RIVER STRUCTURAL WORKS AND OPERATION. Diversion Works (by Bizuneh Asfaw) Date of the project report

Transcription

1 RIVER STRUCTURAL WORKS AND OPERATION Diversion Works (by Bizuneh Asfaw) Date of the project report 1

2 Table of Content 3.1 Introduction General definition of headworks Classification of diversion work Site selection and interaction with other structures The layout of River Diversions 7 3. Weirs and barrages Weir Weirs classification based on material of construction Basic design consideration Hydraulic Design of the Weir Effect of constructing a weir Data required Shape of the Weir Clear waterway Theoretical Rating Curve Discharge and Head over the Weir Afflux Weir Height Water surface profile at the weir site Water Surface Profile at the upstream of the Weir Water Surface Profile downstream of the Weir Energy Dissipation below the Weir Structural Design of the Weir Design of Weir Wall Top Width Bottom Width Stability Analysis Impervious Floor a Theories of Subsurface flow Causes of failures of weirs on permeable foundations a Depth of pile below the bed level b Floor length Divide Wall Design of protection work Silt Excluder Head Regulator De-Silting Basin Protecting Side Wall Sediment Control Sedimentation at diversion works Degradation of the river bed 43 1

3 Aggradation of the river bed General Features of Flow Diversion Sediment Control at Diversion Weirs OPERATION & MAINTENANCE Operation of Headwork Maintenance of Headwork 57 REFERENCES 58 The sources for the figures in this document are the reference list indicated at the end of the document.

4 3.0 River Diversion works 3.1 Introduction General definition of headworks Headworks are defined as the facility which diverts water from a river (lakes and marsh, excluding reservoir) in to a canal for irrigation or water supply. In the Multilingual Technical Dictionary on irrigation and drainage issued by the International Commission in Irrigation and Drainage (ICID), Headwork are defined as A collective term for all works (weirs or diversion dams, head regulators, upstream and downstream river training work and their related structures) required at intakes of main or principal canals to divert and control river flows and to regulates water supplies in to the main canals. [1] Figure 3.1 Headwork Generally, a river originates in the mountain area, and flows down to the plain area collecting tributaries on its way. And thus, rivers are the major source of water supply. The river water usually meets the various demands because, unlike the lakes, they are reasonably evenly distributed over earth s surface but their flow rates vary enormously from river to river and of a given river with precipitation and season (Raudkiwi, 1993). At the beginning of the design process, different aspects have to be considered to divert a certain quantity of water from a river, such as; The flow rate in the river has to be assessed as a function of time and compared with the demand of water which is also a function of time. 3

5 The diversion demands have to be decided taking into account the multitude of interacting factors of technical, environmental, political and aesthetic nature. The process see figure 3.. WATER DIVERSION WATER DEMANDS- Availability: amount, distribution in time, quality parameters, etc WATER RESOURSES- Availability: amount, distribution in time, quality parameters, etc LARGE SCALE WATER DIVERSION SCHEMES SMALL SCALE WATER DIVERSION SCHEMES River System and its Response to Diversion: Hydrological Aspects Hydraulic Aspects Morphological Aspects Sediment Transport Water Quality Environmental Aspects DIVERSION STRUCTURE Type Location SEDIMENT EXCLUDER SETTLLING BASIN WATER CONVEYANCE SYSTEM Figure 3. Schematic illustrations of Interacting aspects in water diversions 3.1. Classification of diversion work Different ways of classifying diversion structures were developed along the time. Among these the most important are according to: a) Hydraulic functions Diversion weirs 4

6 Barrages Intake structure without regulator works b) The source River intakes In takes from reservoirs and lakes c) The slope of the river they are on, which usually also indicates the size of the intake and the sediment size carried by the river that is boulder, gravel, sand and silt respectively: Mountain intakes on steep rivers with slopes greater than about 1:1000 Intakes on plain rivers with slopes 10-4 <s<10-3 In takes on large rivers with slopes less than 1:10,000 [] Figure 3.3 Components of head work Site selection and interaction with other structures The site for constructing a headwork s must be selected in consideration of the river condition and the irrigation benefit area, and should be such that the required water intake function as well as the stability of the structure and convenience for operational maintenance are achieved. Moreover, the pondage requirement and interference with the existing structures such as bridges, urban development, valuable farmland, etc., must be considered, as well as available options to divert the flow during construction. Therefore, the site selected for the Headworks should be the optimum after studying the following items: 5

7 Availability of a stable gut 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 expected with economical construction costs Convenience for operation and maintenance. Basically, it is extremely important to satisfy item above for the selection of the position of the Headworks.[1] After selecting the weir site depending on the topographic and geological condition, the layout is made considering: Reducing engineering quantity & cost of weir, and to enable normal, uniform flow through all bays of the weir with a minimum of shoal formation, the weir is aligned at right angle to the direction of flow in the river. [6] To prevent peak discharge scouring near banks of river upstream & downstream of weir during flood period and ensure safety & normal operation of each hydraulic structure, while making general layout, bank protection arrangement on upstream & downstream of the weir is provided based on the geological condition of the river bank at weir site. The weir alignment may be; straight along a single line, curved, or along two lines, straight or curved as shown in figure 3.4. Figure 3.4 Different aligment of weir axis The first is best suited for straight reach of a river. The lines of flow on the downstream will be parallel to the banks. The concave upstream alignment is well suited when the weir is sited at a point where the river after having widened is narrowing down to its normal course. The current will converge to the center. The later is not recommended as it diverges the current towards the banks exposing them to scour. Besides it is structurally weak. It is preferable to adopt straight alignment whenever possible.[5] 6

8 3.1.4 The layout of River Diversions Figure 3.5 Diversion weir- conceptual diagram River diversion project includes the intake, the diversion weir, and the approach channel and its training work, the tail water channel, its training works, and appurtenant training works (Boeriu, 003). The layout design defines. A diversion site in the river. The relative position of the intake and the diversion weir in the river. The geometry of the approach and downstream. The diversion intakes, gated spans, spillway. The main components of a river diversion (Figure 3.6) are: Diversion works Intake Head works Figure 3.6 Main components of a river diversion Main components are: a) Headworks b) Diversion works c) intakes 7

9 3. Weirs and barrages Weirs and barrages are relatively low-level dams constructed across a river to raise the river level sufficiently or to divert the flow in full, or in part, into a supply canal or conduit for the purposes of irrigation, power generation, navigation, flood control, domestic and industrial uses, etc. These diversion structures usually provide a small storage capacity. In general, weirs (with or without gates) are bulkier than barrages, whereas barrages are always gate controlled. Barrages generally include canal regulators, low-level sluices to maintain a proper approach flow to the regulators, silt excluder tunnels to control silt entry into the canal and fish ladders for migratory fish movements. Weirs are also used to divert flash floods to the irrigated areas (Spate irrigation) or for ground water recharging purposes. They are also sometimes used as flow measuring structures. [4] The basic difference between Weir and Barrage are summarized in table 1. Table 1. Weir and barrage difference 3..1 Weir One of the most important Structures is weirs (fixed or movable). Except in storage schemes, these structures are usually available at intake locations. [8] Movable Weir A movable weir (figure 3.7) is a structure to secure the required water level for water intake and safe flow of water by means of gate operation and ensure safety against the action of flowing water, etc and other relating external forces 8

10 No. description No. description No. description 1 Weir body 8 Upstream apron 16 End sill Retention structure 9 Downstream apron 17 Baffle wall 3 Crest of the weir 10 Weir saddle 18 D/S maintenance gate 4 Rare side of the weir 11 Weir gate 19 cutoff 5 Side wall 1 Support of weir gate 0 Jet splitter 6 Sill 13 Storage 1 Flap gate 7 Inlet flow 14 Stilling basin Figure 3.7 Movable weir and its component parts 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 in to an irrigation canal by timely removal of the sediment accumulated therein. 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. Span length of movable portion of weir. I. Sill elevation of movable weir To determine the sill elevation of a movable weir, careful attention must be paid in principle to the naturally formed river bed configuration. A problem in providing an intake weir on a river is a back water 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 supercritical flow domain, backwater doesn t cause a serious problem. The fundamental requirements are that neither back water nor silt deposit shall be caused by construction of the weir. The following matters should be taken in to account from the view point of design. The front configuration of the intake weir should be almost similar in figure to the section of the natural river bed. The sill elevation should be so determined as to prevent backwater and silt deposit by the roughness effect of the intake weir. 9

