Irrigation Layout and Canal Design

ETHIOPIAN WATER TECHNOLOGY INSTITUTE Irrigation system layout and canal design By Abdurehman Ahmed(Msc) [email protected] December, 2018 i Irrigation System Layout and Canal Design Table of Contents 1. INTRODUCTION ...................................................................................................................................................6 2. OBJECTIVES ..........................................................................................................................................................6 3.1 Some Elements Of The System .......................................................................................................................7 3.2 The Irrigation Network ..................................................................................................................................7 3.2.1 The Irrigation Blocks ................................................................................................................................8 3.2.2 The Fields .................................................................................................................................................8 3.2.3 The Tertiary Units ....................................................................................................................................9 3.2.4 The Secondary Units ............................................................................................................................ 11 3.2.5 The Command Area .............................................................................................................................. 12 3.3 The Preliminary Layout Of An Irrigation System.......................................................................................... 13 3.3.1 Determination Of Full Supply Level (FSL).............................................................................................. 16 3.3.1 Working Head ...................................................................................................................................... 16 3.3.3 The Horizontal Alignment Of Canals ..................................................................................................... 18 3.3.4 The Vertical Alignment Of Canals ......................................................................................................... 20 3.3.5 Water Levels In The Major Supply Canals............................................................................................. 22 3.3.5 Areas Served By The Canals .................................................................................................................. 26 2.3.6 Design flows in the main system .......................................................................................................... 26 3.4 Introduction to Structures in Irrigation Canals ............................................................................................ 29 3.4.1 Structures For Crossings ....................................................................................................................... 29 4.2 Structures For Water Level Regulation (Level Control Structures) .......................................................... 30 3.4.3 Structures For Division Of Water (Flow Control Structures) ................................................................ 31 3.4.4 Structures For The Distribution Of Water To The Fields....................................................................... 31 3.4.5 Structures For Safety ............................................................................................................................ 31 4. CLASSIFICATION AND ALIGNMENT OF CANALS ........................................................................................... 32 4.1 Classification of canals ............................................................................................................................... 32 4.2 Alignment of canals ................................................................................................................................... 34 4.3 Canal losses ................................................................................................................................................ 36 5. DESIGN OF IRRIGATION CANALS .................................................................................................................... 38 5.1 Design Criteria For Irrigation Canals .......................................................................................................... 41 5. 2 Design Procedure ....................................................................................................................................... 43 5.3 Design of unlined canals ............................................................................................................................ 43 ii Irrigation System Layout and Canal Design 5.3.1 Design of non-erodible (stable bed) canals ......................................................................................... 43 5.3.2 Method Of Tractive Force .................................................................................................................... 44 5.4 Design of lined canals ................................................................................................................................ 64 5.4.1 Design procedure for lined canal .......................................................................................................... 65 5.4.2 Best Hydraulic Cross-Section ................................................................................................................ 66 5.6 Canals on non-alluvial soils ......................................................................................................................... 76 5.6 Canals on alluvial soils ................................................................................................................................ 77 5.6.1 Kennedy's theory .................................................................................................................................. 77 5.6.2 Lacey's theory ...................................................................................................................................... 81 iii Irrigation System Layout and Canal Design List of tables Table 1 Working head for different canals ..................................................................... 16 Table 2 Canal Head Losses ............................................................................................. 18 Table 3 Efficiencies for various water supply methods ................................................. 28 Table 4 Conveyance loss ................................................................................................ 38 Table 5 Coefficient of rugosity for different bed material ............................................. 40 Table 6 Critical velocity for various soil formations ....................................................... 41 Table 7 Recommended Hydraulic Design Formulae ....................................................... 41 Table 8 Manning’s coefficient “n” for Unlined Canals .................................................... 42 Table 9 Manning’s coefficient “n” for Lined Canals ........................................................ 42 Table 10 Correction factor for sinuousness of the channel ............................................. 46 Table 11 Particle size and silt factor for various materials ............................................. 47 Table 12 Permissible shear stress for lining material ..................................................... 50 Table 13 Manning coeffecent ........................................................................................ 64 Table 14 Values for Z and b/d for lined trapezoidal canals. For rectangular b/d = 2 ...... 65 Table 15 Suitable side slopes for channels excavated through different types of material ......................................................................................................................... 71 Table 16 Recommended Bottom Widths for Lined Trapezoidal Canals ......................... 73 Table 17 Values of k for various material ....................................................................... 77 Table 18 Values of C for various materials ..................................................................... 78 Table 19 Recommended values of m.............................................................................. 79 iv Irrigation System Layout and Canal Design List of Figures Figure 1 Plot by plot irrigation and drainage ...................................................................8 Figure 2 Layout of secondary fields ............................................................................... 11 Figure 3 Right of way ..................................................................................................... 12 Figure 4 Cutting the main canal through a hill, or not: loss of commanded area ........... 13 Figure 5 Working heads at relative points in the canal system; Source: Asawa, 1993 .... 17 Figure 6 Long intake canals and/or marginal areas ........................................................ 20 Figure 7 Longitudinal profiles with chainage ................................................................. 23 Figure 8 Required Normal Water Levels (NWL) in supply canals .................................... 23 Figure 9 Nomencleture for blocks, canals and structures .............................................. 27 Figure 10 Contour canall ................................................................................................ 34 Figure 11 Watershed canal (ridge canal) ....................................................................... 35 Figure 12 Side slope canal .............................................................................................. 35 Figure 13 Tractive force distribution obtained using membrane analogy ...................... 45 Figure 14 Maximum shear on bed and sides for alluvial channel based on Normal's Method. ......................................................................................................................... 45 Figure 15 Maximum unit tractive force in terms of γySo................................................. 46 Figure 16 Angle of repose for non cohesive material ........ Error! Bookmark not defined. Figure 17 Analysis of forces acting on a particle resting on the surface of a channel bed ...................................................................................................................................... 49 Figure 18 Lined channel section for Q < 55 m3/s ............................................................. 71 Figure 19 Lined channel for Q > 55 m3/s ........................................................................ 71 v 1. INTRODUCTION To fulfill a project purpose of producing crops or increasing crop production, water delivery to the land must be provided by a reliable and efficient irrigation system. A sun-drenched, parched soil may need only water to change it from a sparsely vegetated, thirsty desert to a high-yield crop. An increased crop yield of premium quality is very likely if the proper amount of moisture is made available to the crop when needed. A canal is frequently used to convey water for farmland irrigation. In addition to transporting irrigation water, a canal may also transport water to meet requirements for municipal, industrial, and outdoor recreational uses. The conveyance canal and its related structures should perform their functions efficiently and competently with minimum maintenance, ease of operation, and minimum water loss. A direct irrigation scheme which makes use of weir or a barrage, as well as a storage irrigation scheme, necessitates the construction of a network of canals. A canal is an artificial channel, generally trapezoidal in shape constructed on the ground to carry water to the fields either from the river or from a tank of reservoir. The entire system of main canals, branch canals, distributaries and minors is to be designed properly for certain realistic value of peak discharge that must pass through them, so as to provide sufficient irrigation water to commanded areas. These canals have to be aligned and excavated either in alluvial soils or non-alluvial soils; depending upon which they are alluvial canals or non-alluvial canals. At the end of this course trainers shall be able to understand • • • • • The types of structures and their layout of an irrigation system Canal types and cross-section The alignment of canals Design of lined canal Canal design procedures in both alluvial and non alluvial soil 2. OBJECTIVES The objective of this training on irrigation system layout and canal design are: 1. To introduce participants with the concept of irrigation system layout and canal design. 2. To create a platform for sharing knowledge and experiences. 6 Irrigation System Layout and Canal Design .3. IRRIGATION SYSTEM LAYOUT 3.1 Some Elements Of The System Field irrigation and drainage requirements and the method of irrigation have to be known before the size of tertiary blocks can be determined and the layout can be prepared for such blocks. For the present discussion, concentrated on the (preliminary) design of the network, it is assumed that the design flows to be delivered to the tertiary blocks and the tertiary block sizes are known. Two main delivery types to the tertiary blocks will be considered: ➢ full supply during 24 hours per day; and ➢ full supply during 12 hours per day. The number of days in a week that those supplies will be delivered is, of course, a function of the irrigation requirements at the various stages of crop growth. For the design of the network, however, this is not a governing factor. The capacity of all canals and structures should be such that the maximum envisaged flows can be handled conveniently. Irrigation from a river, for instance, is made possible by the construction of a dam in the form of a fixed weir or a barrage, with or without movable gates, supplemented by an intake with gates that allows controlled quantities of water to be diverted to the conveyance system. A pumping station would form the headwork when water levels in the river are too low, even with a dam, to command the area by gravity, or when a dam is not feasible. Scouring sluices, fish ladders, sand traps and navigation locks can form part of this complex of structures. The complex is called the diversion works or headwork. There exist headworks without a dam and/or a gated intake. Such headworks are usually constructed by the farmers of the area themselves. 3.2 The Irrigation Network The irrigation network comprises a network of canals with appurtenant structures. Those canals are called (in downstream direction from the intake) main, branch, secondary (or lateral), sub-lateral, tertiary (and possibly) quaternary canals. Structures in these canals serve to divide the flow, maintain water levels, measure flows, discharge water that would endanger the system, and, eventually, distribute water to the fields and evacuate water. In short, the flows of water are controlled. The network is a water control system. Two main functions of the irrigation network can be distinguished, as mentioned already before: conveyance and distribution of water. The conveyance canals pass water through a main network (main, branch and secondary canals) to lower order canals (tertiary and possibly quaternary canals) that form the distribution network. Division of water is needed over branch canals and laterals and no water is distributed directly to the fields from canals that belong to the conveyance network. The distribution network passes the flow to even smaller canals and ditches that directly serve (a number of) fields. 