed slope Havg: = 2 A / L (m) A: Individual area An = (Hn + Hn+1) / 2 Ln (m 2 ) H: Accumulative height H = ELn EL0 (m) L: Distance (m) EL: Elevation (m) EL(m) EL3 EL2 EL1 H1 H2 (A2) H3 (A0) (A1) EL0 L0 L1 L2 No.0 No.1 No.2 No.3 Fig. 2.5 Layout for Riverbed slope calculation L(m) < Supplementary explanation > The riverbed slope data helps for design and to estimate the river bed change. When there are no data and no time for measuring information has to be gathered from local community (elders). Actually, it is difficult to estimate the river bed situation and change, even if there is data. (3) Riverbed Materials Riverbed materials are the channel bottom of a stream or river where the normal water flow occurs. Sampling should be done at several points at 200 to 500 m intervals upstream and downstream of the proposed 18

33 headworks in order to get grain size distribution, specific gravity, maximum grain size (90 percent passing by weight) and average grain size (60 percent passing by weight). All sampling points should be indicated on a plan. The results of riverbed materials analysis are used to estimate the roughness coefficient of the river section, to presume riverbed change, to decide on the necessity of a settling basin, to design of scouring sluice and detachability of the silt. < Supplementary explanation > When there are no data or difficult to investigate during the design study time due to flood in the river, the investigation should be done at construction stage. Until that time, design can be done by using assumed values. 2.2 Studies on the Influence of Flood Control and Water Use Studies on the Influence of flood control and water use shall include flood control plans determined by the basin/river authorities (like Awash basin Authority). The study need to investigate the condition of upstream and downstream drainage, condition of existing river structures and present condition of river water utilization Flood Control Plans Flood control plans are required to be designed by the basin/river authorities in the river where the new headworks is proposed. The design of high water discharge, high water level, cross section and the annual schedule of the river improvement works, etc. have to be investigated for sustainability of the scheme. < Supplementary explanation > When there are no flood controls plans, design can be done freely or the plan can be made by engineers responsible for design of headworks. It is necessary to consider the situation of upstream and downstream to avoid damage to those areas Situation of Upstream and Downstream Drainage The situation of upstream and downstream drainage discharges into rivers during the ordinary flow should be investigated in order to discover the influence of rise of water level by weir upon upstream and 19

34 downstream drain ability. For this purpose, it is necessary to study the function, scale and capacity of existing drainage facilities and the situation of in-flow to the river. < Supplementary explanation > The upstream and downstream drainage investigation may be easy but consultation with local community is necessary Dikes, Bridges and Other Structures The scale and dimensions of the structure of foot protection, span, etc. of dikes, bridges and other structures should be investigated to clarify the influence of flood. < Supplementary explanation > Investigation on dikes, bridges and other structures are easy. Additional information has to be gathered from local community and administration. The result needs to get strict attention Present Condition of River Water Utilization Present condition of river water utilization such as irrigation, hydropower, fishery and others need to be investigated: 1 The customary agreement on the construction of a weir, water balance of upstream, downstream and flow restriction in the downstream should be clarified. If it is necessary, then the present condition and the customary agreement should be compared and cheeked, 2 Fluctuation of water level due to power generation and customs of rotational irrigation in the present water utilization condition, etc, 3 In the river, where the conservation of fishes are necessary, kind of fishes, their quantity, migratory season of fishes etc. should be investigated in order to use for the design of fish ways, 4 Other data related to current condition of river water utilization, such as utilization of underflow, should be collected. < Supplementary explanation > Present condition of river water utilization item is used for planning and design. The investigation may be easy however discussion with the local 20

35 community and administration has to be done. (Give attention only on applicable condition which are in the context of Ethiopia) 2.3 Geotechnical and Geological Investigations Geological and geotechnical investigations shall include the type of foundation material (e.g. rocks), chemical and physical condition of foundation material, relevant geologic structures, the thickness, bearing capacity, compressibility and hydraulic conductivity of overburden, the ground water level and its condition. Investigation on the foundations shall be carried out so as to establish the suitability of the site for the structure, determine geotechnical design parameters to design suitable foundation structures according to the existing foundation material, and also to design superstructures rationally considering ground conditions. The methods of these investigations and tests are described hereunder. < Supplementary explanation > This information is important for stability of the structures. The following investigations shall be carried out in consideration of the cost and the scale of project Drilling The purpose of drilling is to identify the type of riverbed material and stratifications of the ground, to take samples for geotechnical analysis, and to execute in situ tests in boreholes. To supplement geological information obtained by drilling, geophysical techniques like electrical resistivity test or a seismic survey, particularly for large-scale headworks located on a wide river, is often applied effectively together with drilling. Drilling sites shall be located at proper intervals not only at the centerline of the diversion weir, but also at the apron ウォータースイベル Water swivel 櫓 ( 鋼製又は木製 ) ワイヤーロープ Wire rope Stage ウインチ Winch ボーリングマシン Drilling machine デリバリーホース Delivery hose ガードフェンス Guard fence エンジン Engine ボーリングポンプ Drilling pump エンジン Engine Suction サクションホース hose 掘削用水 Water for ( 清水又は泥水 drilling ) Casing pipe ケーシングパイプ又はドライブパイプ or Drive pipe ボーリングロッド Drilling rod Core コアチューブ tube Reamer リーマー Metal メタルクラウン crown 又はダイヤビツト or Diamond bit GT-04 Fig. 2.6 Drilling image 21

36 downstream and at the scouring sluices upstream and downstream of the proposed structure. It is desirable that drilling sites be set in a rectangular pattern so as to enable a geological map to be drawn. To stop underflow of water, the area of investigation should be extended sufficiently towards the abutments. For example, even when a floating foundation is preferable, confirmation of foundation rock will be useful for comparison of construction methods. Furthermore, in case of well or caisson foundation, drilling at each location of base is effective in saving construction cost by over sinking or cutting. < Supplementary explanation > Laboratory test can be done by taking pit investigation since drilling test is more or less costly for SSIP projects. This item is very useful for foundation and bank design. When it is not possible to collect data using drilling, pit opening manually can give important useful data. The cost of this investigation method is relatively cheap. The design has to be done by laboratory result for undisturbed & disturbed samples or estimating value from information of test pitting and other information. Resistivity test also can be an option Test Pitting Test pits allow direct observation and appraisal of the soil strata and geology of the foundation. Furthermore they enable sample collection (both disturbed and undisturbed) and in situ bearing capacity tests. The number of test pits shall be at least 3 (three) points (located two at banks and one at river center). Additional pits upstream and downstream along the center of weir axes helps. < Supplementary explanation > This investigation gives very practical data. If a foundation and bank investigation was not done during the study time, the foundation information has to be collected by test pitting before construction. The designer has to take into account not only data collected from test pit but also information from secondary data sources such as discussion with the local community. 22

37 2.3.3 Bearing Capacity Tests The bearing capacity test is very important in design of headworks and when conditions allow the following methods can be applied, some of them described hereunder. (1) Standard Penetration Test (SPT) The standard penetration test is the simplest method to find the properties of ground. It is commonly executed together with rotary core drilling. This method drives a sampling tube (splitspoon sampler), outside diameter 51 mm and inside 35 mm, separable in two, driven at the head of the drilling rod with a weight of 6.35 Kg falling 75 cm. The number of blows required to penetrate 30 cm is counted, and called the N-value. A disturbed soil samples for mechanical analysis can also be collected in the tube during the test. Picture 2.2 Standard Penetration The relationship between N-value and the relative density of the ground both for sand and clay by Terzaghi-Peck is shown in Table 2.5. In general, the standard penetration test gives accurate results for sandy ground. For clayey ground, the bearing capacity should be calculated using the results of an unconfined compressive strength test on samples. Table 2.5 Judgment of N value Sand Clay N value Consistency N value Consistency 0~4 4~10 10~30 30~50 More than 50 Very loose Loose Medium Dense Very dense 0~2 2~4 4~8 8~15 15~30 More than 30 Very soft soft Medium stiff Very stiff Hard The relationship between the unconfined compressive strength qu and the N-value is empirically explained as follows. 2 qu 1.22N( t / m ).... (F. 2.2) 23

38 If the length of the drilling rod is long, this may influence the results of the standard penetration test. Correction of the measured N-value shall be made by the following formula: N = N (l 20m) N = ( l)N (l>20m) (F. 2.3) where, N': corrected N-value N: measured N-value l : length of rod In addition, the N-value is useable to estimate the internal frictional angle of sandy ground and so on which is useful for foundation design. Therefore, the N-value shall always be measured at the time of drilling. Long-term allowable bearing capacity in relation to N-value is shown in Section

39 Fig. 2.7 Sample of boring log sheet 25

40 (2) Loading Test There are two types of loading tests: the plate bearing test and the pile head loading test. Both are to confirm artificial failure of the ground by loading up to ultimate bearing capacity. The loading methods include; 1 Direct loading, 2 Jacking method using anchor frame, and 3 Loading method using lever. In the test, loading is gradually increased, and load intensity, time, and settling depth are observed. Then, a record diagram should be drawn to be analyzed. At the investigation of headworks, plate loading test is rare, but a pile head loading test is desirable after pile driving test in case of a pile foundation, especially of steel pipe piles. < Supplementary explanation > This is also very important for design. In this study, bearing capacity of the foundation can be known. There are two methods to do this standard penetration test and loading test. The later needs machine and it is expensive. When these tests are not practicable, design should be done using standard value of Table 3.11, other project data or text values Investigation on Riverbed Deposit Over a long period, the course of the main channel and thalweg of rivers may move due to fluctuation flood discharges. Riverbed deposits should be investigated based on geological data and information in order to prevent construction failure. Dewatering could prove to be impossible during excavation for the foundation because of the water path in the trail of the thalweg where stone and gravel layers exist, and/or the lower layer below the cofferdam is broken by piping Groundwater Investigation To assess the influence of construction of headworks on drainage of nearby farmland and the water level in wells, the ground water conditions should be investigated by a field permeability test, etc. < Supplementary explanation > This is very important for design and to select construction method. When these tests are not done but groundwater is observed, it is necessary to 26

41 consider dewatering at construction time. 2.4 Assessment & Planning for Construction Works Assessment for construction works should be conducted on the following items, which are necessary for construction planning. 1 Meteorology, surface water, groundwater, riverbed conditions 2 Construction equipment and materials 3 Transportation of equipment and materials (write down for materials such as quality, quantity, location as well as its availability) 4 Power source for construction 5 Availability of adequate labor for construction 6 Construction material quantity & quality Meteorology, Surface Water, Groundwater and Riverbed Conditions Meteorology and surface water should be thoroughly investigated because of their importance in deciding the construction schedule. Based on the assessment on annual rainfall and river runoff, workable days and periods, water level and discharge of diversion channels, elevation of cofferdam, etc. should be determined. Flood runoff pattern should also be investigated in order to decide timing of interruption of works during flood. Data on runoff, water level and flow velocity of floods in the construction period of the cofferdam are essential to make a proper construction plan. For example, the construction method for a cofferdam will differ depending on the characteristics of water level changes whether they are almost constant or fluctuate widely, and if fluctuation is large, what the cycle period is. Rainfall and temperature, etc. should be fully investigated in order to estimate workable days and to make a construction plan. Especially in the cold areas, snowfall and temperature in winter season are important. Investigations on the condition of the riverbed deposit and groundwater which are helpful for construction are described in Section & < Supplementary explanation > This item is in consideration of construction schedule. These investigations may be easy. Construction time of headworks is better in dry season. 27

42 2.4.2 Construction Equipment and Materials Construction materials include ready mixed concrete, steel materials and timber, etc. The supply situation of factory products should be checked on the possibility to cope with sudden requirement for large quantities as well as urgent requirements by the change of design. Availability of construction equipment considerably influences the construction schedule and results. Investigation should be done on the availability of construction equipment with the function and capacity to suit the site conditions, availability of spare parts, and necessity of stand-by equipment during construction period. < Supplementary explanation > This item is in consideration of construction. In the present situation, it is easy to get equipment and materials for construction of headworks of small scale irrigation project. However, it is necessary to collect the information frequently, sometimes, price change happens, supply of material becomes difficult. Particularly industrial input. If it is necessary, supplying main industrial inputs like cement by the client should be considered Transportation of Equipment and Materials Local roads and bridges should be thoroughly checked in order to transport a lot of heavy and/or long equipment and materials including construction materials, factory products such as steel gates, construction machinery, etc. If necessary, roads or bridges have to be repaired or newly constructed. Otherwise, heavy and/or long equipment and materials may be transported disassembled. < Supplementary explanation > This makes construction material transportation easy. In the present situation, most of the site is far from asphalt road. Necessity of access road should be considered Power Source for Construction Considerable power demand may arise during the construction of headworks for a short period, and power source is generally required. Therefore, power source, location of distribution lines and diverting points should be investigated. When electric power cannot be obtained easily, 28

43 independent power generation or an internal-combustion engine may be used. < Supplementary explanation > In current Ethiopian condition, electric power can be generated from small or medium generators for some construction works Availability of Enough Labor for Construction Household survey to know the availability of productive forces; labor at a reasonable price, in which month and so on has to be studied for the intended work. Works also has to be classified for community participation. 2.5 Topographical Survey Topographical survey for headworks should be done on the following items. 1 Topographic survey 2 Longitudinal and cross sectional survey at headworks 3 Necessary survey for temporary works 4 Collection of related topographic maps Topographic Survey Topographic surveys are necessary not only for construction of the headworks itself but also for the planning and design of cofferdams and temporary facilities, etc. Curvature conditions of the river thalweg and elevation of the riverbed near the site should be surveyed as follows: Scope: 1.5 times upstream meander length 1.0 times downstream meander length Scale: river upstream and downstream: 1/1,000~1/3,000 At the headworks site: 1/200~1/500 for detailed survey. The contour lines on the map should be drawn at m intervals for the riverbed and 1.0 m interval for the other parts. 29

44 2.5.2 Longitudinal and Cross-Section Survey Profile and cross-section maps of the river are used for the design headworks and for hydraulic calculations such as of backwater. Prior to the commencement of surveying, the benchmark elevation to be used for the planning of the headworks should be adjusted to the benchmark used by organization in charge of benchmark or basin/river administrator. Picture 2.3 Cross-Section survey During the cross-section surveys, the flood mark of both side of the river around headworks should be indicated and incorporated for hydraulic calculations. This can be done based on elderly flood mark and some safety factor. The extent of this surveying along the river courses at least has to be 500m to each side of upstream and downstream of the site of headworks but effects shall be seen to decide the effect of flood. Scale and interval: Three cross sections; one weir axis, two upstream and downstream at 20m upstream and downstream of the site of headworks has to be done. Every natural change useful for the structure design with its profile and cross section survey for the headworks has to be done. Drawing scale for cross section; horizontally 1/100~1/500 and vertically 1/100~1/200 is acceptable. Cross-section surveys of at upstream and downstream for 200 m interval and the drawing scale by 1/100~1/500 has to be done. Drawing scale for longitudinal profile; vertically 1/100 and horizontally 1/1,000~l/2,000 adjusting the scale of the topographic map. In addition, benchmarks are recommended to be set at both sides of the headworks and at least two in the range to be able to seen from headworks. Please confirm with EMA Bench marks located around it Surveys for Other Temporary Works and Compensations Surveying for topographic maps at a scale of 1/200-1/500 should be done for planning and design of cofferdams, temporary facilities for concrete works, construction roads, and excavation of aggregate materials. The contour interval should be the same as for the topographic survey mentioned above. 30

45 Profiles and cross-sections required for estimation of earthwork volume are prepared following the standard longitudinal and cross section survey mentioned above Collection of Topographic and Other Related Maps Prior to field investigations and surveys, topographic maps and other related maps around the site of headworks should be collected in order to prepare schedule to make a preliminary design using a topographic map (scale 1/25,000~1/50,000) and present land use maps. However, the accuracy of these maps should be checked to minimize the mistakes. Map from Ethiopian Mapping Agency, topographic and other related maps issued by the national survey organization should be obtained. < Supplementary explanation > This item helps to for design consideration and construction. During construction, the bench mark used at survey time should be checked and strengthen or transferred to suitable place, make it concrete etc. 2.6 Data for Temporary Works The collection of hydro metrological data such as rainfall and river water level prior to construction is very important not only for the headworks construction but also for temporary constructions Annual Maximum Daily Rainfall and Annual Maximum Hourly Rainfall These data are necessary to estimate flood discharge and flood stage for planning of temporary works. Since the estimation will be done by statistical analysis, as much data as possible should be collected. The number of years required for the estimation varies depending on required provable value, but data more than 30 to 50 years is desirable. Further, the smaller the catchment area is, the more records on annual maximum hourly rainfall or annual maximum 10 minutes rainfall are necessary. < Supplementary explanation > In our case this has to be done easily by area-velocity method for construction period months and the temporary diversion has to be decided in this context. 31

46 2.6.2 Annual Daily Rainfall Data of annual daily rainfall at least over 10 years are necessary to presume flow regime. When discharge is analyzed using a tank or other model (please refer to Manual for Runoff Analysis for the details of the tank model), data for the design year and 3 additional years are recommended to obtain stability of the model Environmental Impact Assessment The Construction of' headworks may give variety to the natural environment (scenery, ecosystem of animals and plants, etc.) and life of people living around. Therefore, the environment around the site should be investigated in advance. The results of the study from previous sections should be effectively utilized for planning, design, construction and maintenance of headworks. In addition, muddy water may flow into the river downstream during construction of the headworks and may cause adverse effects on inhabitants downstream and life in the water by mud increasing and siltation. Therefore, the degree of adverse effects on fish habitat in the river and other aspects (resting and feeding area of fish and birds) should also be investigated. Special care should be taken in selection of construction methods and periods which cause noise pollution and vibration, etc. to residents around the site. The routes for transportation of equipment and construction materials by large-sized dump trucks should be carefully studied to prevent noise and vibration effects. 32

47 3. DESIGN OF HEADWORKS 3.1 Basic Design At this stage, appropriate basic design criteria must be established to ensure the structure that can perform the intended functions Design Conditions Design water intake discharge, design intake water levels, design flood discharge and design flood levels must be known for the design of headworks. The height of fixed structures and the future level at which the riverbed will be stable must be studied in order to set appropriate design conditions. (1) Design water intake discharge The design water intake discharge is the intake discharge at maximum design intake. The intake discharge that governs the weir design and the dimensions of the intake must be set considering the design maximum intake of the overall irrigation. In other words, Irrigation plan is decided according to beneficiary area, crop pattern, crop water requirment, the base flow and the amount of usable water from the river. Design water intake discharge is decided based on the following flow chart, 33

48 Fig. 3.1 Flowchart for determining design water intake discharge Please refer to Guideline for Irrigation Master Plan Study Preparation on Surface Water Resources. < Supplementary explanation > Base flow calculation at the point of intake can be conducted by watershed ratio method (Catchment area method) to design water intake discharge Based on the standard base flow, the base flow is calculated by watershed ratio method. The formula of this method is as follows, D = Ds Ap / Ar 34

49 D : Base flow (m 3 /s) Ds : Standard base flow in river basin of project site (m 3 /s) Ar : River basin (km 2 ) Ap : Catchment area of the project (km 2 ) Based on the base flow, the amount of usable water is calculated. The formula is as follows, W = D K Qdw W : The amount of usable water (m 3 /s), D : Base flow (m 3 /s), K : Coefficient of released flow for downstream ecology (0.7~0.9), Qdw : The existing design water intake discharge in the downstream of the project (downstream demanded water). In case of using actual measurement data, it should be subtracted from base flow. The design water intake discharge is decided based on the amount of usable water, The design water intake discharge should be within the amount of usable water and be considered from the relationship between beneficiary area, crop pattern and crop water requirement. (2) Design intake water level The absolute requirement for the design intake water level is given as the water level required to be secured at the start of the canal set in the irrigation plan. In case of intake on weir, the design intake water level must be the highest of the following: 1 The total of the water level required to be secured at the canal start plus the total of the head loss between the intake and the canal start; or 2 the total of the intake threshold height from the bottom the silt scouring sluice to prevent sediment inflow and the intake depth. In case of natural water intake, consideration is given to fluctuation of river water levels and base flow during the irrigation period occurring at the probability of once in every 10 years is adopted in Japan as the design intake level. Assuming that this base level flow coincides with the timing when the design intake volume is required, final design intake level is then determined so as to satisfy the requirements of 1 and 2 mentioned previously. As stated earlier, the choice of design intake level is not only related to the location of 35

50 the headworks, but it is also affected by loss of head that varies depending on the distance between the intake and the canal start, existence of a slit basin and the flow velocity of the intake (size of the intake). Therefore, the final decision must be made after a series of trial calculations and reviews of various factors. At the preliminary design stage, it is recommended to take 1.8 to 2.3 times of the intake velocity head (V 2 /2g) as the loss of head at the intake. < Supplementary explanation > Design intake water level is used for consideration of the headworks position. The headworks position has to be located upper than this level. (3) Design flood discharge The design flood discharge is decided in accordance with the river control plan. In other cases, the design flood discharge is decided on the basis of the known discharge capacity of the target river. If there is an experience of flood greater than discharge capacity of the target river in the past, then the design dischage is decided on the basis of the flood. 1) Decision of design flood discharge Headworks structures (the weir in particular being a structure across the river) need to be stable enough to withstand floods while at the same time not being a serious obstacle to disturb the flow of floods. The design flood discharge and the design flood level explained in the following sections are the basic values for calculations stability of external force and stress. They are also basic values in deciding the features of the structure. Design flood discharge has to be decided from the 1 past flood discharge data, 2 the maximum flood in the past based on flood mark or discharge capacity of the river by slope area method or 3 flood (1/50 probability year) by run-off analysis. 36

51 Fig. 3.2 Flowchart for selecting design flood discharge 37

52 Wherever the proposed siting of the headworks may form an obstacle to the normal flow of the river it is necessary to take measures to expand river sections in the vicinity of the proposed site. < Supplementary explanation > There has to be sound Judgement on the basis of regionalization; condition of the site and in consultion with other design etc. (4) Design flood level If the target river is under a river control plan or it is likely to be incorporated in such plan in the near future, then the design flood level is set in accordance with the target plan. In other cases, the design flood level should be the level at the proposed site when the design flood discharge flows. < Supplementary explanation > Design flood level is an input for design of stability, safety and consideration for influence to upstream in the absence of river control plan. Design flood level is calculated from design discharge on condition of after headworks is constructed. H e = (Q d /CL) 2/3 from Q d = CL H 3/2 e H e = H d +H av H av = V a2 /2g V a = Q d / L (h+h d ) H e = H d +(Q d ) 2 /[L(h+H d )] 2 /(2g), Design flood level at weir = H d + Elevation of weir crest Where H e : Total energy head (m) H d :Design head (Water depth on the crest) (m) H av : Approach velocity head (m) V a: Approach velocity (m/s) Qd : Design flood discharge (m 3 /s) C : 1.7 Discharge coefficient L : Length of weir (m) h: weir height (m) g: 9.8m/s 2 gravity acceleration 38

53 H av=va 2 /2g Qmax H e H d Sediment h Weir Design flood levels at other place are calculated by Manning formula. However, it is possible to take some cross section and use non-uniform flow equation. dh i dx Q d 1 n V 2g dx A R 0 Where i :Bed slope h: Water depth (m) X: Length towards the downstream along the canal bed (m) Q: Discharge (m 3 / s) A: Cross-section area (m 2 ) g: Acceleration due to gravity (m/ s 2 ) : Coefficient of energy correction ( = 1.1, is used in general but = 1.0 may be used for simple calculation ) n: Coefficient of Manning roughness V: Mean velocity (m/s) R: Hydraulic Mean Radius (m) (5) Study of riverbed evolution To prevent the function of the headworks from deteriorating due to riverbed evolution, it is necessary to study future riverbed evolution in the vicinity of the proposed location of the headworks. 39

54 1) Summary It is extremely important to predict the future stable riverbed to decide the height of the water intake sill. 1 First, it is necessary to investigate the existing condition whether there is balanced retrogression or aggrandization, 2 Second, it is necessary to assess the variation of the bed that would occur when other structures are constructed upstream or downstream or when river improvement is carried out. 2) Natural conditions of riverbed formation The natural condition of the riverbed is the result of the following three elements. 1 Horizontal distance and the head from the source to the estuary, 2 Topographical, geotechnological and botanical conditions and erosion characteristics of the river basin, 3 Characteristics of rainfall distribution, discharge, volume, wave pattern and its frequency. Out of the above three conditions, condition 1 is a significant restraint in that the surface level of the tributary river must always be higher than the parent river or sea surface level. Rainfall Agro ecology or characteristic of the source of a river Discharge Collapsed sediment amount Channel erosion Sediment amount Riverbed formation downstream Fig. 3.3 Flow chart of riverbed formation downstream The flow chart in Fig. 3.3 describes the conditions 2 and 3 above. Usually local characteristics such as those of rainfall distribution and collapse of sediment amount are extremely indefinite. But the effect of the occurrence probability of these phenomena is rather 40

55 significant in upstream areas. Due to the slow speed in the migration of sediment downstream, the amount of sediment below the point of collapse gradually attains an average value. The change in flow from supercritical flow to subcritical flow would also promote the balance of sediment amount, with the downstream riverbed formed by a sediment amount controlled by the flow strength. Furthermore, the variation of local tractional forces would cause variation in grain distribution of sediment downstream. In this respect, grain size distribution of erosion products as well as wear occurring to the material in the course of moving downstream must be taken into account. 3) Effect of river structures on the riverbed Construction of any structure in the river (such as headworks, check dam, or bed compaction work, the execution of river improvement work or quarrying of gravels) will cause secondary variation of the riverbed (not due to the natural phenomenon, but rather artificial causes). This results in the formation of a newly balanced bed in response to the new condtions of flow. Generally, such secondary formation of the riverbed has the following tendencies: 1 Downstream of such a structures a lowering of the riverbed may occur as a result of change in river hydraulics forming the riverbed. This trend will gradually extend to downstream, 2 The bed upstream of such a structure tends to rise, 3 If the existing water intake weir downstream is demolished due to integration of the water intake with other facilities, etc., then the settlement of the riverbed would occur. This settlement will gradually will be extended upstream, 4 If a large volume of gravel is quarried from the riverbed, then settlement of the riverbed will gradually occur both upstream and downstreams, 5 If the tractive force is changed due to shortening of the flow route by river improvement, then it will affect riverbed formation, 4) Points in investigating riverbed evolution(it has to be in Ethiopia condition and simple way) It is necessary to check the following points to understand the riverbed evolution: 41

56 1 Planar contour of river The stability of the watercourse needs to get special attention. Rivers have a habit to stabilize their courses through meandering. In the investigation, it is necessary to check whether the existing river section is a simple section or a complex section, whether the course at the time of normal flow is regular or braided (forming a type of network), and where discharge passes at the time of flooding. Understanding ofthe existing conditions such as the formation of gravel dunes, etc in detail would help significantly in the study of riverbed evolution. The use of aerial photographs is helpful. The gravel dunes mentioned in this section may be defined as a contour of the riverbed beginning from the pool and ending at the sand bank. One half of the meandering shape is called a gravel dune. 2 Profile gradient Profiel gradient is desirable to obtain the correct profile of the river. Riverbed evolution tends to occur at the points where the profile varies, the profile gradient has an important value in forecasting future riverbed variation. In addition to the study of the profile gradient it is valuable to obtain information on the past variation of the gradient as well as the changes in grain diameters of the sediments in relation to distribution along the longitudinal axis of the river. 3 Profile gradient is also desirable to obtain the results of any sediment sampling tests conducted in the past. Such information can be used for analysis of the watercourse as well as providing valuable clues to understand changes in the watercourse. When taking samples take a record of the location, time and date of course. The information on past floods including the scale and number of recurrences and comparative information on the changes in the riverbed materials in the past and at present is very important to understand variation in the watercourse. 5) Method of assessing riverbed evolution (a) Necessary information Generally, the following items are necessary for analyzing river flow that contains sediment. 42

57 1 Flow section and existing riverbed gradient, 2 Roughness coefficient and grain size distribution of bed material, 3 If any structures for water use exist, then features of such structures, 4 Control section H-Q relations, 5 Assumed quantity of sediment, and grain distribution. Besides, consideration should be given to past records of discharge, bed evolution and the administration and operation of other water use facilities. However, all of this detailed information is rarely available. So it is important to examine the reason and justification of the results and the method of analysis of each item based on the purpose of the analysis and the available information. (b) Types of analysis There is an analytic solution when the flow is uniform flow or non-uniform with simple alteration of cross-section. In reality, it is rare to be able to adopt this kind of simplification. In general, the most commonly adopted method is to divide the river into short sections and to carry out the calculation on each section. This method may be classified into four types of calculations combining of various elements, i.e. either uniform or non-uniform flow for the basic flow formula, and either assumption of uniform grain diameter or calculation by each grain diameter for sediment load. The combination and the primary aims of these analyses are summarized in Table 3.1. ( I ) (II ) (III) Table 3.1 Analysis methods for riverbed evolution Points of Analysis Basic equations Transient fluctuation Basic equation of unsteady phenomena by time and by flow, Equation of tractive location of flow, riverbed sediment volume by particle and grain size distribution size, Continuous equation of Transient fluctuation phenomena by time and by location of flow and riverbed Fluctuation of riverbed and Tendency of predominance riverbed and grain size Basic equation of unsteady flow, Equation of tractive sediment volume by same particle size Continuous equation of riverbed Basic equation of non-uniform flow, Equation 43