11 II. Spillway The spillway should have a structure to release flood water smoothly, and to prevent the effect of back water as much as possible and to maintain the thalweg. Figure 3.8 Explanatory profile of spill way III. Scouring sluice A scouring sluice must be provided at the intake side and be such that it can prevent sediment inflow in to the canal and can smoothly remove off sediment deposited in front of the intake. The scouring sluice must be provided at the intake side and be such that the sediment accumulated in front of the intake can be removed in a short time and sediment in flow to the canal can be prevented as much as possible when in taking water. IV. Sediment inflow The first requisite for the intake site is that the river channel, in other words the gut, should be stable. Sediment inflow to the canal would take place in the following cases; If the intake site is not correctly sited in relation to meandering of the river If water intake is required even during floods. If the intake is located too close to the riverbed elevation and intake flow velocity is large. V. Sediment deposited in front of intake When the force of flowing water exceeds the limits of tractive force, sand particles start moving. As a result the riverbed surface takes various forms, such as ripples, dunes, and plane beds untidunes. Fixed weir A fixed weir (figure 3.9) is such structure as to secure the required water level at the time of intake, to avoid a considerable obstacle to floods and to have a section which is safe enough against external forces and is advantageous from the hydraulic point of view. 10

12 No. description No. description No. description 1 Weir body 5 Side wall 14 Stilling basin Retention structure 8 Upstream apron 15 End sill 3 Crest of the weir 9 Downstream apron 17 Baffle wall 4 Rare side of the weir 13 Storage 19 cutoff Figure 3.8 Fixed weir and its component parts Weirs classification based on material of construction Weirs may be classified according to the material of construction and certain design features as i. Masonry weirs with vertical drop or vertical drop weirs ii. Rock fills weirs with sloping aprons iii. Concrete weirs with a downstream glacis 1) Masonry weirs (Vertical drop weirs) consists of: An impervious horizontal floor or apron A masonry weir wall (with both upstream and downstream faces vertical; or both faces inclined; or upstream face vertical and downstream face inclined) Block protection at upstream end of floor, and a graded inverted filter at the downstream end of floor Launching aprons or impervious aprons (or floors) after block protection and inverted filters. This type of weirs are very old and becoming obsolete Pond Crest Shutte Up stream cut Up stream apron Down stream Down stream cut Figure 3.10 Masonry Vertical Drop weir 11

13 ) Rock fill weir with sloping aprons: It is the simplest type of construction. It consists of Masonry weir well Dry packed bounders laid in the form of glacis or sloping aprons in the upstream and downstream sides of the weir wall Such type of weir requires huge quantity of rock(stone) and is economical only when stone is easily available at the site Crest level Down stream slopping apron Rock fill Masonry Figure 3.11 Rock fill weir with sloping Aprons 3) Concrete weir with d/s glacis: - It is of recent origin and its design is based on sub-surface flow concept. Hydraulic jump is developed on the glacis due to which considerable energy is dissipated. Protection works such as inverted filter; block protection and lunching apron are provided. It can be constructed on pervious foundation. Sheet piles of sufficient depths are provided both at upstream and downstream ends of the floor. Providing shutters controls the flow over weir. This type of weir is best suited on pervious foundation. Pond level Down stream glacis Upstream cut off Intermediate cut off Figure 3.1 Typical x-section of concrete weir with d/s glacis on permeable foundation 3.3 Basic design consideration Down stream Detailed design deals with hydrological, hydraulic and structural design of intake weirs, inlets, related structures and operation & maintenance facilities, based on the design criteria and dimensions which are set up in the basic design. 1

14 3.3.1 Hydraulic Design of the Weir Effect of constructing a weir Before starting the design of weir it will be interesting to know the effect the construction of a weir has on the regime of the river. The following are the main consequences: The heading up of water upstream during the major part of the year leads to flattering of the surface slope on the upstream side. As a result of a flattering of slope, the silt carrying capacity of the water decreases, causing the heavier grade to deposit in the upstream pond & form Irregular shoals. In the first few years while the upstream side is silted the portion of the river up to considerable distance below the weir not receiving adequate quantity of silt goes on scouring. As the water passing over the weir is deprived of some of the silt it can carry on account of settling in the pond, when it goes on the downstream side it picks up some material from the bed & goes on scouring. This scouring of bed on the downstream of the weir is known as retrogression of the levels and may be of the order of 1. to.1m in low water season, while in flood season it is 0.3 to 0.75m.[5] Data required The data required for a design of a weir are: Area of the drainage basin, length & slope of the main river/gully up to the weir site, one day maximum point rainfall data, Maximum Flood discharge that is likely to occur, Minimum discharge, Full supply level of canals, cross section of the river at the weir site, longitudinal section of the river, median size of the river bed material etc Shape of the Weir The shape of the weir is decided based on the practicability and economy of the structure. Whether to use rock fill, masonry or concrete for the construction of the weir & which weir section to adopt, broad/sharp crested or ogee weir depends on the availability of the construction material, the workmanship & the cost of construction. Normally a broad crested type of weir with vertical upstream & slopping downstream face is best for small scale irrigation projects for the local builders can easily construct it. Again considering economy and availability of construction material a masonry type weir will be adopted and also constructed in most part the country so far Clear waterway The shorter the weir the less will be the cost of the main structure but on account of the increase in discharge per unit run, dynamic action on the river bed downstream will be sever, to guard against which the thickness & Length the impervious & pervious floors will have to be increased. Also a shorter weir will cause higher afflux which on the one hand entails higher & longer training works & consequently the cost. The scour action 13

15 on the river bed due to concentration of discharge increases & jeopardizes the safety of the structure. While the weir is relatively long with shallow depth of water over the crest, the cost of training work & bed protection downstream will be less but the cost of weir will be more. Thus there is an optimal length for which the overall cost will be minimum. Several alternatives may consider during site investigation to decide the most economical crest length. The length of waterway is usually determined from Lacey s wetted perimeter. Lacey developed the equations based on the analysis of large amount of data collected on several river gullies. The wetted perimeter, p is given by Where: Q = design discharge P =Wetted perimeter For deep & confined rivers with stable banks, the overall waterway should be approximately equal to the actual width of the river at the design flood discharge.[6] Hence, if the existing physical feature of the site limits the waterway to be less than the one specified by Lacey, the existing waterway has to be adopted Theoretical Rating Curve P=4.75* Q Most of the catchments in the country are un-gauged. It is, therefore necessary to establish the river channel property with respect to the generated flood from the catchment. This can be achieved by establishing a theoretical Stage-discharge relationship using Area-Velocity method, the velocity being estimated from Manning s equation. From this curve the tail water depth, D 3 corresponding to the design flood has to be determined. NW Q d Figure 3.13 Theoretical Rating Curve 14

16 Discharge and Head over the Weir When a weir is constructed across the river, head is produced above the crest of the weir. This head is an important factor in the design of hydraulic structures. Discharge over the weir is generally expressed as: Q= C d * L e * H 3 / e...3. Where: Q = Design discharge (m 3 /s) C = Discharge Coefficient (Usually C = for broad crested type of weir,. for ogee type). H e = Height of energy line above the crest (m) L e = Effective Length of the weir (m). Figure 3.14 Head over the weir For designing purpose it is assumed the peak discharge passes through the weir wall Afflux It is the difference in water level at any point upstream of the weir before & after the construction of the weir. The Afflux affects the water level appreciably long upstream of the weir. For high afflux the length of the weir is decreased but the cost of training works shoots up so that the risk of failure by scouring & outflanking increases. While if an attempt is made to have only negligible afflux, the formation of standing wave will not be assured and the length of the weir will have to be inordinately large. The afflux should therefore be enough to enable the standing wave to be formed at all stages of discharge but not so great s to entail high and long training works. In steeps reaches of rivers for boulder or rocky bed the afflux may be kept within 1.0 to 1.5m & for alluvial and deltaic regions it may be only 0.60m.[5] Silt Factor f = 1.76 mr Where: m r = Median size of the river bed material as determined from the sieve analysis All irrigation structures should be designed against scour which occurs due to surface flow. Suitable protection works are provided to check the possibility of scour holes traveling close to the impervious floor & damage the main structure. The sheet piles & at the upstream & downstream ends of the impervious floor should be provided up to the deepest scour level. According to Lacey, the Normal Scour R in alluvial soil is given by: 15