7 Irrigation System Layout and Canal Design 3.2.1 The Irrigation Blocks The ultimate aim of irrigation is to provide fields with water. So, one argument goes, one should start from the fields and the water Users, when thinking about a new system or rehabilitation. But if the possible extent of the area to be irrigated is not yet known, then the engineer does not know which fields and which farmers will possibly be included in the scheme. So he will make a preliminary design before going to see the farmers (and raise expectations that may not be fulfilled). Whatever one does, it will always be necessary 'to go up and down' and 'to go down and up'. When starting from the general and making assumptions along the road during the preliminary design, one will have to go up the system again with the knowledge gained during the verification of the assumptions. And vice versa. But let us start the discussion with the fields, working our way up through the tertiary blocks to the secondary blocks, and to the intake. 3.2.2 The Fields An irrigated area comprises a sizeable number of fields or plots (quartiers, parcelles), terraced or not terraced, each of which should be supplied with water, be drained and be reachable in a convenient manner. Sometimes water is conveyed through one field to the next lower one, and so on, but this will generally result in difficulties in water control, certainly when the distance between supply canal and drain increases (Figure 2.7). The control of water in the lower plot may be hindered by the required water control in the higher plot or plots, or the owner of the upper plot may not be very cooperative so that the lower situated owner becomes more or less dependent on the upper one. In a rice-irrigated area, however, these drawbacks could be less serious. Figure 1 Plot by plot irrigation and drainage The above mentioned problems are also less serious when the same person cultivates all plots, and with the same crop. Also would it be possible to locate ‘friendly’ owners together in case 8 Irrigation System Layout and Canal Design land consolidation forms part of the project. This would certainly be practiced in the case of very small ownerships. The thus created not-so-unfavorable situation may, however, not last very long: ownerships may be transferred or the compulsory co-operation may result in a quick termination of the friendliness. The situation would be acceptable -temporarily- if economic or financial constraints would make it impossible to construct the optimum density of the network in the initial period of project construction and operation. The design should then be such that the better solution can be implemented at a later stage without major upheavals and interruptions in agricultural practices: "don't close the way" for future developments (Koller, 1972, personal communication). This principle also applies, of course, to other designs, not only to irrigation and drainage networks. 3.2.3 The Tertiary Units A greater or smaller number of plots are united in a small complex called tertiary block, tertiary unit, irrigation unit or service unit. The number of plots in such a unit depends, among others, on the method of irrigation, the physical conditions of the irrigated area (ground slopes) and the possible cooperation between a number of farmers. It is customary in most irrigation schemes for farmers that own or cultivate fields in such a tertiary block to arrange themselves all matters related to water management within the block (Water Users Association). The supply canals from which water is taken directly to the fields are called tertiary canals or main ditches. These canals deliver water to quaternaries, or branch tertiaries, or field ditches from which plots are irrigated. (In some countries, however, field ditches or on-farm ditches are the ditches within the farmer's field.) This network forms the distribution network. Tertiary canals receive their water from a conveyance network that is -usually- operated by an irrigation Authority. Tertiary canals, irrespective of their size, convey water to, or distribute water within one tertiary block only. Otherwise such a canal would be a secondary canal (lateral or sub-lateral). A tertiary drain or drains will collect water from the field drains in the tertiary block and convey this water to a secondary drain. Drains are logically located in the lower parts of the block. Water will never be supplied directly to the fields from a conveyance canal, but only from canals that belong to the distribution network. Water is supplied from the conveyance canal to an irrigation unit in one place only in a well-organized irrigation scheme. Quantities can sometimes be regulated and, in exceptional cases, be measured at this outlet but normally an ‘on/ off’ (‘open/ closed’) structure -or even an open structure that cannot be closed- is all that is needed. It is advantageous to locate the outlet from the canal at or near the highest point of the block. The tertiary canals then remain at higher natural elevations. A certain amount of water is allocated to each tertiary block, usually one or more unit rates of flow or stream sizes (mains d'eau), continuous or at pre-determined times (rotation between blocks). The required volumes of water to be supplied to the fields are obtained by varying the application time of the fixed flow rate. (Alternatively one could vary the flow rate while keeping the application 9 Irrigation System Layout and Canal Design time constant. This will result in greater losses, in less water use efficiency. This method, is therefore, not advocated.) The internal distribution of this water over the farms, continuous or in rotation, is usually left to the farmers. Difficulties within the tertiary block boundaries thus have no major repercussions on the operation of other tertiary blocks so long as the farmers do not physically interfere with the operation of the conveyance network (as they sometimes tend to do). In many schemes there are less equity problems at the distribution network level than there are at the conveyance network level. The tertiary unit is for the civil engineer/ designer the basis for the layout of the major irrigation and drainage network. A thorough understanding of all problems related to activities within the tertiary unit is of primordial importance in order to arrive at a good design. The detailed design of tertiary units will, therefore, be the subject of a separate course. A few remarks on some points that have an immediate bearing on the general layout of the system have, however, already to be made now: ✓ A rectangular layout is very convenient for both design and execution but usually more canals and ditches have to be constructed in fill and drains would be in deeper cut. More crossings with existing watercourses and roads are needed, resulting in more structures or in the shifting of existing alignments with subsequent loss of agricultural land. (One could consider in such a case to utilize buried pipelines or elevated canals. These should, however, never cross fields.) Normally one follows the contours of the natural terrain and accepts irregularly shaped blocks. Blocks should not differ in size from the given or computed optimal block size. The size of the blocks should in any case never be greater than the computed size because in that case the off-take structures from the conveyance network to the distribution network cannot be standardized. Smaller blocks will result in water losses (or very complicated operations, and still water losses). Block boundaries should be clear and understandable, following creeks, small streams, roads or tracks, or limits of built-up areas. Artificial drains or roads should form the boundary incase natural boundaries do not exist. ✓ The blocks should not be too long in relation to their width: main ditches will be long in this case, resulting in increased loss of water and a more difficult social control amongst farmers. In addition it should be noted that the slope of drains is usually greater than the slope of supply canals: long tertiary drains require lower design water levels at the point where they discharge their water into the main drains. This increases the cost of the scheme. 10 Irrigation System Layout and Canal Design ✓ The extent of village areas has an impact on the internal organization of the farmers in the block. It is recommended not to have persons from different villages in one block and to aim instead at having one block, or several blocks, allocated to farmers from the same village. ✓ A combination of tertiary outlets for several blocks in one structure is economic from both investment point of view and ease of operation but one should not exaggerate: five outlets in one structure should be a maximum. ✓ High spots in a block that would require canals in high fill should be designated to other purposes, such as extension of villages or wooded areas for fuel, or should be irrigated by small pumps. Similarly should low areas that cannot be drained be allocated to ponds if health requirements do not preclude such a solution. 3.2.4 The Secondary Units The canals that draw water from a main canal or branch canal to serve a number of tertiary blocks are called secondary canals or laterals; the blocks they serve are called secondary blocks. Secondary canals are conveyance canals. When a tertiary canal runs along the boundaries of another block and the possibility exists for unauthorized diversion of water before the water reaches the tertiary block it is destined for. Figure 2 Layout of secondary fields A major canal serving several secondary blocks after receiving water from a main canal at a bifurcation point is called a branch canal. Similarly one calls a canal that is bifurcating from a lateral, in order to serve a number of tertiary blocks, a sub-lateral Boundaries of secondary blocks are defined by prominent features in the terrain, such as the main canal, main or secondary drains, roads or a railroad. Secondary drains discharge into one or more main drains which evacuate all water out of the area. In practice one will many times find secondary drains (and main drains) to be existing rivers or natural watercourses. Such drains should naturally be located in the lower parts of the area. Most secondary canals have roads or tracks for inspection, operation and maintenance of the canal and its structures. The road network may be open to the public or may even be included in the general road network of the area. Usually inspection paths are, officially, not available to the public (which will use these paths anyway). 11 Irrigation System Layout and Canal Design An extra strip of land along the canals is included in the right-of-way, in addition to the width of roads and tracks, for emergency passage, borrow pits and the like. It is recommended to keep this right of way as narrow as possible. Land that is located in the right-of-way may sometimes be cultivated by the farmers on certain conditions. This authorized solution is to be preferred because unauthorized use of this land will otherwise certainly be made (and would result in the establishment of ‘rights’ in some countries). Figure 3 Right of way 3.2.5 The Command Area The scheme area is the area that will be served by the canal network. It not necessarily so that all grounds within this area can be irrigated. The irrigated area is called the command area. The drainage of the whole project area and of adjacent areas should be ascertained, however, by the drainage network. This includes the drainage of villages and roads and provisions for cross-drainage. One or more main drains can cross the command area or should constitute a boundary but such drains should, of course, never cross irrigation blocks. One may initially assume that around 10% of the area will be occupied by roads, canals and villages. The exact extent of these areas can be found later when the complete system has been designed and accepted. Corrections may then be needed for canal capacities when actually served areas differ substantially from assumed ones. The main supply canal conveys water to the irrigated area. The main canal is in many instances a contour canal in order to command as large an area as possible. Economic considerations, such as minimum head loss, smaller investment and/ or maintenance costs, could lead to a solution to construct part of the canal in the form of a tunnel, a siphon or an aqueduct, even while, in general, the canal follows the contour. It is not uncommon to find in a sizeable system a number of small areas with good soils that can be irrigated from the first reach of a main canal while it is considered not to be appropriate to construct a separate secondary canal for these small areas. Such areas are then subdivided into tertiary blocks which are supplied directly from the main canal, by gravity or by small pumping stations. The needed quantities and flows of water for these blocks would not usually interfere with operation of the main canal to any major extent because these flows are minor in comparison with the flow in the main canal. This solution is then preferred to excluding these areas from the scheme and 12 Irrigation System Layout and Canal Design running the risk that the adjoining farmers will find their own solutions to their water supply problems. 3.3 The Preliminary Layout Of An Irrigation System The three most important requirements for a sound irrigation scheme, and hence for its layout, are: 1. the scheme should be physically feasible; 2. the scheme should be economically feasible; and 3. the scheme should be socially and environmentally feasible. The social and environmental feasibility will not be discussed here but it should be realized that the layout should be acceptable to both the operators and the users of the scheme (and to the general public). The scheme will never work properly if this condition is not met. Recent research in Swaziland (Lankford 1998) confirmed again the importance that the operators of the system attributed to the layout and correct water control. It will be necessary at all stages of design to be aware of the wishes of those groups and to try to meet legitimate requests for specific technical solutions. It will be good to explain the reasons why certain desired solutions cannot be implemented. The economic feasibility is determined after having compared several possible alternatives for costs and benefits. It should be realized, however, that economic computations will also have to be performed during the layout and design for a quick comparison between two technically equivalent solutions. For instance, cutting through the hill with the main canal or following the contour around the hill with a subsequent considerable loss of commanded area. Figure 4 Cutting the main canal through a hill, or not: loss of commanded area The design of the irrigation and drainage network is done on maps or aerial photographs with contour lines. The contour lines should be marked correctly with the top of the numbers pointing up the slope. 