58 (IV) large-diameter grain by location Fluctuation of riverbed assuming that constant-diameter of grains predominate in a section of the river of tractive sediment volume by particle size, Continuous equation of riverbed and grain size Basic equation of non-uniform flow, equation of tractive sediment volume by a particle size and Continuous equation of riverbed. Among the four combinations given in Table 3.1, (I) is appropriate to analyze the situation taking into consideration the flood characteristics at the time of a large scale flood or abnormal phenomenon generated by operation of the scouring sluice gate on a river in which the grain size distribution of the bed ranges widely. (II) is the proper method to assess, abnormal variation of the bed and flow when the gate is operated at points where the grain diameter is comparatively uniform, such as an estuary. (III) is for analysis of a relatively wide section of river or in case of retrogression with the armoring due to the disruption of the movement of sediments by a structure such as a dam in the upstream section. This situation assumes a local deviation of grain size. (IV) is the most common and the most concise and proper method for analysis or assessment of short sections. It should be sufficient to use either (III) or (IV) in the study of riverbed evolution or stabilization in an ordinary river. 6) Method of sampling of riverbed materials Samples of the riverbed materials provide the basic data and information for the estimation of the roughness coefficient, variation of riverbed and for decisions on the necessity of a settling basin. Although it depends on the scale of the river being investigated, the normal method of sampling is carried out at intervals of meters for several samples (2-3 points horizontally and 2 3 points vertically). Samples taken should Picture 3.1 Sampling of riverbed materials be subjected to the grain size 44

59 analysis and a check on specific gravities. The location of sample taking should be clearly marked on the relevant plan or map. The selection of the sampling spot and the method of sample taking should be in accordance with the criteria shown in Table 3.2. Selection or sampling point Sampling method Table 3.2 Key points 1 To avoid the curve portion of river where curvature radius is small, 2 To select the point near thalweg (lowest part of valley), 3 To avoid sandy gravel dunes and the place where scouring happens 1 To avoid collection of cobbles the site, which is outside the grain size distribution of the riverbed surface, To record particle size and distribution condition of these stones. 2 To collect only the riverbed surface portion after excluding cobbles. Surface layer thickness is that for the maximum grain size (90 percent diameter by weight). Average space is to be m. The sample taken is to be divided by quartering method and enough volume of sample to yield a representative value must be used, If grain size is small, 35 kg is the sample. However, if grain size is larger, larger sample must be taken relative to the grain size. 3 To collect sub-layer part of riverbed. Two sampling points are required at 1. 0m and 2.0m deep from surface of riverbed. Plan dimension of sampling is 1.0m 1.0m and thickness of sampling at each point is to be the nearly same as maximum grain size. Remarks Flow of river is not uniform, and the thalweg fluctuates from left to right swingingly. Due to the effect of secondary flows and locational difference of tractive force, significant change of particle size in cross section occurs. It is required to avoid it. Distribution condition of large-sized cobbles, which greatly affects the grain size distribution, is important data. Sampling from two portions, i.e., the riverbed surface and subsurface layer and analysis is of these two portions are especially important in case of considering selective traction and estimation of stable riverbed. 45

60 < Supplementary explanation > Riverbed materials is used for design and consideration of influence to upstream. Implementation will be site specific Position of Headworks The site for constructing a headworks must be selected in consideration of the river condition and the irrigation command area. The site should be such that the required water intake function as well as the stability of the structure and convenience for operation and maintenance are achieved. (1) Points of selecting Headworks position The site selected for the headworks should be the best possible place after studying the following points: 1 Availability of a stable thalweg close to the bank at the proposed position of water intake, 2 Sufficient water intake must be feasible even during the dry season, 3 Least sediment inflow during water intake, 4 Least effect of weir construction on up and downstream sides, 5 Stability of the structure can be expected with economical construction costs, 6 Conveniente for operation and maintenance, 7 Look for additional other referances such as IDD manual, Civil engineering hand book, etc. Basically, it is extremely important to satisfy item 1 from the above points for the selection of the position of the headworks. It is reasonable to consider the energy of flood. It is so enormous that any artificial means such as the adjustment of gates to divert the working direction of such energy or to attempt to divert the thalweg is practically impossible. Therefore, any sites where the position of the existing thalweg is remote from the bank or tends to move are not appropriate for the water intake. A stable thalweg can be regarded as having a stable riverbed against flood for many years and the 46

61 situation is expected to continue barring the implementation of large scale river channel works, and thus a consistent water intake can be anticipated. Therefore, the stability of a thalweg is crucial in siting a headworks. In investigating the stability of a thalweg, it is important to study conditions to a considerable extent both upstream and downstream concerning the history of the main flow channel in particular the condition of gravel dunes. The construction cost of the headworks will generally depend on the volume of intake, width of the weir and the condition of the ground on which the foundations of the structure will be set. The cost variation due to variation in the intake volume caused by the position of the headworks, variation in the river width, the geotechnical conditions for the foundations and the construction cost of the headrace channel must be carefully compared before the position of the headworks is finally decided. (2) Process of selecting Headworks position The following process must be checked to select the position of the headworks: 1 The headrace is planned when the maximum required water to the irrigation area is established. The position at which the water level in the river becomes equal to the one at the upstream end of the proposed headrace is found out. This position is the first assumption for the proposed headworks, 2 In the up stream of the first assumed position, the spot to ensure sufficient intake and satisfy the condition of sediment control during flood is generally the vicinity just downstream center point of the outer side of a loop in the river. This would be the position of the intake. Fig. 3.4 Appropriate Intake Position 3 The next step is to decide the design intake level. It should satisfy the requirement of 1 above as well as the required conditions for preventing sediment influx and for the settling basin, 47

62 4 To prevent sediment influx during water intake, the weir must be designed to make the flow velocity upstream of the headworks at 0.2m/sec. This condition can be achieved with a minimum of consideration and is recommended to be studied, 5 For the settling basin, it is safe to consider the necessary water head to enable natural desilting to be 3.0 meters or more, 6 For the prevention of sediment influx, it is desirable to set the height of the intake sill higher than the original bed at 1/6 of the maximum flood depth. It is important to decide the position and the intake level after comprehensive review of the above items. Attention should be given to the fact that even if the intake level is higher than the temporary level in procedure 1 above, it is still possible to absorb flow energy midway along the headrace by way of drop structures etc. It is further advantageous in that other structures (such as diversion, branching or inverted siphons, etc.) can be planned. Most problems in agricultural irrigation are caused from sediment inflow. Therefore, it is essential to place maximum emphasis on preventing sediment influx Method, Location and Type of Water Intake The arrangement of the selected water intake, position of intake and type of weir etc need to take into consideration the proposed area and its vicinity. (1) Method of water intake The method of water intake can be either natural intake, which takes in water at the natural water level of the lake or river or intake by weir constructed to maintain a constant water level. The selection must be either one of these two in consideration of various conditions of the river. It is important to adopt the natural intake including a stable water level in case of a lake. In case of a river, along with the requirement of a stable intake level, consideration should be given to the possibility of a drop in the water level due to scouring of the bed in front of the intake or inflow of sediment due to a rise of the bed. When intake is adopted by way of a weir, consideration must be given to a structure which secures the design 48

63 intake volume, prevents influx of sediment during the intake period, and that does not disturb the movement of sediment during flood. (2) Location of intake 1) In principle, the intake is installed directly upstream of the weir to take in water from the thalweg. In case of intake from a rivers it is common to get a considerable amount of sediment during the irrigation period and the piling of sediment at the scouring sluice in front of the intake at the time of flood. It is essential to select the location of the intake that can ensure flushing of sediment and easy the maintenance of the thalweg. Where there is no possibility of sediment inflow, there are cases when the intake may be established at a distant point while a weir constructed to maintain the required intake level. In this case, comprehensive study should be made in respect to future changes in the flow regime or thalweg due to the installation of a weir, 2) The intake of water at both banks is considered possible when the intake levels of both sides are almost equal, the intake site is at a straight segment of the river, river width is relatively narrow and sediment load is very low or when raising of water level by a movable weir is relatively large. However, in such cases problems occur on one side through large deposits of sediment despite the fact that the intake of water on the other side performs favorably. When the stability of the thalweg is sufficiently confirmed, however, intakes at both sides may be considered feasible as shown in Fig. 3.5, and merits study. Fig. 3.5 Example of both side intake (Inuyama headworks, Japan) 49

64 (3) Type of weir and direction of weir axis The decision on whether to adopt a fixed type or floating type weir will depend on the level of bedrock, necessity for a complete cut-off, and the condition of scouring. The weir can be constructed directly on the bedrock when the bedrock is relatively shallow or directly on the natural sediment or gravels on the riverbed when the bedrock is deep or does not exist at allthe former weir is called the fixed type while the latter is called the floating type. The first priority to be considered in selecting the weir type is the construction cost. The construction cost must be based on a thorough study on the necessity of complete cut-off and the safety against scouring. In principle, the weir should be constructed on a straight axis perpendicular to the river. The type should be selected taking into account the conditions of water control and utilization. The reason that the weir should be constructed at right angles to the river flow is due to advantages of economy and ensuring the flushing function of the scouring sluice. Weir types are: 1 fully movable, 2 combined weir which is partially movable and fixed, 3 fully fixed. The final selection of the weir type depends on the consideration of economy and ensuring the intake function. Classification of weir could be due to stability factors, construction material, control of surface flow, function and geometry of control section. Usually the economics is the main reason for the selection of weir type, which is influenced by the availability of sufficent construction material at close proximity and availability of skilled and unskilled labor at site and duration of construction time. It is to the designer to select the best one satisfying the maximum explained conditions. 1) Stability Factor (a) Gravity Weir: The weir depends on its weight for countering uplift pressure due to seepage. The weight of the weir body and/or floor is higher than the uplift pressure due to head of seepage water (subsurface flow) under weir. It is used on permeable soil. (b)non Gravity Weir: This type of weirs constructed on the piles (cut offs) and other pressure defusing mechanisms for it's stability against uplift force from the subsurface flow. It needs careful design 50

65 and reduces material cost for construction. 2) Construction Material factor (a) Rock-fill weir: Upstream and down stream rock fill laid in the form of glacis with few intervening core walls. This is economical when enough rock is available at site. It is simple in construction. Fig. 3.6 Rock-fill weir (b) Gabion weir: Gabion boxes filled with rocks. It is economical and easy in construction where rock is available in required amount at or nearby site. It is widely used for river training diversion, storage etc. Fig. 3.7 Gabion weir (c) Masonry weirs: Ti1e weir wall and the solid apron constructed from masonry wall embedded in cement mortar. It is easy in construction to provided the material and the skilled (mason) available at site. Fig. 3.8 Masonry weir (d) Cyclopean Concrete/Concrete weirs: The whole structure constructed out of concrete or for economical purposes. The inner most part is filled with cyclopean concrete provided that the structure is stabile. This type of weir is recommended for flow with high velocity and on permeable foundations. 51

66 Fig. 3.9 Cyclopean Concrete weir 3) Control of Surface Flow factor (a) Uncontrolled Flow: The incoming flow passes freely to the downstream apron from the uncontrolled weir. It is used in seasonal river or on river where regulation is not highly required. (b) Controlled (Barrage): The whole crest is provided with a gate to control (regulate) the flow. It is costly structure but efficiently regulates the flood with low afflux. It is used in seasonal on rivers where regulation is highly required. 4) Function factor (a) Storage: Construction for storage purpose. It is termed as low dam. (b) Waste Water: Spilling flood excess of pond capacity and is constructed to safeguard the main weir. (c) Pickup: Constructed across the river downstream of storage to raise the level of water released from the storage and divert it for utilization. (d) Diversion: Part of a headworks to raise water level in the river and divert supplies in to the off taking canal. 5) Geometry of Control Section (shape) (a) Sharp Crested: Thin walled over flow weir. Fig Sharp Crested weir 52

67 (b) Broad Crested: Thick walled over flow weir, constructed mostly on pervious foundation. (Rectangular, Trapezoidal) Fig Rectangular weir Fig Trapezoidal weir (c) Ogee Crested: The top and bottom downstream part of weir having a smooth curve is mostly constructed on a rocky foundation. Fig.3.13 Ogee Crested weir < Supplementary explanation > In principle, the weir should be constructed on a straight axis perpendicular to the river flow. However, the weir axis can be bended according to river and bed rock sediment situation.. Normal case The weir should be constructed on a straight axis perpendicular to the river flow. Special case According to condition of foundation and landform, the most economically axis should be selected. 53

68 3.1.4 Design Dimensions The dimensions of headworks such as the design intake level, design crest height of the weir, height of the sill and span length of the movable portion of the weir must be decided from consideration of design conditions. This design conditions are design flood discharge and in consideration of their effects up and downstream. (1) Design water intake level The minimum requirement of the intake level can be calculated based on the elevation of the fields to be irrigated. Alternatively, as the function of intake it should satisfy two points: ensuring the required intake volume and sediment control including flushing of sediment. This means after taking the required intake level for the irrigation area, the intake level should be decided to satisfy both sand protection and flushing functions. If any hydraulic surplus energy is expected, then it is better to consider to use such surplus for the improvement of the functions of the settling basin, diversion or branching. Adjustment of such hydraulic energy surplus may be considered by way of drop, etc. 1) Intake level required by irrigation area The data requirements for deciding the intake level from the irrigation area perspective comprise the following points: 1 Water level at the fields of the highest elevation of the irrigation area, 2 Design maximum intake discharge for the irrigation area, 3 Water level at the starting point of the headrace, 4 The hydraulic loss between the intake and the starting point of the headrace, 5 Other structural losses at the intake (hydraulic loss of entrance, hydraulic loss of exit, vertical gap of sill, screen and pier, etc.) 2) Intake level required for sand (sediment)-flushing function The following points should be taken into consideration to fix the intake level of sediment flushing: 1 From the view of sand protection, the approach velocity should not scour sediment that particle size greater than 0.3mm. In case 54

69 of a water intake with weir, it is recommended to design the average velocity upstream of the weir less than 0.4m/sec, 2 An intersection angle of intake streamline with river streamline is essential for sand protection during flood. Furthermore, the vertical height of the intake threshold above riverbed should in general be at least 1/6 of the maximum flood depth, 3 The upstream water level at the weir should be determined so as to satisfy the water level required to scour out the sediment. In general, such water level would coincide with the height of the intake threshold at rapid flow and the design intake level at sluggish flow, 4 Where a settling basin is required (in most cases, the water intake is necessary even during flood), the water level difference to satisfy the function of natural flushing must be decided accordingly. The design intake level must be decided after sufficient investigation of the previous points. < Supplementary explanation > It is necessary to consider both level: Intake level required by irrigation area, Intake level required for sand (sediment)-flushing function (2) Elevation of crest height of weir The crest height of a weir (top of the gate for movable weir or the top of fixed weir) can be obtained from the design intake level with necessary margin. About 10cm is usually given as the margin in consideration of head loss due to waves, blockage of screens of the intake and abrasion on the top of a fixed weir. In case of a combined weir consisting of a movable weir and a fixed weir, the crest levels of both types must be same or that of the fixed weir shall be made higher. However, in case of a weir constructed on a torrent where there is no possibility of erosion of the major bed caused by the constant overflow of the fixed weir, the crest height of the fixed portion may be lower than that of the movable portion. In the case of a combined weir, it may be advantageous and convenient for weir operation if the crest height of the fixed portion is lower than that of the movable portion to allow discharge 55

70 downstream during intake of water. Generally, the riverbed of a fixed weir is the major bed or similar that tends to be eroded by constant overflow. However, in the case of a weir installed on a mountain stream where the bed downstream of the fixed weir is formed of large-sized gravels etc. the height of the fixed weir is usually made lower for economy and easier operation to allow constant overflow. Even in this case, it is necessary to consider some marginal height for abrasion. < Supplementary explanation > Elevation of crest height of weir: = Design intake water level + Margin(10cm generally) (3) Ensuring creep length When constructing an intake weir on a permeable foundationthe creep lengths must be ensured to safeguard the weir: 1 Provision of creep length is necessary to control the flow velocity that prevent the destruction of the foundation caused by the action of seepage flow (i.e. "piping"), 2 The creep length control the volume of seepage resulted from the volume of water leakage as a result of percolation of water blocked by the weir,. 1) Prevention of pipingwhen running water is blocked by a weir constructed on permeable foundation, the difference of water head across the weir (ΔH) can act to move soil of minimum grain size as the water permeates through the ground. This can create voids in the ground that leads to the destruction of the foundation. This action is called piping. To prevent this phenomenon, a safe creep length must be ensured under the foundation of a weir and along the back of a retaining wall. The creep length to be ensured must be the larger of the values calculated by following two methods. 1 Bligh's method L C ΔH (F. 3.1) Where L : length of creep length measured along the foundation face of the weir (which may differ from the actual 56

71 percolation path) (m). C : coefficient which varies depending on the type of the foundation ground material (Table 3.3) ΔH: maximum head difference at upstream and downstream sides (m) 2 Lane's method Lane defined the effect of the horizontal creep length as 1/3 of the vertical creep length. He has established the weighted creep length by dividing the total of the vertical and horizontal creep length by the difference between water heads and defined the ratio as shown in Table 3.3. L' C' ΔH (F.3.2) Where L' : length of weighted creep Length (m), L' = Σv +1/3Σh v : creep length of vertical direction (inclination of more than 45 degrees) h : creep length of horizontal direction (inclination below 45 degrees) C : Coefficient which varies by the type of ground (Table 3.3) ΔH : maximum difference between water heads (m) ΔH adopts a value of big one among H 1 and H 2. 57

72 Fig Creep length Table 3.3 Coefficient for Bligh's and Lane's method Foundation Bligh's coefficient (c) Lane's coefficient (c') Silty sand or clay Finesand Medium sand Coarse sand Gravel Coarse Gravel Sandy Gravel 9 - Cobble stone with Gravel Rocks with Cobble stone and gravel Rocks with gravel and sand 4~6 - Soft lay Medium clay Heavy clay Hard clay (Average grain size of each foundation is shown in the Table 3.4) 58

73 Table3.4 Values of permeability coefficient Classification Clay Silty clay Silty sand Fine sand Medium sand Coarse sand Gravel d(mm) 0~ ~ ~ ~ ~ ~ ~5.0 k(cm/sec) Note) d: Average grain size in mm To ensure a creep length longer than L or L' obtained from the previous two calculations methods a cut off wall is normally provided. This cut off wall increases the vertical creep length in addition to the horizontal creep length on the weir structure and a downstream apron. The vertical creep is not only effective in controlling seepage but it is alsoeffective in countering uplift. Therefore, it is recommended to construct a cut off wall from the upstream end of the weir vertically into the foundation ground. The lengths of the weir structure and the downstream apron are given, as explained in Section "Spillway", as the minimum required lengths from the hydraulic design against the overflow of the weir. Thus, the required depth of the cut-off wall must be the balance of the required creep length less the lengths of the weir structure and the downstream apron. The depth of cut-off wall to be installed at the upstream end of the weir body depends on the grain sizes of the subsoil. If coarser than fine gravels, then the required depth would be equal to the water depth of uprise or more and if finer than coarse sand, then it should be about 1.5 times the water depth of uprise or more. Wherever sheet piling is difficult due to the presence of larger gravels, etc. the minimum length must be ensured by a cut-off through excavation. If the creep length is not sufficiently ensured due to difficulty or impracticability, then the required lengths must be ensured by extending the downstream apron, installation of more than two cut-off walls in parallel or by taking creep length on the upstream apron into account. In case of parallel walls, the spacing between the two walls must be more than the total length of the walls. If the spacing between the two walls is smaller, then the vertical creep length must not be taken as twice the height of the walls installed. The purpose of wall and sheet piling at the downstream apron end is to protect the weir body from scour. Weep holes must be 59

74 provided in the wall to reduce the uplift pressure.. These wall and sheet piles are not included in the creep length. Normally, the upstream apron is not included in the creep length. However, where the apron has to be included in the creep length caused by trouble in driving sheet piles, etc., water stops must be inserted in construction joint. In addition to this, dowel bars or key have to be made to withstand uneven settlement. Water stops and similar mechanism are necessary for construction joints or some other joints in the downstream apron. The difference in head ΔH is usually adopts a value of big one among H 1 and H 2, but consideration should be given to having some safety margin on rivers where retrogression is anticipated if possible. The abutment foundations of a weir with banks or embankments will usually be permeable. Therefore, a safe creep length must be ensured by way of cut-off walls on the extended line of the weir axis and on the retaining wall or at the foot of the upstream retaining wall. In this case, it is necessary to study the percolation path three-dimensionally. Installation of a cut-off wall in the direction of the weir axis is to ensure a safe creep length along the route of A - B - C shown in Fig The foot of the cut-off wall must be at the same level as that of the cut-off wall installed beneath the weir body with its top being at the same level as the crest height of the gate. On the intake side, the intake structure can form a part of the wall. If the construction of a wall between A - B is not practical due to the existence of the embankment, etc., then a cut-off wall between A and A' is required to ensure the required creep length from A' to C. It is not recommended to make weep holes below the seepage line of the downstream side retaining wall within the range of the required creep length. When the upstream side retaining wall is used for the cut-off wall, it is better not to provide weep holes below retention level if within the creep length range. < Supplementary explanation > When constructing an intake weir on a permeable foundation, it is the most important to consider protection of the destruction of facilities. 60

75 (4) Study of possible effect on the river control of upstream Carrying out hydraulic calculations is necessary to obtain the water level that will occur upstream of the weir with the design flood discharge. It confirms the absence of interference with water control and provide appropriate measures to ensure water control, if necessary. 1) One of the appropriate measuresis heightening the embankment, within the range of maximum height of 60 cm (considering acceptable height) or making the river wider so that design high discharge can flow under the condition that flap gates stand up. This height of 60 cm is also adopted as freeboard for wing wall. 2) Upstream backwater calculation (a) Method by calculation a) Standard step method This method is to repeat calculations for each section using the theory of Bernoulli. It starts from the location of the weir, based on the profile and cross sectional drawings, divide the upstream river into numbers of sections with each section generally having a uniform section andbed gradient and assume respective sections x 1, x 2, x 3, x n. To obtain the water level of each section, use the known value of the depth of the weir location, which is equal to the height of the water blocked by the weir, (h 0 ), as the basis of calculation. By repeating a 'series of calculations, h 1, h 2, h 3, hn should be established. To get the water depth at the end of the first section (h1), use formula (F. 3.3), and apply h, as temporarily assumed (h1'). By using trial and error method, repeat the calculations until h 1 h 1 ' or h 1 h 1 '<0.01 m is obtained. Then, using this h, as the basis, seek the value of h2 in the same method hereafter, continue the repetition of this method until h3. hn are obtained. For establishing a temporary value, it is convenient to carry out approximate calculations using the methods developed by Tolkmitt, Rühlmann and Bresse. The curve linking the calculated water levels of individual sections must be the backwater curve. 61

76 h i h i1 h i Z i Z i1 Q a g Ai A 2 R i R 4 / i A 4 / i i 1 A 1 i1 1 1 n 2 1 Q 2 x i Where (F. 3.3) Q : design flood discharge (m 3 /sec) Ai : flow area at (i) section (m 2 ) Ri : hydraulic mean depth at (i) section (m) ni : roughness coefficient at (i) section xi : distance between sections (m) zi : distance from the base line of (i) section to riverbed (m) a : correctional coefficient of the velocity head g : acceleration of gravity (m/sec 2 } Fig Values at section i The average roughness coefficient between sections where the roughness of the wetted perimeter varies in part obtained through the equivalent roughness coefficient at both ends of up- and downstream of each section. The average of two equivalent coefficients at respective sectional ends may be made the average roughness coefficient for the respective sections. The equivalent roughness coefficient (n e ) can be obtained with the exponential formula as follows: From Manning's formula, V = 1/n R 2/3 I 1/2, ne p1n1 p2n2 pnnn (F. 3.4) p Where, p is wetted perimeter 2/ 3 62

77 Technical Guideline for Design of Headworks p p p p p n Also, the following approximate formula may be used. n e 1 p n p n p p n n n (F. 3.5) Table 3.5 parts, roughness coefficient and wetted perimeter PART Roughness Wetted coefficient perimeter A B n 1 P 1 B C n 2 P 2 C D n 3 P 3 D E E F n n P n Fig roughness of each wetted perimeter is different The process of calculating the backwater by step calculation method is illustrated in the form of flow chart in Fig

78 Fig Flowchart of calculating back water < Supplementary explanation > Calculation of backwater curve helps to design wing wall. The height of wing wall has to be higher than design flood level. Wing wall protects overflow and property like house, crop land and it also protects erosion of the bank. When there is no property at flood influence area the purpose of wing wall is only protection of the bank. In this case, height of wing wall doesn t have to be equal to design flood level for cost minimization. Stability has to be checked for all case. 64

79 3.2 Detail Design Detail design deals with hydraulic and structural design of intake weirs, inlets, related structures and operation & maintenance facilities on the basis of the design criteria and dimensions, which are set up in the basic design Movable Weir A movable weir is a structure to secure the required water level for water intake and safe flow of water by means of gate operation. It ensure safety against the action of flowing water etc. and other relating external forces. A movable weir consists of a spillway and scouring sluice functioning to secure the required water level for water intake and to keep flood water flow smooth. In addition, the scouring sluice provided in front of the intake has an important role in preventing sediment inflow into an irrigation canal by timely removal of the sediment accumulated around intake. Important factors to determine the functions of the spillway include the sill elevation and its span. On the other hand, important factors in terms of functions of the scouring sluice are the sill elevation, flow per unit width required for removal of sediment, canal slope and height of guide wall. (1) Sill elevation of movable weir In order to fix the sill elevation of a movable weir attention must be given in principle to the naturally formed riverbed configuration. One of the problem in providing an intake weir on a river is the backwater of the weir. However, it is known empirically that there is supercritical flow domain in a river where the river slope is 1/140 or steeper. In the supercritical flow domain, backwater does not cause a serious problem. Also, as a concrete intake weir becomes much smoother in roughness than that of the river, and construction, taking into account this change in resistance (roughness effect) on the flow, permits raising of the weir elevation within a range above the original riverbed elevation without backwater occurrence. The fundamental requirements are that neither backwater nor silt deposit shall be caused by construction of the weir. The following matters should be taken into account from the design point of view: 1 The front configuration of the intake weir should be almost 65

80 similar in figure to the section of the natural riverbed, 2 The sill elevation should be so determined as to prevent backwater and silt deposit by the roughness effect of the intake weir. < Supplementary explanation > Moveable weir is expensive in current Ethiopian condtion fixed weir are preferable. (2) Spillway by movable weir The spillway should have a structure to release flood water smoothly and to prevent the effect of backwater as much as possible and to maintain the thalweg. 1) Sill elevation See Section (1). 2) Width See paragraph 3.2.7(4). 3) Weir body and apron In Fig. 3.18, lo should be more than twice the upstream water depth h 1 at the design flood level in order to have the weir function as a broad crested weir and to give the roughness effect. Also, the slope should be taken in parallel with the proposed site slope at the riverbed. Fig Explanatory profile of spillway 66

81 The length g, out of the length 0 at upstream side of the pier should be taken about three times the width of the pier and given an adverse slope of 30/100, the tip of which is penetrated about d 1 = 1.5m into the natural riverbed. The thickness of upstream apron t u should be about 1/2~2/3 of the thickness of downstream apron. The length 1 of the downstream weir body from guide frame of gate to the downstream apron can be obtained by formula (F. 3.6) and the thickness t by formula (F. 3.7). =0.9C.(F. 3.6) 1 D 1 Where 1 : length of downstream apron (m) D1 : height from apron surface of downstream end to gate crest (m) C : (Bligh's) C (according to Table 3.3) 4 H - H T 3 γ -1 f.(f. 3.7) Where T : thickness at an assumed point (m) ΔH: Water level difference between the top and bottom Hf : Head loss of seepage to optional point (m) γ : specific weight of materials of the weir and apron 4/3 : safety factor ΔH-H f shows the strength of uplift at an optional point, H f = (ΔH/S)S' where the length of path of percolation is S and the total length of the path of percolation is S. It is possible that the thickness at the downstream end would become extremely small in accordance with formula (F. 3.7). In this case the minimum thickness is assumed to be 50cm, which depends upon the configuration of the river and the headworks are given. The downstream end should be set at an elevation somewhat lower than the existing riverbed level to prevent scouring and riverbed degradation and should be terminated with sheet pile works. However, it should be noted that the sheet pile works are not for watertight purpose. As the weir body is absolutely protected from differential settlement, it is necessary to provide a suitable foundation work. 67

82 Fig Diagram for thickness of apron < Supplementary explanation This item can be used for both Movable and Fixed weir, if it is necessary to decrease the design flood level. It should be operated according to flood. (3) Scouring sluice A scouring sluice must be provided at the intake side. It can prevent sediment inflow into the canal as much as possible when drawing water. Scouring sluice gate can be removed sediment in a short time. 1) Sediment inflow The first requisite for the intake site is that the river channel, in Picture 3.2 Scouring sluice other words the thalweg, should be stable. Sediment inflow to the canal would take place in the following cases: 1 If the intake site is not correctly sited in relation to meandering of the river, 2 If water intake is required even during floods, 3 If the intake is located too close to the riverbed elevation and the intake flow velocity is large. To avoid 1 mentioned above, fix scouring sluice just downstream of a concave bank center as a suitable site for a smooth removal of sediment. Item 2 mentioned above is a multipurpose water intake 68

83 case requiring water intake even during flood such as for water supply, hydropower, etc. In case of item 3, when the intake flow velocity exceeds 40cm/Sec, sediment grain sizes of 0.3mm tend to move. Hence, it is desirable to dam up water by the intake weir so as to make the intake flow velocity smaller. It is necessary to study the relationship with construction cost including provision of a settling basin. 2) Sediment deposition in front of intake Sand particles start moving when the force of flowing water exceeds the limit of tractive force.. As a result the riverbed surface takes various forms, such as ripples, dunes and plane-bed antidunes. The most typical one may be considered to be a dune. When the sand particles of the riverbed move in the form of a dune they make continuous movement on the riverbed not as single particles but as a mass. In such a case, much roughness effect by concrete cannot be expected even with a lower concrete floor provided on the riverbed. However, with a decrease in flood discharge, the traction effect on the concrete floor gradually appears although not perfectly. In order to ensure the complete traction effect, it is best to make the concrete floor elevation the same as the existing riverbed elevation and to provide a canal style sluice. In other words, there is a need to provide the scouring sluice with a canal in front of the intake and this has been confirmed empirically. However, if the scouring sluice is provided at a much lower elevation than the existing riverbed, then sediment will be accumulated there. Consequently, making the sill elevation lower than the existing riverbed level must be avoided. However, if a lower sill elevation is unavoidable, then the sill elevation must be determined so as not to cause deposition through examination by hydraulic model experiments, etc. 3) Basic concept of hydraulic design for scouring sluice The basic concept of the hydraulic design for a scouring sluice to be provided in front of the intake is as follows: 1 The flow for removing sediment should be supercritical flow, 2 The scouring sluice should have the capacity to remove sediment distributed around the site of headworks at rapid stream river, 3 The elevation of the scouring sluice must be determined in 69