17 Weir Height q R= 1.35( f ) 1/ Q Where: q= L The top of crest be such as to store water up to the pond level. It has to be sufficiently above the highest full supply level of canals so as to allow for loss of head required through head regulators, for the fluctuation in the pond and to providing balancing effects for periodical variations. With respect to the adjoining land surface, the elevation of the water surface, upstream of the weir should not be so low as to require an excessively high weir to divert the water at the intake. Therefore the crest height is fixed taking the following considerations: average level of the highest flood mark observed (HFL), average River bed level, Normal Water Depth (NWD), permissible afflux. There are two approaches to fix the crest level of a weir. I. To start from the High Flood Level (HFL) of the River as determined from the theoretical rating curve. Velocity of approach: V a = Head Due to Velocity of approach: q R V H a = g Total Energy line: Downstream TEL = Downstream HFL + Ha 3.8 Upstream TEL = Downstream TEL + Afflux..3.9 Upstream HFL = Upstream TEL Ha Crest level of weir = Upstream HFL H d II. The other method to fix the Crest level is to start from the Full supply level of the canal: Crest Level = Full Supply Level (FSL) of Canal+ Working Head.3.1 Weir Height = Crest Level River Bed Center Water surface profile at the weir site The water profile of the weir at the upstream and downstream should be determined which will be used in design of the appurtenant structures of the weir and infixing the 16

18 dimension of these structures. Therefore, the water profile upstream and downstream of the weir is determined as follows Water Surface Profile at the upstream of the Weir The level of the back water curve is needed to determine the safe level of the embankment and super structures so as to know the length of the wing wall and to know whether the embankment needs some treatment measures to protect the overtopping of water over the river bank due to the construction of the weir. Therefore it is important to know where the effect of backwater curve will cease. There are several methods to determine the water profile upstream of the weirs, out of these for particular scheme; the profile can determine by approximate method, which is given as: ( xs o) Y = o Where: y = water rise at a distance X upstream of the weir above the normal water depth X = distance from the crest to the pt where y is required to be determined s= slope of the river bed (from tail water depth determination Annex A1) o = rise of water above the normal water depth at the weir site. (x = 0) First let s determine the end point where y becomes zero, then: ( xs 0) 4 0 = Y ( xs o) = xs o= o Where: o= (P + H d )-NWD o x= s Therefore the effect of construction of weir on the water profile during peak flood ceases at around xm back from the axis. Then by applying the above equation, the level of the back water curve has to be determined till x m from the weir axis to check whether the existing bank level can accommodate the new water level arising from the construction of the weir. 17

19 Water Surface Profile downstream of the Weir Once the crest level is decided, it is required to draw the water profile for the design flood, Q max (For 50 year Return period usually) discharge. The water profile is required to - Carry out the stability analysis of the weir, - Design the weir structurally, - Design the downstream wing wall and protection works downstream of the apron. 0 1 H L Fig. 4.3: D/S water profile 0 1 Figure 3.15 flow over the broadcrest weir Determine the sequent depth D 1 and D : Applying Bernoulli s equation between section 0 and 1 & ignoring the head loss on the sloping surface due to friction, V1 (H + H e ) = D g q But V 1 = Then solve for D 1 by trial & error. This results three solution sets for D 1 ; two positive values & one negative value. Ignore the negative value & adopt the positive one that is less than the critical depth, Dc. D 1 must be less than Dc since the flow before the jump is super critical. The critical depth Dc is expressed by: q D c = g V1 F r1 =...3. gd 1 D1 D = ( 1+ 8Fr1 1) D 1 18

20 This depth has to be compared with the Normal Water Depth (NWD) estimated from the theoretical rating curve. If this value is greater than the NWD, a depression in the downstream apron has to be provided in order to accommodate D. The dissipated energy as a result of the jump becomes 3 ( D D1 ) H L = DD Energy Dissipation below the Weir Water flowing over the weir has a very high kinetic energy because of the conversion of the entire potential energy to the kinetic energy. If the water flowing with such a high velocity is discharged directly into the channel downstream, serious scour of the channel bed may occur. In order to protect the channel bed against scour the kinetic energy of the water should be dissipated before it is discharged into the downstream channel. The energy-dissipating device can be broadly classified into two types. [6] 1. Devices using a hydraulic jump for the dissipation of energy. Devices using a bucket for the dissipation of energy. The choice of energy-dissipating device is governed by the tail water depth & the characteristics of the hydraulic Jump, if formed, at the toe. The hydraulic jump type energy dissipaters dissipate excess energy through formation of highly turbulent rollers within the jump. (refer any open channel hydraulic books) 19

21 3.3. Structural Design of the Weir General Diversion weirs are constructed from a variety of materials. The most commonly used materials are reinforced concrete, masonry, and gabions. However, whatever materials are used, the structural analysis remains almost the same. Acting Forces on Weirs All external forces acting on a weir are the result of flowing water in the canal or river on which the structure is constructed. A typical force system of a weir consists of the following components: 1. Static water pressure of the surface water. Uplift water pressure 3. Soil reaction at the weir base 4. Friction forces at the base which develop to balance the horizontal forces 5. Weight of the weir and water wedges. Usually in structural analysis of weirs the dynamic force is neglected, since water behind the weir is built up gradually, and the uplift pressure which results from the arrival of a new wave does not develop instantly. In seasonal rivers there is a little or no uplift pressure when the first wave hits the weir, thus the force system which occurs at this moment, is not the most critical one, especially in weirs which are constructed monolithically with the apron. [9] Figure 3.16 presents typical sloped face weir, showing all the forces acting on them. Figure 3.16 Force which act on weir (general case) The components of the force system are discussed in the following: 1. Weight of the weir This is calculated simply by multiplying unit weight of the weir by its volume.. Weight of the water wedges The water wedges present the weight of water that is on the weir body and act, either against or in favour of the weir stability, it depends on the slope of the weir and the water surface downstream. 0

22 3. Upstream water pressure Its value can be easily calculated if the effect of changing the static pressure, upstream, to the dynamic one downstream is neglected. In high dams this effect can be substantial and it causes an over estimate of the design if it is ignored, i.e. the structure will have a higher factor of safety against overturning. 4. Downstream water pressure If a weir is designed to match the lower profile of a free water overflow, theoretically, the water pressure on the face of the weir should be nil, as it is the case in WES ogeeshaped weir. However, in practice this is hardly the case since the structured weir may not match the designed shape 100%. Therefore the water curve downstream of the weir cannot be determined theoretically since the combined effects of the weir geometry and the condition of the tail water are unpredictable. Theoretical determination of the water pressure on the face of the weir and weight of the water wedges downstream is not a straightforward process. The designers therefore have two choices: (i) Either to ignore pressure on the downstream face of the weir, in this case increase slightly the toppling safety factor, or (ii) Approximately draw the water surface and calculate the water weight. Figure 3.16 shows a typical weir with the rapidly variable flow at their downstream side. General Stability Conditions For a structure to remain stable the following conditions must be fulfilled. 1. Summation of all moments about a point must be equal to zero: ΣM a = Summation of all horizontal forces must be equal to zero: ΣH f = Summation of all vertical forces must be equal to zero: ΣV f = The above conditions need to be explained in relation to diversion weirs. For a structure to remain stable, the moments which tend to topple it must be equal to the moments which balance it. In practice, this condition does not satisfy design engineers, since unpredictable situations are likely to occur and cause the toppling moment to exceed the balancing one and hence the structure fails. Usually a safety factor of about 1.5 to is applied. 3.8 in order to avoid lifting up the structure s heel and tension occurrence at the base, the forces must pass through the middle third of the structures base, i.e. Eccentricity e < 6/B 3.9 or e= B/ X <B/ Where:

23 ΣM= Summation of all moments about the structure toe (Figure 3.17) ΣVf=Summation of vertical forces excluding the base reaction. X = Distance of the resultant of the forces from the toe B = Width of the weir base Figure 3.17 critical case for the design of slope face weir Design of Weir Wall The design of the wall involves the determination of its top and bottom widths such that the section will be stable under the condition of maximum stress Top Width As the main weir body is broad crested weir with vertical upstream & slopping downstream side, the top width is determined considering no tension & no sliding criteria. a) On the consideration of no tension criteria which is given by d B 1 = G H B 1 B Where, B 1 = Top width of the weir wall d = Max. Depth of the water above the weir crest, which is equal to H d G =Specific gravity of the material of the weir (for masonry G =.50KN/m

24 b) On the consideration of no sliding criteria which is given by d B1 = , µ G Where: µ is coefficient of friction (=/3) Bottom Width The bottom width should be sufficient so that the maximum comprehensive stresses are within the allowable limit & tension does not develop. The bottom width B of the weir wall is determined by equating the overturning moments to the resisting moments about the outer middle third of the bottom width of the weir wall taking the following critical states of flow. State I: when the upstream water or headwater is at crest level and there is no flow. The overturning moment is given by 3 Mo = w * H Where, δ w = 9.81KN/m 3 γw. HG. M r = ( B + B1B B1 ) Equate the Mo & Mr & solve for B. State II: when water is flowing over the weir crest and the weir is submerged. The overturning moment at the base is obtained from the water pressure diagram from the two faces. δ Mo = W hh Where: h is difference of water levels on the upstream & downstream (=Afflux). The moment of resistance about the middle third of the bottom width, again assuming the tail water at the weir crest is given by: γ M r = H w.( G 1) ( B + B1B B1 ) Equate the Mo & Mr & solve for B. State III: When water is flowing over the weir crest and weir is discharging with a clear over flow. The overturning moment is given by: δw 3 3 Mo = ( H + 3HdH D ) The moment of resistance is given by: γw. HG. Mr = ( B + B1B B1 ) H B1 B 3

25 Equate the Mo & Mr & solve for B. The highest of the three values is adopted & the section is checked for stability under the no flow condition Stability Analysis It has been observed over the years that diversion weirs collapse, initially not because of the unbalanced moment, but mainly due to the foundation scouring. The stability analysis becomes important where the structure and the apron are of two different materials and acts as two independent units, i.e. non monolithically or as this particular scheme the structure is built on alluvial soil. Therefore, the weir as a whole should be structurally safe and stable. It should be able to withstand the stress developed due to imposed loads & at the same time the foundation should be strong enough to carry the loads. Taking this into account the stability analysis is done based on the following assumptions: i) The base of the weir is pervious i.e. water seeps from the upstream side freely through the base to the downstream side ii) No water is flowing over the weir and no water parading is in downstream. Figure 3.18 Forces involved at the stability analysis The weir as a whole should be structurally safe and stable. It should be able to withstand the stress developed due to imposed loads. The foundations should be strong enough to carry the loads. The following points have to be considered while analyzing the stability of the weir body: A. Factor of safety against overturning Overturning failure occurs when the overturning moment exceeds the resisting moment. Thus the failure of the weir by overturning is usually preceded by Tension Failure or Crushing Failure. Therefore, a weir may be considered safe against overturning if the criterion of no tension at any pint in the weir body is satisfied & also the maximum compressive stress does not exceed the allowable limit. To be on the safer side the resisting moment should exceed the overturning moment at least by 100%. F OT = M M R O

26 Where: F OT = Factor of safety against overturning ΣM R = Summation of the resisting moment ΣM O = Summation of the overturning moment B. Factor of safety against sliding The sliding failure occurs when the weir slides over its base or when part of the weir lying above the horizontal plane slides over that plane. To avoid the failure of the weir due to sliding at any horizontal section or at the base, the weir should be designed so that the sliding forces do not exceed the resisting force. F S V = H Where: Fs = Factor of safety against sliding ΣV = Summation of vertical forces ΣH = Summation of horizontal forces C. Check for the development of tension failure The resultant force should pass through the middle third section of the base if not tension cracks develop & worsen the risk of failure by overturning. Masonry sections are very week in resisting tension stress. B e B e= x Mn x= V Where: ΣM n = Net moment (=ΣM R -ΣM OT ) e = Eccentricity developed B = Bottom width D. Check for crushing failure Crushing failure occurs when the compressive stress in the weir or foundation exceeds the safe limit. V 6e P= [ 1± ] Pa B B Where: P = Vertical stress at the toe/heel P a = Allowable bearing capacity of the foundation material Impervious Floor a Theories of Subsurface flow Hydraulic Gradient Theory 5

27 According to this theory the hydraulic gradient in the structure should be less than the allowable value. Bligh s Theory Bligh assumed that the interface between the relatively smooth base of a hydraulic structure and the sub-grade forms an easy path through which water can flow. This is called the creep path and its length the creep length. Further, it is assumed that in this theory, that the loss of head is proportional to the total creep length. If HL is the total head loss between the up stream and down stream and L is the length of creep, the loss of head per unit of creep length (i.e. H L /L) is called the hydraulic gradient Figure 3.19 Bligh s Creep length The total creep length according to Bligh s is calculated as L=d 1 +B+d =B+(d 1 +d ).3.46 Head loss per unit length, is given by: H = L B+ ( d 1 + d ) L The reciprocal of hydraulic gradient i.e. C (= ) is called Bligh s coefficient of creep (C). Refer table 1 for the value of C H Table 1 Recommended values of Bligh s coefficient of creep C and safe hydraulic gradient Soil type Value of C Safe hydraulic Gradient (1/C) Very fine sand or silt 18 1/18 Fine sand 15 1/15 Coarse sand 1 1/1 Gravel and sand 9 1/9 Boulders 4-6 1/4-6 Therefore, the length of creep should be sufficient to provide a safe hydraulic gradient according to Bligh s theory, in another word it may be stated that the hydraulic gradient must be kept under a safe limit in order to insure safety against piping. 6

28 L H C= L= CH For safety against piping, L CH L H C H L 1 C.3.49 Khosla s theory According to this theory, it is absolutely essential to have a reasonably deep cutoff at the downstream end of the floor to prevent piping. Khosla and his associates gave the mathematical solution for a composite floor. Usually, a hydraulic structure consists of a combination of a number of elementary forms. For the determination of uplift pressure at the key points of a structure, Khosla et al gave the theory of independent variables. According to this theory, a composite profile is split into a number of simple elementary standard forms for which the mathematical solutions can be easily obtained. Each elementary form is then treated independent of the other, and the pressures at its key points are obtained from the solution already available. Then the solutions of these elementary forms are superposed to obtain the pressure distribution at all the key points of the entire structure. The uplift pressures obtained from the superposition of the individual forms are to be corrected because the individual pressures have been obtained based on the following assumptions: The floor is of negligible thickness There is only one cutoff wall The floor is horizontal (a) (b) 7