13 Irrigation System Layout and Canal Design The first and foremost principle for a proper layout is that drains will be located in the lower parts of the area; the hydraulic gradient of drains is down the general slope of the area. The layout process is, therefore, started with the layout of main drains for which existing drainage channels or valley bottoms form an alignment. Additional secondary drains will be located between the natural ones, if needed. Secondary supply canals will preferably be projected halfway between these drains. Land levelling, which is expensive and may destroy valuable agricultural soil, is minimized in this manner. The distance between drains is a function of the tertiary block size and shape. The alignment of the main canal forms in many instances the boundary of the command area. A first estimate of the design water level near the intake point and of the required bed slope is made in order to be able to proceed with the layout in case such a canal is a contour canal. Otherwise the main canal can be projected right away. The area to be irrigated from a contour canal can be estimated directly from the map in case the available quantities of water do not. The area to be irrigated from a contour canal can be estimated directly from the map in case the available quantities of water do not permit to irrigate the whole command area. Two possibilities exist in such a case: (a) the whole command area will be included in the scheme but each farmer will receive water only for part of his holding, at least during periods of peak demand; or (b) the commanded area will be irrigated fully and will, therefore, be smaller than the area in case (a). It is necessary in case (b) to determine the alignment of the canal in such a way that the canal commands the exact area for which water is available. The area to be irrigated is, however, also a function of the bed slope of the canal but the design flow Q and, consequently, the bed slope S are initially unknown. The area to be commanded is then first estimated and a flow and slope are computed; tracing the alignment on the map will result in an area that can be irrigated; corrections will be needed if the measured area will be significantly different from the estimated one. A decision, whether to irrigate the whole command area or not, should be based on economic and social considerations, i.e. the objectives of the project have to be clear. The boundary of a secondary unit is delineated by the boundaries of the tertiary units that will be supplied with water from a particular secondary canal. It will be necessary to adjust the boundaries of the secondary units while fitting tertiary units in an optimal way. (This adjustment is another example of the obligatory ‘up and down and up again’ procedure in 14 Irrigation System Layout and Canal Design irrigation system design.) It is recommended to have secondary blocks of approximately equal size but this can seldom be realized in practice. The use of colors facilitates the layout work. The areas between contours on the base map could be colored in such a way that the dominant slope of the terrain becomes more evident to the eye. It could be efficient, at a later stage, to shade the secondary blocks with different colors while at the same time drawing a heavy line in the same color as the secondary block color along the tertiary block boundaries within such a secondary block. Similarly, could all blocks that are directly irrigated from the main canal be colored in the same color and shade? The drawing of the horizontal alignments of supply and drainage canals on the map is done most efficiently with colored pencils. Standard colors are blue and red. Structures are shown on the same map, indicating at the same time which type of structure is required at that particular site. The need for such structures follows partly from the horizontal alignment but mostly from the vertical alignment of the canals. The horizontal alignment of the irrigation and drainage canals follows in general the topography of the terrain. Alignments are further strongly influenced by soil conditions. Secondary supply canals are preferably located on high grounds, such as ridges. This is not always possible, for instance when the main canal is designed on such a ridge: the secondary canals would then run down the slope. Water could in both cases be supplied to both sides of the secondary canal. Tertiary outlets will be located on only one side of the canal in case the secondary canal is a contour canal. The layout is traced on maps with scale 1/50,000 (the military map), 1/20,000 or greater, depending on availability of base maps and needed detail. It is strongly recommended to inspect, as soon as possible, any preliminary canal alignments in the field. The information shown on maps or aerial photographs may not represent the actual citation any more, and maps may have been incomplete -not necessarily incorrect- in the first place. In addition will it be necessary to verify soil and subsoil conditions along the proposed alignments, both from the point of view of canal construction and the foundation of structures. It would be necessary to modify preliminary canal alignments when very permeable soils are encountered and no lining is foreseen. Similarly should areas subject to landslides be avoided as much as possible. Areas may already have a natural tendency for sliding, or it can be expected that (unavoidable) canal seepage and losses may in future cause such slides. (It should be realized that lined canals do leak too.) It is recommended to avoid soils that would cause degradation of concrete lining in case concrete lining is to be installed. Canals and roads must be protected against floods unless regular flooding is foreseen in the design. Embankments of a canal may then have to be designed as flood protection levees. Suitable places for cross-drainage structures have to be identified. The practice of allowing cross-drainage water into a canal should be limited to a maximum of 10% of the canal capacity, 15 Irrigation System Layout and Canal Design which is in practice very difficult to. A high total cost of (main) canal construction and maintenance, together with the cost of the diversion works – and/or smaller benefits in the marginal area – could be a reason to exclude part of the area that could physically be commanded. An economic analysis of various alternatives of possible canal layouts is certainly needed when water has to be lifted by pumping. It would possibly be advantageous in such a case to construct a number of smaller canals at different elevations than to construct one main canal along the most elevated alignment. 3.3.1 Determination Of Full Supply Level (FSL) While planning and designing for the layout of a canal system, fixing the design water levels (FSL) at various points of two canal system has to be finalized. This will help to ensure the desired flow of water from canal to canal and finally onto the land surface. Generally FSL and working head of a canal system is calculated from the field ground level, back through the system to all canals and structures, up to the source location such as river diversion point or vice versa. i) Critical point: it is the spot, which requires highest water level due to the combined impact of spot level in terms of elevation and its distance from the irrigation channel/outlet, and ii) Head over the field: it has been assumed that the depth of water should be a minimum range of 0.10 to 0.20 m over the critical spot level. Therefore, the minimum FSL at the field level of the project was fixed to 0.20m. 3.3.1 Working Head Working head is the difference in the FSL of the parent channel and that of the off taking channel. It is provided in order to facilitate the flow of the design discharge. For this phase of study purpose the working heads used are listed in Table 1, and Fig. 2 explains the locations in the representative canal profile. However, these values need to be checked with the actual required working head during the detail design study. Table 1 Working head for different canals Location in canal system At outlet head from minor or distributary At minor head (a) from a distributary (a) from a major distributary or sub-branch At distributary or major distributary head (a) from sub-branch or branch canal (b) from main canal At branch head (a) from main branch (b) from main canal 16 Working head (m) 0.15 0.25 0.30 0.50 0.60 0.50 0.60 Irrigation System Layout and Canal Design At main branch head from main canal At main canal head from barrage or weir 0.60 1.00 Figure 5 Working heads at relative points in the canal system; Source: Asawa, 1993 The actual working head at the off-take will be the difference of heads between prevailing pond level and the full supply level in the main canal. However, when the river flows in minimum quantity the minimum working head should not be less 0.50 mm. The minimum working head required at the tertiary canal off take can be fixed based on the maximum water depth (y) required at the critical point of the field supplied, the elevation or level of the critical point in the tertiary unit above project datum (A), the sum of head loss (zinlet) over the farm inlet. The starting point should be the critical point of the field. At the off take secondary canal the minimum working head required can be fixed based on the sum of the working head requirement at the tertiary off take plus the head loss as a result of canal bed slope over the length of the supply canal reach up to the tertiary off take plus all the losses on structures along the reach up to the tertiary off take. The main system should not only be able to allocate a peak flow (QTU) to the tertiary unit according to the operational objectives, but also this peak flow should have sufficient head (h) to irrigate all parts of the tertiary unit. The required head (h) in the main system (secondary canal off take) to irrigate the critical point (the spot in the field which requires highest water level due to the combined impact of spot level in terms of elevation and its distance from the distribution or outlet channel) is calculated with the following formula: 17 Irrigation System Layout and Canal Design ℎ = 𝐴 + 𝑦 + 𝑧𝑖𝑛𝑙𝑒𝑡 + ∑(𝐿 × 𝑆𝑐𝑎𝑛𝑎𝑙 ) + (𝑗 × 𝑧𝑏𝑜𝑥 ) + 𝑧𝑜𝑓𝑓𝑡𝑎𝑘𝑒 Equation 2 Where: h is the required head in the main system above project datum (m), A is the critical point level in the tertiary unit above project datum in m, y is the maximum water depth y at the critical point in m, zinlet is the head loss over the farm inlet in m, L is the canal length to reach the critical point in m Scanal is gradient (when the highest or critical point is further away from the off take), zbox is the head loss over each of the j division boxes, and zofftake is the head loss over the tertiary off take in m. It should be noted that the calculated required head in the main system is a minimum value. Therefore, the water level in the secondary canal may be higher so that more head loss is possible over the tertiary off take. In order not to lose any command area it is crucial to consider the minimum practical head losses. However there are list of choices to select from Table 2. Table 2 Canal Head Losses No. Description 1 2 3 4 5 6 Cross Regulators and Control Structures (Main Canal) Secondary Canal Control Structures Tertiary Canal Off take Structures Flow Measurement Structures Culverts (Pipe/Box) Inverted Siphons and other crossing structures for carrying canal over/above drainage canals Head Loss (m) 0.20 to 0.40 0.30 to 0.40 0.15 to 0.20 0.10 to 0.20 0.05 to 0.10 0.30 to 0.50 Remark In general the full supply levels of the subsequent canal were fixed based on the requirements listed in Table 2. During the detailed design phase the actual design for water level will be computed based on the types of out let structures to be provided. The full supply level will be checked against the minimum requirements in Table 9. 3.3.3 The Horizontal Alignment Of Canals The horizontal alignment of the irrigation and drainage canals follows in general the topography of the terrain. Alignments are further strongly influenced by soil conditions. Secondary supply canals are preferably located on high grounds, such as ridges. This is not always possible, for instance when the main canal is designed on such a ridge: the secondary canals would then run down the slope. Water could in both cases be supplied to both sides of 18 Irrigation System Layout and Canal Design the secondary canal. Tertiary outlets will be located on only one side of the canal in case the secondary canal is a contour canal. The layout is traced on maps with scale 1/50,000 (the military map), 1/20,000 or greater, depending on availability of base maps and needed detail. The alignments consist basically of a number of straight lines connected by curves. The minimum curvature is related to water depth or bottom width, sometimes with the flow Q as a second parameter. Such curves are usually not shown on general layout plans, not even on final ones, unless the scale of the drawing would allow for this. The exact curvatures are only computed and traced in construction drawings after the alignments have been surveyed and the detail design of the canal has been prepared. It is strongly recommended to inspect, as soon as possible, any preliminary canal alignments in the field. The information shown on maps or aerial photographs may not represent the actual citation any more, and maps may have been incomplete -not necessarily incorrect- in the first place. In addition will it be necessary to verify soil and subsoil conditions along the proposed alignments, both from the point of view of canal construction and the foundation of structures. It would be necessary to modify preliminary canal alignments when very permeable soils are encountered and no lining is foreseen. Similarly should areas subject to landslides be avoided as much as possible. Areas may already have a natural tendency for sliding, or it can be expected that (unavoidable) canal seepage and losses may in future cause such slides. (It should be realized that lined canals do leak too.) It is recommended to avoid soils that would cause degradation of concrete lining in case concrete lining is to be installed. Canals and roads must be protected against floods unless regular flooding is foreseen in the design. Embankments of a canal may then have to be designed as flood protection levees. Suitable places for cross-drainage structures have to be identified. The practice of allowing cross-drainage water into a canal should be limited to a maximum of 10% of the canal capacity, which is in practice very difficult to achieve. Alignments are the more economic ones when water levels are kept as low as possible in irrigation canals and as high as possible in drainage canals. The method of road construction engineers to balance cut and fill is in practice not feasible in irrigation schemes. Excess cut can be used for roads or levees, alongside or in the neighbourhood of the canals. An advantage of constructing the road network along the drains is that soil for the road base is directly available from the drain excavation. The main disadvantages of this practice are 1. roads are needed along many supply canals (and less along drains) for the operation of the system; and 2. drains are naturally located in the lower parts of the system; they are, therefore, located in a more wet environment that increases the chances of road damages, 19 Irrigation System Layout and Canal Design and thus increases, eventually, the cost of the operation of the scheme. The semicontrolled flows from the irrigation blocks will also cause relatively more damage. ‘ Figure 6 Long intake canals and/or marginal areas A high total cost of (main) canal construction and maintenance, together with the cost of the diversion works – and/or smaller benefits in the marginal area – could be a reason to exclude part of the area that could physically be commanded. This would particularly be the case when water levels near the intake are sufficiently high but the canal would be long and/or in high fill over an appreciable part of its length. An economic analysis of various alternatives of possible canal layouts is certainly needed when water has to be lifted by pumping. It would possibly be advantageous in such a case to construct a number of smaller canals at different elevations than to construct one main canal along the most elevated alignment. 