Chapter 1 Irrigation Engineering

Transcription

1 1.1 Introduction Chapter 1 Irrigation Engineering Irrigation is a science of artificial application of water to the land, in accordance with the crop water requirement throughout the cropping period. Irrigation can also be defined as a systematically developed knowledge based on long-term observation and experiments of handling available sources of water for economic growth of bumper crops. It includes: Training and tapping of source of water Storing of water Conveying that water efficiently to the field (it includes also draining of surplus water) Or in different words is a science of survival for mankind. A crop requires a certain amount of water at some fixed time interval throughout its period of growth. If the water requirement of the crop is met by natural rainfall during the period of growth, there is no need of irrigation. In a tropical country like Ethiopia and India, where the natural rainfall occurs mainly during the monsoon season from June to September, irrigation is essential for the proper growth of plants. Irrigation engineering is a broad-based science which deals not only with the application of water to the land but also with the planning and design of various structures required for an irrigation system. It deals with the harnessing of various natural water resources for irrigation and all aspects and problems starting from the watershed to the agricultural fields. The basic objective of irrigation is to supplement the natural supply of water to land so as to obtain an optimum crop yield. In order to achieve this objective, an irrigation system is developed. It involves planning, designing, construction, operation and maintenance of various irrigation structures required to bring water from the watershed to the agricultural field. It is the duty of the professional engineer to develop and execute an efficient and economic irrigation system to suit the need of the region. Merits and demerits of irrigation There are various direct and indirect advantages of irrigation a. Increase in food production: irrigation helps in increasing crop yield through controlled and timely supply of water to the crop. b. Optimum benefits: optimum utilization of water, implies obtaining maximum crop yield with any amount of water. in other words yield will be smaller for any quantity lesser than or in excess of the optimal quantity. c. Elimination of mixed cropping: in areas where irrigation is not ensured, generally mixed cropping is adopted. Mixed cropping is sowing together of two or more crops in the same field. If the weather condition is not suitable for the other, and those at least some yield is obtained. Mixed cropping is thus necessary and economical when irrigation facilities are lacking. But if irrigation is assured, mixed cropping can be eliminated or reduced. Mixed cropping is not generally accepted; because different crops require different types of field preparation and different amount of watering, manuring, etc. if two crops are mixed together it will be Chapter 1 Irrigation Engineering 1/10

2 difficult to meet special needs of either. Moreover, during the time of harvesting, the crops get intermixed with each other, reducing the purity of each other. d. General prosperity: revenue returns are sometimes quite high and helps in all around development of the country. e. Facilities for transportation f. Generation of hydropower Some of demerits of irrigation are: a. Long-term application of pesticides under large-scale: irrigation system might have a negative impact on the soil microbial activities, on the quality of surface and subsurface water resources and survival of the surrounding vegetation. b. Over irrigation may lead to water logging and may reduce crop yield. c. Population displacement 1.2 Irrigation potential of Ethiopia Ethiopia is the water tower of northeastern Africa. Many rivers arising in Ethiopia are also the sources of the major water resources in neighboring countries. The country is endowed with water resources that could easily be tapped and used for irrigation. Ironically this country is already suffering from food shortage because of the increasing population and chronic drought occurring in the most parts of the eastern and northern part of the country. There is annual food deficit to the extent of 1.0 to 1.5 million tones the country. During the period from 1984 to 1992 the food aid annually received was around 0.9 to 1.0 million tones (World Bank Report, 1992) to meet the demand of the ever-growing population (currently ~ 75 million). The need for utilization these resources are most urgent, in particular, in areas of the country where the length of growing period is short and precipitation is erratic. In Ethiopia, rainfed agriculture contributes the largest share of the total production. Prior to mid 1980s, irrigation in Ethiopia was concentrated on the production of commercial crops, principally cotton and sugarcane on large state farms. The country s irrigation potential ranges from 1 to 3.5 million hectare of irrigable land of which between thousand hectare (5-10 %) is estimated to be currently irrigated. It should be noted that the assessment of irrigation potential is to a large degree subjective as it is dependant on physical resources of land and water, but also on the economic and social feasibility of there exploitation. The irrigation potential of the 12 major river basins is given in the table below: No Basin Basin are km 2 Mean annual vol 10 9 m 3 Chapter 1 Irrigation Engineering 2/10 Ground water potential 10 9 m 3 Potential gross irrigable area ha Net area under irrigation ha 1 Awash Abay Baro-Akobo Rift Valley lakes 5 Omo-Gibe Genale dawa Wabishebelle Tekeze Ogaden Denakil