29 (c) Figure 3.19 Elementary Forms (d) The most usual elementary forms: (a) a straight horizontal floor of negligible thickness with cutoff wall at the upstream end; (b) a straight horizontal floor of negligible thickness with a cutoff wall at the downstream end; (c) a straight horizontal floor of negligible thickness with a cutoff wall at some intermediate point; (d) a straight horizontal thick floor depressed below the bed but with no cutoff wall Causes of failures of weirs on permeable foundations The most common causes of failures of weirs constructed on permeable foundations may be broadly classified into two categories: I. Failure due to subsurface flow II. Failure due to surface flow I. Failure due to subsurface flow The main failures due to the sub surface flow: piping failure, rupture failure of the apron floor & the exit gradient may be steeper than the safe G E. II. Failure due to surface flow The main failures as a result of surface flow includes: development of scour holes that travels close to the impervious floor and damage the main structure & failure of the structure due to absence of appropriate energy dissipation downstream of the weir. Measures to be adopted for the above mentioned failures: The thickness of the floor should be sufficient to resist the uplift pressure due to the subsurface flow as the floor is usually designed as gravity section. A suitable graded filter should be provided at the downstream end of the impervious floor to prevent piping & sufficient creep length has to be provided to keep the exit gradient safe. A device is required at the downstream to dissipate & accommodate the energy due to the surface flow. Cutoff walls at the upstream and downstream ends of the impervious floor should be provided up to the maximum scour level to prevent the main structure against scour. 8

30 3.3..6a Depth of pile below the bed level When the natural waterway of the river is contracted, the waterway scours the bed both at upstream and downstream of the structure. The scour holes so formed may progress towards the structure causing its failure. Therefore, piles should be provided for the above-mentioned situation in a form of masonry cutoff wall of minimum thickness 0.50m. The upstream scour depth is fixed as = 1.5 R Hence, depth of upstream pile, d 1 = (P+H d )-upstream scour depth 3.51 The downstream scour depth is also fixed as = 1.5R.3.5 Hence, depth of downstream pile, d = NWD-downstream scour depth 3.53 These values are the maximum pile depths but the piles may be less than the mentioned value if rock is available at shallow depth. Figure 3.0 different cutoff piles b Floor length Length of percolation Cutoff walls and aprons are usually provided to prevent the piping under the structure, and to limit the intensity of the uplift so that the stability of the structure will not be threatened. Care must be exercised to ensure that the joint b/n the weir body and upstream apron are properly tied. According to Bligh theory the required creep length is computed as: L CH Where: L = Percolation distance (m) H = Maximum head (m) C = Bligh s percolation Coeff

31 Therefore, the length of percolation becomes: L CH As per Bligh s recommendation - Downstream impervious apron (L 1 ) H L 1 =.1 C Upstream impervious apron (L ) L = L - L 1 (b+d 1 +d + d 3 ) 3.56 Where: d1, d, d3=depths of upstream, intermediate & downstream cutoff Thickness of the floor Upstream floor thickness The downstream water pressure is always higher than the uplift pressure in the region of the upstream side of the weir. The thickness of the upstream apron can be based on the practice of the construction and perfection of leakage proofing m thick is usually sufficient for this purpose in case of masonry. Downstream floor thickness For the downstream apron, the thickness to be determined depends on whether static or dynamic cases are being considered. The lower parts of the apron will generally require larger thickness when the static case is selected, while the opposite is true for the toe section of the weir. But as per Bligh s theory the floor thickness at different points will be computed using 4 h T = g 1 Where: h = Ordinate of hydraulic gradient line measured above the water surface of the floor. g = Specific gravity of the floor material (for masonry, g =.5) Then length of percolation up to the weir toe (i.e. Pt. A), L A, became L A = *d 1 +L +B L Unbalanced head, = 1 A ha H L The uplift pressure = δ w *h A h Therefore, thickness, t A = A g 1 Check by Khosla s theory In the above portion the thickness of the floor is designed based on the Bligh s creep theory. So the safe exit gradient and the floor thickness of the structure will be checked by Khosla s theory. 30

32 I) Pressure at Key points For upstream cutoff b α = Now, φ E becomes d λ φ E = cos π λ α Where: λ = Uncorrected φ C1 = φ E Similarly 1 1 λ 1 φ D = cos π λ Uncorrected φ D1 = 100-φ D For Intermediate cutoff b1 α 1 = d b α = d 1+ α α 1+ α1 1+ α λ = and λ1 = φ E φ D λ1 1 = cos π λ λ1 = cos π λ λ1 + 1 φ C = cos π λ For downstream Cutoff α = b d α λ = λ Uncorrected φ E = cos π λ λ 1 Uncorrected φ D = cos π λ These pressures at key points must then be corrected for two cases: 31

33 A. Correction for floor thickness φd 1 φc1 Upstream Cutoff: Correction for φ C = + * t d 1 φe φd Intermediate Cutoff: Correction for φ E = *t d φd φc Correction for φ C = *t d φe φd Downstream Cutoff: Correction for φ E = * t d B. Correction for mutual interference The corrections for the mutual interference between two pile is given by d+d D C = ±19* ( ) ' b b Where: C = Percentage correction b ' = The distance between the two piles D= Depth of the pile whose effect is required to be determined on the neighboring pile of depth d. d = Depth of upstream pile being influenced b =Horizontal length of the Impervious apron b 1 = Horizontal length of the Impervious apron up to the intermediate cutoff wall Then the above corrections are applied to the pressures at the key points; Corrected pressure Upstream cutoff φ C = Uncorrected φ C1 + Floor correction + C.3.83 Intermediate cutoff φ E = Uncorrected φ E + Floor correction + C φ C = Uncorrected φ C + Floor correction + C Downstream cutoff φ E = Uncorrected φ E + Floor correction + C Then the uplift pressure is determined as follows: Upstream cutoff, P C = (Corrected pressure φ C )*H 3.87 Intermediate cutoff, P E = (Corrected pressure φ E )*H P C = (Corrected pressure φ C )*H 3.89 downstream cutoff, P E = (Corrected pressure φ E )*H.3.90 Pc PE Pressure at P B = Pc ( ) * LA B b Floor thickness required: Pc PE Pressure at P C = Pc + ( ) * LC D b Thickness of floor required at point A 3

34 P C 1 = G 1 Thickness of floor required at point B P E 1 = G 1 If this thickness is less than the minimum thickness specified using Bligh s theory, the former is adopted. II) Check for the Exit gradient b α = d Hence, from Khosla s exit gradient formula H 1 GE = * d π λ If this is steeper than the permissible exit gradient provided by Bligh s the structure is not safe against piping failure. In this case the creep length needs to be increased & the exit gradient checked again Divide Wall It is a wall constructed parallel to the direction of flow of river to separate the weir proper section & the silt excluder section there by facilitates scouring operations. If there are head regulator on both sides, silt excluders & there by divide walls have to be provided on both sides. Without divide walls currents approach the scouring sluices from all directions and their effectiveness is reduced. With the provision of divide walls flow can be concentrated along the regulator face. The second objective of the wall is to separate the floor of the scouring sluices which is generally at lower level than of the remaining portion and thus prevent turbulent action. The third reason is to prevent cross currents and flow parallel to the weir which will cause scouring. The length of the divide wall on the upstream side extends a little bit the head regulator, usually up to the end of the upstream impervious floor. On the downstream side, though it is common to extend the wall to the end of the downstream impervious floor, it can be extended till the toe of the weir wall. 33

35 Design consideration While designing the wall the following forces has to be taken into consideration: (a) Silt pressure up to the full tank level on the pocket side when the river level is low. At this time there is no water on the river side or the pocket side. Figure 3.1 Silt pressure difference on the two sides. (b) Difference of water pressure of about 0.5 to.0m on both sides depending on the height of the wall. γ γ γ Figure 3. Water pressure difference A minimum of 0.50m thickness of the wall has to be provided at the top. 34