84 principle at about the same as the water course elevation of the existing riverbed, 4 In general the slope of the scouring sluice should be constant, but the slope in the downstream portion could be larger depending on the riverbed slope and the water level downstream, 5 In the case of rapid stream river (River slope 1/800), Standard flow quantity of sand washout is the river discharge at the movement limit of the average grain size (60 percent passing by weight). In the case of slow stream river (River slope < 1/800), Standard flow quantity of sand washout is the average discharge Qm during the irrigation period for gentle streams. It is necessary that the removal of sediment is performed in a short time in case of 1. The supercritical flow must be straight and plane. For 3, the water course elevation must be carefully examined for implementation of the river improvement. 4) Method of hydraulic design of scouring sluice for rapid streams (a) Design of scouring sluice intake In the condition that the flow within the scouring sluice should be supercritical, the critical flow will be caused at the intake. And the design should be made to transport the maximum size particles of the riverbed materials with this critical flow. The critical velocity required to transport sediment (Vc) can be given by the, formula (F. 3.7) experimentally. The critical water depth (hc) and the flow per unit width (qe) can be given by the formula (F. 3.8) and (F. 3.9), respectively. Ve = 20d e..(f. 3.7) h c =20de /g..(f. 3.8) qe = (20d e) 3 /g 2..(F. 3.9) Where de : maximum grain size of riverbed material (90 percent passing by weight) (m) g : acceleration of gravity (m/sec 2 ) Ve: critical velocity (m/sec) hc: critical water depth (m) qe: critical flow per unit width (m 3 /sec/m) 70

85 The height of the guide wall H required to form a channel for the scouring sluice is made 1.5hc at the point of intake (Fig. 3.20). Fig Explanatory profile for upstream canal of scouring sluice (b) Design of upstream portion of scouring sluice The length of upstream scouring sluice 1, in Fig can be obtained from the formula (F. 3.10). 1 =S++1.5Hs..(F. 3.10) Where : width of intake (m), S : space between upstream end of scouring sluice and downstream end of intake (m) Hs : difference between elevation of the bottom of channel for scouring sluice and design intake level (m) The elevation at point A of the inflow of the scouring sluice in principle should be almost the same as the existing riverbed elevation. The inverting slope of 30/100 should be given to upstream from point A in penetrating into the depth of about 1.5m from the surface of the riverbed. The condition in the canal must be given to a slope i so as to secure the supercritical flow closer to the critical flow between sections I - II, although depending upon the downstream water level. The value i can be obtained by the following formula (F. 3.11): 71

86 h c n ghc i h 1.5h 2 c 10/ 3 1 2h (F. 3.11) hm The second term of the right side of the formula (F. 3.11) represents the energy gradient I e required for traction of the deposited sediment within the canal of the scouring sluice. The value I e should be larger by the energy loss to transport a volume of sediment than the critical flow I e corresponding to the hc in the formula (F. 3.12). On the other hand, I e at the fixed floor will be smaller than the energy gradient I tg for the critical traction against the average grain size. Where, I e from the Manning's formula of the average flow and I tg from the simplified formula by Iwagaki can be given in the formulae (F. 3.12) and (F. 3.13) respectively. Ie = n 2 g/hc 1/3 (F. 3.12) Itg = d m /h m (F. 3.13) Consequently, as the scope of condition to represent item 2 of the formula (F. 3.11) as the formula of (F. 3.14), i can be obtained by the formula (F. 3.11). Ie 2 3 n g hc 10/ 3 hm I tg..(f. 3.14) where n : Manning's roughness coefficient hc : critical water depth at the intake of the scouring sluice (m) which can be obtained by the formula (F. 3.8) h : depth of the section (at guide frame of the gate) on the downstream side of the canal (m) hm : (hc+h)/2 (m) dm : average grain size of riverbed materials (m) The water depth of downstream side h should be equal to the maximum grain size d 1 or larger of the riverbed materials. If the water depth is too small, the top of the gravel to be traced would be exposed above the water, and thus the tractive force will be reduced. The n value that determines Ie can be in a range of 0.017~ However, the water depth of downstream side h should be studied under the condition as n = in i obtained by using the formula (F. 3.11) to confirm that h d 1 is established and finally the slope 72

87 of the upstream side of the scouring sluice should be determined. The slope i is commonly about 1/100. (c) Design of downstream portion of scouring sluice For the downstream portion of the scouring sluice, the length 2 in Fig should be about 1.5 times of the channel width. Also the length from upstream to downstream end of channel should be designed about times of the design 'water depth h 3 of the downstream side as flushing (assuming that h 3 ' h 3 ) must be C and the channel bed between CD must be level. The length e 3 is the distance of the hydraulic jump when the water depth h 3 at the downstream side is at a conjugate water depth. The slope i 2 between B and C can be determined so as to conjugate the water depth h 2 and the downstream water depth h 3. In other words, the design should be made to satisfy the formula (F. 3.15). h 2 2 i h x h 3 F 2 (F. 3.15) 2 Where i 2 and 2 are given by the energy formula and continuity formula and Manning s velocity formula (F. 3.16). i qe n qe h h / 3 2 g (F. 3.16) h h2 h h2 Where h : water depth at the section II (m) x : difference between downstream water level and water level of depth above gate bed (m) h2 : water depth at section III (m) F2 : Froude number to h2, F2 2 = qe 2 /gh To obtain i 2 first h 2 is calculated by above formula for F 2. Assumed F 2 = 1.75 and calculation is carried out by the formula (F. 3.16), which is necessary to satisfy i 2 i (slope upstream from point B). The length of the guide wall on the downstream side is defined as the distance to the downstream end of the channel. The minimum height is downstream water level plus 0.50m in sand flushing while the maximum is the water level of the critical water depth hci at the 73

88 scouring sluice gate base when the gate is opened to the full width at the intake water level. 5) Hydraulic design of scouring sluice for moderate stream For a moderate stream, the grains size of the riverbed materials is small and the slope is extremely lenient. Small sand particles tend to move by small force of water and because of the lenient riverbed slope it also means' that the difference in energy between upstream and downstream is too small to take the required slope. Hence, the design should be based on supercritical inflow the scouring sluice. Fig Explanatory profile of scouring sluice In Fig. 3.22, the canal base hydraulic head y to maintain supercritical flow can be obtained by the formula (F. 3.17) ghc 10 / 3 h m v n y h 1.5hc.(F. 3.17) 2g Where hm = (hc+h)/2, qc = V h (flow per unit width) hc = (qc 2 /g) 1/3 i =y/ Although it is desirable to make the Froude number more than 1.75 for the water depth h in the formula (F. 3.17), assuming to be

89 temporarily h -0.5h, and V = 1.75 gh can be established. In other words, by giving values hs, n and the value y can be obtained. In case of the moderate stream, the storage volume upstream of the weir is generally large. It can be considered to flush sediment by means of the gate operation with the storage energy utilized. Consequently, when the width of the scouring sluice is so small that it is treated as a dead cross sectional area of the river, it is necessary to examine thoroughly the possibility of the storage energy to satisfy the formula (F. 3.17). When the storage energy is a prerequisite, the upstream guide wall must be made higher than the intake water level. The yardstick of the hydraulic condition that does not affect the intake function by this method is the width of the scouring sluice is larger than the width of the intake. hm 10/3 Fig Profile of scouring sluice in sluggish stream 6) The width of scouring sluice The unit discharge qe (rapid stream river : River slope 1/800) or qc (slow stream river : River slope < 1/800), should be determined first by formula (F. 3.17), and then the river discharge Qs at the movement limit of the average grain size (60 percent passing by weight) and the average discharge Q m during the irrigation period for moderate streams. In other words, assuming that the width of the scouring sluice is B s and B m for rapid stream and moderate stream, respectively, the width can be given by the following formula: Bs Qs/qe.(F. 3.18) Bm Qm/qc.(F. 3.19) It is desirable to make the width less than 1/2 of its length. 75

90 7) Data required for designing 1 Flood levels in the past and their probability, 2 Seasonal river discharge fluctuation, 3 Water level-dischargecurve (H - Q curve), 4 Grain size (90% size and average size), 5 Downstream water level in sand flushing, 6 River slope. [Reference 1] The river discharge at the transportation limit of average grain size; first the water depth of the average grain size transportation limit must be obtained by the formula (F. 3.20). hsc=u*c 2 /gi.(f. 3.20) Where U*c is friction velocity at the limit of the average grain size transportation, and can be obtained by Iwagaki's simplified formula: U * c 2 =80.9dm dm 0.303cm (F. 3.21) Where dm : average grain size of riverbed (m) Next, in the H-Q curve obtained from h = (q 2 /gf 2 ) 1/3 the value q where h = h sc is the discharge per unit width for the limit of average grain size transportation. The approximate value of the river discharge can be obtained by multiplying discharge per unit width value by the width of the target river. [Reference 2] Example of calculation of scouring sluice canal for a rapid stream; (i) Design conditions River width B = 250m, river slope i = 1/300, average grain size of the river dm = 0.05m, maximum grain size (grain size of 90% passing by weight) d 1 = 0.30m, ordinary river discharge Q = 130~170m 3 /sec, water level of downstream of the weir at the ordinary water level EL = 34.00m, length of scouring sluice canal at the upstream side of the weir = 40m (planned), original riverbed elevation at the inlet of scouring sluice EL = 34.00~34.50m and length of the channel for the scouring sluice at the downstream side of the weir = 20m (planned). The flood discharge of the river is 4.0 m 3 /sec. 76

91 (ii) Assumption of the river discharge at the limit of average grain size transportation. The Froude number Fr can be approximately calculated by the formula below: F r i i (F. 3.22) The friction velocity at the limit of average grain size transportation U * c 2 = 80.9dm = (cm/sec) 2 (Iwagaki's simplified formula) The water depth at the limit of average grain size transportation hsc = U * c 2 /gi =124cm Estimation of water depth hs and roughness coefficient n for discharge per unit width: h3=(q 2 /gfr 2 ) 1/3 (F. 3.23) n = h3 5/3 i 1/2 /q (F. 3.24) Figure 3.23 shows the results of h 3, n and q estimation in a graphic form. From the graph it can be seen that the discharge per unit width at the limit of the average grain size transportation q and the roughness coefficient n, are q = 2.9m 3 /sec and n = , respectively. Assuming that the river width is 250m, the river discharge at the limit of average grain size transportation can be estimated at 725m 3 /sec. 77

92 Fig Water depth and roughness coefficient for unit width discharge (iii) Study of sand flushing capacity at the inlet of scouring sluice and channel width The limit water depth hc and discharge per unit width qe at the inlet of flood sluice, are 3 / 20dl g hc = 20 dl/g, qe= 2 By substituting dl=0.30m and g=9.8m/sec 2 for the above formula, hc = 0.612m, qe = 1.5m 3 /sec/m can be obtained. The height of the guide wall is H= 1.5 hc = 0.918m. And the correction is made intaking into account the execution and safety factor, as follows: 3 H=10m, he=0.666m, qc= ghc = 1.7m 3 /sec/m It becomes necessary that the channel width of scouring sluice should be made smaller than 100m and 426m, respectively, under ordinary discharge of the river 170m 3 /sec and the discharge 725m 3 /sec at the limit of the average grain size transportation. It is understood that designing can be made within 20m (B 20m). When the width is smaller than a half of its Length, from the viewpoint of sand flushing function of the scouring sluice, the direction of the flow can be controlled more easily. From this viewpoint, it is desirable to make the width of scouring sluice about 20m in the example. 78

93 (iv) Design of upstream slope of gate The condition for determining the slope of a supercritical flow canal can be obtained by the formula (F. 3.12), (F. 3.13) and (F. 3.14). n h 2 g 1/ 3 c n h 2 3 ghc 10/ 3 m h m 2 d m Where hm = (hc + h)/2, dm = 0.05m, hc = 0.666m. In assuming the water depth at the downstream end of the supercritical flow canal h and the n value of the canal, Table 3.6 can be obtained. Table 3.6 Relationship between Roughness Coefficient and Water Depth (n-h) h d m n hm > ghc 2 /hm 10/3 (m) hm n=0.017 n=0.018 n= /153 1/262 1/229 1/ /147 1/228 1/204 1/ /141 1/199 1/177 1/ /135 1/172 1/153 1/ /129 1/147 1/131 1/ /123 1/125 1/112 1/ /117 1/160 1/95 1/77 In Table 3.6, the water depth at the downstream end of the scouring sluice channel h becomes h = 0.40m to give a value of grain size larger than that with maximum grain size in the sediment to be flushed, taking into account the energy loss due to sediment transportation and the roughness coefficient n = Consequently, the length of channel 1 is 1 = 40m and the slope of the channel bed is obtained from the formula below: n i h hc ghc / hc 10/ 3 h hm Also, the critical slope becomes ic = n 2 g/hc 1/3 = 1/275 thoroughly satisfying the supercritical flow conditions. The downstream end of the supercritical flow or the gate base elevation is as follows; i 1 = 40/64 = 0.625m = m (where the inlet base elevation = 34.00m.) 79

94 (v) Design of scouring sluice channel at the downstream side of the gate The condition for the supercritical flow canal with the required tractive force can be shown as follows. h2 2 i 2 2 h x 1 8F2 1.(F. 3.25) 2 Here, the difference in elevation between water level on the gate sill and that of the downstream. The water level on the gate sill is = m x = = 0.225m where the length of downstream channel 2 is made 2 = 20m, and the channel slope i 2 with the required tractive force can be obtained from the formula shown below in assuming the water depth at the end of the channel h 2 ; i qe 1 1 n q e h h / 3 2g h h.(f. 3.26) 2 h h2 where qe = 1.7m 3 /sec/m, n= 0.018, F2 2 = qe 2 /gh2 3, h = 0.40m 2 From a trial calculations 2-3, the condition is satisfied where h 2 =0.45m. The slope of channel base i 2 becomes i 2 = 1/71.2. However, since i 2 <i 1, i 2 is made i 2 = i, i.e. is = 1/64. The height of the downstream guide wall H 2, when the sedimentation is deemed to occur at downstream of the weir, can be obtained from the following formula: Provided that H 2 is based on the gate sill as a datum level. H 2 = {(design intake water level) - (gate sill elevation)} 2/3 (F. 3.27) This H 2 is the value that becomes about the critical water level on the gate sill when the upstream water level is at the design intake water level. 80

95 (4) Pier The pier is structure that the gate operation in opening and closing can be easy,stable mechanically and the obstacles due to the flood flow can be minimized. 1) Height of pier The height of pier is determined by the method shown below. (a) Hoisting gate type The top elevation of a pier is determined by the following formula. Top elevation = design flood water level + freeboard1 + gate height+ freeboard2 + thickness of plate Where, design flood water level = design flood water level at the upstream side of the weir Freeboard1 = distance between design flood water level and gate sill, which must be larger than approach velocity head in flood, Freeboard2 = distance between gate crest and the bottom of crest plate. When the gate includes such structures, as spoiler, screen, stopping hook, etc. and freeboard of hoist. Generally, about m will suffice. Fig Explanatory profile for crest elevation of pier 81

96 (b) Flap gate weir The crest elevation of a buttress or pier is determined by the following formula: Crest elevation = gate crest elevation + freeboard 3 Where freeboard 3 = height required for operation of the gate and installation of the sill plate, which is generally m. 2) Thickness and length of pier The thickness and length of pier is determined as to minimize obstruction to the discharge of floods and to secure mechanical stability for various conditions of gate operation. River t p :Thickness : Length Fig Thickness and length of pier (a) Thickness Generally, 1.50~3.00m thickness is sufficient. The thickness is first estimated prior to designing by the empirical formula (F. 3.28) in taking the height and span length of the pier as parameters based on the empirical data and determined after examining the stress, etc. tp = 0.12(Dp + 0.2Bi)±0.25m (F. 3.28) Where tp = thickness of pier (m) Dp = height of pier (m) Bi = span length (m) It is desirable that the blockage factor of the pier against the flow (the ratio of the total of the thickness of the pier into the water surface width at the design flood water level) should be smaller than 10%. (b) Length The length of a pier in the direction of the thalweg should provide sufficient stability for the pier. A trapezoidal section is generally advantageous for the bottom part of the pier. A rectangular section can be adopted for a pier if used also as large scale bridge piers for 82

97 case of concrete construction and other construction works. If a trapezoidal section is used, then the upstream side should be made vertical with a slope of about 1:0.4 on the downstream side. (c) Others In determining the thickness and length of the pier, it is necessary for the engineer to consider the harmony of the whole structure in regard to height, spacing, bridge, etc. 3) Shape of cutwater and section (a) Height of cutwater Generally, the cutwater should have a freeboard of m above the design flood water level on the upstream side. When cutwater is used as scaffolding for inspection, maintenance, repair, etc. of the gate, the height should be determined taking into account the height required for such use. When a bridge is constructed above the downstream side the height of the bottom of the girder is made larger than the space required by the structural ordinance (the same height as the bottom of the lift gate). If no bridge is constructed, then a height of about m from the design flood water level of downstream is enough. (b) Arrangement The arrangement in horizontal section of a pier below the cutwater is semicircular or a spindle shape toward both the upstream and downstream sides of the pier so as not to cause a vertical inflow time of flood. If debris flow is violent in flooding, then the design of the upstream side must be semicircular. In such a case, it is best to take measures such as installing an iron plate to protect the pier from wear or providing vacuum treatment, etc. Fig Example of pier with hoisting type gate 83

98 4) Safety analysis (a) A pier must satisfy the following stability conditions: 1 If the gate is opened during flood, then it is stable against action of wind load from both upstream and downstream, 2 If the gate is closed at the low flow, then it is stable of both upstream and downstream side against seismic forces in the direction of downstream. In other words, when the hydrostatic pressure and the dynamic water pressure due to earthquake are acting on the entire height of the gate on upstream side and the water pressure at the lowest water level on downstream side (normally 0), 3 If the dam is empty, then it is stable from both upstream and downstream against the actions of earthquake in the direction of upstream, 4 If the dam is empty, then it is stable in the direction of the axis of the weir against actions of earthquake in the direction of the axis of the weir, 5 If the gate is opened in wet season, then it is stable in the direction of the axis of the weir against actions of earthquake in the direction of the axis of the weir, 6 Stability against the internal stress of each components of the pier. (b) Pier stability a) Design conditions (i) Unit weight of concrete Reinforced concrete W 1 = 2.5 (t/m 3 ), plain concrete W 1 = 2. 5 (t/m 3 ), (ii) The seismic force acting upon the pier is expressed in the value of the weight of the pier multiplied by the design seismic coefficient and is considered to act horizontally. In Japan, the design seismic coefficient is set at K = 0.2 (severe earthquake area) and 0.1 (light earthquake area), (iii) Allowable compressive stress of concrete is set at σ ca σ ck /3, σ ca σ ck /4 55(kg/cm 2 ) can be taken in case of plain concrete. where, σ ck : design compressive stress of concrete, (iv) Allowable shear stress of concrete must be smaller than the values shown in Table

99 Table 3.7 Allowable shear stress intensity (Kg/cm 2 ) Design compressive stress intensity σck (kg/cm 2 ) or over Without calculation of Beam diagonal tension bar, τa1 Slab With calculation of diagonal tension bar, τa2 Shear stress only* * : In case of considering effect of torsion, it is permitted to increase these values. (v) Allowable tensile stress of reinforcing bars by σsa(sd30) = 1,800 (kg/cm 2 ), σsa (SD35) = 2,000 (kg/cm 2 ) use the following values when the bars are subjected to repeated loads:σsa (SD30) = 1,600 (kg/cm 2 ), (SD35) = 1,800 (kg/cm 2 ) (vi) Bond stress of reinforcing bars must be smaller than the values shown in Table 3.8. Table 3.8 Allowable bond stress, σ oa (Kg/cm 2 ) Design compressive stress intensity, σck (kg/cm 2 ) or more Round bar Deformed bar (vii) Coefficient of uplift When the pier is founded on rock or the foundation is surrounded by sheet piles that have reached an impermeable stratum μ=0.4 or in other cases than the previous μ=1.0 (viii) Dynamic water pressure strength p 7 w0 kh H h..(f. 3.29) 8 d Where wo : unit weight of water (t/m 2 ) Kh : design seismic coefficient 85

100 0.2 (severe earthquake zone area), 0.15 (weak earthquake zone: Ethiopia) H : total head, but in this case, total height of the gate (m) h : optional water depth from top of the gate to movable section base (m) Resultant force of dynamic water pressure P 7 12 w 1 0 k h Bm d (F. 3.30) H 2 Acting point of the resultant force of dynamic water pressure Yd = (2/5) H (F. 3.31) b) Type and acting point of external force (i) Dead weight W= w V Where w: unit weight (t/m 3 ), refer to Design conditions in section (b) a) (i) above for concrete V: volume of the pier (m 3 ) (ii) Weight of hoist W 1 : to be separately calculated. (iii) Weight of operating bridge W 2, : to be separately calculated, (iv) Weight of the gate W 3 : to be separately calculated, (v) Water pressure acting on the gate 2 H H 0 Bm 1 2 Pg w0 (F. 3.32), 2 (vi) Dynamic water pressure acting on the gate P P B d1 d m... (F. 3.33), (vii) Water pressure working on the pier P 1 2 g w H H tp... (F. 3.34), 2 Where tp: thickness of the buttress(m) (viii)earth pressure due to Sediment 1 2 Pe C0γ eh e (Bm tp)... (F. 3.35), 2 Where Co : coefficient of earth pressure = 0.4~0.5 γe : unit weight of deposited silt in water (t/m 3 ) He : height of deposited silt (m) 86

101 Fig Water pressure received by gate Fig External force and arm length (ix) Wind load The wind load is assumed to be acting on the net projection area of body and the value is multiplied by the form factor of the following: for vertical projection: area 300 (kg/m 2 ) where form factor is made as follows: for plane form : 1.2 for truss form, on the wind ward : 1.6 for truss form, on the leeward : 1.2 for cylindrical form (one piece) : 0.7 (xi) Horizontal forces due to earthquake, KW, KW 1, KW 2, KW 3 (xii) Uplift 1 U ( H Ho) μ w o A (F. 3.36) 2 Where μ : uplift coefficient A : bottom area of the pier (m 2 ) However, in the case where the uplift remains at the end of the pier, p is calculated as a trapezoidal section. The X-X' axis of datum line 87

102 is given to 'the pier section and shows the acting points, of external force as X and Y, respectively. This distance is called the acting point distance and obtain Y value for horizontal force and X value for vertical force. c) Stability analysis Moment is calculated from the above external forces and the acting point distance as well as internal stress and the following items are examined: 1 Safety against overturning 2 Safety against sliding 3 Safety against settlement 4 The stress of each member within the allowable stress First, the resultant force is obtained from the formula below: ( V X ) Xo = (F. 3.37) V Yo = (H H Y ) (F. 3.38) M= (V X)+ (H Y).(F. 3.39) Where Xo: distance between acting line of resultant force of vertical force and datum point (m) Yo: distance between acting line of resultant force of horizontal force and datum point (m) ΣV: resultant force of vertical force (t) ΣH: resultant force of horizontal force (t) Σ(V X) : total of the moments due to vertical force (t m) Σ(H Y) : total of the moments due to horizontal force (t m) ΣM: total of moments or the moment due to resultant force (t m) (i) Analysis on overturning In the case where the pier is of plain concrete, it will be safe as long as the acting point of the resultant force against bottom surface is within 113 of the center. It is necessary that the eccentric distance obtained from the formula (F. 3.40) satisfies the condition of the formula (F. 3.41). 88

103 M L e -.(F. 3.40) V 2 L e.(f. 3.41) 6 Where e= eccentric distance (m) L: length of the base (m) If the pier is of reinforced concrete, the condition of the formula (F. 3.42) must be satisfied at each support. ΣMr > ΣMt..(F. 3.42) Where ΣMr : total of the resisting moments against overturning (t m) ΣMt : total of the moments against overturning (t m) (ii) Analysis on sliding If the foundation is of concrete, the shear stress τ max acting on a construction joint face is obtained from the formula (F. 3.43). It is safe as long as the value obtained is within the allowable shear stress of concrete. 3 H τ max=..(f. 3.43) 2 A Where τmax : maximum shear stress (t/m 2 ) A : bottom area(m 2 ) This is sufficiently safe if the foundation is on bedrock or gravel, but the condition of the formula (F. 3.44) must be satisfied against sliding. α V> H (F. 3.44) Where α : coefficient of friction against ground (iii) Analysis on settlement The compressive strength at the bottom is obtained from the formula (F. 3.45) and it is normally safe as long as the value is within the safe bearing capacity of the foundation ground or foundation concrete. V 6e p= 1 (F. 3.45) A L 89

104 Where p : compressive strength caused at both ends of the bottom (t/m 2 ) (iv) Analysis of stress of each member The stress intensities of concrete, reinforcing bars, etc. that constitute the pier are obtained, and then it is confirmed that respective values obtained are within the allowable stress. With regard to the overturning analysis (i), although no problem is involved in the directions of upstream and downstream, there may be a case where the above condition cannot be satisfied in the direction of the weir axis due to the constricted width of the pier as the width of it may be determined to some extent not to constitute an obstacle at the time of flood. In such a case, the footing must be designed to keep the vector of the resultant forces within the base, and the tensile or compressive forces in the reinforcing bars used in both sides of the pier within the range of the allowable stress, if the vector of the resultant force cannot be kept within the middle third of the base. According to actual cases, the diameters of reinforcing bars in many cases are φ16-25mm for main bars and φ13-16mm for distribution bars. It is best to determine the spacing at about 10-30cm generally for main bars. Types of reinforcing bars used mainly are SD-30 or SD-35 according to actual cases, but it is desirable to select them based on a thorough investigation of production quantities, prices, etc. < Supplementary explanation > This stability item can be used for not only movable weir but also fixed weir. 90

105 3.2.2 Fixed Weir A fixed weir is a structure to secure the required water level at the time of intake, to avoid a considerable obstacle to floods and to have a section that is safe enough against external forces and is advantageous from the hydraulic point of view. (1) Section shape The sectional shape of a fixed weir body is usually trapezoidal, being vertical or close to vertical on the upstream face and with a gentle slope on the downstream face, giving some width on the weir crest. Where there is a flow of stones, it may be desirable to make the slope of the upstream face gentle and that on the downstream face steep as in case of a debris barrier so as to protect the weir body from damage due to falling stones. It may also be considered that for the purpose of hydraulic energy dissipation the overflow water is allowed to drop vertically to the apron without providing deflector blocks at the toe of the downstream slope. The basic shape is supposed as the shape stable dynamically based on height of the fixed weir (h) and water depth on the crest (H d ). There are Bligh s formula (F. 3.46, F. 3.48) and Etcheverry s formula (F. 3.47) to assume the shape. 1) The top width of weir (supposition of section dimension) Bligh s formula ; B = (H d + H av ) / γ..(f. 3.46) Etcheverry s formula ; B = 0.552( h + (H d + H av ))..(F. 3.47) Where B : Top width of weir (m) H d : Water depth on the crest (m) H av :Approach velocity head = V 2 /2g (m/s) γ : Specific gravity of the material of weir (2.35) h: Weir height 2) The bottom width of the weir (supposition of section dimension) Bligh s formula ; L = (h + H d + H av ) / γ..(f. 3.48) Where L : Bottom width of the weir (m) These formulas are estimating formula and it has to be considered these dimension increasing in reference to the conventional results appropriately. 91

106 (2) Type of fixed weir There are two types of fixed weir, one is fixed type the other is floating type. 1) Fixed type : When the bedrock is not scoured by running water, the purpose is achieved only with a weir. Therefore, in this case it is unnecessary to construct apron and riprap. This case is the safest and the cost of construction is cheap too. 2) Floating type : When the foundation is permeable ground such as grit and the bedrock that is easy to get scouring, in this case it has to construct apron and riprap, which is connected with weir to prevent scoring of downstream of weir and piping for safety. (3) External forces The external forces acting on the weir body include dead weight, hydrostatic pressure, dynamic water pressure, uplift, seismic force, sedimentation water pressure, etc. 1) Dead weight In the calculation of the dead weight of the weir body concrete to be used for stability calculation, it is desirable to determine the specific gravity by measuring the sizes of aggregates. Generally, approximate specific gravities are as follows: reinforced concrete: 2.5 (24.5KN/m 3 ), plain concrete: 2.35 (23KN/m 3 ), and cement mortar: 2.15 (21KN/m 3 ). To calculate the dead weight of the weir body, it is convenient to calculate by dividing the trapezoidal section into triangles and a rectangle. In Fig. 3.29, the specific gravity of weir should be taken as γ, the unit weight of water as w 0, (9.8KN/m 3 ), dead weight of weir body as W(t), dead weight of each divided portion and respectively as W 1 (t), W 2 (t), W 3 (t) and n = tanβ, m = tanβ, the dead weight of divided portions and that of the whole section can be obtained form the formula (F. 3.49). 92

107 ndf B 1 mdf W 1 W 2 W 3 Fig Division of weir body W1 woγb1d f W = 2 W 1 +W +W 2 3 W 1 2 2= woγm D 2 f 1 2 W3 = woγnd f..(f. 3.49) 2 2) Hydrostatic pressure 1 In case of non-overflow weir and a weir that has a movable weir on its crest The condition of the largest difference in water level between upstream and downstream should be employed for analysis i.e. the water level upstream is assumed to reach the weir crest and the water level downstream is assumed to be as high as base level. 2 In case of an overflow weir In case of the upstream water depth being h 1 and the downstream water depth being below the weir crest, calculation must be performed under those conditions in consideration of the design flood discharge for an overflow weir. But if the downstream water level is above the weir by h 2 as it is considered that the flow above the weir becomes a supercritical inflow the range of h 2 2/3h 1 in Fig. 3.30, then the hydrostatic pressures on the upstream face, weir crest and downstream side are assumed to be as the low pressure occurs at the weir crest because of the bending of water stream line. It is assumed that the upstream face ab is under a trapezoidal load (h 1 + V 12 /2g) at b and (h 1 + V 12 /2g+Df) at α, the weir crest bc is under a trapezoidal load h 1 at b and 0 at c and the downstream face cd is under a hydrostatic pressure 0 at c and (h 2 +Df) at d. However, as the hydrostatic pressure on the weir crest is unstable, it would be safer to omit this if the width bc is small and the overflow weir is used in 93