3.3.4 The Vertical Alignment Of Canals The vertical alignment is a compromise between the following: (a) the water level in supply canals should be sufficiently high to irrigate the highest areas for which irrigation is envisaged; in drains the level should be low enough to drain the lowest areas that are to be drained; (b) maintenance costs should be as low as possible: they are lower when the water level in the canals is below ground surface; it is then also more difficult to divert water illegally from supply canals; and 20 Irrigation System Layout and Canal Design (c) a balance between cut and fill is economic for construction but canals in high fill are more difficult to construct and would in general lose more water by seepage; this would certainly be so when also the bottom of the canal is above the original ground surface: at least the bottom of a canal -but preferably the whole canal- should be in cut after stripping 0.10 to 0.15m of the topsoil; seepage could also lead to land slides and water logging problems in fields along the canal. It is clear from the above that the water level and the ground surface elevation along the alignment are important parameters. Ground surface elevations are obtained from the map or a survey along the proposed alignment of the canal. The determination of the design water levels is discussed in Section 3.3.1. The vertical alignment is drawn in a longitudinal profile along the axis of the canal. There is no need, as yet, to design the canal. Original Ground Surface (OGS) and water levels is all that is needed for a vertical layout. Blatant errors in the horizontal layout will show up immediately when the vertical layout is prepared. Necessary corrections in the horizontal layout can then be made. Such corrections nearly always result in different design capacities of the canal, be it an irrigation or drainage canal. It would, therefore, be rather useless to compute design flows and to prepare cross sections of canals before the horizontal layout has been optimized in relation to the vertical layout. Kilometer distances are added to the profile. These distances facilitate the computation of the fall in water levels along the canal for various assumed slopes. Multiplying distances in kilometer by slope in per thousand, %, results directly in a difference in elevation in meter. Example: Distance between A and B = 2.35 km Slope = 0.2 ‰ Then Difference in elevation Between A and B = 2.35 * 0.2 = 0.47 m The zero point, or lower kilometer number, of the longitudinal section is always at the left hand side of the drawing (except, possibly, when the customary direction of writing in the country is from the right to the left). The zero point of supply canals is located at the intake point, the zero point of drains at the outfall. Water is, consequently, flowing from left to right in a supply canal and from right to left in a drainage canal in a longitudinal section (except, possibly, when the customary direction of writing in the country is from the right to the left). See Figure 3.7. 21 Irrigation System Layout and Canal Design 3.3.5 Water Levels In The Major Supply Canals Initially it may be assumed that for irrigation by gravity a head loss of some 0.15 to 0.20m is sufficient at the turnout from the distribution system to the field if the highest area of the field that is to be irrigated is located near that turnout. The required 'head' in the field may be taken as 0.1 m (allowing water storage in the field in the case of wet-land rice irrigation and some additional head for basin or furrow irrigation, taking, in both cases, into account that the field may not be perfectly level or perfectly graded (Figure 7, lower right hand side). Thus the water level just outside the field turnout should be 0.25 to 0.3 m above the Original Ground Surface, OGS. 22 Irrigation System Layout and Canal Design Figure 7 Longitudinal profiles with chainage Figure 8 Required Normal Water Levels (NWL) in supply canals 23 Irrigation System Layout and Canal Design A reasonable first assumption for the head loss in the outlet structure from the secondary canal to the distribution system in the tertiary block is 0.15 m. (Figure 3.8, lower left hand side). This drop should guarantee free flow at the off-take even if the downstream water level would be manipulated by the water users in the block. This then raises the minimum required water level in the conveyance system at the site of an offtake structure to a tertiary block to 0.4 to 0.45 m above the highest point to be irrigated in that tertiary block if the highest point of the block that is to be irrigated is located near that turnout (distance to the highest point, 1, equals 0, layout in the upper left corner of Figure 3.8). The layout in Figure 8 in the upper right hand corner is not a very desirable one but sometimes such a situation is unavoidable. The cost of the system increases significantly in this case. Adjustments now have to be made to the required minimum water levels because the highest point is located further away from the off-take: the slope of the tertiary canal over the distance l and the head loss in control structures along that canal will now have to be taken into account too. The head loss in these control structures at the tertiary and quaternary level may initially be taken as 0.1m. Example (see also Figure 3.8): 'normal' head at off-take = 0.4 m distance to turnout, l = 0.2 km tertiary canal gradient = 0.4 ‰ head loss in additional control structure = 0.1 m then additional head loss = 0.2 * 0.4 + 0.1 = 0.24 m total head required at the off-take = 0.4 + 0.24 = 0.64 m The minimum required water level upstream of the off-take structure in the conveyance canal is thus 0.4 to 0.45 m above OGS plus the possible additional head that is required to irrigate the highest point in the tertiary block if that point is located at a distance from the off-take. This minimum required level is to be indicated clearly for each tertiary outlet in the longitudinal profile of the conveyance canal (Figure 8). The required computations for each off-take are most conveniently done in a table, as will be discussed during the design exercise. An additional 0.10 to 0.30 m would be needed for some types of flow measurement structures downstream of the off-take structure if such a measurement structure would be planned at this place. In general, it is very expensive to locate such measurement structures downstream of tertiary outlets, unless sufficient head is (naturally) available. The use of modules as outlets may be a less costly alternative. 24 Irrigation System Layout and Canal Design The cheapest construction of the canal would require that the slope of the water level in the secondary canals follows approximately the average ground slope of the reach under consideration. However, this is not possible in most cases: hydraulic considerations preclude this practice in most circumstances, except in very flat terrain. It will then be derived later that gradients should be increasing with decreasing discharges in order to keep the sediment moving: the greater the discharge, the flatter the hydraulic gradient can be. And the smaller the discharge, the steeper that gradient has to be. Gradients will, consequently, be increasing in downstream direction in supply canals, and will be decreasing in downstream direction in drain canals. (If the zero-point of the longitudinal section is taken at the left hand side of the section then gradients are always increasing from left to right.) The following gradients could be used for a preliminary layout and design of the water control system: bigger canals, say upwards from 15 m3/s 0.2 - 0.1 ‰ intermediate canals 0.3 - 0.2 ‰ smaller supply canals, say till 0.3 m3/s 0.4 - 0.3 ‰ Ease of construction is another parameter that has to be taken into account. The staking out of hundreds of small tertiary and quaternary canals is significantly facilitated if all gradients in the area are the same: no discussions, less mistakes. (If your measuring tape is 30 metre long then use a gradient of 1/3 %: the drop in bottom level then is 0.01m (or 1 centimetre) per tapelength. A tape-length of 25 metre would indicate a gradient of 0.4 ‰, etc.) For biggerorder canals this requirement is much less stringent. More experienced surveyors will be needed in any case for staking out such canals. Drop structures (and control structures at off-takes) offer the possibility to design canal slopes that are smaller than the average slope of the terrain. Such drops would certainly be needed when canals are flowing down along very steep slopes. A series of such structures, one after the other, could in such a case be required. Drops greater than 1 metre require (designed) stilling basins. For smaller drops a simple bottom protection is usually sufficient. A number of smaller drops are, therefore, many times cheaper to construct and maintain than one bigger drop. A number of structures are needed to control water levels in the canal at the tertiary outlets, particularly when discharges in the lateral are -during part of the irrigation season- smaller than the design discharge. It is sometimes stipulated that it should be possible to pass the full tertiary design flow to individual irrigation units in case the flow in the secondary canal is smaller than the full design flow. It is then, of course, not possible to serve all irrigation units with their full individual design flows at the same time and operation will be much more complicated. Some Irrigation Departments require the capacity of a structure to be 10 to 15% 25 Irrigation System Layout and Canal Design greater than the capacity of the canal in which the structure is placed. This in order to build-in a certain measure of flexibility in case a flow greater than the originally envisaged design flow will have to be passed, incidentally (by encroaching on the freeboard) or permanently (after re-designing the canal). It is easier to enlarge a canal than to enlarge a structure when the system capacity has to be increased. This practice is more difficult to apply in the case of fixed structures. The head loss in secondary canal control structures can conveniently be taken at 0.15m during the preliminary design stage when all hydraulic characteristics of the structures are not yet known. 3.3.5 Areas Served By The Canals The areas served by the various canals, be it for supply or for drainage, can be determined from the map with the horizontal layout. (A planimeter would be useful for the determination of the contributing drainage and the net irrigable areas but for a preliminary design a quick measurement with a ruler is usually good enough.) 2.3.6 Design flows in the main system Drain flows can be computed by multiplying the adopted drainage module with the area served by the canal. Application efficiencies of the irrigation water at the farm level may be low and a considerable amount of water would then flow directly to the drains after it has entered the irrigation units. During prolonged rainfalls and when the supply system cannot be closed, the total flows diverted through tertiary outlets will be routed to the drainage system by the farmers. Operational losses in the main system and flows from escapes will also enter the drainage system. The magnitude of such flows, with seepage and percolation added, has to be compared with the drainage module. Usually the drain flows from the adopted drainage module are much more important than the other flows but care should be exercised when, for instance, including the possibility to divert - in an emergency- a considerable flow from a main or secondary canal to a relatively small drain. Rotation between irrigation blocks and staggering of the maximum water demands in the various blocks could reduce peak flows in the main system. This could especially be the case when only one and the same crop will be grown over the whole area. It is very difficult, however, to achieve such a reduction in the case of two or more crops per year. And there is usually a requirement, for agronomic reasons, that similar crops will be planted more or less at the same time. The resulting cropping calendar will be very tight when more than one crop per year occupies the same land. Possible reductions in peak flows will, consequently, be negligible. The design capacity of the supply canals consists basically of the sum of the maximum flows that have to be diverted at the same moment at the downstream end of the section under consideration, plus the conveyance losses. This design capacity is, therefore, greater than the sum of the maximum flows that have to be diverted at the same moment to 26 Irrigation System Layout and Canal Design the tertiary units: conveyance losses, evaporation losses (nearly always negligible) and seepage losses will have to be added to the capacity of each reach. Figure 9 Nomencleture for blocks, canals and structures Efficiency figures in handbooks, particularly for those schemes in which supplementary 27 Irrigation System Layout and Canal Design irrigation during the wet season is included, are very misleading. Usually seasonal figures are given and not the efficiency during peak discharges. This seasonal efficiency also includes periods during which the system cannot be operated at very high efficiencies, i.e. when flows are reduced to save water during periods that the crop or the land requires less water. Moreover, rainfall is assumed to be effective to varying degrees, thus 'blaming' the system, the operators and the farmers when more water is used than would follow from an assumed rainfall contribution to water use. Field irrigation efficiencies depend on the method of irrigation, the soils and the capabilities of the farmers. The conveyance efficiency depends on the design and the capabilities and working conditions of the irrigation staff. (The operating efficiency is usually included in the conveyance efficiency.) Overall efficiencies in a scheme depend also very strongly on the irrigation schedules. The following Table 3 is always presented during discussions on the subject but it should be kept in mind that the table is based on total seasonal figures and is, therefore, of no use whatsoever for the design of the system. (The data could be useful for water resources planning purposes.) Table 3 Efficiencies for various water supply methods Water supply demand Continuous Rotation On Demand No f samples 12 20 6 Conveyance efficiency 0.91 0.7 0.53 Tertiary unit efficiency 0.27 0.41 0.53 Overall efficiency 0.25 0.29 0.28 During peak demand, a tertiary unit overall efficiency (distribution and application) of 90% for flooded rice irrigation and of 80% for other crops can be achieved, unless soils would be very unfavourable for an efficient field application of water. A conveyance efficiency of 95% should then also be possible (there is nothing to operate), resulting in an overall efficiency of some 75% in non-rice schemes and 85% in rice schemes during the period of peak demand, (excluding seepage losses, see below). And this is the period on which design flows and, consequently, canal capacities are based. Whatever efficiency will be assumed during design, the designer should realize that it is forever impossible to obtain high efficiencies if the system has been designed assuming a low efficiency, i.e. if the system has been over-designed. Seepage losses, which do not enter in the efficiency calculations as such, range from 5-10% in well constructed canals in heavy soils to 50% or more in poorly constructed canal systems. Also lined canals and pipelines have losses of at least 5%. As mentioned above, both operation and seepage losses will eventually show up in the drainage system, as will water losses at the irrigation unit level. 