36 Design of protection work Downstream protection work Riprap stones are placed on the channel bottom downstream of the stilling basin to prevent bank erosion caused by surges & residual energy from the stilling basin to reduce the possible undermining of the structure by the erosive currents. Factors affecting design of the riprap include size or weight of the individual stones, the shape of the large stones, the gradation of the entire mass of riprap, the thickness of the layer, the type of the filter or bedding material place beneath the riprap, the slope of the riprap layer, velocity & direction of currents and eddy action and waves, etc. Based on published material, laboratory observations and field experience, a design curve was developed for the determination the individual stone size to resist a range of velocities (Larry W. Mays, 1999). Inverted Filter Length of the inverted filter = 1.5*D 3.97 To prevent washing of fine soil particles by seepage an inverted filter consisting of graded material overlaid with heavy stone blocks is laid downstream of the impervious floor. It incidentally also prevents erosion of the soil by direct action of the flowing water. The length of the inverted filter is 1.5 times the conjugate depth, D. Basic requirements of the Filter: There are two basic requirements which should be satisfied: (1) The filter material should be fine enough to prevent the particles of the protected soil from being wasted into its voids, () The filter material should be coarse enough so that it acts as a drain for the protected material. Procedure: D15of filtermaterial To fulfill the first requirement: D of protectedmaterial 85 D15of filtermaterial To fulfill the second requirement: = 5to D of protectedmaterial 15 A multilayer filter having or 3 layers of the filter material should be provided in order to have more stages of transition from the protected material to the filter material if the difference of size of size of the two materials is quite large. The filter should be sufficiently thick to provide a good distribution of particles of all size in it and to carry the seepage discharge safely. A minimum thickness of the layers of a horizontal filter is 15cm for sand and 30cm for gravel. 35

37 Block stone with a thickness of 0.60m is provided over the inverted filter & the length of the launching apron is taken to be the same as the length of the block stone protection & its thickness in the horizontal position is usually fixed as 1.0m. upstream protection works On the upstream of the upstream cutoff wall, a stone block of minimum size 00mm has to be provided for a length of about 1.5D 1 where D 1 is the depth of the upstream cutoff from the upstream bed. A curtain wall between the stone block and the launching apron is provided to keep both structures as solid mass. It is provided in the upstream & downstream part of the weir. The height of the wall extends up to the bottom of the inverted filter with a minimum thickness of 0.30m. A footing depth of 0.40m with a 0.30m extension in both sides is provided. Size of riprap stones for protection work Different investigators relate the minimum size of stones used in the riprap to the bottom velocity. Use the estimated bottom velocity or the average velocity at the end sill of the stilling basin to find the minimum stone size. Bos (1989) states that the bottom velocity can be obtained by the formula q V b = D 3 Where: V b = Bottom velocity in m/s q = Discharge intensity (m 3 /sec/m) D 3 = NWD (m) The minimum size of stone for rip rap corresponding to bottom velocity is read from the curve developed by the US Bureau of reclamation & the maximum stone size from the gradation curve. Alternatively, the following empirical equation, which was developed, based on studies performed by Mavis and Laushey, and Berry as reported by USBR, may also be used to determine the stone size with reasonable accuracy (Larry W. Mays, 1999): V b = 5.05 d Where: d = diameter of rock (m). Note that the rock is assumed to have a specific gravity of about.65. The accuracy of the equation for velocities above 4.9m/s is unknown. Place the riprap in a layer at least 1.5 times as thick as the maximum stone size. It is recommended that the riprap be placed over a filter, or bedding, composed of gravel or graded gravel having the larger particles on the surface. 36

38 Silt Excluder Silt excluder is device located just in front of canal head regulator and its function is to exclude the lower layer of river water which carry comparatively coarser particles & prevent them from being diverted into the canal. The basic requirements for satisfactory silt exclusion are: It draws relatively silt free water from the top-layers and excludes heavy laden from bottom layers, Entry of water is smooth so s to void turbulence in water & agitating of silt, It provides a smooth surface by paving or plastering the bed and sides to reduce friction & give chance to the silt in the upper layers to settle down in layers adjacent to the bed from where it is led away downstream, & Velocity is reduced in the pocket so s to attract silt load in lower layer. Design Procedure To facilitate silt-exclusion through the silt excluder, the flood that can pass through the gates is usually taken as 10-0% of the design flood. The clear water way required to pass this discharge is fixed using discharge formula for broad crested weir in submerged condition: Q * * [( ) ] 3 / s = C1 L g h+ Ha Ha + C * d * L g( h+ Ha ) Where: Qs = Discharge through the silt excluder, m 3 /s C 1 = Coefficient (=0.577) i.e. discharge coefficient for the free portion h = Head causing flow i.e. upstream HFL downstream HFL L = Clear water way D = Depth of water over crest on the downstream Ha = Head due to velocity of approach C = Discharge coefficient for the drowned portion (=0.80) The number of gates usually depends on the type of lifting device arranged. In small scale irrigation project as the gate is designed to be operated manually by the beneficiaries, it is better to minimize the number of gates to be provided as the larger the number of gates, the smaller the width of the gates become but the higher the leakage through the gate groove will become as a good water tight groove cannot be achieved during construction Head Regulator The purpose of the weir is to create sufficient head to supply the min irrigation canal with the design discharge. The design of head regulator should serve the following objects: (a) To make the regulation of supply in the canal easy (b) To control silt entry into the canal (c) To shut out river floods. 37

39 The canal head regulator is usually gated to control the mount of flow into the canal. Out of the most common types of head regulators, the Bureau uses closed or culvert intake type in case of small scale irrigation projects. The characteristics of flow in culvert are very complicated since the flow is controlled by many factors. In designing a culvert for the intake, it is convenient to assume that the pipe is fully flowing with both ends submerged and to include all the head losses in the orifice coefficient C. Q= CA gh To obtain the full flow condition, the pipe inlet must be submerged to a depth not less than the sum of the velocity head and the head losses in the pipe. For concrete pipe with square-cornered entrance, L C = [ D + ] 1. D Where: D = Pipe diameter, ft L = Length of pipe, ft Note that 1ft = 0.308m. Lay Out of Head regulator The angle of diversion, the angle between the stream channel and the diversion channel, has considerable effect on the amount of sediment attracted into the diversion channel. An angle of diversion between 30 o and 45 o can be adopted when a model study is not available, and sediment excluder is not to be provided. Figure 3.3 Layout of the head regulator. 38

40 Design Procedure In order to avoid constructing large canals, the design capacity of the main & secondary canals is fixed based on the surface flow measured t the end of the rainy season so that it can also be used to irrigate for supplement. Head loss calculation The following losses have to be taken into consideration. i) Trash Rack loss A trash rack has to be provided using reinforcing bars, usually φ10mm bars 50mm c/c in front of the gate both ways. It can be assumed that 50% of the area is clogged: h t = K t Vn * g Where: h t = Trash rack loss V n = Velocity through the net trash rack (= Q ) A n An A K t = 1.45 n 0.45* ( ) A A g g ii) Inlet Loss h i = Vn Ki * g Where: K i = 0.50 iii) Outlet Loss h o = K Where: K o = 1.0 o Vn * g iv) Gate loss h g 1 = [ C D Vn 1] g 39

41 Where: C D = 0.96 v) Friction loss Vn h f = [ ] * L / 3 R Where: R = Hydraulic Radius, m n = Manning s roughness coeff. (Usually for concrete pipe) L = Length of pipe, m Therefore, the total head loss through the pipe is the sum of all losses. H = i h Then the discharge through the pipe becomes, Q= CA gh Where: h = Net driving head, m (Pond level Pipe Invert level - φ/ H) φ = Pipe internal diameter, m If the discharge so calculated is equal to or greater than the one adopted for design (by small amount ), the driving head is sufficient, so is the weir height. If not adjust the pipe invert level or the weir height De-Silting Basin To avoid accumulation of sediment in the canal & costly maintenance, construction of de-silting basin downstream of the intake is recommended. Design Procedure Length of the basin can be calculated s follow: D L b b = V V s A 5% additional safety factor is added to the calculated L b. For an assumed γ s =.65gm/cm 3 & water at 0 o C, the settling velocity, Vs, can be calculated as: Vs = 0.9d The flow velocity in the basin should not be too high to cause lifting of settled particles. The maximum allowable flow velocity can be determined from the shear velocity. Avci has given the following formula for the maximum permissible velocity & claims that it has been investigated & can be applied satisfactorily (Rozgar Babah, 1995): V = p Co d (mm) Co d< d