108 proportion to h 1. Fig Hydrostatic pressure of complete overflow Fig Hydrostatic submerged weir Next, if the weir takes the form of a submerged weir when h 2 >2/3 (h 1 + V 12 /2g), then the condition where h 2 = 2/3 (h 1 + V 12 /2g) is dangerous. The section is determined to satisfy stability under hydrostatic pressure with the above procedure and to make sure that the stability can be maintained assuming bc and cd may be subject to hydrostatic pressure in proportion to the water depth, when the water level becomes the maximum. Fig shows the hydrostatic pressure in case of a submerged weir. 3) Dynamic water pressure No particular consideration of dynamic water pressure is required in dynamic water pressure. 4) Seismic force The seismic force is assumed to be acting horizontally to the center of gravity of the weir body. The design seismic coefficient applies correspondingly to the case of Section (4) pier. As in the case of the pier, the seismic force is considered only at the time of the ordinary water level but not at the time of maximum flood. Fig Seismic force acts on weir body 94

109 The seismic force per unit length F is obtained by the formula (F. 3.50) 1 F KW ( B1 B2 ) D f 2.(F. 3.50) Where F : seismic force per unit length (KN/m) K : design seismic coefficient (0.15) W : unit weight of weir body (KN/m 3 ) B1 : crest width(m) B2 : weir bottom width (m) Df : weir height (m) 5) Earth pressure due to sediment The earth pressure is added to hydrostatic pressure. The depth of sediment should be determined depending on the condition of the weir body. It would be safe to assume that the sediment rises to the crest. The unit weight of sediment is 8.2KN/m 3 in water and Rankine's earth pressure is used with the coefficient of earth pressure as The earth pressure pe is: Pe ( W1 wo) CoHe.(F. 3.51) Where W1 : unit weight of deposited silt (18 KN/m 3 ) w0 : unit weight of water (9.8 KN/m 3 ) C0 : coefficient of earth pressure He : height of deposited silt (in principle it is treated to be deposited on the crest) 6) Uplift The trapezoidal load of each water depth multiplied by the coefficient of uplift is considered to act against the bottom at the upstream and downstream ends. The coefficient of uplift is μ = 0.4 an rock foundation case or a case using sheet piles reaching an impermeable stratum, otherwise μ = 1.0. In calculation, it is convenient to divide the trapezoid into two and the uplift on the upstream side is made U 1 and that of the downstream side U 2. 95

110 U U w B w B The total uplift 2 h 1 h 2.(F. 3.52) U 1 U1 U 2 w0 B2 h1 h2.(f. 3.53) 2 Where wo : unit weight of water (9.8 KN/m 3 ) B2 : width of bottom of weir body (m) h1, h2 : water depths of upstream and downstream (m) μ : uplift coefficient The uplift of the weir on the permeable ground varies depending on how the apron is designed. Generally, when the river water level is equal to the weir crest, the uplift becomes the maximum. But sometimes the uplift is larger than the difference between the upstream water level and the downstream water level. Therefore, in designing the cross section, up- and downstream water elevations and the worst condition then the largest hydraulic head difference should be considered. There are two methods of designing the cross section. One is to determine the section above the apron floor slab first, followed by the apron thickness from the uplift, which is added thereto. The other is to determine the weir body first and front and rear aprons are added thereto. The most suitable method should be chosen depending on the method of excavation. If the weir body is to be constructed first and the apron floor slab later, then the latter must be selected. 96

111 Fig Uplift water level Fig The way of taking water level (4) Determination of section (Stability analysis) The section is determined so as to satisfy the following conditions: For the bottom 1 no overturning 2 no sliding 3 no settlement These condition should be considered the case of dynamic (flood), static (dry) and earthquake. 1) Stability for the overturning In regard to the overturning, the following conditions must be satisfied: e = M / V B 2 / 2 B2 / 6 (Normally), B2 / 3 (Earthquake).(F ) Where ΣV : resultant force of vertical force (KN) ΣH : resultant force of horizontal force (KN) e : eccentric distance (m) To avoid tensile stress in the weir body, the resultant force must pass within the middle 1/3 of the center of the bottom length as a limiting stress. Thus the condition against the overturning can be satisfied. 2) Stability for the sliding In regard to the sliding, the following conditions must be satisfied: S L = M f / V 1.5 (Normally), 1.2 (Earthquake).(F. 3.55) 97

112 Where S L : slide coefficient f : friction coefficient = 0.7~0.75 However, when it is constructed on bedrock, the shear sliding resistance is taken with the safety factor (n) of larger than 4. In the case f = 0.6 is used: Z 0 B2 fv n 4.0 (F. 3.56) H Where Z0 : shear resistance at the bottom 3) Stability for the settlement (over stress) In regard to the settlement, the following conditions must be satisfied: q= V 6e 1 B2 B < q a (F. 3.57) 2 Where q : compressive strength caused at both ends of the bottom (KN/m 2 ) q a : allowable stress of the graund (KN/ m 2 ) (refer to table 3.11) (5) Correction of trapezoidal section In the case of construction of large scale weir, it is desirable to modify sectional form of the weir body so that weir body is not separated from overflow water streamline. In this case, it should be considered easiness of the construction. It is desirable to employ quarter perimeter of an oval for upstream edge line of the weir crest and a quarter circle also can be used.. The downstream edge of the weir crest employs a parabolic shape and joins the downstream slope smoothly. The bucket requires a radius of 1/2 ~ 1/3 of the crest height (D f ) and must have a tangent line to the downstream apron (Fig. 3.35). 98

113 1) Edge of upstream weir crest The edge of the upstream weir crest employs a quarter perimeter of an oval so that the formula below can be satisfied, where hi + hv = h a =0.125h b=0.28h (F. 3.58) Fig Modification of trapezoidal section 2) Downstream face Although there are various corrected sectional forms for the downstream face, it is necessary that the overflow water streamline should not separate from the weir body. For this reason, the downstream face is shaped as a parabola with x 2 = αhy. The value of α is made larger than 1.78 and the parabolic line comes in contact with the downstream face. Therefore, in Fig. 3.35, axes x and y are taken as shown in the figure and where a point of intersection between y axis and the slope of downstream is O, and OE = d The curve formula comes in contact with the downstream face below the weir crest by d. x 2 = 4m 2 dy..(f. 3.59) The following condition must also be satisfied; 4( d / h) m (F. 3.60) Where it is required to give a certain width to the weir crest, a horizontal section may be suitably provided from point E on axis x. 3) Bucket The bucket is provided to divert the falling water streamline to the horizontal direction at the toe of the slope of the body of the overflow weir except for a weir with small flow and constructed on relatively 99

114 good foundation bedrock. The purpose is to prevent scouring caused by the falling and crushing of water. Etcheverry defined the curve to be a radius of 1/2-1/3 of the weir height. To make the bucket really effective, it is necessary to make a tangent to the ground and make the radius of the bucket larger as the weir is higher and the overflow volume is larger so as not to create a sudden change at the junction point with the bedrock. Also, the radius may be changed depending on the quality of the bedrock and it is necessary to provide a system to prevent scouring downstream of the weir body in case of poor bedrock or gravel ground. (6) Apron 1) Downstream apron To prevent scouring downstream of the weir due to the overflow water a downstream apron is provided at the downstream face. The thickness of apron is determined in the same manner as that of the movable weir. The length can be obtained from the formula below: 0 C D... (F. 3.61) Where 1 = length of downstream apron (m) D 1 : height from top surface of downstream end of apron to weir crest (m) C : Bligh's C (Table 3.3) About the thickness of downstream of apron, please refer to (2) 3). 2) Upstream apron The apron to be provided upstream is to prevent scouring of the riverbed by vortical flow caused by the overflow water. Therefore, the thickness may be thinner than the downstream apron. Normally the thickness is made about 1/2-2/3 of that of the downstream apron, but on a river where vortical flow may occur, the thickness must be increased. As with the upstream apron of a movable weir, a reverse slope of about 30% may also be applied for the upstream apron of a fixed weir. However, when the weir height (D f ) is more than 2 ~ 3m and deposited silt at the upstream face is foreseen, It is not needed to provide. The calculation of creep length should not be taken this 100

115 into account for safety. Where it is unavoidably taken into account in the calculation of creep length, reinforcing bars, structural steel, etc. must be inserted in the joint with the weir body so as to prevent settlement and a water stop must be used to make the structure thoroughly watertight. < Supplementary explanation > Apron, if river bed is hard rock, may not be necessary to construct. When floating type is selected, the unification of apron and weir for creep length is needed and the reinforcement bar should be put in bottom slab for protecting the destruction by uneven subsidence. Creep length has to be considered with only weir and downstream apron Riprap Riprap is provided where there is fear that local scouring would be caused on the riverbed taking into account the condition of the riverbed and of the flow caused both upstream and downstream, and of construction that is safe against the flow, and will have an energy dissipating effect. Riprap is provided continuously on the downstream apron to prevent the scouring of the riverbed, because it is apparent that the scouring is caused due to removal of deposited silt or vortical inflow compensation of the local dissipation of the water energy. (1) Basic concept for engineering of riprap work The fundamental concept for preventing local scour downstream of a weir is to dissipate the energy of the high velocity flow that passes the weir body gradually using the resistance of the riprap, making the flow velocity of the downstream section of the riprap the same as that of the downstream river following the riprap. The condition of the flow at that time is given by the hydrological quantity at the time of the critical movement of the average grain size of the riverbed material. This is premised on the fact that the flow of sediment in excess of the critical movement has continuity and the equilibrium of the riverbed can be maintained. 101

116 (2) Conditions required for riprap 1) Dissipating effect and prevention of local scouring The construction of riprap is such that it dissipates the energy of the high velocity overflow and allows it to flow gently downstream. It prevents differential settlement due to erosion etc. of riverbed materials below the riprap and the occurrence of local scouring at the end of the riprap. 2) Curvature (adaptability to settlement) The adaptability to settlement is given to riprap so as to adapt it to changes in the riverbed when the riverbed on the downstream side of the weir should become lower in the future through retrogression of the riverbed. It also allows continuity over the whole riprap. 3) Wear resistance As there can be a large movement of sediment at the time of flood, the riprap must have wear resistance against the flow of sediment. 4) Flow-through ability of sediment In the case of a natural river, there is a continuous movement of sediment. Therefore, the construction of riprap should be such that the movement of sediment should not be hindered. If the hydrological construction of riprap is made such that it stops the movement of sediment abruptly, then the sedimentation would occur on the weir body being affected by the deposited silt on the riprap when the weir height is low. In addition, the abrupt dissipation of energy would result in a destructive force being given to the riprap itself. (3) The shape of riprap 1) Riprap of downstream side Riprap of downstream side is established in consideration of the following point to an action of a flood. (a) Riprap of downstream side of headworks is considered hydraulic phenomenon at the time of a flood (hydraulic jump and disorder of flow after the jump) and shares with two sections of downstream riprap A and B, (b) Riprap A should be made of concrete structure or connected 102

117 structure of blocks because the flow of this section is disordered by the high-speed flow, (c) Riprap B is desirably made of a flexible structure to follow the unexpected river bed fluctuation of up and downstream. 2) Riprap of upstream side Riprap of upstream side is installed for prevention of scouring which formed by eddy current at the direct upper reaches of headworks. The structure of riprap should be flexible type to follow the river bed fluctuation. Moreover it is desirable to make a slope in the direction of upper stream in order to correspond to the degradation of river bed. (4) The length of riprap of upstream side Riprap in the upstream side of headworks has its objective to prevent the local scouring that forms at the direct upper reaches of headworks and protect the body and an installation retaining wall in the riverbank part. According to a hydraulic experiment and the past case, the local scouring depth formed by water-route of sandbar formation in an upstream riverbed and the eddy that occurs by a decline of the upstream riverbed amount bring is around the water depth. Therefore, it's better to be an upstream side riprap longer than design flood depth. But, a decline of the upstream side riverbed will be prime factor of eddy occurrence at the river of narrow width and that doesn't generate a sandbar. The riprap necessary to a local scouring measure is about 2 times of length of the decline amount of the expected upstream riverbed in this case. (5) The length of riprap A of downstream side The length of riprap A of downstream should be made the length that cuts the energy of the running water with energy dissipator surely by the thing, which makes hydraulic jump occur by riprap. The phenomenon of hydraulic jump is different in the form depending on changes in the flow rate, the riverbed slope and the gap. The perfect hydraulic jump that winds a big vortex gives a large force to riverbed part. Therefore, a riprap A section should be beyond the length of perfect hydraulic jump in consideration of hydraulic phenomenon. In case of rapid river, because hydraulic jumping length become long, it is possible to install the help structure that had energy dissipator 103

118 and shares the dynamic water pressure and to reduce a riprap A section by the thing, which makes hydraulic jump occur compulsorily. Apron Riprap B L2 Riprap A L1 Fig Riprap 1) Calculation method for fixed weir The length of riprap A section is calculated by the following formula: L= L 1 + L 2..(F. 3.62) Where L 1 :The hypercritical flow section of from dropping point of stream to start point of hydraulic jump L 2 :The section of hydraulic jump occurred Z Fig Riprap detail Hypercritical flow section(l 1 ) and hydraulic jump section(l 2 ) should be calculated repeatedly by increasing flow discharge from low flow to design flow while water depth of dropping point(h 1a ) is lower than conjugate depth(h 1b ) of down stream natural depth. 104

119 In the case of "h 1a >h 1b ",because the perfect hydraulic jump changes to imperfect jump, turbulent of river bed is decreased. The calculation of riprap A section is made according to the following procedures. 1 Calculation of water depth at the dropping point from overflowing, 2 Calculation of water depth at the beginning point of hydraulic jump, 3 Comparison with h 1a and h 1b (a) Calculation of water depth at the weir toe The water depth of weir toe can be calculated by the following equation about preservation of energy between the section Ⅰ -Ⅱ. V 2 c 2g 2 1a V Z+hc= h1 a..(f. 3.63) 2g Where Vc :Velocity of critical depth (m/sec) = q / hc g : Acceleration of gravity (m/sec 2 ) h c : critical depth (m) = 3 (q 2 /g) h 1a : water depth at the dropping point from overflowing(m) V 1a : velocity at the weir toe (m/sec) Z : height between crest and apron (m) q: Unit width flood discharge (m 3 /s/m) = Q d / W Q d : Designed flood discharge (m 3 /s) W: River width (m) In the substitution of V 1a =q/h 1a for this equation, and assumption as the polynomial of h 1a, h 1a and V 1a are obtained by the trial and error calculation. (b) Calculation of water depth at the beginning point of hydraulic jump The water depth at the beginning point of hydraulic jump(sectionⅢ-Ⅳ ) is obtained by following equation. 105

120 h 1 1 b ( 1 8F 2 2 1)..(F. 3.64) h 2 2 Where h1b : water depth at the beginning point of hydraulic jump h2 water depth of downstream F2 : Froude's number of downstream F2 V2 / gh2 V2 : Velocity of downstream (obtained by normal flow calculation) (c) Comparison with h 1a and h 1b a) In the case of h 1a = h 1b In this case, hydraulic jump occurs from the dropping point from overflowing. It is assumed that the length of hydraulic jump section is about times of water depth of downstream, the length of riprap A is calculated by following formula. L=L2=(4.5~6) h2..(f. 3.65) b) In the case of h 1a > h 1b In this case, it is not necessary to install riprap A because hydraulic jump is submerged. However, it is better to adopt riprap B rather longer because it is possible to occur jet flow on the riverbed. c) In the case of h 1a < h 1b In this case, as the beginning point of hydraulic jump moves to downstream, it is better to make riprap A longer. Therefore, the length of riprap A is calculated following formula. L=L1+L2 L 1 is the length while the water depth h 1a rises to h 1b. So the L 1 can be obtained by the following formula seeking for water surface profile. 2 q dh 3 3 ( h hc )..(F. 3.66) 2 C dx q x a h hc 3 h..(f. 3.67) 2 C 4 Where q : flow per unit width of design flood discharge (m 3 /sec/m) C: Chezy's formula (=h 1/6 /n) n: Coefficient of roughness of weir (=0.04) 106

121 x : sectional length a: constant When h 1b is substituted for this equation(h = h 1b ) after first water depth h 1a is substituted for this equation(h = h 1a, x=0) and constant "a" is obtained, the sectional length x=l 1 is obtained. Consequently, necessary length of riprap A is following formula added up hydraulic jump length. L=L 1 +L 2 = L 1 +(4.5~6) h 2..(F. 3.68) (6) Riprap B of downstream side Flow distribution of vertical direction after energy dissipator by hydraulic jump by riprap A becomes uniform mostly. Therefore, the shearing force is bigger than a downstream riverbed and it becomes easy to bring about scouring by a riverbed base. Riprap B must be a buffer structure that has large roughness to reduce shearing force of riverbed to that of downstream. When the length of the riprap B section necessary to this rectification is judged from an existing case, it's proper to make it 3 to 5 times of design water depth. The way of thinking to decide about the length of the riprap was described above, but it's desirable that the specification and characteristics of the river course by which the width of a rivers are the riverbed material and the flow rate, etc. Whenever possible make sure of its validity, do model test about an important structure, consider these overall and decide a riprap length from a case in the same river mostly. (7) Use of Bligh's formula (Reference) Although Bligh's formula has been used frequently, the flow resistance due to the flow situation and the riprap itself is not taken into consideration in the formula. It is scope of construction of the riprap. L..(F. 3.69) L B a L B 0. 67C H q f..(f. 3.70) a 107

122 Where L : length of riprap (m) LB : total length of protection including length of apron l a and length of riprap L (m) Ha : height from above water level of downstream side to weir crest in dry season (m) q : flow per unit width of design flood discharge (m 3 /sec/m) f : safety factor, 1.5 in case of movable weir 1.0 in case of fixed weir C : Bligh's coefficient by type of foundation ground (refer to Table 3.3) (8) Structural engineering of riprap 1) Size of riprap block It is necessary that riprap block resists the flowing force and is stable. The size of each block is desirable to satisfy the formula below. 2 W 3.75 AV / 2 g.(f. 3.71) Where W : weight of each block (t) A : area of collision with flowing water (m 2 ) v : velocity at which flowing water collides with (m/sec) g : acceleration of gravity (m/sec 2 ) Formerly, the sizes of blocks used in the river works were 2t/blocks for sluggish stream sloping at less than 1/1000 ; 3t/blocks for 1/1000~1/500 and 4t/blocks for rapid streams more than 1/500. Unlike a foot protection of river, the size of each block should be larger than in case of riprap for the headworks. 2) Jointing of blocks and prevention of suction (a) It is preferable that blocks have the ability of interlocking with each other and resist against the flow as a whole. Particularity, it is desirable to secure the ability of interlocking by making use of the construction of the block itself. The end position of riprap is desirable to be equal throughout the whole riprap. If an irregular 108

123 downstream limit of riprap is unavoidable, then it is necessary to give consideration not to change the length of construction abruptly or to provide a partition wall, etc. so as not to generate vortical flow due to whether or not blocks exist to the left or right sides of the direction of flow. (b) To prevent erosion of riverbed sediment due to the flowing water, a suitable construction method must be selected. The danger of causing erosion is high in the supercritical flow section and the colliding force of the flowing water is also large. Attention must be paid to prevent suction and to preventing the blocks from moving. The usual method of construction considered is to construct the concrete floor on the riverbed where blocks are installed or a partition wall in the boundary with the ordinary flow area (Refer to Fig. 3.38). Also cobble stone fill is provided between the blocks. Although the measures of preventing erosion of sediment in the ordinary flow section differ depending on the flow velocity, the method in which the bed where blocks are installed is left as the undisturbed riverbed and a filter using cobble stone, etc. is provided between blocks or the method by which a mat is provided on the bed where blocks are installed for prevention of erosion are methods to be considered. There is also method by which a partition wall and sheet piles are constructed to prevent blocks from moving due to local scouring and to maintain the function of the riprap for a longer period. Fig Flow over riprap < Supplementary explanation > Riprap, if river bed is hard rock, may not be necessary to construct. Riprap dimensions becomes long expensive when this design method is used. 109

124 3.2.4 Foundation Work Foundations have to support the structural loads, such as a pier, weir body, etc. The appropriate construction method must be selected in consideration of the situation of the foundation ground at the point where the headworks is to be constructed and also the functions of the superstructure. (1) Functions of foundation work The foundations of a weir must transmit the loads of pier, weir body, etc. safely to the bedrock below or in subsoil stratum having sufficient bearing capacity..in addition it reduces the ground water flow below the weir body owing to difference in water level between upstream and downstream. It serves as a cut-off wall to secure required creep length preventing the piping of the gravel layer (refer to Section 3.2.5). It also functions, as a partition wall (refer to Section 3.2.5) to prevent scour occurring on the riverbed at the upstream and downstream ends of the weir body owing to the, flowing water from reaching the base of the weir body. Therefore, in the design of the foundation of the headworks, care must be taken to understand the objective thoroughly, adopt the construction method that is suitable for the site, make design chioces be economical and durable, and thoroughly execute said construction method. (2) Types of foundation work Foundation work includes spread foundations, constructed directly on bedrock or gravel, pile foundations may be bearing loads with piles resting on a firm layer(end bearing) or by friction piles; and well foundations and caisson foundations as special foundations. 1) Spread foundations Although there is no problem when headworks are constructed on bedrock, footings are required whereby the base of a fixed weir, abutment and piers are enlarged when the weir is founded directly on gravel. The design of footing is performed by the following procedure: 110

125 1 Calculate the load on the base of the footing. 2 Obtain the allowable bearing capacity of the supporting ground. 3 Determine the configuration and sizes of footings so that the maximum contact pressure acting on the base of the footing is less than the allowable bearing capacity. 4 Determine the section and arrangement of reinforcing bar for the footings. (a) Contact pressure a)considering that where the vertical load from the superstructure acts on the center of the base of the footing, the contact pressure is distributed uniformly over the entire base area, the following formula is applied. P σ α = q a (F. 3.72) A Where, σα.: contact pressure (KN/m 2 ) P : vertical load acting on footing (including dead load of footing) (KN) A : base area of footing (m 2 ) qα : allowable bearing capacity of ground (KN/m 2 ) When an eccentric load acts on the base of the footing as shown in Fig. 3.39, one end of the footing is lifted up as the eccentricity of the load exceeds a certain value and the contact pressure at that portion becomes zero. Assuming that the contact pressure is distributed linearly, the maximum contact pressure is examined by the formula (F. 3.73). αp σ max= q a.(f. 3.73) A Where, αmax : maximum contact pressure (KN/m 2 ) α : coefficient determined by eccentricity of load and configuration of bottom. It is 1 where there is no eccentricity. Generally, the extent of eccentricity in an independent footing should be limited to e/l = 1/3. Fig shows the value of α in the range of 0 e/l 1/3. 111

126 P : vertical load acting on footing (including dead load of footing). A: base area of footing (m 2 ) qα: allowable bearing capacity of ground (KN/m 2 ) Fig Contact pressure distribution of rectangular foundation Fig Graph for calculation of α, α When an eccentric load and moment act on the base simultaneously as shown in Fig. 3.41, the equivalent eccentric load indicated by the broken line is substituted, and using MG M+Pε M e = = +ε..(f. 3.74) P P P α is obtained from Fig If the case where the vertical load is acting on an particular point on the base of a rectangular footing, the coefficient when the vertical resultant force is acting on the shaded portion as shown in Fig (a) can be calculated from the formula below. 6 e L 6 e B α = 1+ +.(F. 3.75) L B 112

127 Fig Where eccentric load and moment work at the same time Fig Contact pressure distribution when resultant vertical force acts on every point of rectangle bottom Fig Coefficient of contact pressure when resultant vertical force acts on every point of rectangular bottom When the vertical load is acting on the outside of the shaded portion as shown in Fig (b), the compressive stress becomes zero at a part of the base. The value in this case can be obtained from Fig

128 (b) Allowable Bearing Capacity of ground To obtain the allowable bearing capacity of the ground there are the following methods: 1 Allowable bearing capacity is determined from the ultimate bearing capacity formula using the results of soil test. 2 Allowable bearing capacity is determined directly from the results of a plate bearing test. 3 Allowable bearing capacity is determined from the conventionally adopted table of bearing capacity of ground. a) Formula to calculate allowable bearing capacity To calculate the allowable bearing capacity, it is suggested to use the formula below, which has been developed from Terzaghi's ultimate bearing capacity formula. The long-term allowable bearing capacity: 1 q a a C N c 1 B N +γ 2 D f N q (F ) 3 The short-term allowable bearing capacity: 2 1 q a a C Nc 1 B N+ γ2 Df Nq (F. 3.77) 3 2 Where qα : allowable bearing capacity (KN/m 2 ) C : cohesion of ground below foundation load surface (KN/m 2 ) γ1 : unit weight of ground below foundation load surface (KN/m 3 ). When it is below ground water level, use the submerged unit weight. γ2 : average unit weight of ground above foundation load surface (KN/m 3 ). When it is below ground water level, use the submerged unit weight. α, β : shape factor (refer to Table 3.9) Nc, Nr, Nq : coefficient of bearing capacity, which is a function of angle of internal friction and can be obtained from Table 3.10 or Fig Df : depth from deepest ground surface adjacent to foundation to foundation load surface (m) (refer to Fig. 3.44). 114

129 B : minimum width of foundation load surface. In case of circular shape, use diameter (m) Table 3.9 Form factor Shape of foundation plate Continuous Square Rectangle Circle Coefficient Α B/L Β B/L <Remark>B : Length of short side of rectangle L : Length of long side of rectangle The value of cohesion C and angle of internal friction should be determined by a direct shear test or by a triaxial compression test. However, as it is difficult to take undisturbed samples from sandy ground, the value of the angle of internal friction is assumed from the results of the standard penetration test(spt), and cohesion is assumed at C = 0. Table 3.10 Bearing capacity factor Φ Nc Nr Nq or more

130 Fig Investigation of contact pressure Fig Relation between internal friction angle and f coefficient of bearing capacity As a formula to infer the angle of internal friction φ of sandy ground from the N-value of the standard penetration test, Peck proposes the following formula within the range of N <40: φ = 0.3N + 27 (F. 3.78) 116

131 Dunham gives the following relational expressions in consideration of particle shape of sand and grain size distribution: For round particle with uniform grain size: Φ= 12 N +15 (F. 3.79) For round particle with good grain size distribution: Φ= 12 N +20 (F. 3.80) For angular particle with uniform grain size: Φ= 12 N +20 (F. 3.81) For angular particle with good grain size distribution: Φ= 12 N +25 (F. 3.82) b) In the case the footing is subjected to inclined load and eccentric load: When the load is inclined or eccentric, the bearing capacity of the ground decreases. When the load is inclined as shown in Fig the unit allowable bearing capacity is obtained approximately by multiplying each term of the formula (F. 3.76) by the correction, respectively, as follows: q a i 1 C c 1 +q 2 D f N q 3 γ ca N ir B N i.(f. 3.83) Where ic = iq = (1 - θ/90) 2 ir = (1 -θ/φ) 2 θ : angle of inclination (degree) φ: angle of internal friction (degree) Fig Inclined load 117

132 When the load is eccentric from the center of the footing, there is a method by which the contact pressure distribution of trapezoidal or triangular load balanced with the eccentric load is considered as shown in Fig. 3.42, and designed is such that the maximum contact pressure σ max of the foundation base is within the allowable bearing capacity (q a ), and another method by which design is performed assuming that the effective width of the foundation base decreases by two times the eccentricity. In this guideline, the former method is used. When the load is inclined or eccentric, design is such that the maximum contact pressure is within the unit allowable bearing capacity with inclined load. c) Method by plate bearing test Where spread foundation is used, the ground is generally considered sound, there is generally no need for a plate bearing test to be performed. However, where the plate bearing test is carried out, the smallest of the three types of loads, 1/2 of yield load, 1/3 of ultimate bearing capacity and 1/2 of load when the gross settlement reaches 20mm, is used as the long-term allowable bearing capacity of the ground. d) Table of bearing capacity of normal ground (normal bearing capacity table) When the outline of ground condition is known and investigation according to the above is not carried out, the normal bearing capacity table shown in Table 3.11 can be referred to for the empirical estimation of the long-term unit bearing capacity of the ground. 118

133 Table 3.11 Long-term allowable bearing capacity Long-term Remarks Condition of allowable N value Unconfined Foundation bearing compression plate capacity strength (kn/m 2 ) qu(kn/m 2 ) Bedrock 1, or more Cemented sand or more Mudstone or more Gravel, dense 600 non-dens 300 Sand, dense ~50 Medium ~ ~20 loose 50 5~10 Very 0 less than 5 Clay, very hard ~ or more Hard 100 8~15 100~250 medium 50 4~8 50~100 Soft 20 2~4 25~50 very soft 0 0~2 less than 25 It is possible for short-term allowable bearing capacity to be applied double value of long-term one Upstream and Downstream Cut-off Walls A cut-off wall must be an impermeable structure with sufficient depth. A dwarf wall must have enough depth against the anticipated depth of scour. (1) Upstream cut-off wall An upstream cut-off wall is installed for the purposes of prevention of piping by percolating water, stabilization of headworks and reduction of percolation. The wall must be located at the upstream extremity of a weir body and beneath the weir. The wall is required to be installed at least at the upstream side of the weir body axis so 119