28 Irrigation System Layout and Canal Design Greater than design flows would have to be diverted to a certain area when, for instance, a fast flooding of all blocks is needed to combat rats. Encroaching on the freeboard of the canal would be allowed in such a case (and only in such a case). Design flows could be reduced in the main and branch canals if an area greater than some 300400 ha is being served from such a canal. It has been observed that never all land will be under cultivation at any given moment in such an extent of irrigated area This would result in a smaller demand for water in case of continuous irrigation and in theoretically shorter delivery periods of water in case of rotational irrigation. It is difficult to take this possible reduction into account during the preliminary design stage. It is certainly worthwhile to consider this effect for very large schemes where quite a number of blocks are irrigated from the same main canal. It is not economic to change design flows downstream of each and every outlet or every drain outfall. Changes of 10-20% in theoretically required discharge capacities can be disregarded. The canal capacities are only adjusted when this percentage is exceeded. 3.4 Introduction to Structures in Irrigation Canals This section only serves to provide a first overview of the types and the functions of various structures in an irrigation and drainage system. Quite a number of structures are needed for the orderly conveyance, division and distribution of irrigation water over the area. Single functions of structures are described in the following subsections; a combination of functions in one structure is common practice (such as in a check-drop). One is also referred to Section 2.1.1 3.4.1 Structures For Crossings ✓ Crossings are required between canals, canals and roads and canals and obstacles in general. ✓ Crossings between canals should be avoided by preparing a proper layout; culverts, inverted siphons or aqueducts are provided when such crossings are really unavoidable. ✓ Crossings between canals and roads cannot be avoided although a careful layout may reduce the number of such crossings without diminishing access to all parts of the area. Culverts, inverted siphons and bridges are normally used to provide this type of crossing. In the Netherlands aqueducts have also been constructed for the purpose. ✓ Crossings of roads and drains can sometimes be executed in the form of so called Irish or half-Irish bridges when possibility is not required during 100 % of the time . 29 Irrigation System Layout and Canal Design The need for crossing obstacles in general is due to the topography. Such crossings are for instance tunnels to cut through a hill or mountain to reach a certain part of the area or to reduce the length and fall of the canal, thereby increasing the irrigable area or decreasing pump lifts. The crossing of existing water courses (cross-drainage structures) can be achieved by constructing an inverted siphon under the water course or the water course may be channeled under the supply canal through an inverted siphon. An aqueduct or culvert may serve the purpose too. It is also possible to impound the runoff water temporarily on the high side of the canal or to let it enter into the canal. The added drain water flow should, however, not exceed 10% of the design flow of the canal, as mentioned before. 4.2 Structures For Water Level Regulation (Level Control Structures) Water levels will have to be regulated in the supply canals when flows lower than the design flow are conveyed through the system. The system capacity is based on a certain peak flow. This peak flow may only be needed during a limited number of hours per day (rotational irrigation) or a limited number of days or weeks per year. Lower water requirements of the crops during the major part of the season, non-availability of water for part of the area and/ or the optimum utilization of rainfall reduce the required flows in the system during the remainder of the time. It is possible to design a system without control structures or with fixed control structures only. Such a system will, however, not operate satisfactorily for other flows than the design flow. Such a system could be considered though when possibilities for proper operation are nonexistent or very weak and/ or when water is plentiful. The same remarks apply to drainage canals. Here it may be required to maintain a certain water level in the canal during all or part of the year. This will certainly be the case in combined or semi-combined systems. Wind setup in canals may also necessitate the construction of water level control structures along the canal. Water levels will have to be rather constant in the system or should at least not vary too rapidly for the following reasons: a ) to secure the stability of earth canals (rapid drawdown) and to limit erosion; and/ or b ) to secure the proper operation of certain division and outlet structures. The regulation of water levels can be done by keeping the upstream or the downstream level near the structure constant, within limits, as discussed before. The former may be achieved by weirs or gates, the latter by gates only. A combination of gates and weirs may be useful to achieve the purpose. Gates may be operated automatically. Flows should certainly not vary too much when the system is equipped with hand-operated gates: the operator would continually be trying to find an optimum setting, without any real 30 Irrigation System Layout and Canal Design chance of achieving his goal. Or he may not care anymore, knowing that he will not succeed in reaching that optimum solution anyhow. Water control will be poor in both cases. 3.4.3 Structures For Division Of Water (Flow Control Structures) The flow will have to be divided according to variable rules at certain points in the system. Division structures serve this purpose. The division may be according to the following principles: ✓ the magnitude of the diverted flow is independent (within limits) of the upstream discharge; or ✓ the flow is divided proportionally, irrespective of the discharge. The former result is obtained by the use of culverts or gates in combination with a water level regulating structure. The inlet structure may be of the on-off type or may also be able to regulate the quantity of flow passing. The latter is done with the aid of weirs with a fixed crest elevation and width, or movable flow dividers. Structures for the division of water are many times combined with a water level regulating structure. The general term 'water control structure' may thus indicate either of the two types or a specific type of structure. Water control structures may be combined or supplemented with discharge measurement structures for the determination or registration of flows at various points in the system. 3.4.4 Structures For The Distribution Of Water To The Fields These structures can be fixed (weirs) or movable (weirs, siphons). 3.4.5 Structures For Safety All structures mentioned till now should perform satisfactorily within the range of maximum and minimum discharges they are designed for. The maximum discharge can normally not be exceeded without endangering the canals or the structures in the canals. Additional measures and structures are, therefore, needed to reduce this risk of damage. The more important safety measures and structures are: ➢ Tail structures at the end of supply canals to evacuate any surplus flow from the supply system to the drainage system. Overflow weirs and automatic siphons are used, possibly combined with a bottom outlet to empty the canal for maintenance. ➢ Emergency escapes (waste ways) in supply canals upstream of water level regulators or at points where the capacity of the downstream canal is limited. The incoming flow may be too great as a result of operational errors or because uncontrolled quantities of 31 Irrigation System Layout and Canal Design ➢ ➢ ➢ ➢ water enter the supply system. Lateral weirs or automatic siphons will have to be installed where the excess flow has to be discharged into the drainage system. Emergency escapes upstream of canals in high fill where any overtopping of the canal banks would be disastrous. Waste-ways before inverted siphons. See also (b). Transitions between canals and between canals and structures in order to guide the flow efficiently without causing erosion. Drop structures in supply or drain canals serve the same purpose. Energy dissipators downstream of all mentioned structures. Canal lining, either by compacted soil or any other less permeable material may be classified as a safety structure if thereby the seepage will be reduced to avoid sliding of the subsoil. 4. CLASSIFICATION AND ALIGNMENT OF CANALS 4.1 Classification of canals 4.1.1 Classification on the basis of the nature of their source of supply and distribution system The irrigation canals can be classified into following four subdivisions: 1. Permanent canals 2. Inundation canals 3. Perennial canals 4. Non-perennial canals. A canal is said to be permanent when its source of supply is sufficiently assured to warrant construction of protective and regulatory works with a regular permanent and graded channel. A canal is said to be of inundation type when the supply of water to the canal depends on a periodical rise in the water level of a river from which the canal takes off. Generally it does not require construction of a permanent regulatory work, where possible natural depressions are made use of to convey the water. The irrigation canals may also be classified as perennial or non-perennial depending upon the water requirements of the various crops grown in a year in a particular region. Since the inundation canals run only during flood season they are essentially of non-perennial type. When a permanent canal is run all the year round to meet the water demands of two to three crops in different seasons the canal is said to be a perennial canal. On the contrary when a permanent canal is run only in one crop season it is called non-perennial canal. 4.1.2 Classification of canals based on financial basis 32 Irrigation System Layout and Canal Design 1. Productive canals. Productive canals are those which are introduced with an idea of recovering all the expenditure incurred during the construction in a specified time limit. The following explanation clarifies the meaning: After full development of the canal system the revenue obtained from the cultivators is such that it is more than the maintenance expenditure. This exceeding amount of revenue is so adjusted that it is about 2 percent of the cost of the canal system. With this rate the total cost of the project will be recovered in about 50 years. Thus the productive canal is that which adds to the wealth of nation directly. 2. Protective canals. Protective canal is one which is not actually remunerative as the previous type. The construction work of this type of canal and its development may be started during the famine. It helps in giving employment to the cultivators at the time of the famine. Thus the protective canal is a sort of a relief work. 4.1.3 Classification based on the function served by the canal With the advancement in the field of Irrigation Engineering and to fulfill multi- objective needs the canals are constructed to perform different functions. Such canals are: 1. 2. 3. 4. 5. Irrigation canal Carrier canals Feeder canals, Navigation canals and Power canals. Carrier canals are those which are constructed to join two rivers. The canals may or may not provide direct irrigation. Feeder canals convey water from a river or a storage project to feed another water deficient irrigation system. Like link canals these canals also may or may not provide direct irrigation along its course. Power canals are generally meant to feed the hydroelectric power station. Generally such canals do not provide irrigation before water is utilized in power generation. A tail race channel which leads the water away from the power house may discharge into the river or another canal system for subsequent utilization. 4.1.4 Classification based on the capacity of the canal Depending upon the size and capacity of the canal different, canals are classified as 1. Main canals 2. Branch canals 33 Irrigation System Layout and Canal Design 3. Major distributor 4. Minor distributor 5. Watercourse 4.2 Alignment of canals An irrigation canal is aligned in such a way that the water gets proper command over the whole irrigable area. When canal follows a watershed or a ridge of the area it gets necessary gravity flow. The water-shed or the ridge is a dividing line between two drainage areas. Thus a canal which runs over the ridge gets command of the areas on both the sides of the ridge. Another advantage is that the drains do not cross the bridge or the water-shed. The canals are taken off from the river or from the storage reservoirs or tanks created or constructed in the upper or hilly reaches of the river. A river is nothing but a big drain and the canal takes off from it. Naturally the head of the canal is in a valley portion and not on the ridge. Obviously first thing to be done after takeoff is to bring the canal on the watershed of the catchment. 3.2.1 Types of canals-alignment There are three types of canal alignments Contour canal It is a canal aligned nearly parallel to the contour lines. However, the canal should have sufficient slope along the flow for the required velocity of flow. Such a canal irrigates only on one side of the canal (lower side).Would cross maximum number on natural drainages. Figure 10 Contour canall Watershed canal (ridge canal): 34 Irrigation System Layout and Canal Design Aligned along the watershed of the area to be irrigated. Watershed in this case is the line dividing the area into two small sub-catchments. Irrigation from a watershed canal is possible by gravity on both sides. Cross drainage works will be avoided by this alignment as no natural drainage can cross the watershed. Figure 11 Watershed canal (ridge canal) Side slope canal: It is a canal running roughly at right angles to the contour lines. Such canals are aligned parallel to natural drainages and will not cross them. Irrigation may be on either side of the canal. Figure 12 Side slope canal 35 Irrigation System Layout and Canal Design 4.3 Canal losses The canal section is never made hundred percent of water tight. The canal water tries to seep into the soil. Moreover the canals are exposed to the atmosphere at top. The water also goes to the atmosphere in the form of vapor. Thus irrigation water, when taken in the canal system for conveying it to the fields, is liable to be lost in various ways. Loss of canal discharge occurs due to two main reasons, namely: ▪ Evaporation, and ▪ Seepage. Out of these two types of losses, seepage loss is the one which is of most concern. It is observed that in initial stages total losses may amount to 40 percent of the total discharge of the canal system. If the tract in which the canal system is introduced is made up of porous soil the seepage loss will be exceptionally high in initial stage. Then it is just possible that the whole supply diverted in the canal system, may be lost if the length of the canal system is appreciable. After running the canal for a sufficient period the loss is minimized to a considerable extent. The reduction in loss with time is due to two main reasons. Firstly when the subsoil formation below the canal gets saturated entry of seeping water does not remain resistance free. The reason for this resistance lies in the fact that the soil pores are already filled with water. Obviously the rate of seepage will be lowered. Secondly in course of time soil pores also get filled up with silt and sand load carried by the canal water. It may be mentioned here that evaporation loss cannot be checked whereas seepage loss goes on reducing with the age of canal system and it can also be checked by adopting suitable measures. Thus it is clear that when a canal section is to be designed proper allowance should be made for these losses. Generally losses due to evaporation and seepage are combined while making allowance for them. The water loss is generally expressed in m3/sec per million square metres of the exposed water surface or in m3/sec per million square metres of the wetted surface. Loss due to evaporation; in most of the cases evaporation loss is not significant. It may range 0.25 to 1 % of the total canal discharge. The rate of loss of water by the process of evaporation depends mainly on the following factors: Canal losses are generally expressed in m3/sec per km length of the canal. There can be several empirical formulas for estimation one such formula is: 𝑄= 1 200 ∗ (𝑏 + 𝑑)2/3 36 Irrigation System Layout and Canal Design Where Q is canal loss m3/sec per km, b and d are bottom width and depth of the canal respectively. Seepage loss; the loss due to seepage is the one which is most significant so far as irrigation water loss from canals is concerned. The seepage loss depends mainly on the following factors: 1. Underground water-table condition 2. Porosity of the soil 3. Physical properties of canal water for example its temperature and quantity of suspended load carried by the water (turbidity of water). 