42 0.1<d< d> The above formula for calculating the settling velocity assumes that the water in the basin is stagnant, obviously this is not true. Avci (1991) proposes the following correction for the settling in the flowing water: Corrected V s = V s V s Where: V s = a o V a o = D The flow velocity in the basin, V, should be less than or equal to the maximum permissible flow velocity in the basin, V P, in order not to cause the settled silt particles lifted up. The bottom width of the basin is calculated as: Q B= DV b Protecting Side Wall Protecting side wall is an essential part of a diversion weir engineering design. During flood period, flush flood with high velocity give extreme scour to both banks of the river where diversion weir is located. The scouring damage can bring bout bank failure, increase river sediment content and threaten the safety of min hydraulic structures in the diversion weir project. Therefore it is important to design properly protecting side wall. Design Principle Protecting side wall layout should be determined by comprehensive studying on channel current condition, bank geological condition & project general layout. The design major should be ginger to bank regulation on upstream and downstream of weir within some range to provide good diversion condition for intake and ensure overflow weir can discharge smoothly design flood during flood period. Structural steadiness and normal life span & other factors should be considered to determine protecting side wall type & construction materiel & make it can meet the requirement of easy construction and relatively low cost as possible. Design flood standard for protecting side wall is determined s the flood frequency of 50 Yr. Return periods. Protecting Side Wall Type As for earth bank, when bank height is.0-5.0m, gravity masonry side wall is usually used in small & medium diversion weir engineering. Gravity side wall is simple structure, 41

43 easy for construction and relatively low cost. Hence it is suitable to adopt gravity retaining walls. Bank Protection Length Bank protection length depends on: upstream & downstream channel water level of weir site, Channel current direction & velocity during flood process, Bank geological condition, Bank protection type. But in the downstream side it need be provided at least for a distance equal to the length of the Jump. Sectional Size The top elevation of protecting side wall is treated separately on upstream & downstream of overflow weir. In both cases the top level is: Upstream protecting Wall = upstream HFL + Free board Downstream protecting Wall = downstream HFL + Free board A minimum of 0.50m free board has to be provided. The water face of side walls is generally vertical face. After fixing the sectional size, the section has to be checked for stability using the standard procedure. Selected material has to be used for back fill material & Weep holes have to be provided at proper points to drain out excess water. Proper grading materiel has to be provided behind the weep holes to protect the back fill material from being washed out. 3.4 Sediment Control An important part of any water resources or water diversion project is the design of effective sediment control measures aimed at reduction of sediment yield from the catchment. The topic of soil conservation commands a literature in its own right, and the section is included here only to emphasise the need and importance of works to reduce soil erosion. The most important of all the measures to reduce soil erosion is the land use management and cropping practices, and these range from arable farming to forestry. Depending on topography and intensity of land use additional engineering works may be necessary. Terracing, check dam, percolation pond, contour farming, etc. are some of the age-old methods to reduce soil loss that are still in use. In the upper parts of many catchments debris dams are often effective in trapping and retaining sediment and reducing sediment runoff. 4

44 3.4.1 Sedimentation at diversion works Figure 3.4 Watershed and River Processes Setting and Terminology The lowering (degradation) or raising (aggradation) of a river frequently results in problems to infrastructure such as at bridges, bank protected reaches, reservoirs upstream from dams and scour downstream from weirs. During planning and design of river works it is essential to recognize potential problems and this can be done by assessing vertical river processes Degradation of the river bed Degradation involves the lowering of a river bed over a long river reach or several reaches and is often progressive (ongoing). The lowering is achieved by the reduction of sediment load or by the lowering of a downstream control level such as a lake level. The degradation process can move in a downstream direction as well as upstream. 43

45 Figure 3.5 Combinations of degradation processes along a main river and along a major tributary. There are several factors that result in the process of bed lowering (degradation). Also, we should clarify that the position of the river-bed is dependent upon the removal of sediment from the bed which is technically called bed erosion and involves the transport of sediment out of the river reach. It is only the bed-material removal that causes degradation the movement of fine materials such as silt and clay (wash load) through the river reach does not have any effect on the degradation process. Generally, degradation occurs in the form of a wedge downstream from dams or weirs Aggradation of the river bed Aggradation involves the rising of a river-bed over a fairly long river reach and is usually the result of an increase in bed load from a diversion or from sand/gravel dumping into a river system, or from increased bank erosion upstream. Alluvial fans are aggradation zones due to the fact that there is a sharp reduction in velocity where the river and sediment exit from a gorge onto an alluvial plain. There are two distinctly different processes of aggradation as shown below: 44

46 Figure 3.6 downstream and upstream progressing aggradation Many rivers exit mountain gorges and, upon entering an alluvial plain reach, dropout most of the coarse sands and gravels. In many parts of the world these zones are fertile agricultural lands and flood dikes are constructed to reduce the extent of flooding. Usually this results in an accelerated rate of bed rise because the surface deposition area is significantly reduced by dikes: Figure.7 downstream progressing aggradation 3.4. General Features of Flow Diversion a) Flow patterns One of the primary design tasks of diversion structures is to keep the amount of sediment diverted to a minimum. For this two well-known hydraulic phenomena are basic Curved flow paths are associated with secondary currents; Increased turbulence increases the sediment content of flow. If a boundary forces the flow to change direction, the resulting impact (centrifugal) force, and the force exerted on the flow by the boundary are in balance, but since velocity decreases from surface downwards, due to the boundary layer effect, a dynamic pressure gradient is created. This leads to a downward flow, which together with the main flow produces the spiral (helicoidally), current typical of flow in all bends, as illustrated in figure 3.8. Secondary currents can also be produced by a convergence of flow, as illustrated in figure

47 Figure 3.8 Illustration of spiral (helicoidal) flow through a river bend Figure 3.9 Secondary currents produced by convergence of flow 3.4. Sediment Control at Diversion Weirs In most river systems that are used for supplying water for irrigation and water supply in case of semi-arid region; where the rainfall is seasonal and erratic, river flows are characterized by flashy as well as transport large quantities of suspended sediment along with bed material, which can cause serious silting problems in irrigation canals, particularly if flows are being diverted during the rainy season flood flows. A partial solution is to control the sediment at the canal intake or in the main river channel and if sediment enters the canal then canal structures, such as sediment ejectors, can be incorporated. The various methods are summarized and grouped as 46

48 a) Sediment Control within the River Curvature of a River One of the simplest methods for control of sediment movement at weirs is to locate a weir near the end of a river bend with the intake on the outside of the bend, because sediment (bed-load) generally moves to the inside of the bend. One typical intake and weir layout as shown in figure It is considered an improvement in weir location compared to on a straight stretch. Note: The location of an intake at the outside of a river bend generally does not solve the problem of sediment control it is usually necessary to add some form of under sluices to move sediment through the weir. Figure 3.30 Importance of river bends in controlling sediment movement Several weirs in mountainous river reach are experiencing sediment problems due to intakes being located at the inside of the river bend as shown in figure Typical example is Zamera diversion weir located in southern tigray near Hawane town along Addis Ababa to Mekelle road. 47

49 Figure 3.31 Wrong position of canal intake b) Role of intake structure in sediment control The intake structure (or head regulator) is a hydraulic device constructed at the head of an irrigation or power canal, or a tunnel conduit through which the flow is diverted from the original source such as a reservoir or a river. The main purposes of the intake structure are To admit and regulate water from the source, and possibly to meter the flow rate, To minimize the silting of the canal, i.e. to control the sediment entry into the canal at its intake, and To prevent the clogging of the entrance with floating debris. In high-head structures the intake can be either an integral part of a dam or separate; for example, in the form of a tower with entry ports at various levels which may aid flow regulation when there is a wide range of fluctuations of reservoir water level. Such a provision of multilevel entry also permits the withdrawal of water of a desired quality. The layout of a typical intake structure on a river carrying a heavy bed load is shown in Figure 3.3. The following are its major appurtenances: 48