134 that it can prevent scouring especially by the floods just after the completion of construction and as a water-seal during construction. The walls may be extended from the weir body to abutments of the headworks to complete the seal if it is necessary. In this case, the wall should be called a water-cut wall or wing wall to distinguish it from cut-off wall. Fig Cut-off wall (example) A upstream cut-off wall should be constructed on the firm rocks exposed by earth excavation as long as dewatering is possible and the rock mass is at shallow depth. If dewatering is not possible, the repacked concrete method is recommended. When firm rocks are too deep to be reached and the river bed consists of sand or gravel, sheet piles may be used. Sheet piles include concrete piles, steel sheet piles etc. Prior to construction, test driving is required to confirm the possibility of piling. Since sheet piles are used also for coffering, the work schedule should be well arranged in order to reduce the total costof the construction. Re-usage of sheet piles and partial pre-construction of a cut-off wall to be used for dewatering or drainageof the site are the examples of such use. Currently improved cast-in-place concrete piling methods have made it possible to install a continuous concrete wall by combination of piles (see Fig. 3.48). These methods are applicable to sand or sand gravel river 120

135 beds in the same way as sheet piles. When the riverbed contains large gravel and neither sheet piles nor cast-in-place concrete piles are applicable, wells or caissons are sunk and used as a cut-off wall or a dwarf wall.in this case, special attention must be given to construction of joint portion in order to ensure the water-cut-off function. Grouting work may follow the above mentioned sealing works if necessary. Fig Diaphragm wall by overlapping piles Water-seals should be carefully scaled at the inlets, the junctions between fixed weirs and banks, and major bed(flood plane), etc. Sheet piles are the most recommendable method for water-seal at inlets. Extension of weir body up to the river banks is considered to be the most safe method. The choice should be made after considering the site conditions. There is Lacey s score depth formula to assume the length of vertical cut-off. Vertical cut-off must go below the depth of scour. R = 1.35 (q 2 / f ) 1/3..(F. 3.84) where, R : Depth of scour (m) q : Discharge per unit width (q = Q d / L) Q d : Designed flood discharge (m 3 /s) L : Length of weir (m) f : Silt factor ( f = 1.76 m r ) m r : Mean diameter of silt particles (mm) When it is difficult to investigate the mean diameter of silt particles, it can be adopted the value of silt factor on Table

136 Table 3.12 Silt factor for various grain size Types of soil particles Silt factor (f) Coarse sand 1.50 Medium sand 1.25 Standard silt of lacey 1.00 Medium silt 0.85 Fine silt 0.70 to 0.50 The above-mentioned depth of scour may adopt a practical value based on geological condition because of in some cases to become the big value. (2) Downstream cut-off wall A downstream cut-off wall is installed to stabilize a weir body against riverbed scouring at the downstream end of the apron. The wall should extend the entire width of the weir and the retaining wall at both sides. The depth of the wall must be decided after consideration of the anticipated scour depth. Scour depth can assume by formula (F. 3.84). Since a downstream cut-off wall does not require to be watertight, weep holes are introduced to reduce uplift. Occasionally weep holes are also provided in the apron to reduce uplift and to prevent piping. Drainage filter materials must be carefully selected to prevent basement soil from flowing out of the holes. Prior to the construction of weep holes locations and combination of weep holes must be designed according to the length of the apron, location and depth of upstream cut--off wall and downstream cut-off wall. Hydraulic model tests or flow network analysis must be executed to estimate the uplift pressures which act on a weir body and apron. The escape gradient at the downstream end of the apron can be calculated from these results. It is necessary to assess the risk of piping based on the above results. 122

137 Downstream cut-off Fig Downstream cut-off wall < Supplementary explanation > Cut-off wall is constructed according creep length at floating type. During construction time, it is not necessary to use form work. Doing form work, gives rooms and it should be filled up. But it is difficult to fill up fully. The room will be a water path, it makes difficult to keep creep length Inlet An inlet must be designed to prevent soil, sand and floating material, which are undesirable in irrigation water, from entering the canals. Inlets must allow the designed flow-rate and be safe fromexternal forces. The principle of inlet design is to ensure the consistent intake of water and prevent inflow of sand. Stability of the river course, river meandering condition and runoff condition should be studied to fulfill the principles. (1) Function of Inlet Inlet must have sufficient capacity to draw the design discharge from a river and transfer the flow to the irrigation canal. Generally, discharges in rivers are subject to change and at the time of flood, large amount of soil, sand, and floating debris materials are carried by the river flow. Thus, the inlet is expected to enable easy control of intake discharge and prevention of the materials carried by flood waters from flowing into the canal. For these purposes discharge control gate, gate pillar, screen(trash rack), conduit (bank crossing culvert etc), intake apron sand drain may have to be installed. Minimization of maintenance and operation costs for these facilities is important. 123

138 (2) Location of inlet In case of natural intake method, inlet should be installed where the thalweg is near to the river bank, stable and water depth is deep enough to draw water. For example, the outside of a bend in a river is the most preferable from the viewpoint of preventing inflow of sediment. It is important to make sure that the water level in the river be not lower than design intake level in the future. A diversion weir has the merit of controlling the river surface level artificially. When installation of inlet is possible at a river bend where the thalweg is stable, a scouring sluice will not be required. But if the preferred point mentioned above is not available, a scouring sluice is required and the inlet should be installed within the effective Fig. 3.4 Apporopriate Intake Position part of it. When intakes are necessary at both sides, installation of inlets with the scouring sluice at both sides of intake weir may cause sediment accumulation and an unstable thalweg, which may cause unstable intakes. It is better to install one intake at point where the grit is stable even if it is far from the intake weir. In addition, the front part of inlet has to be matched with bank of river and it has to avoid sticking out and standing back from bank of river. Because, if the front part of inlet sticks out from bank of river, the part receives big destructive power. On the other hand, if the front part of inlet stands back from bank of river, it will occur sedimentation due to the weak flow.. (3) Features of Inlet Design 1) Sill elevation In case of the natural intake method, the elevation of the inlet sill shall be decided based on the lowest base flow level ever observed. The sill is preferred to be higher than 0.4H=h l from the lowest drought water surface (see Fig (a)). As the water level fluctuates greatly throughout the year, double leaf slide gate is recommended in order to avoid sediment inflow. At the normal stage, intake of surface water by overflowing the lower half of the gate is desirable. 124

139 Fig Inlet sill elevation The inlet elevation should be 1.0m higher than the base elevation of the scouring sluice for prevention of sediment, if a diversion weir is installed. There is a standard that the height from the riverbed to the inlet elevation should be more than 1/6 of maximum flood depth of the river, which is based on the consideration of the critical height of sedimentation on the riverbed by flooding. The inlet elevation should also be the same level or higher than the guide wall height of the sediment sluiceway. However, if the width of the sediment sluiceway is wider than the width of inlet, this does not have to be considered strictly. 2) Intake Velocity Generally an intake velocity of 0.6 ~ 1.0m/sec above inlet apron is standard, which is decided based on the following two conditions'. First, the approach velocity (velocity of the river flow just upstream of the inlet) must not exceed 0.4m/sec so that soil particles bigger than 0.30mm in diameter, which might be harmful for irrigation water, will not inflow. Another condition requires that the velocity in the main canal is higher than 0.75m/sec to prevent the growth of aquatic plants. 3) Width of Inlet Width of inlet is calculated by the formula (F. 3.85) with inlet apron, elevation, design intake water level and inflow velocity. 125

140 B=Q/h1V..(F. 3.85) where B: inlet width(m) Q: design intake discharge (m 3 /sec) h1: water depth (design intake water level - inlet apron elevation) (m) V : inflow velocity (m/sec) Large spans must be avoided by dividing them into reasonable numbers the bays, considering the height-width ratio of the gate and intake operation. Shape of pier must be designed to reduce shrinkage by approaching velocity and head loss due to shrinkage. 4) Screen A trashrack is attached just in front of the regulation gate with an inclination of about 1:0.3 so that trash is easily evacuated. The top of the trashrack is made round and workability of cleaning devices must be considered (see Fig. 3.51). A fish screen that prevents fry from entering into canals is installed right after the trashrack if necessary. Fig Screen 5) Inlet Basin An inlet basin is a transitional structure between an inlet and a head race for putting the inflow uniform. A guide wall is usually provided to avoid nonuniformity of flow. During floods, sedimentation usually occurs in this section. The inlet basin must be as short as possible and covered. (4) Flow Discharge at Inlet The configuration of an inlet may be of two hydraulic categories as follows : 1 Overflow type and 2 Orifice type (see Fig. 3.52) 1) Overflow type inlet Discharge is calculated using the submerged overflow formula for 126

141 weirs (F. 3.86). Q m( B n k h ) h 2g ( h h ).(F. 3.86) where Q : inflow discharge(m 3 /sec) m : head loss coefficient, * when there exists a gap in base line as inlet. (However, it becomes small when the flow is affected by the guide wall of sand sluiceway.) n : number of end contraction k : coefficient for end contraction, when the shape of front side of. pier is circular B : inlet width(m) h1: river flow depth between the surface and inlet apron elevation (m) h2 : flow depth at inlet (m) g : acceleration of gravity(m/sec 2 ) if 2 2 v 1 h2 h1 then the discharge is calculated by the 3 2g formula for complete overflow. 2) Orifice type inlet Generally a surface intake (overflow type) is preferred since this type can prevent sediment inflow better than that of an orifice type. An orifice type inlet has the merit of lower sensitivity to change of discharge because the discharge is in proportion to the square root of the upstream depth. Thus the orifice type is used together with the overflow type so that water is diverted by overflow type during low water level and by orifice type during high water level (like a double boarded slide gate). Discharge is calculated by the following formula (F. 3.87) when the downstream water depth does not affect the discharge. h 0 Q m B h 2gh 0 1..(F. 3.87) 2 where Q : inflow discharge(m 3 /sec) m : coefficient, (0.65 is used for design and an adjustment should be made by measurement after the completion of construction) h0 : gate opening (m) 127

142 h1 : upstream water depth (m) B : inlet width (m) g : acceleration of gravity (m/sec 2 ) Fig Configuration of inlet (5) Water Level Calculation for Inlet The design intake elevation is calculated using the given elevation at the head of the main canal and the following series of head losses if necessary. The head losses are due to ; 1 inflow, 2 gap of base elevation, 3 pier, 4 screen, 5 friction, 6 abrupt or gradual increase in sectional area, 7 abrupt or gradual decrease in sectional area and 8 curve. Calculation procedure is; 1 Basic design of the shape of inlet with given diversion discharge, 2 Calculation of each head losses due to the shape designed, and 3 Calculation of intake water elevation by adding the total head loss to the elevation at the beginning of main canal, detailed design of inlet structure and check calculation of head loss for the entire facilities. 1) Head loss and change of water level by inflow V2 V1 V2 V2 V1 he he f e. (F. 3.88) 2g 2g 2g 2g 2g where Δhe: difference in water level by inflow (m) he : head loss (m) fe : head loss coefficient V1 : average velocity before inflow (m/sec) V2 : average velocity after inflow (m/sec) 128

143 Head loss coefficient (f e ) is due to plan feature of inlet (see Fig. 3.53) Fig Shape of inlet and head loss coefficient 2) Head loss and change of water level due to elevation gap of base line V2 V1 V2 V2 V1 hc hc fc 2g 2g (F. 3.89) 2g 2g where Δhc: difference in water level due to elevation gap of base line (m) hc : head loss due to elevation gap fc : head loss coefficient V1 : average upstream velocity of gap (m/sec) V2 : average downstream velocity of gap (m/sec) Fig Change of water level by gap Head loss coefficient f c is determined by the ratio of the flow sectional areas at both sides of the gap (see Table 3.13). Cross sectional areas are calculated as A 1 =B H 1 and A 2 =B H 2 respectively where H 1 and H 2 are upstream and downstream water depths of the gap. Table 3.13 Head loss coefficient by elevation differnce A2/A (1.0) fe (0) 129

144 Head loss and difference in water level due to drop in elevation are calculated by the formulae (F and F. 3.91). 2 2 V2 V1 h t h t..(f. 3.90) 2g 2g h t Z f Fr 2,..(F. 3.91) h 2 h2 Fig Flow on drop where Δht : difference in water level (m) ht : head loss (m) ΔZ : elevation gap (m) Fr2 : Froude number at section 2 h2 : flow depth at section 2 V1 : average upstream velocity of gap (m/sec) V2 : average downstream velocity of gap (m/sec) g : acceleration of gravity (m/sec 2 ) Head loss h t is a function of F r2 and ΔZ / h 2 and is shown in Fig Fig shows the critical condition for the control section (line-a is for steep gap and line-b is for inclined or gradual gap). For a given ΔZ / h 2, if Froude number F r2 exceeds a certain limit, the section above the gap becomes a control section. For the area below the lines where there is no control section, the formula (F. 3.91) is applicable. Further, section 2 in the Fig is supposed to be 30 4Z downstream from the gap. 130

145 Fig Head loss (h r ) calculation of drop Fig Critical condition of occurrence of controlling section at drop Fig Change of water level by piers 3) Change of water level due to piers D'Aubuisson's formula (F. 3.92) is used to calculate the change of water level due to piers. h 2 Q g c b2 ( H hp ) b1 H p 2 1..(F. 3.92) where Ahp: difference in water level (m) Q : flow discharge (m 3 /sec) c : coefficient of plan feature (see Fig. 3.59) 131

146 b1 : anal width upstream side of piers (m) b2 : b1-σt ; sectional flow area at piers t : pier width (m) H1 : flow depth upstream side of piers (m) Fig Shape of pier and C value Formula (F. 3.92) is implicit so trial and error method is necessary. 4) Head loss and change of water level due to screen V2 V1 V1 V2 V1 h f r hr r (F. 3.93) 2g 2g 2g 2g 2g 4/3 2 h t V1 sin r b 2 (F. 3.94) g Where Δhr: difference in water level (m) hr : head loss (m) fr : head loss coefficient β : coefficient for sectional shape of screen bar (see Fig. 3.60) θ: inclination of screen (degree), t : bar screen thickness (m) b : bar interval (m) V1 : upstream average velocity (m/sec) V2 : downstream average velocity (m/sec) g : acceleration of gravity (m/sec 2 ) 132

147 Fig Coefficient of section shape of screen and bar Formula (F. 3.93) treats only a case without trash obstructing the screen and an adjustment calculation for actual conditions is necessary (see Fig. 3.61). Formula (F. 3.95) is used to calculate head loss by trash*. f r 4 / 3 t a 6.69 sin exp 0.074γ w. (F. 3.95) b H where Hs: head loss due to trash (m) a : trash sticking height(m) γw : unit weight of wet trash** (kg/m 3 ) Other notations are same as those in formulae (F. 3.93) and (F. 3.94). Fig is the calculation condition. Trash sticking height ratio a/h is usually assumed to be 0.1 as a standard. Since this ratio may affect the intake discharge and the choice of weir crest elevation, the ratio has to be decided after considering river conditions. 133

148 Fig Change of water level by trash Fig Latticed type screen 5) Head loss due to friction 2 2 h 2gn L V f 1/3 R R 2..(F. 3.96) g where hf: frictional head loss (m) n : roughness coefficient (0.014 ~ for concrete lining canal ) V : average velocity (m/sec) L : canal length(m) R : hydraulic mean depth(m) If uniform flow is assumed to occur, the head loss by friction is calculated as; h f =IL, where I=canal bed slope and L=canal length. 6) Head loss and change of water level due to abrupt increase in flow area V2 V1 V2 V1 h h h se hse e f.(f. 3.97) 2g 2g 2g 2g h e 2 V2 fe.(f. 3.98) 2g 134

149 h f 2 2 n V1 V 4 / 3 2 R1 R / 3.(F. 3.99) where Δhse : water level difference (m) hse : head loss due to increase in flow area (m) he : head loss due to abrupt change of shape (m) hf : head loss due to friction between section 1 and 2 (m) fe : head loss coefficient (see Fig. 3.64) V1 : average velocity before increase in f-low area (m/sec) V2 : average velocity after increase in flow area (m/sec) g : acceleration of gravity (m/sec 2 ) R1 : hydraulic mean depth before (m) R2 : hydraulic mean depth after (m) n : roughness coefficient l : length between section 1 and 2 Fig Water flow in abrupt increase of flow area The head loss coefficient f e is due to width ratio b1/b2 (see Fig. 3.64). If the Froude number F r 2 V 2 / gh 2 is bigger than a certain value, the depth at flow area changing point becomes a critical depth and there occurs a control section. The area above each line in Fig

150 is the area where flow is subcritical. The Lower area represent's supercritical flow. Fig shows fe for subcritical flow. Further the length e between section 1 and 2 is suggested to be about 30(b2-b1). Fig Head loss coefficient by abrupt increase of flow area Fig Critical condition by controlling section of abrupt increase of flow area 7) Head loss due to gradual increase in flow area h ge ( V1 V2) A1 V1 V1 f ge f ge1 f ge fse 2g A..(F ) 2 2g 2g where 136

151 hge: head loss due to gradual increase inflow area (m) fge: head loss coefficient for gradual increase (see Fig. 3.66) fse : head loss coefficient due to abrupt increase (see Table 3.14) V1 : average velocity before change of flow area (m/sec) V2 : average velocity after change of flow area (m/sec) Fig Head loss coefficient for gradual increase Table 3.14 Head loss coefficient by sudden enlargement A1/A (1.0) fse (0) 8) Head loss and change of water level due to abrupt shrinkage of flow area V2 V1 V2 V1 h h h sc hsc c f.(f ) 2g 2g 2g 2g h c h f 2 V2 fc.(f ) 2g 2 2 n V1 V 4 / 3 2 R1 R / 3.(F ) where Δhsc : water level difference due to abrupt shrinkage of flow area (m) hsc : head loss due to shrinkage (m) 137

152 hc : head loss due to shape change (m) hf : head loss due to friction through the transition section with length of e (m) fc : head loss coefficient for shape change (see Fig. 3.68) V1 : average velocity before shrinkage (m/sec) V2 : average velocity after shrinkage (m/sec) g : acceleration of gravity (m/see 2 ) R1 : hydraulic mean depth before shrinkage (m) R2 : hydraulic mean depth after shrinkage (m) n : roughness coefficient l : length of shrinkage section (m) Fig Abrupt shrinkage flow Fig Energy loss at abrupt shrinkage Head loss coefficient for abrupt shrinkage f c is related to the Froude number F r2 and width ratio b 2 /b 1 (see Fig. 3.68). The flow condition is judged from Fig The coefficient f c in Fig can be applied to all conditions. Shrinkage coefficient C C as shown in Fig is obtained from Fig ) Head loss due to gradual shrinkage of flow h gc 2 V2 f gc.(f ) 2g where hgc : head loss(m) fgc : head loss coefficient (see Fig. 3.70) 138

153 V2 : average velocity after shrinkage (m/sec) Fig Flow condition in abrupt shrinkage (open flow channel) Fig Head loss of gradual shrinkage (pipe) 10) Change of water level due to curvature of flow Boussinesq-Lahmeyer's formula is used to calculate change of water level by curvature effect. I I 0 3 b 1..(F ) 4 R where I0 : bed slope I : surface slope b : flow width (m) R : radius of curvature (m) L( I I0) (F ) h b 139

154 where Δhb : water level difference (m) L : radius of curvature (m) < Supplementary explanation > In principel, the inlet elevation should be 1.0m higher than the base elevation of the scouring sluice for prevention of sand if a diversion weir is constructed. However, it is difficult to keep 1.0m for consideration of influence on upstream, this height can be reduced but as much as possible to take the difference between inlet elevation and the base elevation of the scouring sluice. Refer to (3). Intake level required for sand flushing function. Formula is below; Loss calculation he=he+v 22 (2 g)-v 12 (2 g) he=fe V 22 (2 g) he: Change of water level due to inflow he: Head loss due to inflow fe: inflow head loss coefficient=0.5 V1: Just in front of intake flow in Velocity=0m/s V2: flow in Velocity Q A2 g: gravity acceleration h1:water depth of intake B:wide of intake Q:amount of intake A2:Area of after intake=b (h1- he) Calculation Example B=3.5m, h1=0.6m, Q=1.5m 3 /s At first he is assume 0.046m h1- he=0.6m-0.05m=0.554m A1=3.5m 0.6m=2.1m 2 A2=3.5m 0.554m=1.939m 2 V2=Q A2=1.5m3/s 1.939m2=0.77m/s he=fe V2 2 (2 g)=0.5 (0.77m/s) 2 (2 9.8m/s 2 )=0.0153m 140

155 he=he+v2 2 (2 g)-v1 2 (2 g) =0.0153m+(0.77m/s) 2 (2 9.8m/s 2 )-(0m/s)2 (2 9.8m/s 2 ) =0.0458m Result of calculation is almost same as assumed he. So assumed he is proper. This calculation needs trial and error. For easy calculation the width of inlet, the goal seek function of excel is useful Gate The gate structure must be watertight to ensure stable water intake and firm enough against several external forces like flowing water. Steady and smooth operational workability is also required for its function. A gate is one of the most important facilities to control both water utilization and floods. The following conditions are to be fulfilled for the purpose of water use and flood control. Conditions required from the view point of Water use are; 1 to keep a constant required water level and to control intake level and discharge and 2 to have water tightness. On the other hand, conditions required from the view point of flood control are; 1 capability of being operated quickly and smoothly so as to release water safely and 2 to remain workable without accumulation of material carried by water. Structural stability and endurability are also required. (1) Selection of type of gate Type of gate is decided after considering its purpose, installation location, cases of operation, safety, dependability and economy of water intake function, especially from the view point of effective usage of the water resources, appropriate style and operation method to reduce over diversion. Materials available for gates include steel (including east steel), aluminum, stainless steel, rubber and FRP (fiberglass reinforced 141

156 plastic). This section covers a steel gate. When using materials other than steel for gate, the characteristic features of the material for gate should be studied carefully. 1) Type of gate (a) Vertical lift gate ; Fixed wheel type gate Slide gate Double leaf gate Stop-log (b) Hinged type gate; Radial gate Sector gate Mitre gate Flap gate Swing gate (c) Other type 2) Definition of Gate The gate structure consists of a gate leaf, guide frame, pivot and hoisting equipment. The gate leaf is the part which receives the hydraulic load and conveys it to pivot. The guide frame is the embedded part in concrete and adjacent to the sealing part of the leaf to prevent water leakage. The pivot is part of a hinged type gate which transmit the external force (load) to the concrete. 'The guide frame covers this function for a vertical lift type gate. The hoist is the equipment which operates a gate leaf. 3) Specific features of each gate (a) Fixed wheel type gate : With a fixed wheel type gate the hydraulic load is transmitted to a horizontal main girder through the skin plate (sheet metal) and its supporting girder. The load is finally transmitted to the guide frame by way of vertical end girders at each side of the gate leaf and wheel. A shell type transmit the load through its box type body. The gate leaf usually raised vertically by wire rope or spindle hoist. Since this type gate is mechanically and structurally simple, hoisting load is lighter than for slide gate and is more dependable, this type is the most frequently used as a barrage gate. The applied range is also been wide from small gates to long span gates. 142

157 Fig Fixed wheel type gate Fig Types of fixed wheel type gate Three types are classified by leaf structure and generally defined by height of leaf(h), width (L) and their ratio (H/L). Stiffness must be considered for long span gate. H/L should be kept large to maintain gate body safety against leakage by bending due to direct sunshine. The shape of rubber seal should also be carefully considered. Vibration at partial opening flow should also be cheeked. Upstream and downstream water levels and gate opening heights affect vibration occurring conditions. Perfect outflow never causes vibration but submerged outflow may cause it for reasons of interference by hydraulic jump, waves right after the gate and swirl over the bottom plate. Interference by hydraulic jump can be avoided by changing the angle of the bottom plate to about 20 degrees. Long term operation with a small opening and submerged outflow should be avoided. (b) Double leaf gate : A double leaf gate can control discharges and suspended load can be easily passed through. Control discharge and reduced height of piers are significant features. If control of discharge is the main purpose, the range of intake discharge must be decided in considering the intake method, operation method and the river condition. A double leaf gate has a complex mechanism for sealing, guide frame and operation and H/L ratio have to be smaller than the other types. Stability of these mechanism should be checked carefully. A spoiler 143

158 (that separates vein of water and supplies air behind the leaf) or other reasonable treatment is required to minimize vibration. Double leaf gates are divided into three types due to structural differences. a) Double leaf gate ; Two shell type or girder type gates are combined. Track rail and a hoist are installed to operate each leaf. b) Hook shape type gate ; Hook type gate leaf is installed for upper leaf and combined with lower wheel gate. Bottom wheel of upper leaf transmits load by rolling on the skin plate (sheet metal) of the lower leaf. A hoist is usually used. c) Shell type with flap ; This is a combination type of a flap gate for the upper gate and a shell type gate for the lower gate. Since the sealing mechanism and hoisting mechanism are comparatively simple, this type has been adapted as regulating gate for the diversion weir. The hoist part consists of both upper and lower operative winches and has a mechanism that makes it possible for the two hoists to work together. Further, the leaf height of the upper part flap gate should be less than one third of the total height of the gate. Fig Types of double leaf gate (c) Slide gate: This type is suitable for a relatively small span and water level difference. The mechanism is simple as a metal plate can be used for guide frame. Operation under hydraulic load causes a large load for hoisting since the gate leaf has to slide on the guide frame. Thus, this type is not suitable for large gate leaves unless operated under balanced water pressure. The spindle of an oil-pressure- operated 144

159 cylinder is generally used as the servomotor system. (d) Stop-log : Stop-log is a kind of gate that is used for small gate and discrete operation when main gate is under repair. Horizontally divided leaves are placed on top of each stair. A portable winch also used for storage of the stop logs is also desirable. A floating type and a shield type leaf are also used during temporally gate repair. Fig Slide type gate Fig Gate types for repairing (e) Radial Gate: This gate has a circular-arc-shape leaf with a trunnion pin at the center of the circular arc.hydraulic load is transmitted through the skin-plate, support beam, main beam, gate arm and trunnion pin to pedestal. A radial gate does not have a track rail and is hydraulically and mechanically advanced. It rotates around the trunnion pins and it operated economically as the hoisting load is relatively small. On the other hand, its mechanism is complex as compared with a wheel gate. Since it has many parts of low-stiffness and the total load is concentrated on the trunnion pin, discussion and alternative studies are required for its design, construction and installation. Since over-flowing water interferes with its parts, mechanical weakness and restriction of over-flow height, this type is not prevalent for intake gates. 145

160 Fig Radial gate Fig Miter gate (f) Mitre Gate and Swing Gate : Mitre gates are vertically divided into two leaves and a swing gate has one leaf. Both gate open by swinging horizontally around vertical axes at the end of the leaf(s). Since it has a relatively small operation load and no restriction for pass-through height, this type is widely used as a navigation gate. Symmetrical design for reverse pressure is required and this type has the disadvantage that operating parts are always under-water, that maintenance is not easy and that operation in a river with sand or mud pile is dangerous. Location of its installation is the most important point to be considered. (g) Sector Gate : This is a kind of radial gate that is operated horizontally with a pair of leaves. Rotating operation makes the operating load small compared with mitre gates this type is more complex. This type has the advantage that it is suitable for reverse hydraulic load, that there is no restriction for passing through height and that it does not requires a water level control facility to keep a suitable balance between inside and outside of heads. The disadvantage is that operation and maintenance are difficult as with a mitre gate and that it requires large operation space for leafs. This type has been adopted for large span gates that have to allow the passage of a large vessel passing because wheel gates cannot fulfill the height requirement. 146

161 Fig Sector gate (h) Flap gate : This type of gate is installed on the overflow portion of a weir or dam with the hinge pin beneath a leaf of shell type or plate girder type. It rotates around the pin for operation. It may also be installed over a long span wheel gate. In comparison with a wheel gate, a smaller sectional area for the leaf is made possible by a multi-point hinge system and it is applicable to the case of small ratio between height of leaf and span. Regulation by overflow makes it easy to control the downstream discharge and automatic operation is also possible by using hydraulic load. Further, this type requires-fewer and shorter piers. On the other hand, when as being installed directly on the riverbed, a 0.2 to 1.0m (or about 30% of leaf height) gap is required where hinges are installed. Sand piles or residue at the back surface of a leaf may cause imperfect flapping or damage to the leaf and cylinder. A spoiler is required to avoid dangerous vibration by overflow. A sand removal facility has to be considered if sand flow is significant. Stainless-clad steel is preferred to be used as skin plate (sheet metal) because of the inefficiency of coating. Flap gates are classified into the following types according to leaf structure, operation system and operation mechanism. 147

162 a) Leaf structure; i) Torque shaft type ii) Horizontal main girder type iii) Shell type (Fish belly type) b) Operating system; i) End leaf operation type that has operation system at the end of leaf. ii) Middle support type that has operation system at the middle of leaf. c) Operating mechanism; i) Mechanical type ii) Servomotor type Final design is decided from a combination of these types and categories and according to the site conditions installation location and operating condition. Fig Types of flap gate (i) Sluice Valve (Gate Valve) : This type consists of a valve body that corresponds to the leaf of agate and casing as well as operation system that corresponds to guide frame or pivot. Since this has a simple mechanism and is cheap, it is used as the control valve for a conduit. It must be installed at the end of the conduit to allow enough air supply and a single taper valve is helpful in preventing valve vibration when operated at partial openings. Careful maintenance is required for residues like sand in and around the guide rail. Fig Sluice valve 148

163 (j) Rubber Gate: This is a kind of weir made of rubber that is directly installed upon the riverbed. Air or water is used to form inflate of the rubber tube. Picture 3.3 Rubber dam Fig Facilities of rubber dam (2) Lifting Height The gate lifting height must be decided to allow safe flow down of the design discharge. The height of the lower end of the leaf for a vertical opening gate is calculated (as freeboard of each bank plus) release design high water level. Clearance in a conduit should be as high as top height of the conduit. (3) Material Materials must be selected to be suitable for the specific purpose of each gate type. 149

164 (4) Dimension of gate for Slide gate and Stop-log The standard of the relationship between height, width and thickness of skin plate (sheet metal) and upstream water height of Slide gate and Stop-log is shown in Table Table Dimension of gate for Slide gate and Stop-log Upstream water height of gate (m) Skin plate (Height x Width) 1.0 x x x x x x x x x x Note; Inside of is thickness of skin plate (mm) Upstream water height of gate Gate (Skin plate) < Supplementary explanation > In Ethiopian case a gate should consider the water and debris pressure not to be bended and it has to consider the operation. If the skin plate (sheet metal) is big and heavy at one gate, it should be separated two gate. 150