3 11 Aysha Mereb-Gash Ethiopia has not developed irrigation to the potential it has. The development of irrigated areas in the countries has also been unevenly spread. On going irrigation projects 1. Kesem & Tendeho Irrigation Projects The projects are found in the Awash Basin. A total of 90,000ha Irrigation area with all its infrastructure & Dams design completed. With in the coming one year it is assumed that all of is construction works will be completed. 2. Ethiopian Nile Irrigation & Drainage Projects These projects are found in the Abbay & Tekeze Basins. Arjo- Dedesa Irrigation Projects. This project area is located in the Abbay Basin. It has a total area of 14,300 ha. Previously it was studied to a reconnaissance level. Humara Irrigation Project This project area is located in the Tekeze Basin. It has a total area of 43,000 ha. Gumara Irrigation Project. This project area is situated around Lake Tana in the Nile Basin. It has a total area of 14,000 ha. 3. Lake Tana sub basin irrigation project. Ethiopian Government financed projects The projection areas are located in Abbay Basin. It includes North East Lake Tana, North West Lake Tana, South West Lake Tana, Jemma, Megech, Rib and Gilgel Abbay Irrigation Project. They constitutes a total area of 62,457 ha 4. Koga Irrigation Project. This project is located in Abbay Basin. It is under construction. It covers a total area of 7,000ha. 5. Lake Abbaya Sub-basin Irrigation Project. It includes Gelana, Gidabo and Billate Irrigation Projects. They are located in the Rift Valley Lakes Basin. The Total area of this project is 31,920 ha. 6. Ziway Irrigation Project. It is feasibility study and detailed design project. The project area is located in the Rift Valley Lakes Basin. The project area covers 15,500 ha. etc 1.3 Components of an irrigation system In an irrigation system four main functional elements can be distinguished: Headwork- can be described as a complex of structure in and along a river or stream for the diversion of water into a canal system for irrigation. Headworks include facilities to clean the water from excess sediment and to measure the quantities of water taken in. Structure included in the headworks will be diversion structures, energy dissipaters, intake sluice, sediment trap, river works and appurtenant works. Headworks are classified as: Weir or Barrage: A weir or barrage is used to raise the water level in the river to the elevation needed for the water supply to the irrigation canals. This elevation will normally give the upper boundary of the command area. A weir is a solid overflow structure across the river. A barrage is a structure with gated openings which can be opened to let large floods pass and closed during low flows. Weis are the most common structures to divert irrigation water. Reservoirs: Reservoirs for irrigation store water in periods of surplus supply on a river for use in periods with deficient supply so that the function mainly is to regulate the river flow. Large reservoirs often have multipurpose functions as they may be used for irrigation, hydropower, flood regulation, fisheries etc. Chapter 1 Irrigation Engineering 3/10