4. Condition of canal system. The seepage loss may occur in two characteristic ways, namely: (i) Absorption. When the underground water-table is at a considerable depth, the water which has entered the soil is unable to join the saturated zone and wets the subsoil locally immediately below the canal bed. The soil layer which is in immediate contact with the channel section is completely saturated due to absorbed water. It forms a bulb of saturated soil below the channel. The soil layer below the saturated bulb is not fully saturated. Thus the extent of saturation goes on decreasing from the ground level below the soil with depth. These now exists a zone of unsaturation between the underground saturated zone and the saturated bulb. Thus there is no chance of continued and constant flow from the canal to the ground water reservoir. (ii) Percolation. When the underground water table is nearer to the natural surface, the water which has entered the subsoil may join the saturation zone or underground reservoir to maintain a continuous direct flow. In such conditions water directly flows from the canal to the underground reservoir through the soil pores. The pressure responsible for this flow is the difference of level between ground water table and the water level in the canal. Since there is a direct flow this type of loss is more serious than the lots due to absorption. Measurement of seepage loss: Seepage loss is generally measured by a very simple method called inflow and outflow method. In this method a long reach where fairly sufficient and constant discharge flows is selected. Regular and simultaneous discharge measurement is done at the entrance and end of the selected reach. Attempt is made to maintain same gauge and discharge at entrance during the period of observations. The difference between inflow 37 Irrigation System Layout and Canal Design and outflow over the reach gives the quantity of water lost. The discharge measurement can be done either by flumes or weirs or current meters suitable for the selected canal size. The loss is generally expressed in cubic metres per sec per million square meters of wetted perimeter i.e. cumec/M sq m of wetted perimeter. According to Etchevery and Harding the range of water lost in conveyance in different soil types is as given in Table below Table 4 Conveyance loss Soil type Conveyance loss in cumec/ M sq.m of wetted perimeter 5.18 to 6.10 3.66 to 5.18 1.83 to 3.66 1.22 to 1.83 Loose sandy soil Sandy loam Clay loam Clay loam with Hard pan below Impervious clay loam 1.02 to 1.22 5. DESIGN OF IRRIGATION CANALS An irrigation and drainage network should be designed and operated in such a way that: • The required discharge flows are passed at design water levels; • No erosion of canal bottoms and banks will occur, and • Any sediment that enters the system will not settle in the network but will be carried along and discharged through the outlets, to the fields or to the natural drainage system, or is collected in dedicated silt traps if those are constructed. The objective of a canal design is to select such a bottom slope and geometric dimensions of the cross section that during a certain period the sediment flowing into an irrigation canal is equal to the sediment flowing out of the canal. Changes in equilibrium conditions for sediment transport result in periods of deposition or erosion. Moreover, the canal should be able to convey the required quantity of water to satisfy the water demand in the field. Design discharge: Design discharge also called canal capacity is the maximum discharge that the cross section of a canal reach is designed for. This discharge is the maximum expected flow in the canal during peak periods of peak flow. The design discharge of a canal reach is the sum of the following: 38 Irrigation System Layout and Canal Design • The simultaneous maximum flows through the outlets in the reach • The outflow into the next reach of the canal; and • Seepage and other losses (conveyance seepage and operational losses). Determination of design discharge The design discharge of a canal can be determined as follows: 1. Determine the total area irrigated from the canal under consideration; 2. Based on the crop water requirement, determine the water demand for each unit irrigated in l/s/ha; 3. For each outlet in the canal reach, determine the outflow by multiplying the area by the water requirement; 4. Determine the outflow into the next reach f the canal; 5. Add all the outlet discharges and the outflow into the next reach of the canal to determine the design discharge of the canal reach under consideration. The outlet discharges are; Q = A* q Where A= Area to be irrigated from the outlet q= peak water demand rate (l/s/ha) The design discharge of the reach is thus, Q= Q1+Q2+Q3+…+ Qo+ losses Where: Q1, Q2, Q3… are outlet discharges downstream of the reach and Qo outflow into the next reach. In design of an irrigation canal various channel dimensions are to be determined. They are bed width, depth, side slope, longitudinal or bed slope, etc. The canal is designed to carry maximum required discharge safely. Before proceeding with the subject it is essential to understand following terms: Hydraulic mean radius (R). It is also called hydraulic mean depth. It is a ratio of cross-sectional area of flow and wetted perimeter. That is 𝑅= A P Where A is sectional area of flow and P is wetted perimeter. Full Supply Discharge (F.S.D.): It is the maximum discharge for which the canal is designed. It is the sum of maximum irrigation requirement and various losses in conveyance of water. The level of water in the canal at F.S.D. is called a fully supply level (F.S.L.). 39 Irrigation System Layout and Canal Design Coefficient of rugosity: It is generally denoted by a letter 'N'. This coefficient is mainly a function of: ▪ Grain size of bed and bank material; ▪ Bed shear stress; and ▪ Depth of water in the channel. Table 5 Coefficient of rugosity for different bed material Material Coefficient of rugosity (N) Wood Steel Concrete Masonry Earth 0.013 to 0.0165 0.0125 to 0.018 0.013 to 0.018 0.02 to 0.35 0.0225 to 0.035 It depends on the size of ripples formed on the bed of a channel. Table above gives the values of N for different types of bed material. Channel in regime: a channel is said to be in regime when the following conditions are fulfilled: ▪ Channel water flows in "incoherent unlimited alluvium"; ▪ The character of its bed and bank material is same as that of transported material; and ▪ The silt grade and silt charge is constant. The term "incoherent unlimited alluvium" gives information about the character of the bed material and its formation. It represents the bed made of loose grains of small size (silt) and the processes of silting and scouring need no special efforts. Depending upon the existing condition at a site under observation, regime may be initial or final. The channel is said to be in initial regime condition when it has formed its section only and yet not secured the longitudinal slope. The channel after attaining its section and longitudinal slope is said to be in final regime. If the channel is protected at bed and sides with some protecting0 material, there is no possibility of change of section and longitudinal slope. Then the channel is said to be in permanent regime. Thus it is clear that final regime and permanent regime are two different things. Critical Velocity (Vo). When the velocity of water flowing in the channel is such that neither silting nor scouring is taking place then the velocity is said to be critical. Naturally if the velocity is below critical value silting occurs. On the contrary if it is above scouring occurs. 40 Irrigation System Layout and Canal Design It is seen that the critical velocity is dependent on the nature of the soil formation in which a canal flows. Table 4.2 gives various values of critical velocity for different soil formations. Table 6 Critical velocity for various soil formations Type of formation Critical velocity in m/sec Earth 0.3 to 0.6 Ordinary murum 0.6 to 0.9 Hard murum 1.2 Boulders l.5 to l.8 Soft rock l.8 to 2.4 Hard rock more than 3 5.1 Design Criteria For Irrigation Canals Table 7 Recommended Hydraulic Design Formulae No. Situations Recommended Formulae 1 Water flowing without sediment in unlined canals. The canal design is governed by the need to prevent erosion, which depends on the parent material of the canal. For sizing: Manning’s equation; For limiting slope: Tractive force equation and USSCS method (if data is available). 2 Water carrying sediments in lined canals. The design is governed by the need to ensure the sediment load that can be transported. Water carrying sediments in unlined canals. Both nonscouring and non-silting criteria, whereas “ridge” canals are governed by non-scouring criteria since they generally run down the slope. For sizing: Manning’s equation; For sediment transport: Engelund and Hansen equations. 3 41 For sizing: a) Steep terrain – Tractive force method; b) Flat terrain(small canal) – Lacy Eqn; c) Flat terrain(large canal) – Tractive force Eqn; For sediment transport: Engelund and Hansen equations. Irrigation System Layout and Canal Design Water carrying sediment in For sizing: Kennedy formulae in unlined canals. Where the route combination with continuity, of the canals is fully on alluvial chezy and other equations. soils and canals carry For sediment transport: appreciable silt and sand load. Engelund and Hansen equations. This return affects the velocity of flow in the canal considerably. Hence the Manning’s or Chezy’s equations do not consider the condition of picking of picking up of silt material from canal bed & sides in case the water is silt free when entering to the canal. The canals shall be designed using Manning’s equation and for limiting non-silting, nonscouring velocities the canal sections shall be checked by Kennedy, Lacey’s and Tractive force equations as required. 4 Table 8 Manning’s coefficient “n” for Unlined Canals Canal Location On plain terrain On hills Canal Type Primary Condition Values of Roughness Well maintained Poorly maintained Secondary Well maintained Poorly maintained Tertiary Well maintained Poorly maintained All Types Straight & Uniform Well maintained Poorly maintained Stony bed sides Co-efficient “n” 0.025 0.028 0.025 0.03 0.03 0.035 0.025 0.035 0.03 Table 9 Manning’s coefficient “n” for Lined Canals Lining Type Dry Stone Dry Brick Dressed Masonry Brick Max. Velocity (m/s) 1.03 1.0 2.0 1.5 42 Co-efficient “n” 0.025 0.02 0.018 0.017 Irrigation System Layout and Canal Design Random Rubble Un-reinforced Concrete Shortcrete Buried membrane & Earth 1.5 2.5 0.020 0.015 2.5 0.017 Depend on the cover material or the lining material for earth designed as unlined. 5. 2 Design Procedure The following is the procedures in the design of canals: 1. Formulate the conveyance system based on topographic and survey; 2. Determine net irrigable area of each serving canal; 3. Assimilate water demand according to the proposed cropping pattern and crop water requirements; 4. Calculate design discharge capacity at different sections incorporating its off taking canals; 5. Based on the proposed canal control system finalize the location of regulating structures; 6. Determine the required Full Supply Level at different chainages taking in to account the level of Command area and required for off taking canals; 7. Design the different sections using Manning’s formula and also check CVR particularly in main canal where the water may be silt laden 8. Check the canal longitudinal slope and velocity of flow by Lacey’s theory where the water is silt laden; and 9. Determine the CBL (canal bed level), TBL (top bank level) to achieve required full supply level using designed canal parameters. 5.3 Design of unlined canals Unlined canals can be classified into two classes based on the stability of the boundaries of the canal for design purposes: a. Canals with stable bed (non-erodible) b. Canals with erodible bed (Alluvial) with significant amount of sediments flowing 5.3.1 Design of non-erodible (stable bed) canals; Non-erodible canals are canals with fairly stable boundary. The design of such canals should ensure that any sediment entering into the canal go on flowing and avoid settlement. The flow velocity however should be in such a way that does not cause any erosion of the canal boundary. The design of the canals involves the conditions of Steady uniform flow. In steady 43 Irrigation System Layout and Canal Design uniform flow the discharge of the canal reach under consideration will be the same at all sections and the flow depth and velocity are constant. For the design of such canals, the following equations are employed: 1. Continuity equation; 2. Manning’s/ Strikler formula; There are two approaches for the design of such canals: i. The recommended (b/d ratio) approach ii. The Tractive force (permissible velocity approach) 5.3.2 Method Of Tractive Force However, a design methodology based primarily on experience and observation rather than physical principles. The first step in developing a rational design process for unlined, stable, earthen channels is to examine the forces which cause scour. Scour on the perimeter of a channel occurs when the particles on the perimeter are subjected to forces of sufficient magnitude to cause particle movement. when a particle rests on the level bottom of a channel, the force acting to cause movement is the result on the flow of water past the particle. A particle rests on the slope side of a channel is acted on not only by the flow - generated forces, but also by a gravitational component which tends to make the particle roll or slide down the slope. If the resultant of those two forces is larger than the forces resisting movement, gravity, and cohesion, then erosion of the channel perimeter occurs. By definition, the tractive force is the force acting on the particle composing the perimeter of the channel and is the result of the flow of water past these particles. In practice, the tractive force is not the force acting on a single particle, but the force exerted over a certain area of the channel perimeter. This concept was first stated by duBoys( 1879 ) and restated by Lane (1955 ). In most channels, the tractive force is not uniformly distributed over the perimeter 44 Irrigation System Layout and Canal Design Figure 13 Tractive force distribution obtained using membrane analogy Figure 14 Maximum shear on bed and sides for alluvial channel based on Normal's Method. The maximum net tractive force on the sides and bottoms of various channels as determined by mathematical studies are shown as a function of the ratio of the bottom width to the depth of flow. It may be noted that for the trapezoidal section, the maximum tractive force on the bottom is approximately ƔyS and on the sides 0.76 γ γ . The figures show the maximum unit tractive forces in terms of for different b/y ratios. 