165 3.2.8 Related Structures Related Structures are attached to a weir or intake gate to upgrade functions of the intake facility. They are to be installed where necessary to keep the river function and must be functional and reliable for their purposes. (1) Settling Basin Settling basin is used to settle and exclude sand which may flow into an intake facility under certain river conditions and which may obstruct canals and degrade their functions. Picture 3.4 Settling basin A settling basin must be effective in exclusion of sand particles and have a proper hydraulic design for design discharge and site conditions. One is required where the intake discharge is high and harmful sand will inflow. For example, an intake without a diversion dam or a weir constructed at a steep-flow-section of a river will require a settling basin. A settling basin is also one of the most important facilities for a weir which is installed on a mountain stream where the discharge and riverbed changes. A weir with a high intake apron which is constructed at a slow-flow-section of the river and that works as a settling basin does not have to have a separate settling basin. 151

166 1) Selection of site A settling basin should be installed adjacent to the intake and a head of the canal. When it is to be constructed downstream it should be as near to the intake as possible if connective installation to intake is impossible. The slower the inflow velocity, the more effectively sand is settled. To have larger flow area, the headrace should be as wide as the intake width. Transition angles should also be as small as possible so as to regulate the flow. The inflow direction should be the same as the center line of the settling basin. When the headrace has to be curved, uneven flow must be avoided. When a settling basin is installed separately from the intake, velocity control in the headrace is important to ensure that sand does not settle in the headrace bed and to avoid a large velocity which would accelerate sand inflow. Enough drop in head is required for natural clearing of sand which settles in the settling basin. 2) Characteristics and settling capacity of inflow sediment Diameter distribution of inflow sand must be studied by sediment investigation to determine the minimum and maximum diameter of inflow particles. The minimum diameter of the particle acceptable in irrigation water is generally 0.3mm. Multi-purpose water usage including city and industrial water supply has different regulations for maximum acceptable diameter according to usage. For example for city water supply 0.1mm or larger diameter particles must be settled out. Sand particles to be settled in settling basin are conveyed as bed load in general. When water flow in the settling basin, it follows a gradient and is well regulated to avoid uneven flow. Large particles settle at the basin mouth due to an equilibrium between the critical tractive force and the tractive force at the sand surface, and a terrace is formed as shown in Fig The front surface of the terrace moves forward as sediments increase. Fig Sedimentation in settling basin 152

167 Small particles which do not settle at the enterance as well as those which are carried as suspended load form thin bed load in front of the terrace. A location (piling thickness is about 25 cm in depth) where the tractive force in the basin and the critical tractive force for the minimum diameter particle come to their equilibrium and- where there is no refloating, is first determined. The allowable settling limit is defined as the condition when the design diameter particle is settled at the location mentioned above. In reaching the allowable settling limit, the distance for complet settlement of the minimum diameter particle should be completed is about 10 times the terrace height from the terrace front. The total capacity of the settling basin is generally calculated by adding the terrace length to this distance. The capacity of the sedimentation ditch is determined by the total amount of inflow sand. Since the amount changes by season, a separate sedimentation ditch in parallel, which can also be operated properly, helps to make the capacity smaller. 3) Hydraulic structure of settling basin Settling basin must have a proper shape for the design discharge and site conditions, and must be effective for settling of the design diameter particle. It must function to exclude sediment easily and be an appropriate hydraulic structure for operation and maintenance. The sedimentation ditch should preferably be symmetrized and have a rectangular section in the flow direction. Existing basins differing from that described above have been proved to be inefficient and unsuitable for settling as well as exclusion. The side overflow type in particular tends to allow large diameter particles to pass and is not recommended. Parallel separation into more than two rows is preferred for reasons of water usage and operation. A wide sedimentation ditch without any separation wall may have low effectiveness in settling and difficulty in exclusion of sand. A relatively deep basin contributes to high efficiency in settling and exclusion, and exclusion may be possible in normal operation. 4) Transition part Uneven or inverse flow is to be avoided in the transition part. When the subcritical flow area of the headrace increases sharply at the transition part, separation of the stream line and uneven or reverse 153

168 flow occurs and the effective area of settling basin and effectiveness of settling decreases. A wider headrace and smaller angle of transition are preferred. separation of stream line starts to occur when the angle is more than 10 degrees. Transition parts which have a larger angle should have appropriate regulation treatment. There are several ways to regulate flow such as use of a guide wall, regulating grating and multi-hole board. The following methods are most effective. (a) Where a constant water level is possible, reverse gradient transition is recommended to keep the flow area constant through the section (see Fig. 3.83). A short level portion (3 to 4m) follows the reverse gradient section and a 45 degrees inclined drop connects this to the sedimentation ditch. Separating walls should be extended back to end point of the reverse gradient section. A drainage pipe (100mm in diameter) is required to exclude water which is still ponding in the settling basin after stopping of operation. (b) When the intake water level tends to change, and a constant water level is difficult to maintain, a regulating pipe (see Fig. 3.83) which has constant flow area through the pipe is recommended. Fig Method of regulating flow in transition part The bed of this pipe should have level or reverse gradient subject to site conditions. This method is applicable when the transition part crosses a road. This can be used as a spillway by installing a trough at the beginning of the pipe or as a cleaning channel of suspended materials. When separating the sedimentation ditch, separate pipes should be provided and regulation gates are preferred at the 154

169 beginning of the pipe. 5) Width and depth of sedimentation ditch The width and depth of the sedimentation ditch is determined so as to be suitable for site conditions and discharge. Effective dimensions for settling and excluding sediment are selected. For efficient transportation and sedimentation in the sedimentation ditch and for the relationship between the actual tractive force and the critical tractive force in the sedimentation ditch, the width and depth of a sedimentation ditch, which has even rectangular cross section, should be decided as follows. (a) Level bed; B Q hu (F ) where B : width of sedimentation ditch (m) h : water depth at the allowable-critical limit (m) Q : design discharge (m 3 /sec) u : critical tractive force for suspended solid (m/sec) A standard of the critical tractive force for suspended solid is about 80% of the settling velocity of minimum diameter particle. (in case of d=0.03cm, u=0.20m/sec) (b) Supercritical flow flushing; B h 2 Q 2 h 2 1/ 2 h (F ) Where B : width of sedimentation ditch (m) h : water depth at the allowable critical limit (m) Q : design discharge (m 3 /sec) a = 1.0 ~ 1.2, velocity change coefficient k = τc/(pi) τc : critical tractive force (t/m 2 ) p : unit weight of water (t sec 2 /m 4 ) i : gradient of sedimentation ditch ( i > 1/100) Iwagaki's formula m d cm, U*c 2 =τc/p=8.41d 11/32 (cm 2 /sec 2 ) 155

170 Fig shows the relation between B and h under several conditions mentioned in the figure. Values are calculated using Iwagaki's formula. For a case when both formulae (F ) and (F ) are applicable, the larger value is to be taken for B with using a h value. Since the thickness of sediment at the point where the minimum diameter particle has settled completely is D (about 25cm, mentioned above), the depth of settling channel (H) at this point is; H=D+h (F ) If the calculated width of sedimentation ditch is too wide compared with its depth, it can be separated by walls to have a smaller capacity of each ditch so that the basin can be improved from view points of structure and water usage. Fig Relation between width and water depth at critical section of allowable settling The relationship between width and depth of the sedimentation ditch is predominantly determined by site conditions. The depth should preferably be less than 3m to enable it to exclude sediment naturally. 6) Length of sedimentation ditch Length is calculated to be enough for settling of the minimum particle in Paragraph 2) above. Since decision on the length is not 156

171 easy, several formulae should be tried before deciding with allowance for safety factor. Width and depth are calculated by any formulae on the basis that the actual tractive force at the point where minimum particle is settled completely should be less than the critical tractive force for the particle. (a) Formula from sedimentation theory; h Q L K u K (F ) v g Bv g Where L :length of sedimentation ditch (m) K : safety factor ( ) h : water depth at a point where minimum particle is settled completely (m) B : width of sedimentation ditch (m) U : average velocity in the sedimentation ditch (0.2m/sec or so) Vg: critical settling velocity (m/sec) Q : design discharge in sedimentation ditch (m 3 /sec) The formula (F ) is from sedimentation theory. It includes some theoretically irrational points according to actual sedimentation condition of the particles range in settling basin. It gives proper values for width and depth of the ditch. This formula is concluded to be applicable for length design calculation using an appropriate safety factor considering design conditions. Table 3.16 Allowable critical sedimentation speed in muddy water Specific gravity of muddy water grain size (mm) Vg m/sec Vg m/sec l

172 Fig Relation between diameter (d) of sandy gravel and settling velocity (V g ) (b) Formula on the basis of detaching length of the flow on gapped bed (reference); L.(F ) where L : length of sedimentation ditch (m) 1 : length of sediment terrace (m) 2 : distance from terrace front to the point further than whom there is no re-floating of sediment (m) (l2 10W'), W: gap height at the beginning of sedimentation ditch, W': height of terrace front (W' W) 3 : excess length (m), about gap height at the end of the sedimentation ditch 158

173 Fig Each factor on sedimentation in setting basin Terrace length l 1 is an important element for decision of the capacity of sedimentation ditch. This is initially determined roughly according to site condition, inflow volume of and and operation method. Check calculations are required using several other formulae. (c) Empirical formula; L 20 Q.(F ) where L : length of sedimentation ditch (m) Q : design discharge in the ditch (m 3 /sec) The formula above is applicable to discharges between 1 and 5 m 3 /sec and using a supposition that L=20m when Q=1m 3 /sec. This formula has been proved to give appropriate results when the ratio between B and h is nearly equal to 1.25 and the diameter range of minimum particle is 0.3 to 0.5 mm. This formula is applicable for rough calculations. 7) Gradient of sedimentation ditch A settling basin, which excludes sediment naturally, employs supercritical flow to clean sediment out from the basin with a given gradient to sedimentation ditch. The steeper the gradient, the more effective the flushing work. The steeper gradient causes a bigger tractive force so that a longer length is required until the tractive force decreases to design level. Finally the gradient is decided considering both conditions of settling and flushing as well as site and discharge conditions. 1/50 to 1/70 is dominant in general for the gradient. When there is enough drop, a steep gradient of 1/10 at 159

174 the end of sedimentation ditch is preferable to make flushing easy. Cross sectional gradients which make the bed shape like the bottom of a ship may help flushing but its effectiveness is small for a wide ditch. Separation of the basin into narrow ditch is more effective without cross sectional gradients. This type has bigger capacity and high performance with supercritical flow flushing and proper operation. A settling basin which employs artificial flushing should have a level bed and enough width for convenience of machinery operation using a power shovel, bladeless sand pump, etc. When a sand pump is used after collecting sand in a pond, a bed gradient is required. 8) Hydraulic structure of flushing pipe Natural flushing of sediment in a sedimentation ditch requires appropriate values of each items as follows and their combination. The items are gradient of ditch, transition at the end of channel to sand flushing pipe, flushing head between the beginning and the end of sand flushing pipe, gradient of the pipe, attachment angle of the pipe to sedimentation ditch and relations between sand flushing discharge and width of the ditch, and between the discharge and cross sectional shape of the flushing pipe. When there is enough drop head for a flushing pipe, steeper gradient should be applied for the end section (5 ~ 10m) of the channel and the water level at the entrance of the sand flushing pipe should be lower than design flushing water level in the ditch (see Fig. 3.87). Fig Transition from end of ditch to sand flush pipe, with enough drop head 160

175 Fig Transition from end of ditch to sand flush pipe Table 3.17 Schaffernak experiment about relation between grain size (d) and flow velocity (v) Grain size(d)(cm) unit Traction starting flow velocity (v0) m/sec Traction Continuing flow velocity (v1) m/sec Traction Stopping flow velocity (v2) m/sec And the cross sectional area and gradient of the pipe should be large enough to flush sediment out of the pipe. Circular and rectangular shapes are both applicable for the pipe. The shock wave caused by the transition part at the end of the sedimentation ditch and a proper balance between flushing discharge of sediment and width of the ditch and between the discharge and cross section of rectangular shape pipe make supercritical flushing in settling basins possible for any flushing head. A multi-lane sedimentation ditch in a basin and a reverse-gradient-transition part to the basin make simultaneous operation of usual operation and sediment flushing possible. The velocity caused by drop for flushing is 1.2 to 1.5 times faster than usual operation and the effects of the reverse gradient is remarkable. Design flushing discharge of sediment is equal to the discharge of each ditch. The relationship between the width of the sedimentation ditch and of the rectangular-shaped flushing pipe is calculated by the formula (F ). 161

176 b B 2 2 B / B F 2 r / B..(F ) Where B : width of sedimentation ditch (m) b : width of rectangular-shape pipe (m) l : length of transition to pipe (m) Fr1 : Froude number in the sedimentation ditch When h 2 <h c2 between the critical depth (h c2 ) and the depth of shock wave (h 2 ) against the width of flushing pipe (b), the shock wave enters the flushing pipe, maintaining supercritical flow conditions and formula (F ) comes into effect. h h 2 c Fr / B 3 Q gb h 1 (F ) Where Q : sand flushing discharge (m 3 /sec) h1 : critical depth in sedimentation ditch (m) g : acceleration of gravity (m/sec 2 ) Fig shows the relationship between the formulae (F ) and (F ). The ratio b/b is the functions of F r1 and of l/b. In the regime where h 2 /h c2 <1, the values b/b and l/b are read on the specific line of F r1, and b as well as l are determined. The length of transition is to be doubled for safety. The height of flushing pipe (H) is subject to relationship with its width (b). When there is no back-water affect from the river at the outlet of the flushing pipe, the slope of the pipe should be nearly same as that of the settling channel and H should be larger than hc 2 for smooth flushing. When the water level of the river is high at the outlet of the flushing pipe, the hydraulic jump height (h 3 ) at the inlet of flushing pipe may be found in Fig and the height of the pipe (H) should 162

177 be larger than the hydraulic jump height (h 3 ). The ceiling of the pipe should be level. When sedimentation ditch is multi ditch, sediment is gathered into a collecting pond from each ditch using connecting pipes and then flushed through a flushing pipe. The connecting pipes should curve naturally for smooth inflow. Several site condition may permit right angle curving at a transition portion which is installed at a side of each ditch. Fig Relation between Width ratio of sand flush pipe and sedimentation ditch, ratio of length of curve and width of sedimentation ditch, ratio of Froude No. and depth of shock wave 9) Related structures Regulating gate, flushing gate, spillway, etc. may be installed as related structures. A regulating gate for a water intake is installed around intake mouth. When it has to be installed within the headrace the installation location should be sufficiently apart from 163

178 settling basin to avoid a distortion of flow by gate operation on the flow regime in the settling basin. When a flap gate is installed for regulating of the intake discharge in a conduit as a part of the headrace, unexpected noise or vibration may occur according to the flow and weather conditions. These problems have to be taken care of. A regulating gate is installed at the inlet of each sedimentation ditch for controlled flushing of sand. This kind of gate is not required when an adverse slope is introduced to make regulation and flushing possible together with normal operation. At the end of each sedimentation ditch, a flushing gate is installed and a regulating gate or a weir to keep the design water level in the sedimentation ditch. A weir is preferred to keep a stable water level. A spillway is installed in the middle of a headrace or right before a settling basin for the security downstream reaches. If the locations mentioned above are not available for installation due to site conditions, the spillway can be installed in the side of the sedimentation ditch. When the settling basin is designed for disposal of the particles of pollutant affected sand, access roads are required for machinery like a conveyer truck and cleaner, which are required to transport the sand to the specific area for disposal. 164

179 Fig.3.90 Relation between depth ratio of before and after hydraulic jump, Froude No., and gradient on slope < Supplementary explanation > There is some possibility to control grain size to be taken by design of intake, and settlement basin. Observation tells us that, it is necessary to consider to arrange settling basin. In case of settling basin, grain size is defined by inflow velocity at intake which is defined by intake design. (2) Protection of bank and major bed In constructing a weir, bank protection may be required to prevent erosion of the banks or dikes for a certain section of the river. Major bed (Flood plane) protection may also be required to prevent erosion. 1) Protection of transition and dike Discharge and flow direction may change due to construction of a weir. Protection works of riverbed, minor bed, major bed and river bank may be necessary against these changes. Protection of the riverbed is mentioned in the Section of Other protection works 165

180 are mentioned below. (a) Protection work of transition The upstream edge of transition protection should be the upper end compared with 10m upstream from the upstream side apron and 5m upstream from the upstream side riverbed protection. The downstream edge of transition protection should be the lower end compared with 15m downstream from the downstream side apron and 5m downstream from the downstream side riverbed protection. This protection work section is divided into two parts, an apron protection part and a riverbed protection part. Further protection of up- and down-stream river bank in this section may be necessary. When a weir is designed at a curved section of the river, transition protection should be extended further than the distances mentioned above at the downstream side. Bank protection height is higher than design flood level (freeboard 0.5~0.6m) or as high as original river bank height. a)apron part of bank protection This part of protection covers the section between piers and both side apron and is also called transition retaining wall. The wall must have vertical front surface lest there should be any problem when overflowing water comes down from top of gates of from fixed portion of the weir directly to the slope of bank protection. Generally, the wall is to be a reinforced concrete wall or concrete gravity wall. When watertightness is necessary at both sides of the weir, water seal is required at joints between the walls and aprons. b) Riverbed protection part of bank protection This part of protection covers the rest of bank protection. It must employ the same structure as above or joint structure of slope protection work and foundations. Foot protection, which is required for river structure is involved in bed protection work in this case, hence foot protection is required at both side of the ends of bed protection. For both cases, the depth of foot protection has to be studied considering the erosion condition of the river. Deep foot protection or sheet piles must be adopted to make structures stable. Genellay the depth of foot protection is adopted 0.5m to 1.5m. Bank 166

181 protection height is as high as original river bank height when transitive bank protection is constructed at the minor bed slope. Wing width on major bed is 1 to 2m for a standard case. When the protection is constructed adjacent to a fixed weir, wing width must be wider according to the flow condition. Fig shows the general shape of bank protections. If the protection employs reinforced concrete structures, concrete cover on reinforcement bars must be thick enough. (b) Bank protection with dike Bank protection with dike is divided into two cases, one of which is that transition bank protection covers the function of dike protection, when a weir is installed at the single sectional river. The other case is that dike protection is required independently; when the weir is installed at the minor bed section of a multi-section-river. For the former case, the structure must follow the basic standard mentioned in Paragraph (a). For the latter case, the protection area is decided according to the agreement between the authority and the river controller. In this case, when the distance between river bank and the nearest pier is large, this protection might not be required. Height of this protection is to be higher than or at least equal to the design high water level. When weir is installed at the bending part of the river, the height is to be as high as the dike top. 2) Major bed protection The parts where major bed protection is necessary are; 1 around piers and downstream major bed from piers, 2 upper portion of the section between inlet and sluice way and both side of connection conduit, 3 upper portion of cut-off wall when a weir is installed in a multi-section-river and 4 upper portion of wing walls which are installed on both banks. Protection,work is decided Fig Protection of transition and dike 167

182 according to the river scale and structure of the weir. Roughness of the protection surface is preferred to be as same as that of the riverbed. < Supplementary explanation > This item should be designed according to the design flood level. Refer to 3.1.4(4) Control Facilities Control facilities have to be installed to operate and maintain intake facilities and related structures and to facilitate intake of the design discharge reliably and to flush the flood discharge safely. (1) Operation equipment Operation equipment must be selected and installed so as to meet the requirements for smooth operation of all the facilities < Supplementary explanation > It is necessary to consider safety of the operator such as walking slab to operate gates. (2) Power receiving and distributing facilities The power receiving and distributing facilities are installed to receive electric power from the power source available at the site and to distribute it properly to the respective loads. The power supply conditions such as reliability of service and voltage fluctuation range should be considered on planning and design of the facilities. < Supplementary explanation > Don t forget to make small-scale irrigation gates to be manually operated with relative ease and safety. (3) Operation and maintenance bridge 1) An operation and maintenance bridge is installed for safety operation and maintenance of a facility and must have a safety structure. The operation & maintenance bridge is a kind of bridge which is 168

183 installed as an access facility to a control house or so to operate the facilities. It may be installed over piers to carry gate operating equipment. Steel, prestressed concrete and reinforced concrete are materials used for bridges. The width of it must fulfill the needs of operation and maintenance as well as of special operation in an emergency. 2) Span The span of the operation bridge is decided after considering river section, flow direction, features and its economical performance. 3) Location of abutment (a) An abutment of bridge must not be installed within the flow section of the river in the following three (3) cases when installed on the bank of a river (or on the dikes if a sectional shape is designed). The three cases are ; 1 width of the river is more than 50m, 2 installing bridge is within the.back water section and 3 installing bridge is within tidal zone of the river (see Fig. 3.92). Fig Relation between abutment, dike, and river bank (b) When abutments are installed in banks or dikes in the section of the river except the cases mentioned above, the abutments must not be erected inside the slope of the banks or dikes (see Fig. 3.92). (c) Abutments must be installed in parallel with the top-of--slope 169

184 line. If impossible, special treatment is required to avoid extreme hindrance to the structure of the bank or dike (see Fig. 3.93). Fig Example of arrangement of abutment of which surface is oblique to the alignment of dike Fig Position of bottom of abutment (d) Foundations installed in banks or dikes must be founded on firm ground (see Fig. 3.94). 4) The beam seat height(operation slab) The beam seat height must be decided considering the operation height of the gate of the movable portion of the weir, wave height and safety clearance. (4) Other operation facilities Warning equipment, visual and audio detection, lighting, data processing, and trash-raking systems may be installed if necessary. < Supplementary explanation > It is better to construct simple trash-racking systems like screen and stand for taking trash by human power to reduce its cost. (5) Operation Proper operation are necessary for ordinary and flood cases to maintain the weir's function to intake and release design discharge. 1) Intake operation Intake operation must be executed to draw the required discharge, prevent sand from flowing into the canals and to avoid causing 170

185 obstruction to the third parties. Drawing the required water is the first priority. Appropriate operation of gates, maintenance of scouring sluice and prevention of sand inflow are required for the purposes. The following points must be considered. (a) Gate operation must meet fluctuation of river discharge and intake discharge. It is appropriate to operate the intake gate to keep the water level at pondage during operation. Scouring sluice gate(s) is required to regulate water level and an automatic control devices of the sluice gate. Discharge from bottom discharge type gate is smaller than the one from overflow type gate at the same opening in both cases. Namely, precise discharge control for water level regulation by the former type is more difficult than the latter one. The wider the gate, the bigger this difficulty becomes due to the bigger deflection of the gate. Thus, when a flap gate is installed, additionally an appropriate operation is required to maximize the function of the flap gate. (b) When introducing an automatic operation system, precise detection of water level and discharge is necessary, and the function of the detection system must be checked for its sensitivity. (c) For maintaining the function of the scouring sluice, deposited sand in the scouring sluice must be removed and a clear water way must be maintained. Continuous monitoring of the conditions of depositing and timely removal of the sand are necessary. Especially when a flood is abating, an appropriate gate operation is required since functioning of the scouring sluice depends on the discharge. (d) If sand deposition or sand dune is growing in the scouring sluice and, therefore, degradation of the function of the scouring sluice is expected, deposited sand must be removed off shortly. (e) In general, gate operation during an abating flood starts with the spillway gate at the opposite side to the inlet, then works across to the scouring sluice. The reverse order of operations may be selected considering conditions of movement of the sand. (f) A special operation not to cause big fluctuation of water level in front of the inlet is required if the height difference between inlet apron and scouring sluice base is small. (g) Owing to the deposition on the scouring sluice, the flow section of 171

186 the scouring sluice is constricted and several malfunctions such as obstruction of water flow, and enlargement of sand grain size by increasing of inflow velocity, etc. are induced. To avoid such malfunctions, it is necessary to minimize water level fluctuation and prevent vortical inflow front of the inlet. Smooth and quirk operation of gates are also required to prevent sand deposition in front of the inlet during flooding. (h) Trash or other matter which is caught within the gate guide frame or screen when flooding must be excluded as soon as possible. (i) Continuous monitoring of the condition of sand deposition in the settling basin is required for timely flushing the sand. Flushing must be avoided at the peak irrigation stage and should be executed when the irrigation requirement is small (j) Sand deposition at the outlet of the flushing pipe has to be removed off for maintaining the function. 2) Flood time operation. Prevention of decreasing cross-sectional area of the river and protection of joint parts of weir and dike must be considered for the operation while flooding. (a) Detection of discharge change of the rivers by systematic observation is necessary for preparation of suitable and quick operation of gates against floods. (b) Decrease of cross-sectional area of the river must be avoided and the water level must be kept less than the design high water level by suitable operation of gates. (c) When releasing of water from gates is necessary, care has to be taken for river bank protection or riverbed protection from erosion owing to change of flow line. (d) An operation manual for gates is prepared for training on operations, which is necessary for correct understanding about the manual 3) Operation and maintenance of the facilities Checking and monitoring of functions of the facilities are required for operation and maintenance. The following items must be considered for the purposes. 172

187 (a) Operation and maintenance of fixed portion a) The upstream riverbed tends to be scoured when flooding if the weir is constructed on permeable foundations. If the scour depth becomes lower than the weirs foundation level, stability of the weir will be jeopardized. By continuous and timely detection, an upstream apron may be constructed additionally if necessary. b) Uplift force is working at the downstream side of the weir along foundation line of the weir body or of the downstream apron. Breaking of the water stop may cause piping at construction joints. Piping is one of the most serious problems for stability of a weir body and must be repaired when found. Repair of piping comes more difficult near the center of the body. A popular method of repairing of piping is grouting. Research of water pressure and ground conditions is required before repair. c) Uplift force is usually designed to be cancelled at the downstream end of apron. If there is a greater uplift force than designed, which means a shorter creep length than designed, a treatment to lengthen the creep length is required. d) If rolling rocks are expected at the site, monitoring of wear on the top of the weir body and of the apron and proper repair are required. e) The joint between apron and riprap is also a transition of roughness structurally and turbulence of flow may happens near the joint. Monitoring of scour by soil uptake by vortical flow and proper treatment are required. f) The length of riprap is designed longer than the anticipated hydraulic jump length. However, since the hydraulic jump may happen downstream from the riprap by chance, monitoring of the locations of hydraulic jump and proper treatment such as extension or increasing roughness of the riprap are required. g) Monitoring of the scour condition at the joint between the riprap and unprotected river bed. Since complete prevention of scour there is almost impossible, necessity for repair or extension of riprap is decided from consideration of the monitored results as to scour speed and depth at and around the joint. 173

188 (b) Operation and maintenance of movable part of weir a) Operation and maintenance of the apron, Joints between apron and riprap, and joints between riprap and unprotected river bed must follow the directions mentioned above in the section on operation and maintenance of fixed weir. b) Since the apron connected to the movable part and scouring sluice may be significantly worn, monitoring and proper repair are required. c) Gates must be re-painted every 3-5 years for rust protection. Cables and motors must be monitored and lubricated to the required parts for safe operation. d) The oil volume and pressure conditions must be monitored continuously for an inflation weir with hydraulic operator. A packing for the water seal may become significantly worn and must be replaced at intervals as necessary. (c) Revetment a) Since the scouring sluice is usually installed at the lowest elevation of a natural riverbed, the flood may be concentrated there and flood energy may carry its effect further downstream than expected and a revetment there may be damaged. Revetment beside the thalweg must be designed with care. b) The contact line of piers or the fixed weir body with the riprap may be one of the weak points and must be designed with care. < Supplementary explanation > For discharging the sediment in front of intake and keeping water way open, it is necessary to open the gate of scouring sluice. It is necessary to check the situation of sediment especially after flood. 174

189 4. DATA SHEET, CHECK LIST AND OTHERS 4.1 Data Sheet Data sheet is effective method to understand easily the headwork design information. It helps not only for the person in charge of the headwork design but also for other officers participating. It can be used for checking design, explanation, and database management. Format and example of the data sheet is shown below: 175

190 Format of data sheet 176

191 Example of the data sheet 177

192 4.2 Check List The check list is one of the effective methods to prevent oversight and mistake while designing. It makes the condition of designing easily understandable. With the present s situation some of the investigation and design procedures can t be exercised due to the problem of cost and equipment. These necessary matters can be done during construction phase. It is not necessary to check all of the item before construction. There are necessary item that should be checked at construction time. Therefore, check lists are made separately one for before construction and the other for construction time. The format of the check lists are in the next pages. When the answer is yes, check into. And fill in the necessary information. Some of the item is not necessary isn t to be checked according to each situation of the project. 178

193 Design check list (before construction) (1/4) Item(s) Contents Check Remark Rainfall data Annual Maximum daily rainfall Annual Minimum daily rainfall Annual daily rainfall ~ ~ ~ Survey Topographic survey Longitudinal survey Cross-section survey Km section Survey for temporary works Collection of topographic map River Discharge data (Discharge condition, water level and discharge) How to get Long-term records of river discharge Run-off analysis from rain fall data Measuring the flow Collecting information from local community Term of data ~ Sediment data How to get Measuring Observation Collecting information from local community Term of data ~ Condition of riverbed (condition of thalweg, riverbed slope) How to get Measuring Observation Collecting information from local community Condition of riverbed (river bed material) How to get Measuring Observation Collecting information from local community Flood control plan If there is flood Situation of drainage Data is collected Upstream Downstream Dikes, Bridge and Data is collected other structures Present condition of river water utilization Data on current situation of river water utilization from other sector is collected 179

194 Design check list (before construction) (2/4) Item(s) Contents Check Remark Foundation How to investigate Drilling Test pitting Observation Bearing test by standard penetration test Bearing test by Loading test Investigation on river deposit Ground water investigation Investigation for construction works Meteorology Surface water Ground water Riverbed conditions Construction equipment Construction material(cement) Construction material(gravel) Construction material(sand) Construction material(stone for masonry) Transportation system Electric source Other Environmental Impact assessment Design water intake discharge Maximum design discharge Measurement test Design intake water level There is consideration for irrigation area There is consideration for protecting flowing of sediment. Design flood discharge How to achieve Run-off analysis Flood mark Capacity of river There is Safety factor There is probability analysis / Design flood level It is calculated from design flood discharge Study of riverbed evolution Riverbed slope Riverbed material Riverbed condition 180