4 Pumping Station: Pump irrigation is considered when gravity diversion is technically and economically not feasible. Conveyance system /Canal System Irrigation canals: Primary canals convey water from the systems headworks to the secondary canals and to adjacent tertiary units. Secondary canals convey water from the primary canal to the tertiary units. Tertiary canal with their distribution system and drainage collection system; water is distributed and delivered to the fields and the excess water is collected in the drainage system within the tertiary unit; Drainage canals: Outside the irrigation area to evacuate excess water to rivers and natural water sources Division and off-take Structures- A division structure is situated in the primary or secondary canals at bifurcation points and serves to divide the flow between two or more canals. A tertiary off-take conveys water from a primary or secondary canal to the off-taking tertiary canals. Division and off-take structures may be combined into one structural arrangement. Cross-drainage structures- Culverts are the most common cross-drainage structure for external protection. Inverted Siphons are used where a small irrigation canal crosses a large drainage channel in which case it is usually safer and more economic to carry the irrigation water through an inverted siphon underneath the drain. 1.4 Standards of irrigation water Every water is not suitable for irrigation. The quality of irrigation water is very much influenced by the constitutes of the soil, which is to be irrigated. Irrigation water may be said to be unsatisfactory for its intended uses if it contains: Chemicals toxic to plants or to the persons using plant as food Chemical that react with the soil to produce unsatisfactory moisture characteristics Bacterial injuries to persons or animals eating plants irrigated with water Nearly all waters contain dissolved salts, many of which result from the natural weathering of the earth s surface. In addition, drainage waters from irrigated lands and effluent from city sewage and industrial waste water can impact water quality. In most irrigation situations, the primary water quality concern is salinity levels, since salts can affect both the soil structure and crop yield. However, a number of trace elements are found in water which can limit its use for irrigation. Most salinity problems in agriculture result directly from the salts carried in the irrigation water. As water evaporates, the dissolved salts remain, resulting in a solution with a higher concentration of salt. The same process occurs in soils. Salts as well as other dissolved substances begin to accumulate as water evaporates from the surface and as crops withdraw water. Types of salt problems Two types of salt problems exist which are very different: those associated with the total salinity and those associated with sodium. Soils may be affected only by salinity or by a combination of both salinity and sodium. Water with high salinity is toxic to plants and poses a salinity hazard. Soils with high levels Chapter 1 Irrigation Engineering 4/10

5 of total salinity are called saline soils. High concentrations of salt in the soil can result in a physiological drought condition. That is, even though the field appears to have plenty of moisture, the plants wilt because the roots are unable to absorb the water. Water salinity is usually measured by the TDS (total dissolved solids) or the EC (electric conductivity) expressed in parts per million (ppm) or in the equivalent units of milligrams per liter (mg/l). EC is actually a measurement of electric current and is reported in one of three possible units as mmhos/cm, µmhos/cm or deci Siemens per meter at 25 C (ds/m). Irrigation water containing large amounts of sodium is of special concern due to sodium s effects on the soil and poses a sodium hazard. Sodium hazard is usually expressed in terms of SAR or the sodium adsorption ratio. SAR is calculated from the ratio of sodium to calcium and magnesium. If the percentage of sodium ion increases it has an influence on the aggregation of soil grains i.e breaks down. The soil becomes less permeable and of poor tilth. It starts crushing when dry and its PH increases towards that of alkaline soil. High sodium soils are therefore, plastic, sticky when wet, and are prone to form clods and they crust on drying. The proportion of sodium ions present in the soil is generally measured by a factor called sodium absorption ratio (SAR) and represents the sodium hazards of water. SAR is defined as: SAR = Ca Na Mg 2 ++ Where the concentration of the ions is expressed in equivalent per million (epm); epm is obtained by dividing the concentration of salt in mg/l or PPM by its combining weight (i.e atomic weight / valence) The latter two ions are important since they tend to counter the effects of sodium. Continued use of water having a high SAR leads to a breakdown in the physical structure of the soil. Sodium is adsorbed and becomes attached to soil particles. The soil then becomes hard and compact when dry and increasingly impervious to water penetration. Fine textured soils, especially those high in clay, are most subject to this action. Certain amendments may be required to maintain soils under high SAR. Calcium and magnesium, if present in the soil in large enough quantities, will counter the effects of the sodium and help maintain good soil properties. Classification of irrigation water Several different measurements are used to classify the suitability of water for irrigation, including EC, the total dissolved solids, and sodium adsorption ratio SAR. Some permissible limits for classes of irrigation water are given in the table below; the sodium hazard of water is ranked from low to very high based on SAR values. Chapter 1 Irrigation Engineering 5/10

6 Leaching for salinity management Soluble salts that accumulate in soils must be leached below the crop root zone to maintain productivity. Leaching is the basic management tool for controlling salinity. Water is applied in excess of the total amount used by the crop and lost to evaporation. The strategy is to keep the salts in solution and flush them below the root zone. The amount of water needed is referred to as the leaching requirement or the leaching fraction. Shallow water tables complicate salinity management since water may actually move upward into the root zone, carrying with it dissolved salts. Water is then extracted by crops and evaporation, leaving behind the salts. Shallow water tables also contribute to the salinity problem by restricting the downward leaching of salts through the soil profile. Installation of a subsurface drainage system is about the only solution available for this situation. 1.5 Functions of irrigation water The function of water in the plant growth are diversified: 1. addition of water to the soil supplies essential moisture to the plant growth it acts as a solvent for the nutrients, water acts as a nutrient carrier the irrigation water supplies moisture, which is essential for the life of bactria beneficial to plant growth 2. some salts present in the soil react to produce nourishing food products only in the presence of water Chapter 1 Irrigation Engineering 6/10