45 Irrigation System Layout and Canal Design Figure 15 Maximum unit tractive force in terms of γySo When a particle on the perimeter of a channel is in a state of impending motion, the forces acting to cause motion are in equilibrium with the forces resisting motion. A particle on the level bottom of a channel is subject to the unit tractive force on a level surface and effective area. In the above figure the particle size is the diameter of the particle of which 25 percent of all the particles, measured by weight, are larger. Lane (1955) also recognized that sinuous canals scour more easily than canals with straight alignments. To account for this observation in the tractive force design method, Lane developed the following definitions. Straight canals have straight or slightly curved alignments and are typical of canals built in flat plains. Slightly undulating topography. Moderately sinuous canals have a degree of curvature which is typical of moderately rolling topography. Very sinuous canals have a degree of curvature which is typical of canals in foothills or mountainous topography. Then, with these definitions, correction factors can be defined as in Table. Table 10 Correction factor for sinuousness of the channel Degree Of Sinuousness (Stream length/valley length) Straight Channels Correction Factor 1.00 46 Irrigation System Layout and Canal Design Slightly Sinuous Channels 0.90 Moderately Sinuous Channels 0.75 Very Sinuous Channels 0.60 Plasticity index (PI) is the difference in percentage of moisture between plastic limit and liquid limit in Atterberg soil tests. For canal design PI can be taken as 7 as the critical value. In this figure, for the fine non cohesive , i.e.,average diameters less than 5mm , the size specified is the median size of the diameter of a partical of which 50 percent were larger by weight. Lacey developed the following equations based on the analysis of large amount of data collected on several irrigation canals in the India. 𝑃 = 4.75√𝑄 𝑓𝑠 = 1.76 𝑑1/2 𝑄 𝑅 = 0.47( )1/3 𝑓𝑠 𝑆𝑜 = 3 ∗ 10−4 𝑓𝑠 5/2 𝑄1/6 In which P is the wetted perimeter (m), R is tkhe hydraulic mean radius (m), Q is the flow in 3 m /s, d is the diameter of the sediment in mm, fs is the silt factor, S0 is the bed slope. Table 11 Particle size and silt factor for various materials Material Size (mm) Silt Factor Small boulders, cobbles, shingles Coarse gravel Fine Gravel Coarse Sand Medium Sand Fine Sand Silt (colloidal) Fine Silt ( colloidal) 64 – 256 6.12 to 9.75 8 – 64 4–8 0.5 – 2.0 0.25 – 0.5 0.06 – 0.25 Taken from Gupta (1989) 47 4.68 2.0 1.44 – 1.56 1.31 1.1 – 1.3 1.0 0.4 – 0.9 Irrigation System Layout and Canal Design Combining the above equations the following resistance equations similar to the Manning equation based on the regime theory is obtained. V = 10.8 R1/2 So2/3 in which V is the velocity in m/s. Tractive Force Method When water flows in a channel, a force that acts in the direction of flow on the channel bed is developed. This force, which is nothing but the drag of water on the wetted area and is known as the tractive force. A particle on the sloping side of a channel is subject to both a tractive force and a downslope gravitational component. It is noted that the tractive force ratio is a function of both the side slope angle and the angle of repose of the material composing the channel perimeter. In the case of cohesive materials and fine noncohesive materials, the angle of repose is small and can be assumed to be zero; i.e.. for these materials the forces of cohesion are significantly larger than the gravitational component tending to make the particles roll downslope. Consider the shear stress at incipient motion (which just begins to move particles) for uniform flow. The tractive force is equal to the gravity force component acting on the body of water, parallel to the channel bed. Gravity componenet of weight of water in the direction of flow is equal to γALSo in which γ is the unit weight of water. A is the wetted area. L is the length of the channel reach and So is the slope. Thus, the average value of the tractive force per unit wetted area, is equal to 𝜏𝑜 = γALSo 𝑃𝐿 = γRSo In which P is the wetted perimetr and R is the hydrualic mean radius; For wide rectangular channel, it can be written as 𝜏𝑜 = γySo The tractive force is also called Drag Force. Consider a sediment particle submerged in water and resting on the side of a trapezoidal channel. For this case the tractive force (Apτ) must be equal to gravity force components wssin α Let τb be the critical shear stress on bed, τs be the critical shear stress on side-walls Ap be the effective surface area of typical particle on bed or side wall 𝜃𝑜 be the angle of the Side slope and α be the angle of repose (angle of internal friction) of bank material 48 Irrigation System Layout and Canal Design Figure 16 Analysis of forces acting on a particle resting on the surface of a channel bed From Force diagram, resultant Force R: 49 Irrigation System Layout and Canal Design R =√ ( 𝐴𝑝 𝜏𝑆 ) 2 + (𝑊𝑠 sin 𝜃𝑜 ) 2 Resisting force: Ws cos 𝜃𝑜 is the weight component normal to side slope Tan𝛼 is the coefficient of friction (due to angle of internal friction) 𝐹𝑠 = 𝑊𝑠 cos 𝜃𝑜 𝑡𝑎𝑛𝛼 Therefore R = Fs at incipient motion → Wscos𝜃𝑜 tan𝛼 = √𝑊𝑠 𝑠𝑖𝑛2 𝜃𝑜 + 𝐴2𝑝 𝜏𝑠2 Solving for the unit tractive force that cuases impeding motion on a sloping surface → 𝜏𝑠 = 𝑡𝑎𝑛2 𝜃𝑜 𝑊𝑠 tan 𝛼 cos 𝜃𝑜 √1 − 𝑡𝑎𝑛2 𝛼 𝐴𝑝 On the channel bed, with 𝜃𝑜 being zero it reduces to → Ap𝜏𝑏 = Wstan𝛼 → 𝜏𝑏 = Tractive force Ratio → K= 𝜏𝑜 𝜏𝑏 = cos 𝛼√1 − 𝑊𝑠 𝑡𝑎𝑛𝛼 𝐴𝑝 𝑡𝑎𝑛2 𝛼 𝑡𝑎𝑛2 𝜃𝑜 K is the reduction factor of critical stress on the channel side. Thus the ratio is a function of only side slope angle 𝜃𝑜 and angle of repose of the material 𝛼 The Limiting shear stress or limiting velocity procedure is also commonly used. In this approach, the uniform depth is calculated for the maximum discharge Q and this value is to be compared either , and if they satisfy their add the freeboard and the design is complete. Table below lists the values for various lining types. τ max VS τ permissible or Vmax vs. V permissible Table 12 Permissible shear stress for lining material Lining Catagory Lining Type Temporary Woven Paper Net Jute Net Fiberglass Roving Single 50 Permissible Unit Shear Stress (kg/m2) 0.73 2.20 2.93 Irrigation System Layout and Canal Design Vegetative Gravel Riprap Rock Riprap Double Stream with Net Cured Wood Mat Synthetic Mat Class A Class B Class C Class D Class E 2.5 cm 5 cm 15 cm 30 cm 4.15 7.08 7.57 9.76 18.06 10.25 4.88 2.93 1.71 1.61 3.22 9.76 19.52 0 Where 𝜃 the angle of repose of the bed material and Ws is is weight of a soil particle 51 Irrigation System Layout and Canal Design Figure 17 Angle of repose (degrees from horizontal) for non-cohesive earthen materials Design Procedure - The design procedure is based on calculations of maximum depth of flow, h. Separate values are calculated for the channel bed and side slopes, respectively. It is necessary to choose values for inverse side slope, m, and bed width, b to calculate maximum allowable depth in this procedure Limits on the bed width can be set by specifying allowable ranges on the ratio of b/h, where b is the channel base width and h is the flow depth Thus, the procedure involves some trial and error STEP 1 ▪ ▪ ▪ ▪ Specify the desired maximum discharge in the channel Identify the soil characteristics(particle size gradation, cohesion) Determine the angle of repose of the material, 𝜃 Determine the longitudinal bed slope, So, of the channel STEP 2 52 Irrigation System Layout and Canal Design • Determine the critical stress, 𝜏𝑐 (N/m2 , based on the type of material and particle size from Fig. 3 or 4 • Fig 3 is for cohesive material, Fig 4 is for non-cohesive material • Limit ∅ according to 𝜃 (𝐿𝑒𝑡 ∅ ≤ 𝜃) Step 3 • Choose a value for b • Choose a value for m Step 4 • Calculate ∅ from Eq. 16 • Calculate K from Eq. 24 • Determine the max shear stress fraction (dimensionless, Kbed, for the channel bed, based on the b/h ratio and fig 6 • Determine the max shear stress fraction (dimensionless, Kside, for the channel bed, based on the b/h ratio and fig 7 53 Irrigation System Layout and Canal Design Figure 18 Permissible value of critical shear stress in N/m2 for non-cohesive earthen material • The three curves at the left side of Fig. 5 are for the average particle diameter • The straight line at the upper right of Fig. 5 is not for the average particle diameter,” but for the particle size at which 25% of the material is larger in size • This implies that agradation (sieve analysis has been performed on the eartehen material Particle gradation 25 % 75 % Smallest Largest • The three curves at the left side of Fig 5 9d≤ 5 𝑚𝑚) can be approximated as follows: Clear water Low sediment 𝜏𝑐 = 0.0759𝑑 3 − 0.269𝑑 2 + 0.947𝑑 + 1.08 54 Irrigation System Layout and Canal Design 𝜏𝑐 = 0.0756𝑑 3 − 0.24𝑑 2 + 0.872𝑑 + 2.26 High Sediment 𝜏𝑐 = −0.0321𝑑 3 + 0.458𝑑 2 + 0.190𝑑 + 3.83 • Regression analysis can be performed on the plotted data for Kbed & Kside • This is useful to allow interpolations that can be programmed instead of reading values off the curves by eye • The following regression results give sufficient accuracy for the ma shear stress fractors 𝑏 Kbed ≅ 0.792( )0.153 ℎ Kbed ≅ 0.00543 𝑏 ℎ + 0.947 for 1 ≤ b/h ≤ 4 for 4 ≤ b/h ≤ 10 55 Irrigation System Layout and Canal Design Figure 19: Kbed values as a function of the b/h ratio Note: This figure was made using data from USBR Hydrualic Lab Report Hyd-300. 56 Irrigation System Layout and Canal Design Figure 20: Kside values as a function of the b/h ratio. 57 Irrigation System Layout and Canal Design • ` For trapezoidal cross sections and, Kside ≅ Where, 𝐴𝐵+𝐶(𝑏/ℎ)𝐷 𝐵+ (𝑏/ℎ)𝐷 A = -0.592(𝑚)2 + 0.347(𝑚) + 0.193 7.23 B = 2.30 – 1.56 𝑒 −0.000311(𝑚) 5.63 C = 1.14 – 0.395𝑒 −0.000143(𝑚) −3.29 D = 1.58 – 3.06𝑒 −35.2(𝑚) For 1≤ 𝑚 ≤ 3, and where e is the base of natural logarithms • Equations 29 give Kbed to within ± 1% of the values from the USBR data for 1≤ 𝑏 ℎ ≤ 10 • Equations 3-34 give Kside to within ± 2 % of the values from the USBR data for 1 ≤ 𝑚 ≤ 3 (where the graphed values for m = 3 are extrapolated from the lower m values) • The figure below is adapted from the USBR, defining the inverse side slope, and bed width • The figure below also indicates locations measured maximum tractive force on the side slopes, Kside, and the bed, Kbed • These latter two are propotional to the ordinate values of the above two graphs (Figs. 6 &7) Step 5 • Calculate the maximum depth based on Kbed: ℎ𝑚𝑎𝑥 = 58 𝐾𝜏𝑐 𝐾𝑏𝑒𝑑 𝛾𝑆0 Irrigation System Layout and Canal Design ℎ𝑚𝑎𝑥 = 𝐾𝜏𝑐 𝐾𝑠𝑖𝑑𝑒 𝛾𝑆0 • Note that k, Kbed, Kside, and So are all dimensionless; and the result gives units of length • The smaller of the two hmax values from the above equations is applied to the design (i.e the “worst case” scenario) Step 6 • Take the smaller of the two depth, h, values from 35 & 36 • Use the manning or Chezy equations to calculate the flow rate • If the flow rate is sufficiently close to the desired maimum discharge value, the design process is finished • If the flow rate is not the desired value, change the side slope, m, and or bed width, b, checking the m and b/h limits you may have set initially • Return to Step 3 and repeat calculations • There are other ways to attack the problem, but it’s almost always iterative • For a very wide channel the sides of channel neglegible and the critical tractive force on the channel bed can be taken as 𝜏𝑐 = 𝛾ℎ𝑆𝑜 Sample Earthen Design Example • Design an earthen canal section in an alluvial soil such that the wetted bounderies do not become eroded • The canal will follow the natural terrain at an estimated So = 0.000275 m/m with a preliminary design side slope of 1.5:1 (H:V) • The bed material has been determined to be a non-cohesive”coarse light sand” with an average particle diameter of 1 mm, 25 % of which is larger than 15 mm. • Tests have shown that the average angle of repose for the bed material is approimately 34o. Measured from the horizontal. • For the manning equation, use a roughness value of 0.03 • The design discharge is 18.6 m3/s • The source of water is such that there will be a low content of fine sediment • The canal can be assumed to be straight, even though there will be beds at several locations • Design the section using trapizoidal shape with width to depth ratio, b/h, of between 1. And 5.0 • Adjust the side slope if necessary, but keep it within the range .5:1 to 2:1 59 Irrigation System Layout and Canal Design • Note that ∅ < 𝜃 must be true to allow for a stable side slope SOLUTION Critical Tractive Force ➢ The critical tractive force can be estimated from Figure 5 ➢ The material is non-cohesive, and 25% of the particles are larger than 15mm. This gives 𝜏𝑐 ≈ 16.3 N/m2 for the 15 mm abscisa value. Angle Of Repose ➢ The angle of repose, 𝜃 ,is given 340 ➢ Then, the ratio of 𝜏𝑏𝑒𝑑 𝑡𝑜 𝜏𝑠𝑖𝑑𝑒 is: K= 𝜏𝑠𝑖𝑑𝑒 𝜏𝑏𝑒𝑑 = √1 − sin ∅2 sin 𝜃 2 = √1 − 3.2 sin ∅2 ➢ Design requirements for this example call for a side slope between 0.5 & 2.0 ➢ Actually, the range is restricted to 1.5 to 2.0 because the preliminary design side slope of 1.5:1 corresponds to an angle ∅ = 33.70 ➢ This is less than the angle of repose, 𝜃 = 340, but it is very close ➢ Make a table of K values: m 1.5 1.6 1.7 1.8 1.9 2.0 ∅ 33.70 32.00 30.50 29.10 27.80 26.60 k 0.122 0.318 0.419 0.493 0.551 0.599 Maximum Shear Stress Fractions ➢ From figure 7 (see above),the maximumshearstress fraction for the sides,Kside,isapproximately equal to 0.74in the range 1 <b/h<5, and for side slopes from 1.5 to 2.0 ➢ Take Kside as a constant for this problem: Kside ≈ 0.74 ➢ The maximum shear stress fraction on the channel bed, Kbed, will fall on the curve for trapizoidal sections, and will vary from 0.79 to 0.97 within the acceptable range 1.0 <(b/h)< 5.0 ➢ Make a table of Kbed values according to b/h ratio (from figure 13): 60 Irrigation System Layout and Canal Design Table 13 Values of Kbed for different b/h ratios b/h 1 2 3 4 5 Kbed 0.79 0.90 0.94 0.96 0.97 Manning Equation ➢ The Manning roughness, n, is given as 0.03. The longitudinal bed slope is given as 0.000275 m/m ➢ The side slope can be any value between 1.5 and 2.0 ➢ Construct a table of depths (normal depths) for the maximum deisign discharge of 18.6 m3/s, then make another table showing bed width to depth ratios: Table 14 Flow depths (m) for 18.6 m3/s Inverse side slope, m b (m) 1.5 3.1 2.7 2.5 2.2 2.0 1.9 3.0 4.5 6.1 7.6 9.1 10.6 1.6 3.0 2.7 2.4 2.2 2.0 1.9 1.7 3.0 2.6 2.4 2.2 2.0 1.9 1.8 2.9 2.6 2.4 2.2 2.0 1.9 1.9 2.9 2.6 2.3 2.2 2.0 1.8 1.9 9.3 8.4 7.7 7.0 6.5 6.1 2 2.8 2.5 2.3 2.1 2.0 1.8 Table 15 Bed width to depth ratios b (m) 3.0 4.5 6.1 7.6 9.1 10.6 Inverse side slope, m 1.5 1.6 1.7 1.8 1.9 2.0 0.989 1.665 2.469 3.397 4.444 5.600 1.008 1.691 2.743 3.429 4.478 5.636 1.027 1.716 2.766 3.458 4.511 5.672 1.044 1.740 2.554 3.492 4.545 5.700 1.060 1.763 2.581 3.521 4.573 5.738 1.076 1.784 2.608 3.551 4.601 5.766 61 Irrigation System Layout and Canal Design • In table 15 it is seen that the bed width must be less than 10.6m, otherwise the required b/h ratio will be greater than 5.0. • Also, it can be seen that bed widths less than 0.3 m will have problems becaouse the b = 3 and m = 1.5 combination gives b/h < 1.0 Allowable Depth for Tractive Force Side Slopes • Taking Kside from Figure 20, the maximum allowable depth according to the tractive force method for side slopes is: hmax = 𝜏𝑐 𝐾 𝐾𝑠𝑖𝑑𝑒 𝛾𝑆𝑜 = 0.34√1−3.2 sin ∅2 (0.74)(62.4)(0.000275) = 26.8√1 − 3.2 sin ∅2 • Make a table of hmax values (essentially independent of b/h) for different values of the angle ∅ : Table 5. Maximum depth values for different side slopes m ∅ hmax 0 1.5 33.7 3.3 0 1.6 32.0 8.5 0 1.7 30.5 11.2 0 1.8 29.1 13.2 0 1.9 27.8 14.8 0 2.0 26.6 16.0 • • Now the design possibilities will narrow further Compare depths calculated by the Manning equation for 18.6 m3/s with the maximum allowable depths by tractive force method, according to side slope traction ( combine Tables 3 & 5) Table 16 ratio of Manning depths to hmax for 18.6 m3/s Side