195 Design check list (before construction) (3/4) Item (s) Contents Check Remark Position of Headworks Availability of a stable thalweg close to the bank at the proposed position of water intake, Sufficient water intake must be feasible even during the dry season, Least sediment inflow during water intake, Least effect of weir construction on up and downstream, Stability of the structure can be achieved with economical construction costs, Convenient for operation and maintenance. Method of water intake Position of intake Natural intake Intake by weir constructed to maintain a constant water level The position of the intake can ensure flushing of sediment and easy maintenance of the thalweg, One side intake, Both side intake Fixed type Floating type Type of weir Creep Length Creep length is considered Elevation of weir crest Study of possible effect on the river control of upstream Scouring sluice It is enough height 1m difference between inlet and base of scouring sluice, Intake height, 10cm head loss margin There is There is off take discharge, maximum flood discharge Spillway There is maximum flood discharge, dimensions of the spillway Pier There is interval, separate foundation Weir stability Overturning ok Sliding ok Settlement ok Stress analysis ok 181

196 Design check list (before construction) (4/4) Apron Riprap Foundation work Item Content Check Remark Upstream and downstream Cut-off wall Inlet Check the following Thickness is enough, The monolithic construction of apron and weir for creep length at floating type The reinforcement bar is put in bottom slab for protecting destruction at floating type It is Length is calculated by guideline Size of riprap materials The allowable bearing capacity of the foundation is bigger than contact pressure Creep length is enough 1m difference between inlet and base of scouring sluice, Inflow velocity is smaller than 1.0m/s Approach velocity is smaller than 0.4m/s Gate Consider gates that are not bending Consider gates that are not heavy for operation by human Wing wall It has enough height It has enough stability Settling basin Sediment load has to be known The design is done by guideline Protection of bank It has enough capacity Operation Stand There is enough space and safety 182

197 Design check list (At construction term) (1/1) Item Contents Check Remark Survey Longitudinal survey(bed rock) Cross-section survey (bed rock) Survey for temporary works (additional) Sediment data How to get Measuring sediment load Observation Condition of riverbed (condition of thalweg, river slope) How to get Measuring Observation Condition of riverbed (river bed material) How to get Measuring Observation Foundation How to investigate Drilling Test pitting Observation Bearing test by standard penetration test Bearing test by Loading test Investigation on river deposit Ground water (subsurface) investigation Other Environmental impact assessment (hydrology, salinity, etc) Weir stability(bed lock elevation will be changed) Overturning ok Sliding ok Settlement ok Stress analysis ok Foundation work(bed The allowable bearing capacity of the lock elevation will be changed) foundation is bigger than contact pressure Wing wall(bed lock elevation will be changed) It has enough height It has enough stability 183

198 4.3 Coefficients of Roughness (1/3) Lining, retaining walls, tunnels, culverts, siphons, and aqueducts Materials and conditions of canals Concrete (cast-in place flume, culvert, etc.) Concrete (shotcrete ) Concrete (with precast flume, pipe, etc.) Concrete (reinforced concrete pipe ) Concrete block masonry Cement (mortar) Asbestos cement pipe Steel (locked bar or welded ) Steel (rivet ) Smooth steel surface ( not painted) Smooth steel surface and pipe (painted) Corrugated surface (steel sheet ) Cast iron (not painted) Cast iron sheet and pipe (painted) Chloride vinyl pipe Reinforced plastic Ceramic pipe Earth lining Asphalt (smooth surface ) Asphalt (rough stone ) Masonry (rough stone wet masonry) Masonry (rough stone dry masonry) Wood (wooden gutter ) Wood (lined in thin layer, treated with creosote ) Rock tunnel with no lining on overall cross-section area Rock tunnel with no lining except concrete placed on the bottom Vegetation coverage (sodding ) Coefficient of roughness Minimum value Standard value Maximum value

199 4.3 Coefficients of Roughness (2/3) Canals constructed by excavation or dredging Coefficient of roughness Materials and conditions of canals Minimum value Standard value Maximum value Earthen canals, uniform and straight 1) No weeds (immediately after completion of the canal ) 2) No weeds (after the canal has been exposed to weather ) 3) Gravel (no weeds) 4) Few weeds with short grasses Earthen canals, non-uniform and curved 1) No vegetation coverage 2) Some weeds 3) Dense growth of weeds or water weeds 4) The bottom is earth and the side walls are covered by rubble stones 5) The bottom is covered by stones, and the side walls are covered by weeds 6) The bottom is covered by cobble stones, and the side walls have no weed Drag line excavation and dredging 1) No vegetation coverage 2) Some shrubs on shore Rock excavation 1) Smooth and uniform 2) Irregular

200 4.3 Coefficients of Roughness (3/3) Natural flow canals Coefficient of roughness Materials and conditions of canals Minimum value Standard value Maximum value Small canals on flat land 1) No weed and straight. No fracture or deep water spot when the high-water level is reached 2) Same as above, but a lot of stones and weeds ) No weed, but meandering Some shoals and deep water spots 4) Same as above, but some stones and weeds 5) Same as above, but low-water level and few changes in slopes and cross sections 6) Same as the line item above, but more stones 7) Weeds and deep spots in mild flow sections 8) Section with thick vegetation of weed. Many deep water spots and trees Canal in mountainous land, no plant in the canal. River banks are steep. Trees and shrubs along river banks are immersed in the water when the high-water level is reached. 1) River bed is covered by cobble stones and gravels. 2) River bed is covered by large cobble stones Large canals 1) Regular cross section without large cobble stones or shrubs 2) Irregular and rough cross section

201 5. EXAMPLE OF DESIGN FOR HEADWORKS This Chapter shows an example of crucial data and calculation for basic design of headworks. In case of actual design, designer must follow the contents of this Chapter. 5.1 Basic Design Input Data (Chapter2) Discharge through Float Measurement Method (Chapter2.1.1(2)2)) According to the river discharge measurement data, the calculation table can be made. Example; Measurement date 26 / 12 / 2012 Area Velocity Discharge Position Measurement point Measuring L t Velocity V W(m) D(m) A(m2) line (m) (s) coefficient (m/s) Q(m3/s) Left side of No river bank No No No No Right side of river bank Total Ave River discharge = 0.12 x 0.66 = 0.079m3/s = 79l/s Refer to Excel format guide 1 for the details. 187

202 Excel format guide 1. Float measurement method #Please input data into yellow cell. Area W(m) D(m) A(m2) Left side of river bank No No No No No Right side of river bank Measurement date 26 / 12 / 2012 Velocity Discharge Measuring line V(m/s) Q(m3/s) Position Measurement point L(m) t(s) Velocity coefficient Total Ave

203 5.1.2 Riverbed Slope (Chapter2.1.2(2)) According to the river slope survey data, the calculation table can be made. Example; No. River slope survey data Station No. Distance L(m) Elevation H(m) Accumulative Height H(m) Area A(m2) Total H avg = 2 x A / L = 2 x / = 6.09m I (slope) avg = H avg / L = 6.09 / = = 1/35 The result of calculation, average of river slope = = 1/35 Refer to Excel format guide 2 for the details. 189

204 Excel format guide 2. Average River slope #Please input data into yellow cell. No. Station No. Distance L(m) Elevation H(m) Accumulative Height H(m) Area A(m2) Total H avg = 2 A/ L = 6.09 m I (slope) avg = H avg / L = = 1/35 190

205 5.2 Basic Design (Chapter3.1) Design Water Intake Discharge (Chapter3.1.1(1)) (1) In case of getting discharge data in or near river basin of project site 1) Data collection In or near river discharge data on project site is collected from Ministry of Water, Irrigation and Energy, etc. It is better to have more than ten years data. Based on these data, the latest ten years data can be used. 2) Selection of reference year Based on 1) data, reference year occurred minimum discharge in ten years is selected. Example; Take minimum discharge of each year. Year Minimum Date Remark discharge (m3/s) January December December January January December Reference year December December January December The year which is occurred minimum discharge in ten years is ) Selection of standard base flow Based on the reference year data, standard base flow (355/365 days discharge (D s )) is selected. Example; Reference year: 2009 Take small discharge from minimum to 11 th value of 2009 s data Ranking Discharge (m3/s) Date Remark December December December 191

206 December December December December December December December December 355/365 days discharge The standard base flow is 1.58m3/s in 20 December. 4) Calculation of base flow at the point of intake Based on the data of river basin and catchment area of the project, base flow at the point of intake is calculated. Example; River basin (A r ) is 1000km 2 from delineation Catchment area of the project (A p ) is 50Km 2 from delineation Base flow (D) is calculated by watershed ratio method. D = D s A p / A r = / 1000 = 0.079m3/s 5) Calculation of the amount of usable water Based on base flow, the amount of usable water is calculated. W = D 0.8 = = 0.063m3/s (2) In case of getting discharge data by actual measurement 1) Data collection Refer to 2.1.1(2) and ) Making consecutive discharge data Based on 1) data, it makes the consecutive discharge data of ten years by tank model. Refer to Guideline for Irrigation Master Plan Study Preparation on Surface Water Resources for the detail. 3) Selection of reference year Refer to 5.2.1(1)2) 192

207 4) Selection of standard base flow Refer to 5.2.1(1)3) 5) Calculation of base flow at the point of intake Refer to 5.2.1(1)4) 6) Calculation of the amount of usable water Based on base flow, the amount of usable water is calculated. The existing design water intake discharge in the downstream of the project Q dw = 0.01m3/s (This value can be obtained from previous design document and site investigation.) W = D 0.8 Q dw = = 0.053m3/s Design Intake Water Level (Chapter3.1.1(2), 3.1.4(1), 3.2.6) (1) Water level of the field at the highest elevation of the irrigation area The field level at the highest elevation of the irrigation area + irrigation water depth on the farmland = EL m m = EL m (2) Water level at the starting point of the main canal - Canal length from the starting point of the main canal to the starting point of the irrigation area (the highest elevation of the irrigation area) = 1300m - Canal slope (assumption) = 1/1000 It can be changed after decided canal slope if necessary. And it must be calculated again as everything below. - Necessary water head = /1000 = 1.3m - Water level at the starting point of the main canal = Water level of the field at the highest elevation of the irrigation area + Necessary water head = EL m + 1.3m = EL m (3) The hydraulic loss between the intake and the starting point of the main canal There is no structure in this design but this loss assumed 0.1m for safety. 193

208 (4) Other structural losses at the intake (hydraulic loss of entrance) 1) Inlet sill The inlet elevation prefers to be 1.0m higher than scouring sluice sill and also prefers to be more than 1/6 of maximum flood depth of the river from the riverbed for prevention of sand. But in case of small head weir, a minimum inlet elevation is at least 0.5m higher than scouring sluice sill. If the height from scouring sluice sill to inlet elevation is lower than 1.0m, settling basin should be considered. (a) Scouring sluice sill It adopts the elevation of river bed level as the elevation of scouring sluice sill. Inlet sill Scouring sluice sill (EL m) + more than 1m = more than EL m (See 3.2.6(3)1)) (b) Maximum flood depth of the river (See 5.2.4) - Maximum flood depth of the river = Design flood level Riverbed level = EL m EL m = 4.32m Inlet sill 1/6 of maximum flood depth of the river = Riverbed level m 1/6 = EL m = EL m As the comparison of (a) and (b), inlet sill EL m 2) Intake size and hydraulic loss of entrance B = Q / h 1 V (See 3.2.6(3)3)) At the time, 0.6 Intake velocity (V) 1.0m/s (See (3)2)) Approach velocity (Va) 0.4m/s (See (3)2)) he = he + V 22 / 2g - V 12 / 2g = f e V 22 / 2g + V 22 / 2g - V 12 / 2g (See 3.2.6(5)1)) (this case, V 2 1 = 0m/s) By using the above two formula, it calculates he, B and h1 by trial and error calculation. Refer to Excel format guide 3 for the detail. The result of calculation, intake size is B=0.4m, H=0.25m So, hydraulic loss of entrance ( he) = 0.045m 194

209 Excel format guide 3. Intake size (In the case of diversion weir) #Please input data into yellow cell #The input data of green cell is the value assumed yourself. (Goal seek "By changing cell") #Blue cell is the "set sell" for Goal seek. "to value" = 0 1Design intake discharge (Q) = m3/s 2Difference between inlet sill and base of scouring sluice = 1.25 m At first this value is assumpon. 3Head loss coefficient of inlet (fe) = 0.5 When 12,14 and 16 is OK, select the smallest area of 4 x 5 This value is assumption Goal seek "By changing cell" Goal seek ("set sell". "to value" = 0) Inlet width Water depth Head loss Water depth (Inflow) (After Inflow) Calculation Difference between eachhe B(m) h 1 (m) he(m) h 2 (m) A 1 (m2) A 2 (m2) V 2 (m/s) he(m) Check assumption of Head loss Intake velocity Approach velocity (After inflow) 0.6 V 2 1.0(m/s) V a =5 6 8=4 5 9=4 7 10=1/9 11= /2g+10 2 /2g OK or NG 13=1/9 14OK or NG 15=1 ((2+5) 4) 16OK or NG Goal seek OK 1.92 NG 0.11 OK Goal seek OK 0.77 OK 0.11 OK Select Goal seek OK 0.57 NG 0.10 OK Goal seek OK 1.89 NG 0.10 OK Goal seek OK 0.64 OK 0.09 OK Goal seek OK 0.50 NG 0.09 OK Goal seek OK 1.87 NG 0.09 OK Goal seek OK 0.56 NG 0.08 OK Goal seek OK 0.44 NG 0.08 OK Goal seek OK 0.37 NG 0.08 OK Goal seek OK 0.32 NG 0.08 OK #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Goal seek #DIV/0! #DIV/0! #DIV/0! #DIV/0! #DIV/0! Result <Confirmation calculation > Design discharge Q = m * B * h 0 * ( 2 * g * ( h 1 h 0 / 2 ) ) = m3/s > m3/s OK 1m= B= 0.4 3h0= h1=

210 3) Confirmation calculation Q = m B h 0 ( 2 g ( h 1 - h 0 / 2 ) ) = ( ( / 2 ) ) = m3/s > m3/s OK (See 3.2.6(4)2)) (5) Calculation result of design water intake level The result of above calculations, design water intake level is decided as follows. Design water intake level = (2) + (3) + (4) = Water level at the starting point of the main canal + The hydraulic loss between the intake and the starting point of the main canal + hydraulic loss of entrance = EL m + 0.1m m = EL m EL m According to above elevation, Inlet sill = EL.2422m 0.25m = EL m According to (4)1)(b) in this Chapter, Inlet sill = EL m EL m OK So, the difference between inlet sill and base of scouring sluice = Inlet sill (EL m) - Scouring sluice sill (EL m) = 1.25m Based on this value, the approach velocity can be calculated again using Excel format guide 3. The result of calculation, Approach velocity is OK. Design water intake level = EL. 2422m EL m EL m 1.25m Intake 0.4m 0.25m 196

211 5.2.3 Design Flood Discharge (Chapter3.1.1(3)) (1) In case of getting past flood discharge data in or near river basin of project site 1) Data collection of in or near river discharge on project site from Ministry of Water, Irrigation and Energy, etc. It is better to have within 50 years data. 2) Based on the data, select the biggest discharge. Example; From 1984 to 2013 (30 years) Take maximum discharge of each year. Year Maximum discharge (m3/s) Date Remark August July July July August July July July August Flood discharge July August August July July August July August July July August August August July July August July August July August July The biggest discharge (Q max ) is 1765m 3 /s in 20 August,

212 3) Based on the data of river basin and catchment area of the project, flood discharge at the place of headworks is calculated. Example; River basin (A r ) is 1000km 2 from delineation Catchment area of the project (A p ) is 50Km 2 from delineation Flood discharge (Q f ) is calculated by watershed ratio method Q f = Q max A p / A r = / 1000 = 88.25m3/s 4) If the river discharge data is less than 50years, design flood discharge (Q d ) is calculated by the value which flood discharge calculated in 3) above multiplies 1.2 for the safety. Example; Q d = Q f 1.2 = = 106m 3 /s (2) In case of using the maximum flood in the past based on flood mark or discharge capacity of the river by slope area method According to the river cross-section survey data, the calculation table and drawing of cross-section of the river can be made. Example; Cross-section survey data Point ID X (East) Y (North) Distance (m) Accumulated Distance (Width) (m) Elevation (m) RCX Depth (m) RCX RCX RCX RCX RCX RCX RCX RCX RCX RCX RCX RCX RCX RCX RCX Remark Left side River bed Right side 198

213 Based on above data, rating curve by Manning formula can be made. Slope = 0.029, Coefficient of Roughness = 0.04 Elevation (m) Depth (m) Water area (m2) Wetted Perimeter (m) Hydraulic radius (m) velocity (m/s) Discharge (m3/s) Note ; Water area and Wetted Perimeter calculated by Auto CAD Flood mark Rating curve Elevation(m) Discharge(m3/s) According to the rating curve of this river, the river discharge at the flood mark (EL m) is 88.26m 3 /s. In this case, peak flood should be multiplied 1.2 for safety. Design flood discharge = Peak flood discharge 1.2 = = 106 m 3 /s Refer to Excel format guide 4 for the detail. 199

214 Excel format guide 4. Rating curve of the river #Please input data into yellow cell. 1. Basic data for making cross section of the river Point ID X (East) Y (North) Distance (m) Accumulated Distance (Width) (m) Elevation (Z) (m) Depth (m) Auto CAD (x,y) RCX ,0 RCX , RCX , RCX , RCX , RCX , RCX , RCX , RCX , RCX , RCX , RCX , RCX , RCX , RCX , 500 RCX ,0 Copy to Auto CAD <How to make cross section by Auto CAD> 1.Open Auto CAD 2. Turn off Object Snap and Ortho Mode 3.Copy the low of "Auto CAD"(or column H) 3. Input PL to commandline in Auto CAD and push Enter Paste the data which copy from Excel file (the part of "Copy to Auto CAD") to comandline in Auto CAD Push Escap 200

215 Elevation(Depth) (m) Distance(Width)(m) We made drawing of cross section of the river using above date by Auto CAD. 201

216 2. Discharge of the river Slope Coefficient of Roughness 0.04 Elevation (m) Depth (m) Water area (m2) Wetted Perimeter (m) Hydraulic radius (m) velocity (m/s) Discharge (m3/s) Note ; Water area and Wetted Perimeter calculated by Auto CAD Flood mark Rating curve Elevation(m) Discharge(m3/s) 202

217 5.2.4 Design Flood Level (Chapter3.1.1(4)) See (2) Elevation of Crest Height and Length of Weir (Chapter3.1.4(2)) An example calculation on elevation of crest height of weir is as follows, - Design water intake level + margin (+10cm) = EL.2422m + 0.1m = EL m - Crest height of weir = EL m Average river bed level (EL m) = 1.6m And the length of weir can be decided from cross-section survey data and geological data of the bank around weir site. - Based on the survey data of cross-section on weir site and geological data of the bank around weir site (up to the hard rock), the length of weir = 12.5m (EL m) Possible Effect on the River Control of Upstream (Chapter3.1.4(4), 3.1.1(4)) An example calculation on possible effect on the river control of upstream is as follows, (1) Water depth of the river where the place of headworks before construction as design flood discharge (Tail water depth) From 5.2.3(2), the depth of the river at design flood discharge (Q max = 106m 3 /s) = EL m (Reading from the Rating Curve) So, Tail water depth = EL m EL m = 1.9m (2) Water depth on the crest as design flood discharge (See 3.1.1(4)) - Total energy head as design flood discharge He = (Q d / CL) 2/3 = (106m 3 /s / ( m)) 2/3 = 2.92m - Water depth on the crest He = H d + Q d2 / ( ( L ( h + H d ) ) 2 2g) 0 = H d + Q d2 / ( ( L ( h + H d ) ) 2 2g) He = H d / ( ( 12.5 ( H d ) ) ) The value of H d is calculated by trial and error calculation. Refer to Excel format guide 5 for calculation of above formula. The result of calculation, H d = 2.72m, H av = He - H d = = 0.20m 203

218 Excel format guide 5. Water depth on the crest 1. Design head #Please input data into yellow cell #The input data of green cell is the value assumed yourself. (Goal seek "By changing cell") #Blue cell is the "set sell" for Goal seek. "to value" = 0 1Total energy head (He) = 2.92 m 2Design flood discharge (Q d ) = 106 m3/s 3Length of weir (L)= 12.5 m 4Weir height (h) = 1.6 m 5Design head (H d ) = m Goal seek ("By changing cell") 6The result of calculation Goal seek ("set cell". "to value" = 0) 204

219 If upstream of weir is filled up until crest, He = H d + Q d 2 / ( ( L ( 0 + H d ) ) 2 2g ) 0 = H d + Q d 2 / ( ( L H d ) 2 2g ) - He = H d /( ( 12.5 H d ) ) The result of calculation, H d = 2.04m, H av = He - H d = = 0.88m H d value should adopt the biggest one from the result of above calculation. So H d = 2.72m Design flood level at weir = H d + Elevation of weir crest = 2.72m + EL m = EL m H av =V a 2 /2g Q max H e H d Sediment h Weir 3) Study of possible effect on the river control of upstream According to the design flood level at weir (EL m), river slope (1/35) and cross-section data, possible effect on the river control of upstream can be assumed or modeled. 205

220 5.3 Detail Design (Chapter3.2) Fixed Weir (Chapter3.2.2) An example calculation for fixed weir is as follows, (1) Section shape 1) The crest height of weir = 1.6m (from 5.2.5) 2) The top width of weir (Supposition of section dimension) (See 3.2.2(1)1)) - Bligh s method; B = (H d + H av ) / γ = ( ) / 2.35 = 1.90m - Etcheverry s method; B = ( h + ( H d + H av ) ) = ( ( ) ) = 1.64m So, it selects average between Bligh s method and Etcheverry s method ( ) / 2 = 1.77m 2.0m 3) The bottom width of the weir (Supposition of section dimension) (See 3.2.2(1)2)) - Bligh s formula; L = (h + H d + H av ) / γ = ( ) / 2.35 = 2.95m 3.8m (After calculate stability analysis, L is to be 3.8m finally) (2) Determination of section (Stability analysis).. (See 3.2.2(3), (4)) Section shape of weir has to check the stability by stability analysis. The calculation is difference whether downstream water height is higher than crest height. Refer to Excel format guide 6 for the details. 206

221 Excel format guide 6. HW Stability Analysis (Summary) Case1 : H downstream < Hw #Please input data into yellow cell 1.Basic data for calculation Item Value Reference Unit weight of plane concrete γc= 23 kn/m 3 Reinforced concrete: 24.5KN/m3, Plain concrete: 23KN/m3, Cement mortar: 21KN/m3 Unit weight of wet soil γwse= 18 kn/m 3 Unit weight of water W 0 = 9.8 kn/m 3 Unit weight of submerged soil γwe= 8.2 kn/m 3 (γwse-w0) Coefficient of earth pressure C 0 = 0.45 Uplift coefficient μ= 0.4 Rock foundation case or a case using sheet piles reaching an impermeable stratum: 0.4, Otherwise: 1.0 Seismic horizontal acceleration kh= 0.15 Friction coefficient f= Allowable stress of the graund qa= 1000 kn/m 2 Bedrock=100t/m2*9.8m/s2=980KN/m2 See Mesurement of structure 3. The result of calculation Item Value Content Slope of front body m = 1 : 1.13 Overturning e<b/6 or B/ OK OK OK B m Sliding ΣV f/σh 1.5or OK OK OK B m Settlement ΣV/B (1+6e/B)<q a 31kN/m2 < 1,000kN/m2 OK 36kN/m2 < 1,000kN/m2 OK 31kN/m2 < 1,000kN/m2 OK B (B1+B2) 3.80 m ΣV/B (1-6e/B)<q a 26kN/m2 < 1,000kN/m2 OK 14kN/m2 < 1,000kN/m2 OK 19kN/m2 < 1,000kN/m2 OK H w 1.60 m (Without uplift) ΣV/B (1+6e/B)<qa 48kN/m2 < 1,000kN/m2 OK 42kN/m2 < 1,000kN/m2 OK 37kN/m2 < 1,000kN/m2 OK H d 2.72 m (Without uplift) ΣV/B (1-6e/B)<qa 33kN/m2 < 1,000kN/m2 OK 14kN/m2 < 1,000kN/m2 OK 19kN/m2 < 1,000kN/m2 OK H downstream 1.90 m H e 2.92 m Flood Dry Earthquake B2 B1 H e H d H dd H downstream 1:m H w B 207

222 1Stability analysis (The case of flood) 1.Basic calculation 2.Calculation table (1 )Area (unit width) External force Area Vertical Distance Resistance Horizontal Distance Turning Dead load; W1 3.20m2= 2.00m 1.60m force moment force moment W2 1.44m2= 1.80m 1.60m 1/2 V(kN) x(m) V*x(kN m) H(kN) y(m) H*y(kN m) Static water P1 4.67m2= 2.92m 1.60m Dead load W pressure; P2 1.28m2= 1.60m 1.60m 1/2 W P3 2.72m2= 2.72m 2.00m 1/2 P4 2.03m2= 1.90m 2.14m 1/2 P5 1.81m2= 1.90m 1.90m 1/2 water pressure P Earth pressure; Pe 1.28m2= 1.60m 1.60m 1/2 P Uplift; U1 8.21m2= 3.80m (2.72m+ 1.60m) 1/2 P U2 3.61m2= 3.80m 1.90m 1/2 P P (2)Distance External force Dead load; W1 1.00m= 2.00m 1/2 Earth pressure Pe W2 2.60m= 2.00m+ 1.80m 1/3 Uplift U Static water P1 0.80m= 1.60m 1/2 U pressure; P2 0.53m= 1.60m 1/3 Total P3 0.67m= 2.00m 1/3 P4 3.09m= 1.66m+ 2.14m 2/3 P5 0.63m= 1.90m 1/3 Earth pressure; Pe 0.53m= 1.60m 1/3 Uplift; U1 1.27m= 3.80m 1/3 U2 2.53m= 3.80m 2/3 (3)External force by unit width External force Distance External lf force by unit width Dead load; W kN= 3.20m kN/m2 W kN= 1.44m kN/m2 Static water P kN= 4.67m2 9.80kN/m2 pressure; P kN= 1.28m2 9.80kN/m2 P kN= 2.72m2 9.80kN/m2 P kN= 2.03m2 9.80kN/m2 P kN= 1.81m2 9.80kN/m2 Earth pressure; Pe 4.72kN= 1.28m2 8.20kN/m Uplift; U kN= 8.21m2 9.80kN/m2 0.4 U kN= 3.61m2 9.80kN/m2 0.4 P5 H dd H downstream P4 B2 U2 B1 w2 w1 B U1 P3 H d P1 H w Pe H e P2 208

223 2Stability analysis (The case of dry) 1.Basic calculation 2.Calculation table (1 )Area (unit width) External force Area Vertical Distance Resistance Horizontal Distance Turning Dead load; W1 3.20m2= 2.00m 1.60m force moment force moment W2 1.44m2= 1.80m 1.60m 1/2 V(kN) x(m) V*x(kN m) H(kN) y(m) H*y(kN m) Static water P1 Dead load W pressure; P2 1.28m2= 1.60m 1.60m 1/2 W P3 P4 P5 water pressure P Earth pressure; Pe 1.28m2= 1.60m 1.60m 1/2 P Uplift; U1 3.04m2= 3.80m 1.60m 1/2 P U2 P P (2)Distance Distance Dead load; W1 1.00m= 2.00m 1/2 Earth pressure Pe W2 2.60m= 2.00m+ 1.80m 1/3 Uplift U Static water P1 U pressure; P2 0.53m= 1.60m 1/3 Seismic load S1 P3 S2 P4 P5 Total B2 B1 Earth pressure; Pe 0.53m= 1.60m 1/3 Uplift; U1 1.27m= 3.80m 1/3 U2 Seismic load S1 S2 H w w2 w1 (3)External force by unit width Pe P2 External force External force by unit width Dead load; W kN= 3.20m kN/m2 W kN= 1.44m kN/m2 B U1 Static water P1 pressure; P kN= 1.28m2 9.80kN/m2 P3 P4 P5 Earth pressure; Pe 4.72kN= 1.28m2 8.20kN/m Uplift; U kN= 3.04m2 9.80kN/m2 0.4 U2 Seismic load S1 S2 209

224 3Stability analysis (the case of earthquick ) 1.Basic calculation 2.Calculation table (1 )Area (unit width) External force Area Vertical Distance Resistance Horizontal Distance Turning Dead load; W1 3.20m2= 2.00m 1.60m force moment force moment W2 1.44m2= 1.80m 1.60m 1/2 V(kN) x(m) V*x(kN m) H(kN) y(m) H*y(kN m) Static water P1 Dead load W pressure; P2 1.28m2= 1.60m 1.60m 1/2 W P3 P4 P5 water pressure P Earth pressure; Pe 1.28m2= 1.60m 1.60m 1/2 P Uplift; U1 3.04m2= 3.80m 1.60m 1/2 P U2 P P (2)Distance External force Distance Dead load; W1 1.00m= 2.00m 1/2 Earth pressure Pe W2 2.60m= 2.00m+ 1.80m 1/3 Uplift U Static water P1 U pressure; P2 0.53m= 1.60m 1/3 Seismic load S P3 S P4 P5 Total Earth pressure; Pe 0.53m= 1.60m 1/3 Uplift; U1 1.27m= 3.80m 1/3 U2 Seismic load S1 0.80m= 1.60m 1/2 S2 0.53m= 1.60m 1/3 (3)External force by unit width External force External force by unit width Dead load; W kN= 3.20m kN/m2 W kN= 1.44m kN/m2 Static water P1 pressure; P kN= 1.28m2 9.80kN/m2 P3 P4 P5 Earth pressure; Pe 4.72kN= 1.28m2 8.20kN/m Uplift; U kN= 3.04m2 9.80kN/m2 0.4 U2 Seismic load S kN= 73.60kN 0.15 S2 4.97kN= 33.12kN 0.15 B2 B1 s1 s2 w2 w1 B U1 H w Pe P2 210