7 3. water cools the soil and the atmosphere and thus makes more favorable environment for healthy plant growth 4. irrigation water, with controlled supplies, washes out or dilutees salts in the soil 1.6 Data, surveys and investigations for irrigation design Surveys and investigations are conducted to obtain data required to execute the preliminary and final design stage. Hydrological phenomenon like river flow and rainfall vary in time can only be studied properly on the basic data series recorded prior to the study. For information on topography and geotechnical conditions special surveys preceding the design phase will be carried out to obtain the data required for the design. Hydrometeorology The hydrological parameters essential for the design of irrigation systems are rainfall, evapotranspiration, peak discharges, daily discharges and sediment transport. For a large part the above hydrological parameters will be collected, analyzed and evaluated in the study phase of the project. Recorded information for the hydrological analysis consists of maps, steam flow records and meteorological records. It is important that the design engineer inspects the data recording stations, examines the data collection and the processing methods. Topographic Surveys The mapping requirements for the general topographic map are specified as follows: Clear portrayal of landform, micro relief and physical features: these dictate the layout and location of canals, drains, and roads. Accuracy in ground level: in flat areas where canal slopes may be less than 10 cm/km, accuracy in level is extremely important since this indicates whether adequate irrigation command or drainage can be achieved, In steep terrain irrigation command and drainage rarely present a problem Contour intervals: Flat land <2% 0.5 m interval Undulating and rolling land 2-5% 1.0 m intervals Hilly land 5-20% 2.0 m intervals Mountainous >20 % 5.0 m intervals River, weir site and canal alignment surveys For the design of the river headworks detailed topographic information on the river and weir site is required: river map for that part of the river where the headworks will be constructed. Scale of this map should be 1:2,000 or larger and it should cover the river for at least 1 km upstream and 1 km downstream of the headworks and extend for at least 250 m on either side of the river banks. Flood plains should be fully covered. Included in this survey will be the mapping of areas liable to flooding. The map should be provided with contour lines every 1.0 m except in the river bed where contour lines every 0.50 m are required. The irrigation engineer will be responsible for drafting the survey specifications as he is familiar with the sensitivities in the preliminary design and knows the field situation. Canal alignment surveys normally include the irrigation as well as drainage main systems. The canal alignment survey (strip survey) will follow as close as possible the proposed canal alignments of the preliminary layout. The survey and mapping includes the production of: Chapter 1 Irrigation Engineering 7/10

8 Canal alignment map at scale 1:2,000 with contours at 0.5 m interval for flat areas and 1.0 m for hilly land; Longitudinal profiles at horizontal scale 1:2,000 and vertical scale 1:2,00 (or 1:100 for small canals) Cross-sections at horizontal and vertical scale 1:200 (or 1:100 for small canals) at intervals of 50 m in straight sections and of 25 m in curves. 1.7 Engineering Design and Phasing of Irrigation Design The preparation of the engineering design for an irrigation project is divided into three stages, preparation of the Outline design / the study phase Preliminary design Final design The following sections deal with the design of the different elements of an irrigation system. Outline Design The outline design is the final result of the study phase (when no feasibility study is carried out) and is generally based on available topographic information. The map scale may be 1:25,000 or larger if available. No topographic surveys are executed to support this outline design. Existing maps form the basis. The outline design will result in a sketch layout delineating the approximate boundaries of the irrigation area and a canal layout plan. Contour information may give an indication of the ground slopes in the canal alignments. The main structures can already be indicated on the sketch layout. In the study phase provisional decisions are made on type and approximate location of the headworks. Also the type of irrigation canal, earthen or lined will be provisionally decided upon, review of geological and soil conditions will present an insight in the geotechnical conditions to be expected. Presence of sufficient amounts of stone will indicate possibilities for the design of stone masonry structures. Otherwise construction in reinforced concrete will be necessary. Preliminary Design The objective of the preliminary design phase is to determine the location and levels of the headwords, irrigation and drainage canals and structures as well as areas served on preliminary basis. The results of the preliminary design will allow the accurate formulation of the detailed surveys and investigations required for the detailed design. The preliminary design is presented in a preliminary design report of prescribed layout will contain preliminary design drawings showing the approximate dimensions of irrigation structures and their layout. The preliminary design report will be similar to final design report and it shows the justification of the preliminary irrigation scheme and confirms its data base. The preliminary design starts with a review of the conclusions of the study phase. In this review the information from the topographic map and the land capability surveys are incorporated. Validity of the earlier drawn conclusions will be re-examined. Fields of interest are: Chapter 1 Irrigation Engineering 8/10