50 Figure 3.3. Canal intake on a river carrying heavy bedload (after Mosonyi, 1987) 1. The raised inlet sill to prevent entry of the bed load of the river;. The skimmer wall (with splitter pier) at the inlet to trap floating ice and debris; 3. The coarse rack (trash rack) to trap subsurface trash, equipped with either manual or automatic power-driven rack cleaning devices; 4. The settling basin (sand trap) followed by a secondary sill (entrance sill) diverting the bottom (sediment-laden) layers towards the de-silting canal; 5. The flushing (de-silting) sluice to flush the deposited silt; 6. The intake (head regulator) gates to control the flow rate into the canal; 7. The scouring (tunnel) sluices in the diversion weir to flush the bed load upstream of the inlet sill. The de-silting canal with its flushing sluices may be omitted if the sediment load which would settle in the settling basin is negligible; however, smaller grain size sediment (silt) 49

51 is always likely to enter the canal, and maintenance of minimum velocities in the canal is essential to avoid its silting up. c) Location and alignment of an intake The river reach upstream of the intake should be well established with stable banks. As the bottom layers of the flow around a bend are swept towards its inside (convex) bank (see figure 3.30 and 3.31), it is obvious that the best location for an intake (to avoid bed load entry) is the outer (concave) bank, with the intake located towards the downstream end of the bend. This choice of location from the sediment exclusion point of view is not always possible and other considerations such as the pond (command) levels and their variations, navigation hazards, and location of the diversion structure, pump/power house, and outfalls must be considered. An offtake at 90 to the main flow is the least desirable one. The structure should be aligned to produce a suitable curvature of flow into the intake, and a diversion angle of around is usually recommended to produce this effect; in addition, an artificial bend (Figure 3.33), a groyne island (Figure 3.34) or guide vanes (Figure 3.35) may be designed to cause the required curvature of flow (see Avery, 1989). Model tests are desirable in deciding on the location and alignment of any major intake structure (Novak and Cˇ abelka, 1981). 50

52 Figure 3.33 Intake layouts with induced curvature to flow (after Mosonyi,1987) Figure 3.34 Use of artificial groyne (e.g. island) to induce desired curvature to flow at intakes 51

53 Figure 3.35 Guide vanes layouts upstream of intake for sediment exclusion d) Silt control at headworks I. Silt excluder The silt excluder is a device constructed in the river bed just upstream of the regulator to exclude silt from the water (source) entering the canal. It is so designed that the top and bottom layers of flow are separated with the least possible disturbance, the top sediment-free water being led towards the canal while the bottom sediment-laden water is discharged downstream of the diversion structure through under sluices. The device basically consists of a number of tunnels (Figure 3.36) in the floor of the deep pocket of the river, isolated by a dividing wall. The sill level of the regulator is kept the same as that of the top level of the roof slab of the tunnels. The capacity of the tunnel(s) is usually kept at about 0% of the canal discharge, and they are designed to maintain a minimum velocity of 3ms_1 (to avoid deposition in tunnels). 5

54 Figure 3.36 Silt excluder (under tunnel type) II. Silt ejector or extractor The silt ejector is a device constructed on the canal downstream of the head regulator but upstream of the settling basin (if any), by which the silt, after it has entered the canal, is extracted. a) Vane type ejector: The layout of a vane type ejector as shown in Figure A diaphragm at the canal bed separates the top layers from the bottom ones. On entering the depressed area of the canal bed, the bottom sediment-laden layers are diverted by the curved vanes towards the escape chamber. The design should be such that the entry disturbances are minimal; the streamlined vane passages accelerate the flow through them, thus avoiding deposition. 53

55 Figure 3.37 Silt ejectors (vane type) b) Vortex tube type ejector. The vortex tube ejector (Figure 3.38) consists of a pipe with a slit along its top, placed across the bottom of the canal at an angle of around to the direction of flow. The vortex motion within the tube draws the sediment into it, and the wall velocities along the tube eventually eject the 54

56 sediment at its discharge end. A properly designed vortex tube ejector can be more efficient than any other conventional ejector, with less water loss. e) Settling basin Figure 3.38 Vortex tube type silt ejector The settling basin is a device placed on the canal downstream of its head regulator for the removal of sediment load which cannot be trapped by the conventional excluders or ejectors. It consists of an enlarged section of the channel where the flow velocity is sufficiently low so that the fine sediment settles on the bed (Figure 3.39). The settled sediment is removed by sluicing, flushing or dredging. The following equation may be used to design a settling basin: W = W o e wsx q Where W is the weight of sediment leaving the basin, W 0 is the weight of sediment entering the basin, w s is the fall velocity of a sediment particle, q is the discharge per metre width of the basin and x is the length of the settling basin. Alternatively x= cdv s w s Where D s is the depth of the settling basin, V is the mean velocity in the basin, and c is the safety factor (1.5 ). On the basis of the steady-state two-dimensional dispersion equation, the fraction of removal, f, at a distance x in a sedimentation tank of depth y, can be obtained (assuming isotropic dispersion) from [ V log( 1 f) ] 10w x s y= 1 U Where U* is the shear velocity in the basin Equation (3.10) gives reasonably satisfactory results when computing each fraction of removed sediment independently, if the concentration is small. 55

57 Figure 3.39 Arrangement of settling basin 56

58 3.5 Operation & Maintenance The systematic & rational operation of the system goes a great way in achieving the great goal for which the scheme exists. The main purpose of the scheme is to yield best results and achieve the target of food production, social happiness, improvement of economic & financial status and preventing the environment for better ecology, health & happiness Operation of Headwork As it is a diversion scheme, the flow of the stream will maintain the pond level with proper operation of silt excluder gates as indicated below: During dry season irrigation the sluice gates remain closed while the off take gate is opened. Operation of silt excluder gates should be done during the start of the rainy season so that the silt is carried away by the high velocity created in the silt excluder and deep channel formed in its front for a long length. During this time the sluice gate should be remain open while the off take gates closed. In case of supplementary irrigation, after the sediment is flushed out, the opposite holds true. The leakage through the gates (bottom or side ways) has to be stopped, The trash rack behind the sluice gate should be moved during the rainy season or operation of the gates. During lean flow no over flow through the main weir structure is to be encouraged Maintenance of Headwork The efficiency & sustainablity of the scheme lie upon the nature of maintenance of the components involved in the system. The beneficiaries are to play the executive role in the operation and maintenance of the system. After the development and completion of the small scale irrigation scheme by Water Resource Development sectors, it is transferred to the direct beneficiaries through their genuine Water Use Association (WUA). The WUA may do it by contributing voluntary labors (by mobilizing the farmers). The activities involved in the repair & maintenance of the headwork is: Repair & maintenance of the toe of the weir crest, & the downstream stone pitching, Repair & maintenance of the sidewalls General checking of sidewall & function of weep holes Repair & maintain of gates & trash rack Periodical flushing of silt deposited in the weir & silt excluder bays. 57

59 References 1. Engineering manual for Irrigation & Drainage, Head work Volume I&II published by the Japanese institute of Irrigation & Drainage march Dr. P. Boeriu(1998/1999 ) River Intakes And Head works, Lecture Notes, IHE International Institute for The Netherlands Infrastructural, Hydraulic and DELFT Environmental Engineering 3. Boeriu, P., 003, Water supply works part I, HRI Egypt. 4. Novak, P., et al, 4 th ed, Hydraulics structures, London. 5. K.B. Khushalani & Manohar Khushalani IRRIGATION PRACTICE AND DESIGN Diversion and distribution works. Vol. IV. New Delhi, India. 6. K.R. Arora Irrigation Water power and water resource engineering. New Delhi, India 7. Raudkiwi,A., 1993, Sedimentation. 8. Dr.Ing. Seleshi Bekele : Hydropower II: Design of Components of Hydropower Schemes (HE607) Lecture note unpublished 9. Rozgar Baban, Design of Diversion weir,small scale irrigation in hot climate 10. Larry W. Mays Hydraulic Design Handbook. R.R. Donnelley & Sons Company. USA. 11. River Works Design Guidelines For Tigray, Ethiopia, Prepared by Northwest Hydraulic Consultants (NHC) Vancouver, Canada 58

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