225 Excel format guide 6. HW Stability Analysis (Summary) Case2 : H downstream > Hw #Please input data into yellow cell 1.Basic data for calculation Item Value Reference Unit weight of plane concrete γc= 23 kn/m 3 Reinforced concrete: 24.5KN/m3, Plain concrete: 23KN/m3, Cement mortar: 21KN/m3 Unit weight of wet soil γwse= 18 kn/m 3 Unit weight of water W 0 = 9.8 kn/m 3 Unit weight of submerged soil γwe= 8.2 kn/m 3 (γwse-w0) Coefficient of earth pressure C 0 = 0.45 Uplift coefficient μ= 0.4 Rock foundation case or a case using sheet piles reaching an impermeable stratum: 0.4, Otherwise: 1.0 Seismic horizontal acceleration kh= 0.15 Friction coefficient f= Allowable stress of the graund qa= 1000 kn/m 2 Bedrock=1000KN/m2 See Mesurement of structure 3. The result of calculation Item Value Content Slope of front body m = 1 : 1.13 Overturning e<b/6 or B/ OK OK OK B m Sliding ΣV f/σh 1.5or OK OK OK B m Settlement ΣV/B (1+6e/B)<q a 29kN/m2 < 1,000kN/m2 OK 36kN/m2 < 1,000kN/m2 OK 31kN/m2 < 1,000kN/m2 OK B (B1+B2) 3.80 m ΣV/B (1-6e/B)<q a 25kN/m2 < 1,000kN/m2 OK 14kN/m2 < 1,000kN/m2 OK 19kN/m2 < 1,000kN/m2 OK H w 1.60 m (Without uplift) ΣV/B (1+6e/B)<qa 46kN/m2 < 1,000kN/m2 OK 42kN/m2 < 1,000kN/m2 OK 37kN/m2 < 1,000kN/m2 OK H d 2.72 m (Without uplift) ΣV/B (1-6e/B)<qa 33kN/m2 < 1,000kN/m2 OK 14kN/m2 < 1,000kN/m2 OK 19kN/m2 < 1,000kN/m2 OK H downstream 1.90 m H e 2.92 m Dynamic Static Earthquake B2 B1 H e H d H downstream 1:m H w B 211

226 212 Case2 : H downstream > Hw 1Stability analysis (The case of dynamic) 1.Basic calculation 2.Calculation table (1 )Area (unit width) External force Area Vertical Distance Resistance Horizontal Distance Turning Dead load; W1 3.20m2= 2.00m 1.60m force moment force moment W2 1.44m2= 1.80m 1.60m 1/2 V(kN) x(m) V*x(kN m) H(kN) y(m) H*y(kN m) Static water P1 4.67m2= 2.92m 1.60m Dead load W pressure; P2 1.28m2= 1.60m 1.60m 1/2 W P3 2.72m2= 2.72m 2.00m 1/2 P4 1.71m2= 1.90m 1.80m 1/2 P5 1.52m2= 1.90m 1.60m 1/2 water pressure P Earth pressure; Pe 1.28m2= 1.60m 1.60m 1/2 P Uplift; U1 8.21m2= 3.80m (2.72m+ 1.60m) 1/2 P U2 3.61m2= 3.80m 1.90m 1/2 P P (2)Distance External force Dead load; W1 1.00m= Distance 2.00m 1/2 Earth pressure Pe W2 2.60m= 2.00m+ 1.80m 1/3 Uplift U Static water P1 0.80m= 1.60m 1/2 U pressure; P2 0.53m= 1.60m 1/3 Total P3 0.67m= 2.00m 1/3 P4 3.20m= 2.00m+ 1.80m 2/3 P5 0.53m= 1.60m 1/3 Earth pressure; Pe 0.53m= 1.60m 1/3 Uplift; U1 1.27m= 3.80m 1/3 U2 2.53m= 3.80m 2/3 (3)External force by unit width External force External force by unit width Dead load; W kN= 3.20m kN/m2 W kN= 1.44m kN/m2 Static water P kN= 4.67m2 9.80kN/m2 pressure; P kN= 1.28m2 9.80kN/m2 P kN= 2.72m2 9.80kN/m2 P kN= 1.71m2 9.80kN/m2 P kN= 1.52m2-9.80kN/m2 Earth pressure; Pe 4.72kN= 1.28m2 8.20kN/m Uplift; U kN= 8.21m2-9.80kN/m2 0.4 U kN= 3.61m2-9.80kN/m2 0.4 H downstream P5 B2 P4 U2 B1 w2 w1 B U1 P3 H d P1 H w Pe H e P2

227 213 Case2 : H downstream > Hw 2Stability analysis (The case of static) 1.Basic calculation 2.Calculation table (1 )Area (unit width) External force Area Vertical Distance Resistance Horizontal Distance Turning Dead load; W1 3.20m2= 2.00m 1.60m force moment force moment W2 1.44m2= 1.80m 1.60m 1/2 V(kN) x(m) V*x(kN m) H(kN) y(m) H*y(kN m) Static water P1 Dead load W pressure; P2 1.28m2= 1.60m 1.60m 1/2 W P3 P4 P5 water pressure P Earth pressure; Pe 1.28m2= 1.60m 1.60m 1/2 P Uplift; U1 3.04m2= 3.80m 1.60m 1/2 P U2 P P (2)Distance Distance Dead load; W1 1.00m= 2.00m 1/2 Earth pressure Pe W2 2.60m= 2.00m+ 1.80m 1/3 Uplift U Static water P1 U pressure; P2 0.53m= 1.60m 1/3 Seismic load S1 P3 S2 P4 P5 Total Earth pressure; Pe 0.53m= 1.60m 1/3 Uplift; U1 1.27m= 3.80m 1/3 U2 Seismic load S1 S2 B2 B1 (3)External force by unit width External force External force by unit width Dead load; W kN= 3.20m kN/m2 W kN= 1.44m kN/m2 Static water P1 pressure; P kN= 1.28m2 9.80kN/m2 P3 P4 P5 Earth pressure; Pe 4.72kN= 1.28m2 8.20kN/m Uplift; U kN= 3.04m2-9.80kN/m2 0.4 U2 Seismic load S1 S2 w2 w1 B U1 H w Pe P2

228 214 Case2 : H downstream > Hw 3Stability analysis (the case of earthquick ) 1.Basic calculation 2.Calculation table (1 )Area (unit width) External force Area Vertical Distance Resistance Horizontal Distance Turning Dead load; W1 3.20m2= 2.00m 1.60m force moment force moment W2 1.44m2= 1.80m 1.60m 1/2 V(kN) x(m) V*x(kN m) H(kN) y(m) H*y(kN m) Static water P1 Dead load W pressure; P2 1.28m2= 1.60m 1.60m 1/2 W P3 P4 P5 water pressure P Earth pressure; Pe 1.28m2= 1.60m 1.60m 1/2 P Uplift; U1 3.04m2= 3.80m 1.60m 1/2 P U2 P P (2)Distance External force Dead load; W1 1.00m= Distance 2.00m 1/2 Earth pressure Pe W2 2.60m= 2.00m+ 1.80m 1/3 Uplift U Static water P1 U pressure; P2 0.53m= 1.60m 1/3 Seismic load S P3 S P4 P5 Total Earth pressure; Pe 0.53m= 1.60m 1/3 Uplift; U1 1.27m= 3.80m 1/3 U2 Seismic load S1 0.80m= 1.60m 1/2 S2 0.53m= 1.60m 1/3 B2 B1 (3)External force by unit width External force External force by unit width Dead load; W kN= 3.20m kN/m2 W kN= 1.44m kN/m2 Static water P1 pressure; P kN= 1.28m2 9.80kN/m2 P3 P4 P5 Earth pressure; Pe 4.72kN= 1.28m2 8.20kN/m Uplift; U kN= 3.04m2-9.80kN/m2 0.4 U2 Seismic load S kN= 73.60kN 0.15 S2 4.97kN= 33.12kN 0.15 s1 s2 w2 w1 B U1 H w Pe P2

229 (3) Apron.. (See 3.2.2(6), 3.1.4(3), 3.2.5(1), 3.2.1(2)3)) Case 1 : Riverbed is not scoured by overflow water (Riverbed is hard rock) It is unnecessary to consider apron because the riverbed of upstream and downstream of weir is hard rock. And the place of upstream of weir is also expected sedimentation in the future. Case 2 : Riverbed is scoured by overflow water 1) Length of downstream apron (See 3.2.2(6)1)) The elevation of top surface of downstream end of apron is almost same as river bed level of weir. - D 1 = Elevation of crest height of weir River bed level = EL m - EL m = 1.6m - Bligh s coefficient C = 9 (Sandy Gravel) L 1 = 0.6C D 1 = = 6.83m 13.0m (After calculate creep length, L 1 is to be 13.0m finally) 2) Thickness of downstream apron The thickness of downstream apron assumes as 1.7m at point A, 0.35m at point B. After the calculation below, it will be confirmed. 3) Depth of Cut-off wall (See 3.1.4(3), 3.2.5(1), (2)) There are two ways to calculate the depth of cut-off wall. (a) The depth of Cut-off wall at upstream In case of grain sizes of the subsoil is coarser than fine gravels, the depth of Cut-off wall at upstream = equal to the water depth of uprise = H d + h = = 4.32m (b) The depth of Cut-off wall at upstream and downstream (Lacey s formula) R = 1.35 (q 2 / f ) 1/3 = 1.35 ( ( Q d / L ) 2 / f ) 1/3 = 1.35( ( 106 / 12.5 ) 2 / 1.5 ) 1/3 = 5.0m This value may be theoretical to provide practical value based on geological condition because the calculated values above are very big. In this case it adopts 2.5m. 215

230 3) Ensuring creep length (See 3.1.4(3)) 2.72m ΔH 1 = 2.42m 1.6m 2.0m 3.8m Point A 13.0m Point B 1.9m ΔH 2 = 1.6m 2.5m 0.8m 1.7m 0.35m 2.15m 1.0m 1.0m 1.8m 0.3m (a) Bligh s method L C ΔH Where, Bligh s coefficient C = 9 (Sandy Gravel) ΔH 1 = H d + h Tail water depth = = 2.42m ΔH 2 = h = 1.6m So, ΔH = 2.42m (ΔH 1 > ΔH 2 ) C ΔH = m = 21.78m L = (2.5m + 0.8m + 1.7m) + (3.8m m) = 21.8m As the result of calculation, L = 21.8 C ΔH = 21.78m, OK (b) Lane s method L' C' ΔH Where Lane s coefficient C' = 3.5 (Coarse Gravel) ΔH = 2.42m (ΔH 1 > ΔH 2 ) C' ΔH = m = 8.47m L' = (2.5m + 0.8m + 1.7m) + 1/3 ( 3.8m m ) = 10.6m As the result of calculation, L' = 10.6m C' ΔH = 8.47m, OK From the result of (a) and (b), the length of apron and cut-off are OK. 4) Confirmation of thickness on downstream apron (Point A).(See 3.2.1(2)3)) T = 4/3 (ΔH H f ) / (γ - 1) Where ΔH = 2.42m γ = 2.35 (Concrete) 216

231 (a) The case of Bligh s method H f = ( ΔH / S ) S' = ( ΔH / L ) S' = ( 2.42m / 21.8m ) (2.5m + 0.8m + 3.8m) = 0.79m (b) The case of Lane s method H f = ( ΔH / S ) S' = ( ΔH / L' ) S' = ( 2.42m / 10.6m ) ((2.5m + 0.8m ) + 1/3 3.8m) = 1.04m H f value adopts the smallest one from the result of calculation for safety. So, H f = 0.79m T = 4/3 (ΔH H f ) / (γ - 1) = 4/3 ( ) / (2.35-1) = 1.61m It assumed the thickness of point A as 1.7m. So this assumption value does not have any problem. 5) Confirmation of thickness on downstream apron (Point B) T = 4/3 (ΔH H f ) / (γ - 1) Where ΔH = 2.42m γ = 2.35 (Concrete) (a) The case of Bligh s method H f = ( ΔH / S ) S' = ( ΔH / L ) S' = ( 2.42m / 21.8m ) (2.5m + 0.8m m + 3.8m +13.0m) = 2.38m (b) The case of Lane s method H f = ( ΔH / S ) S' = ( ΔH / L' ) S' = (2.42m/10.6m) ((2.5m + 0.8m m) + 1/3 (3.8m m)) = 2.34m H f value adopts the smallest one from the result of calculation for safety. So, H f = 2.34m T = 4/3 (ΔH H f ) / (γ - 1) = 4/3 ( ) / (2.35-1) = 0.08m It assumed the thickness of point B as 0.35m. So this assumption value does not have any problem. 217

232 6) Upstream apron (See 3.2.2(6)2)) If the place of upstream of weir is expected sedimentation in the future, it is unnecessary to construct upstream apron. On the other case, thickness on downstream apron = T 1/2 ~ 2/3 = 1.7 1/2 ~ 2/3 = 0.85 ~ 1.1m The length of upstream apron should decide based on the condition of river. 218

233 5.3.2 Riprap (Chapter 3.2.3) An example calculation for riprap is as follows, Case 1 : Riverbed is not scoured by overflow water (Riverbed is hard rock) It is unnecessary to consider riprap because the riverbed of upstream and downstream of weir is hard rock. Case 2 : Riverbed is scoured by overflow water Z (1) Calculation of water depth at the weir toe as design flood discharge.. (See 3.2.3(5)1)(a)) V 2 c 2g 2 1a V Z+hc= 2g h 1a Where, Q d = 106m 3 /s. (From 5.2.3) W = 12.5m.. (From 5.2.5) Z = h = 1.6m.. (From 5.2.5) q = Q / W = 106 / 12.5 = 8.48m 3 /s/m hc = 3 ( q 2 / g ) = 3 ( / 9.8 ) = 1.94m Vc = q / hc = 8.48 / 1.94 = 4.37m/s Vc 2 /2g + h + hc = h 1a + V 1a 2 / 2g = h 1a + q 2 / (2g h 1a2 ) /( ) = h 1a / (2 9.8 h 1a2 ) 0 = h 1a / (19.6 h 1a2 ) 4.51 The value of h 1a is calculated by trial and error calculation. Refer to Excel format guide 7 for calculation. The result of calculation, h 1a = 1.03m 219

234 Excel format guide 7. Water depth h1a 1. Water depth (h 1a ) #Please input data into yellow cell #The input data of green cell is the value assumed yourself. (Goal seek "By changing cell") #Blue cell is the "set sell" for Goal seek. "to value" = 0 1Design flood discharge (Qd) = 106 m3/s 2Length of weir (W) = 12.5 m 3Flow per unit width of design flood discharge (q) 8.48 m3/s/m 4Weir height (h) = 1.6 m 5Critical depth (hc) = 1.94 m 6Velocity of critical depth (Vc) = 4.37 m 7Water depth on the crest (H d ) = m Goal seek ("By changing cell") 8The result of calculation Goal seek ("set cell". "to value" = 0) 220

235 (2) Calculation of water depth at the beginning point of hydraulic jump.. (See 3.2.3(5)1)(b)) h 1b / h 2 = 1 / 2 ( ( 1 + 8F 2 2 ) -1 ) Where, h 2 = 1.9m (From 5.2.6(1)) F 2 = V 2 / (gh 2 ) = q / ( h 2 (gh 2 ) ) = 8.48 / ( 1.9 ( ) ) = 1.03 h 1b = h 2 / 2 ( ( 1 + 8F 2 2 ) -1 ) = 1.9 / 2 ( ( ) -1) = 1.98m (3) Comparison with h 1a and h 1b.. (See 3.2.3(5)1)(c)) h 1a = 1.03m < h 1b = 1.98m So, it needs riprap. (4) Calculation of supercritical flow length.. (See 3.2.3(5)1)(c)) -q 2 x / C 2 + a = h 4 /4 h c 3 h Where, C = h 1/6 / n - h 1a is substituted for this equation (h = h 1a, x=0) a = h 1a 4 /4 h c 3 h 1a = / = h 1b is substituted for this equation (h = h 1b ) n = 0.04 C = h 1b 1/6 / n = /6 / 0.04 = X = L 1 = -C 2 / q 2 ( h 1b 4 /4 h c3 h 1b a ) = / ( / ) = 36.82m (5) The length of hydraulic jump.. (See 3.2.3(5)1)(c)) L 2 = (4.5~6) h 2 = (4.5~6) 1.9 = 8.55~11.4m (6) Necessary Length of riprap A.. (See 3.2.3(5)1)(c)) L = L1 + L2 = 36.82m ~11.4m = 45.37~48.22m 48.0m (7) Length of riprap B.. (See 3.2.3(6)) L = (3~5) h 2 = (3~5) 1.9 = 5.7~9.5m 9.0m (8) Length of upstream riprap.. (See 3.2.3(4)) The length of upstream riprap is desirably longer than water depth at the flood time (H d = 2.72m). In consideration of water depth, upstream length is designed as follows. Length of upstream riprap : L= 3.00m 221

236 5.3.3 Foundation Work (Chapter 3.2.4) An example calculation for foundation is as follows, Example; Based on geological data, this weir foundation selects spread foundation because there is hard rock (hard brown Ignimbrite rock) at proposed headworks site excavated around 0.5m to 1.5m from the surface. - Average river bed level = EL m - Foundation = EL m 0.5 ~ 1.5m = EL.2420m ~ EL.2419m (Until hard rock) In reference to Chapter (Table3.11), long-term allowable bearing capacity is adopted 1000KN/m2 of bedrock Upstream and Downstream Cut-off Walls (Chapter 3.2.5) An example calculation for cut-off wall is as follows, Case 1 : Foundation is non-permeable foundation (Riverbed is hard rock) The weir can be constructed directly on the bedrock. So it is unnecessary to consider cut-off wall. Case 2 : Foundation is permeable foundation See 5.3.1(3) Inlet (Chapter 3.2.6) See Gate (Chapter 3.2.7) An example calculation for gate is as follows, According to 5.3.9, the size of scouring sluice gate is H = 1.6m, Bs = 1.0m So, Skin plate area = = 1.6m 2 This gate will be opened during rainy season, so the upstream water height of gate assumes height of gate plus half of design flood height for safety. H upstream = 1.6m m/2 = = 2.96m According to the 3.2.7(4), thickness of gate is 10mm. 222

237 5.3.7 Settling Basin (Chapter 3.2.8(1)) An example calculation for settling basin is as follows, Case 1 : In case of enough prevention of sediment inflow into the canal by feature of inlet and scouring sluice Enough prevention of sediment inflow into the canal is as follows: The height from the scouring sluice sill to the inlet sill is higher than 1.0m. The height from the riverbed to the inlet sill is more than 1/6 of maximum flood depth of the river. Installation of Intake gate It is unnecessary to consider settling basin when these three points above are satisfied. Case 2 : In case of lack of prevention of sediment inflow into the canal by scouring sluice and feature of inlet (1) Width and depth of sedimentation ditch.. (See 3.2.8(1)5)(b)) 1) Calculation of κ τ c / ρ = 8.41 d 11/32 Where, d = 0.03cm, ρ = 1.0 τ c = /32 = 2.52cm 2 /sec 2 κ = τ c / ρi Where, i = 1/50 = 0.02 (Assumption value) κ = 2.52 / (1 0.02) = 126 cm 2 /sec 2 = m 2 /sec 2 2) Calculation of B and H 1/ 2 2 B h 2 Q h Where, h = 0.6m (Assumption value) kh 2 Q = 0.063m3/s (From 5.2.1) α = 1.2 B = ( / ( ) ) 1/2 0.6 = 0.59m 0.6m H = D + h Where, D = 0.25m H = = 0.85m 223

238 (2) Length of sedimentation ditch.. (See 3.2.8(1)6)) 1) Formula from sedimentation theory L K h v g u K Q Bv g Where, K = 2.0 Vg = (ρ s = 1.1, d = 0.3mm) L = / ( ) = 8.4m 2) Empirical formula L 20 Q = = 5.02m The result of calculation 1) and 2), the length of sedimentation ditch adopts 8.4m Protection of Bank and Major Bed (Chapter 3.2.8(2)) An example calculation for protection bank is as follows, In the place of headworks, if there is low elevation land than design flood level (EL m), it is necessary to protect this part from flood. This part is protected by gabion starting from EL m (Stable bank. The depth of foot protection from O.G.L is 0.5m.) to EL (Design flood level) + 0.5~0.6m (freeboard) which is equal to EL m. For the length of 10m, bottom width is 1.0m and thickness is 0.5m. EL m O.G.L Stable bank (hard rock) 0.5m EL m EL m 1.0m Scouring Sluice (Chapter 3.2.1(3), 2.1.2(3)) An example calculation for scouring sluice in case of rapid stream (river slope 1/800) is as follows, 224

239 (1) Diameter of riverbed materials.. (See 2.1.2(3)) - Maximum grain size (90 percent passing by weight) de = 0.5m - Average grain size (60 percent passing by weight) dm= 0.15m (From the geological study data) (2) Design of scouring sluice intake (See 3.2.1(3)4)(a)) In the condition that the flow within the scouring sluice should be supercritical, the critical flow will be caused at the intake. And the design should be so made as to transport the maximum size particles of the riverbed materials with this critical flow. The critical velocity required to transport sediment Ve can be given by the following formula experimentally. And the critical water depth hc and the flow per unit width qe can be given by the below formula respectively. Ve = ( 20 de ) = ( ) = 3.16 m/s hc = 20 de / g = / 9.8 = 1.02 m qe = (( 20 de ) 3 / g 2 ) = (( ) 3 / ) = 3.23 m 3 /s/m The height of the guide wall H required to form a channel for the scouring sluice is made 1.5hc at the point of intake. H=1.5 hc= > 1.5m So, the height of the guide wall : H=1.6m = weir height The critical water depth : hc = H / 1.5= 1.6 / 1.5 =1.07m So, qe = (g hc 3 ) = ( ) = 3.46 m 3 /s/m (This formula is from hc = 3 (qe 2 /g) ) (3) Engineering of upstream portion of scouring sluice (See 3.2.1(3)4)(b)) 225

240 1 =S++1.5Hs Where S=0.7m, =0.4m, Hs=1.5m 1 = = 3.35 (m) The upstream length of scouring sluice is 1 = 3.35m on calculation, 1 = 3.7m is adopted as design length. The elevation at point EL2 of the inflow of the scouring sluice in principle should be almost the same as the existing riverbed elevation. The average river slope is So, the riverbed elevation is 3.7m EL2420.5m = EL m. Therefore, the EL2 = m is adopted. (4) Cannel width of scouring sluice. (See 3.2.1(3)6), Reference1, Reference 2) Bs Qs / qe Qs / 3.46 (From 5.3.9(2)) Assumption of the river discharge at the limit of average grain size transportation (Qs) - The Froude number (Fr) Fr = 9.82( i) ( i) = 9.82( 0.029) ( 0.029) 3.5 = The friction velocity at the limit of average grain size transportation (U X c) U X c 2 = 80.9dm = (cm/sec) 2 = (cm/sec) 2 - The water depth at the limit of average grain size transportation (h sc ) h sc = U X c 2 / gi = / ( ) = 42.7cm = 0.43m - The discharge per unit width for the limit of average grain size transportation (q sc ) h = (q 2 /gfr 2 ) 1/3 q = Fr g 1/2 h 3/2 q sc = Fr g 1/2 h sc 3/2 = / /2 = 1.12 m 3 /s/m 226

241 - The river discharge at the limit of average grain size transportation (Qs) Qs = q sc B = = 14 m 3 /s Where, River width B = 12.5m The result of calculation of Qs, Bs Qs / 3.46 = 14 / 3.46 = 4.05m In case of wider cannel width, required discharge can be increased and the sand flushing capacity is declined. When the width is smaller than a half of its length, from the viewpoint of sand flushing function of the scouring sluice, the direction of the flow can be controlled more easily. And also the width of scouring sluice is decided based on the applicable size of skin plate shown in 3.2.7(4). From these viewpoints, the width of scouring sluice is adopted to 1.0m. (See 5.3.6) (5) Design of upstream slope of cannel (See 3.2.1(3)4)(b), Reference2 (iv)) n h 2 g 1/ 3 c n h 2 3 ghc 10 / 3 m h m 2 d m Where n=0.018, hc=1.07(m), hm : (hc+h)/2 (m), dm=0.15 (m) In assuming the water depth at the downstream end of the supercritical flow canal h and then value of the canal, following table can be obtained. Refer to Excel format guide 8 for the details. Example; Test calculation table of water depth h h m 2 n g 2 3 n g hc dm (m) 1 3 hc < 10/3 hm < h m /322 < 1/244 < 1/80 OK /322 < 1/288 < 1/84 OK /322 > 1/337 < 1/88 NG From the above table, the water depth at the downstream end of the scouring sluice channel h becomes h = 1.0m to give a value of grain size larger than that with maximum grain size in the sediment to be flushed, taking into account the energy loss due to sediment transportation and the roughness coefficient n =

242 Consequently, the length of channel 1 is 1 = 3.7m and the slope of the channel bed is obtained from the formula below: i h+ 2h h c n g h c 2-1.5hc + h m 10 3 = 1/3.7 ( / ) / /3 = = 1/182 The elevation of scouring sluice gate sill EL1 is EL m - 3.7m 1/182=EL m on calculation, EL1=EL m is adopted. (same elevation of average of river bed) Consequently, the slope of upstream cannel is I =(EL m-EL2420.5m)/3.7m = 0.11m/3.7m= = 1/34 < Confirming calculation >.. (See 3.2.1(3)4)(b)) - The discharge of scouring slice (Q) Q = B qe = = 3.46 m 3 /s In case of n = 0.015, according to Manning formula, h is Q = B h I 1/2 (Bh/B+2h) 2/3 / n 3.46 = 1 h ( 1 / 34 ) 1/2 ( 1 h/ 1 + 2h ) 2/3 / h = 1.21 (Refer to Excel format guide 8) So, h = 1.21 > de = 0.5 OK. 228

243 Excel format guide 8. Scouring sluice 1.Test calculation table of water depth #Please input data into yellow cell #The input data of green cell is the value assumed yourself. (Goal seek "By changing cell") #Blue cell is the "set sell" for Goal seek. "to value" = Q value <Formula> n 2 g/h c 1/3 < n 2 gh c 3 /h m 10/3 < 8.25*10-2 d m /h m n = g = 9.8 m/s 2 h c = 1.07 m h m = (h c +h)/2 m d m = 0.15 m This value is assumption h(m) h m n 2 1/3 g/h c < n 2 gh 3 10/3 c /h m < 8.25*10-2 d m /h m = 1/ = 1/ = 1/55 OK = 1/ = 1/ = 1/57 OK = 1/ = 1/ = 1/59 OK = 1/ = 1/ = 1/61 OK = 1/ = 1/ = 1/63 OK = 1/ = 1/ = 1/65 OK = 1/ = 1/ = 1/67 OK = 1/ = 1/ = 1/69 OK = 1/ = 1/ = 1/72 OK = 1/ = 1/ = 1/76 OK = 1/ = 1/ = 1/80 OK = 1/ = 1/ = 1/84 OK = 1/ = 1/ = 1/88 NG = 1/ = 1/ = 1/92 NG = 1/ = 1/ = 1/96 NG = 1/ = 1/ = 1/100 NG 229

244 2.Test calculation of upstream slope 1 = 3.7 m h = 1m h m = = = 1/182 3.Confirming a calculation <Formula> Q = B*h* I 1/2 * (Bh/(B+2h)) 2/3 / n n = Q = 3.46 m3/s B = 1 m I = de = 0.5 m Goal seek ("By changing cell") h = 1.21 m > 0.5 OK The result of calculation Goal seek ("set cell". "to value" = Q value) 230

245 (6) Design of downstream cannel - The length of downstream cannel is designed about 1.5 times of width of cannel.. (See 3.2.1(3)4)(c)) l 2 =1.5 B= m =1.50m 2.2m (From gate to end of the bottom width of the weir) - The calculation of h 2 and h 3 (See 3.2.1(3), Reference2) F 2 2 = qe 2 / gh 2 3 = h 2 = (qe 2 / gfr 2 ) 1/3 = ( / ( )) 1/3 = 0.74m h 3 = (h 2 /2) (-1+ (8F 22 +1) ) = (0.74/2) (-1 + ( ) ) = 1.50m - The calculation of i 2.. (See 3.2.1(3)4)(c)) i 2 = - 1/l 2 ((h h 2 ) + qe 2 /2g + (1/h 2 1 / h 2 2 )) + n 2 qe 2 / (( h + h 2 ) / 2 ) 10/3 = -1 / 2.2 ( ( ) / (2 9.8) (1/ / ) ) / ( ( ) / 2 ) 10/3 = = 1/8.6 So, EL 3 = EL m = EL m - The calculation of l 3.. (See 3.2.1(3)4)(c)) l 3 = 4.5~6 h 3 = = m - The calculation of H 2.. (See 3.2.1(3), Reference2(v)) And the height of the downstream guide wall H 2, when the sedimentation is deemed to occur at downstream of the weir, can be obtained from the following formula: Provided that H 2 is based on the gate sill as a datum level. H 2 = {(design intake water level) - (gate sill elevation)} 2/3 = ( EL.2422m - EL m ) 2/3 = 1.0m 231

246 5.4 How to use Goal seek There is a function named Goal Seek on Excel. This function can simplify trial and error calculation. To use this function, it has to go to [Data] tab on Excel, then select [What-IF Analysis], and select [Goal Seek]. Place of item is shown below. After select [Goal Seek], it can be selected Set cell which is having formula. Then enter a figure into To value which is target value. Finally it can be selected By changing cell which is a cell to change figure to get target value. This cell s figure is an assumption value at first. After those three cells are set on Goal Seek and click on OK, Excel calculates to find target value by itself and show that value under By changing cell. There is some guidance about Set cell, To value and By changing cell in Excel format guide on this manual as follows. 232