9 Layout configuration check with the new topographic map; Location of the headworks with a view to the required intake level and site map; Type of irrigation canals, earthen or lined, in view of soil conditions encountered; Suitability of the area for irrigated agriculture; Administrative boundaries; Consultation with village administration and farmers on alignments and boundaries; Existing irrigation; Residential occupation and other non-arable areas identified on the topographic map; Drainage conditions and cross-drainage requirements; Water balance computations with more accurate data on irrigated land and irrigation requirements; Selection of structure types and construction materials. Extensive field checking is required to resolve the above interests. Location of major structures and canal alignments shall be based on a survey of the alignment and the elevation of the canals by instrument. The results of this survey will be checked in the field by the irrigation engineer accompanied by the geotechnical engineer and the topographical engineer. It will be used to ascertain the accuracy of the contour map and it will yield the final layout of the system. Final Design In the final design, the preliminary layout is reviewed on the basis of new data from the topographical and geotechnical canal alignment surveys. Modifications will be necessary depending on discrepancies found between the canal strip and the topographic map. Final figures and final layout map for the irrigation area are established at this point only and final diversion requirements are determined. Final location and intake level at the headworks are determined and incorporated in the headwork design. In this stage the layout, canal and structure drawings will be prepared in final detail. At the start of the final design the results of the previous surveys and investigations will be reviewed. The preliminary design will be checked with the results of the canal alignment surveys. The final canal centre lines and canal design water levels will be determined. If the contour map does not deviate too much from the canal survey results, only minor adjustments in the layout and the canal alignment will be made. Before completing the layout map the irrigation engineer will inspect all canal alignments, the headwords site and the major structure sites in the field. Once layout and levels are finalized the detailed canal and structure design calculations will be completed together with all related drawing work. Detailed headworks design will be carried out once the final intake level and design discharge are determined. The final design will be presented as a design report of prescribed layout and standard dimensions. It will contain the final design presented on drawings with completely detailed layouts, canal and structural drawings. The report will include coverage of subjects such as: Description of the proposed layout Justification of the design proposed Justification of adopted design flood and design discharges Data base and results of survey and investigations Chapter 1 Irrigation Engineering 9/10

10 Land acquisition requirements Bill of quantities and cost estimate Construction methods for special structures Tender documents PHASING OF IRRIGATION DESIGN PHASE/STAGE Outline phase Study Phase Desk study MAIN CHARACTERISTICS The idea for irrigated agriculture and approximate area is formulated in the office based on an inventory of the river basin development potential Identification of the project by name and area, outline of alternative irrigation schemes; notification of relevant government agencies and interested parties Classification to determine the scope for further studies Comparison of project alternatives on basis of approximate cost and benefits, Selection of alternatives for further study Determination of requirements for survey and investigation Technical and economical analysis of the formulated project Establish the need and define precisely the project to meet the need. Recommendation of a program for implementation Accuracy required for engineering aspects is similar as for preliminary design Planning and Design phase Preliminary Design Final Detailed design Aerial photography or topographical map survey, land suitability survey Final layout and preliminary design of head works, canals and structures, water balance computations. Office activity with extensive field checks Canal alignment surveys and detail geotechnical investigations Final layout with tertiary units. Detailed design, tender, drawings, bill of quantities. Office activities with field checks. Chapter 1 Irrigation Engineering 10/10