slope, m b (m) 3.0 4.5 6.1 7.6 9.1 1.5 3.06 2.73 2.45 2.23 2.05 1.6 1.17 1.04 0.94 0.86 0.79 1.7 0.87 0.78 0.71 0.65 0.59 62 1.8 0.73 0.65 0.59 0.54 0.50 1.9 0.64 0.58 0.52 0.48 0.44 2 0.58 .53 0.48 0.44 0.41 Irrigation System Layout and Canal Design • From the table above it is seen that the side slope must now be between 1.6 and 2.0 otherwise the required flow depths will exceed the limit imposed by tractive force method for side slopes. Allowable Depth for Tractive Force: Channel Bed Again taking Kbed from Figure 19, the maximum allowable depth according to the tractive force method fro channel bed is: hmax = 𝐾𝜏𝑐 𝐾𝑏𝑒𝑑 𝛾𝑆𝑜 = 0.34√1−3.2 sin ∅2 𝐾𝑏𝑒𝑑 (62.4)(0.000275) = 19.8 𝐾𝑏𝑒𝑑 √1 − 3.2 sin ∅2 Table 17 Maimum allowable depths according to bed criterion Side slope, m b (m) 1.5 3.0 4.5 6.1 7.6 9.1 1.6 2.4 2.2 2.1 2.0 2.0 1.7 3.2 2.9 2.7 2.6 2.6 1.8 3.8 3.4 3.2 3.1 3.1 1.9 4.2 3.8 3.6 3.5 3.4 2 4.5 4.1 3.9 3.8 3.7 • Comparing with Table 16 from Manning equation, most of the depths required for 18.6 m3/s at m = 1.6 are higher than allowed by tractive force method (bed criterion) • However, the combination of m = 1.6 and b = 30 falls within acceptable limits Final Tractive Force Design • The allowable depths according to the bed criterion are all less than the allowable depths (for the same m values) from the side slope criterion • Therefore, use the bed criterion as the basis for the design • The permissible values for m = 2.0 are all much higher than those from the manning equation • For m = 1.6, only the b = 0.3 m bed width is within limits (less than that required by the manning equation for 18.6 m3/s) • Make a judgment decision based on economics, convenience of construction, area occupied by the channel (channel width), saftey considerations, and other factors • Reject the 9.1 m bed width; it will be wider than necessary • Due to lack of other information, recommend the 20-ft bed width and 1.7 side slope option • At this point it would not be useful to consider other intermidiate values of b and m 63 Irrigation System Layout and Canal Design • For b = 6 m and m = 1.7, the depth will be about 2.4 m (allowable is 2.7 m from Table 15 and the mean flow velocity at 18.6 m3/s is: V = 0.8 m/s 5.4 Design of lined canals In lined canals the cross section of the canals is covered (lined) with some kind of harder material than earth (soil) to provide resistance against erosion and avoid seepage losses. Materials which can be used for lining include cement concrete, plastered stone masonry, precast concrete slabs, Brick with sandwiched mortar and soil cement. Design of lined canals is usually done based on the permissible velocity approach. A minimum permissible velocity is that which will not start sedimentation in the canal and is determined by the sediment transport capacity of the flow. A maximum permissible velocity for lined canals is usually higher for lined canals than earthen canals. This velocity is very uncertain and variable and can be estimated only with experience and judgment (Chow, 1983). Maximum permissible velocities are given depending on the type of bed material. For the design of lined canals, uniform flow equations for open channel flow can be used. This can be the Chezy equation or Manning’s formula: Q =A*V Continuity equation Q = A *C√𝑅𝑆 Chezy equation; Manning’s Formula Q = A*1/n*R2/3S1/2 Where: Q A C R S n design discharge, m3/s the x-sectional area of flow Chezy constant hydraulic radius, m longitudinal slope of the canal Manning’s coefficient The Manning’s n or the Chezy C is functions of the kind of lining and the condition (roughness) of the surface. The following are recommended values of the Manning’s n for different linings. Table 18 Manning coeffecent S.N 1 ‘n’ Value 0.013 to 0.018 Type of material Concrete lining 64 Irrigation System Layout and Canal Design 2 3 4 Masonry lining with random stone Brick lining Asphalt lining 0.017 to 0.020 0.014 to 0.017 0.013 to 0.016 5.4.1 Design procedure for lined canal For a known permissible velocity V, Manning’s n, and longitudinal slope S of the canal, determine the hydraulic radius from Manning’s or Chezy equation; Form two equations with two unknowns, determine bottom width b and depth of flow y, as follows: (One from the relationship between hydraulic radius R, area of flow A and wetter perimeter P and another from continuity equation). Trapezoidal canal: 𝐴 = 𝑏𝑦 ∗ 𝑚𝑦 2 ………………(i) 𝑃 = 𝑏 ∗ 2𝑦 ∗ √𝑚2 ………………(ii) Rectangular canal A =by P = b+2y Then, R = A/p Q = A*V = by *V ………… (iii) Where y is flow depth, b is bottom width, m is side slope, A is the area of flow and P is the wetter perimeter. In the equations above, there are two unknowns b and y, which can be solved simultaneously to determine the flow depth y and bottom width b. The concept of best hydraulic section is also usually used for design of small lined canals. In ordinary lined canals, the steepest satisfactory side slope from construction point of view is z = 1.25 or z = 1.5 (z = cotangent of side slope). The best hydraulic section is the one with minimum wetted perimeter for a given discharge. The best hydraulic section for a trapezoidal canal is one when R= y/2. The bed width/depth ratio, b/y, is usually unity for small lined trapezoidal canals, and somewhat larger for larger sections and b/d = 2 for rectangular canals. Table 19 Values for Z and b/d for lined trapezoidal canals. For rectangular b/d = 2 Q (m3/s) z b/d 0.3 1 or 1.25 1 0.3-2.0 1 or 1.25 65 2.0-30.0 1.25 or 1.5 0.03*Q+1.0 > 30.0 1.5 – 2.0 Irrigation System Layout and Canal Design 5.4.2 Best Hydraulic Cross-Section The best hydraulic (the most efficient) cross-section for a given Q, n, and S0 is the one with a minimum excavation and minimum lining cross-section. A = Amin and P = Pmin. The minimum cross-sectional area and the minimum lining area will reduce construction expenses and therefore that cross-section is economically the most efficient one. V = Q/A Q/Amin = Vmax V = 1/n So1/2 R 2/3 = C*R 2/3 V = Vmax R = Rmax R =A/P R = Rmax P= Pmin The best hydraulic cross-section for a given A, n, and S0 is the cross-section that conveys maximum discharge. 1 2 12 𝑄 = 𝐴 𝑅 3 𝑆𝑜 𝑛 Q = C’ x R2/3 C’ = constant Q = Qmax R = Rmax R= A/P R= Rmax P = Pmin The cross-section with the minimum wetted perimeter is the best hydraulic cross section within the cross-sections with the same area since lining and maintenance expenses will reduce substantially. Trapezoidal Cross-Sections 66 Irrigation System Layout and Canal Design As can be seen from equation, wetted perimeter is a function of side slope m and water depth y of the cross-section. For a given side slope m, what will be the water depth y for best hydraulic trapezoidal crosssection? 67 Irrigation System Layout and Canal Design The hydraulic radius R, channel bottom width B, and free surface width L may be found as, Example: Design the most efficient cross-section of a lined trapezoidal canal to carry a discharge of 15 m3/s when the maximum permissible velocity is 2 m/s. Assume the side slope = 1:1. Also, determine the bed slope for the canal if the \chezy coefficient, C=60 Given Q = m3 / s V = 2 m/s SS = 1:1 C = 60 68 Irrigation System Layout and Canal Design Substitute the value of A = 7.5 m2 and B = 1.828D into the above equation 7.5 = D(0.828D) + D2 = 0.828 D2 + D2 Solution Step 1: Determine the cross-section area, A A = Q/V = 15/2 = 7.5 m2 Ste 2 = Compute the hydruaic radius, R D2 = P = B + 2D√1 + 𝑧 2 = 𝐵 + 2𝐷√1 + 12 P = B + 2√2𝐷 = 𝐵 + 2.828𝐷 √𝑆 = For most efficient trapezoidal cross-section R = D/2 𝐷 (𝐵+𝐷) 𝐵+2.828𝐷 DB + 2.828 D2 = 2DB + 2D2 → DB – 2DB + 2.828 D2 – 2D2 = 0 DB + 1.828D2 = 0 → -B + 1.828D = 0 B = 1.828D → A = DB + D2 = 4.1026 Chezy’s formula V = C√𝑅𝑆 𝐷 (𝐵+𝐷) 𝐵+2.828𝐷 → D/2 = 1.828 D = √4.1028 = 2.03 m = 2.00 m B = 0.828*2 = 1.66 m Determination of bed slope, S A = D(B +zD) = D(B + 1D) = D(B + D) →R= 7.5 𝑉 𝐶 √𝑅 22 S= →𝑆= 602 ∗1 = 1 900 𝑉2 𝐶2𝑅 For trapezoidal section the following holds true. 69 Irrigation System Layout and Canal Design 5.4.3 Practical Lined Canal Sections Lined Canal Design USBR Standard Indian Standard Use trapizoidal with sharp crners Q > 55 m3/s YES NO Use Trapizidal canal with rounded corners Use Triangular Sectin with rounded corners Assume a suitable velocity and use manning equatin Find the value f B r B/D ratio from the table and use manning Assume suitable Velcity and use mannning equatin The most efficient cross-sections obtained in the preceding derivations need angles in practice. For instance, the type of soil through which the channel is carried may not permit the adoption of a 1:1 side slope for a triangular channel. Secondly, sharp corners in cross-section are virtually ones of stagnation and may lead to the deposition of silt. As such it is desirable to have rounded corners. The Indian practice has been to adopt a triangular section (of the permissible side slope) with a rounded bottom (see Fig below) for discharges less than 55 m3/s. (This limit on the discharge varies from state to state, but a value of 55 m3/s is recommended here as a triangular section for such a discharge is unlikely to result in very high velocities.) 70 Irrigation System Layout and Canal Design Figure 21 Lined channel section for Q < 55 m3/s Figure 22 Lined channel for Q > 55 m3/s To avoid damage to the lining, the maximum velocity in lined channels is restricted to 2.0 m/s. Thus, the design is based on the concept of a limiting velocity. Table 20 Suitable side slopes for channels excavated through different types of material Material Side slope (H:V) Rock Muck and peat soil Stiff clay or earth with concrete lining Nearly vertical 0.25 : 1 0.5 : 1 to 1 : 1 Earth with stone lining 1:1 Firm clay Loose, sandy aoil 1.5 : 1 2:1 From triangular section, fig 19 Area, 1 A =2 ( ℎ2 cot 𝜃) + 2 = ℎ2 (𝜃 + cot 𝜃)𝜃 1 2 ℎ2 (2𝜃) Wetted Perimetre, p = 2hcot 𝜃 + ℎ(2𝜃) 71 Irrigation System Layout and Canal Design Hydraulic Radius R =A/P = ℎ2 (𝜃+cot 𝜃) 2ℎ(𝜃+cot 𝜃) = Similarly, for trapezoidal section, fig 2 1 ℎ 2 1 A= Bh + 2( ℎ2 cot 𝜃) + 2( ℎ2 𝜃) 2 2 = Bh + ℎ2 (𝜃 + cot 𝜃) P = B + 2h(𝜃 + cot 𝜃) R= 𝐴 𝑃 In all these expressions for A, P, and R, the value of θ is in radians. For designing a lined channel, one needs to solve these equations alongwith the Manning’s equation. For given Q, n, S, and A, and R expressed in terms of h for known Q, the Manning’s equation will yield, for triangular section, an explicit relation for h as shown below : ℎ 2/3 1 Q = [ℎ2 (𝜃 + cot 𝜃)] [ ] 𝑛 H=[ 𝑛𝑄22/3 √𝑆(𝜃+cot 𝜃) ] 2 3/8 However, in case of trapezoidal section, Fig. 8.2, the design calculations would start with an assumed value of velocity (less than the maximum permissible velocity of 2.0 m/s) and the expression for h will be in the form of a quadratic expression as can be seen from the following : From the Manning’s equation R =( 𝑄 𝑈𝑛 3/2 ( ) [𝐵 + 2ℎ(𝜃 + cot 𝜃)] √𝑆 √𝑆 B= ( ) 𝑈𝑛 𝑈 𝑈𝑛 3/2 ) √𝑆 3/2 – 2h (𝜃 + cot 𝜃) On substituting this value f B in the expression for area of flow A, ne gets, 𝑄 √𝑆 h ( ) 𝑈 𝑈𝑛 3/2 − 2ℎ2 (𝜃 + cot 𝜃) + ℎ2 (𝜃 + cot 𝜃) = 𝐴 = ℎ2 (𝜃 + cot 𝜃) − ℎ 3 2 𝑄 √𝑆 𝑄 ( ) + =0 𝑈 𝑈𝑛 𝑈 72 𝑄 𝑈 Irrigation System Layout and Canal Design h= 3/2 𝑄 𝑄 √𝑆 3/2 √ 𝑄2 √𝑆 ( ) + ( ) −4(𝜃+cot 𝜃) 𝑈 𝑈 𝑈𝑛 𝑈2 𝑈𝑛 Therefore, in rder thave a feasible slution, 𝑄2 √𝑆 ( ) 𝑈 2 𝑈𝑛 𝑆 3/2 𝑈4 U≤ 2(𝜃+cot 𝜃) 3/2 ≥ ≥ 4(𝜃 + cot 𝜃) 4𝑛3 (𝜃+ cot 𝜃) 𝑄 𝑈 𝑄 1/4 𝑄𝑆 3/2 [ 3 (𝜃+cot ] 𝜃) 4𝑛 This means that for designing a trapezoidal section for a lined channel, the velocity will have to be suitably chosen so as not to violate the above criterion in order to have a feasible solution (see Example 8.2 for illustration). A trapezoidal section with rounded corners, as shown below, is used at higher discharges. The side slope is maintained to be the permissible slope of the soil. Brick and concrete tiled linings are commonly used in India and the mean velocity is restricted to 2.0 m/s in these cases to avoid danger to the material forming the lining. Thus the design is based on a limiting velocity rather than on any relation between B, h and z. The procedure of design is illustrated in example 4.4. The U.S.B.R practice is to specify a width-discharge relationship for trapezoidal channels, thereby placing no direct limit on the velocity. Further, no rounding of the bottom is specified by them. The bottom width recommended by U.S.B.R are as follows: Table 21 Recommended Bottom Widths for Lined Trapezoidal Canals Q, m3/s B, m 1.0 10.0 20.0 100.0 2.4 2.9 8.0 The design involves the use of the Manning’s equation and Table 4.1 for the determination of h and B for given values of Q, n and S. 73 Irrigation System Layout and Canal Design Example 4.3 Design a lined canal to carry 30 m3/s on a slope of 1 in 1600. The side slope is to be maintained at 1.25H : 1V and the lining is expected to give a value of n equal to 0.014. Solution Since Q is less than 55 m3/s, a triangular section with a rounded bottom will be used. Refer to fig 1 1 𝐴 = 2 ( ℎ2 cot 𝜃) + ℎ2 2𝜃 2 2 = ℎ2 (𝜃 + cot 𝜃 ) P = 2hcot 𝜃 + 2ℎ𝜃 = 2h (𝜃 + cot 𝜃) R = A/P = h/2 Cot𝜃 = z = 1.25 𝜃 = 38.60 = 0.644 𝑟𝑎𝑑𝑖𝑎𝑛 A = ( 1.25 + 0.644) h2 = 1.894 h2 From mannings equation Q = A/p R2/3 S1/2 h 8/3 = 14.1 h = 2.7 m Example 8.2 Design a lined channel to carry a discharge of 300 m3/s through an alluvium whose angle of repose is 31°. The bed slope of the channel is 7.75 × 10–5 and Manning’s n for the lining material is 0.016. Solution: Since Q > 55 m3/s, trapezoidal section with rounded corners, Fig. 8.2, is to be designed. Here, Side slope 𝜃 = 310 = 0.541 radian Cot 𝜃 = 1.664 𝜃 + cot 𝜃 = 2.205 74 Irrigation System Layout and Canal Design Side slope 𝜃 = 310 = 0.541 radians Cot 𝜃 = 1.664 𝜃 + cot 𝜃 = 2.205 A = Bh + 2.205 h2 P = B + 4.41 h Adopting U = 2 m/s A = 300/2 = 150 m2 Bh + 2.205h2 = 150 ∴ 𝐵ℎ + 2.205 ℎ2 = 150 𝑈 2𝑥0.016 R = ( 𝑛 )3/2 = ( And √𝑆 3/2 ) = 6.93 m −5 √7.75 𝑋 10 ∴ 6.93 (𝐵 + 4.41ℎ) = 150 B = 21.645 – 4.41h ∴ 21.645ℎ − 4.41ℎ2 + 2.205ℎ2 = 150 Or 2.205h2 – 21.645h + 150 = 0 ∴ h= 21645 ± √(21645)2 +4𝑥2.205𝑥150 4.41 Obviously, the roots of h are imaginary. Using the criterion, Eq. (8.4), one gets, U≤ [ 𝑄𝑆 3/2 1/4 ] 4𝑛3 (𝜃+𝑐𝑜𝑡𝜃) 1/4 300𝑋(7.75𝑋10−5 )3/2 ] ≤ [ 4(0.016)3 (2.205) ≤ 1.543 𝑚/𝑠 ∴ Adopt U = 1.5 m/s ∴ A = 200 m2 R=( 1.5 𝑋 0.016 √7.75 𝑋 10 75 3/2 ) = 4.50 𝑚 −5 Irrigation System Layout and Canal Design 4.5x(B + 4.41h) = 200 B = 44.44 – 4.41h = 14.52 m and h = 6.784 m Other value of h (= 13.37 m ) gives negative value of B which is meaningless B = 14.52 m and h = 6.784 m. 5.6 Canals on non-alluvial soils Here non-alluvial soils are supposed to be stable for the purpose of design of an irrigation canal. The dimensions of a canal can be worked out on the basis of the well known hydraulic formula. The bed slope of the canal may be kept anything. Only consideration is that the velocity of flow should be quite close to the critical velocity for the available soil. Generally the side slope of value 1’h: 1 (Horizontal: Vertical) in filling and 1: 1 in cutting is given. Knowing the ratio of bed width B and depth D the dimensions of the canal can be determined uniquely. The hydraulic formulae commonly used are the following: Q = A.V where; Q A V is design discharge in m3/sec is cross-sectional area of the canal in m2 and is mean velocity of flow in ml/sec (2) Chezy's formula: V= C√𝑅𝑆 where; R is hydraulic mean depth in m S is bed slope of the canal and C is Chezy’s constant The value of Chezy's 'C' can be calculated from the two formulae given below: (a) Kutter's formula: 0.000155 1 𝑠 𝐶 = 23 + + 0.000155 𝑁 1 + (23 + ) 𝑁/√𝑅 𝑠 Where N is coefficient of rugosity. (b) Bazin's formula: 76 Irrigation System Layout and Canal Design 𝐶= 87 1+𝐾/√𝑅 Where K is roughness constant. The value of K depends on the character of the canal bed and side material Table 22 Values of k for various material Soil Type Earth Wood Dirty Stone Smooth Stone K 1.30 to 1.75 0.16 0.46 0.06 (3) Manning's formula: 𝑉= 1 𝑁 𝑅2/3 𝑆1/2 Once V is fixed using Chezy's or Manning's formula, sectional area can be determined from the fundamental equation Q = A. V. 5.6 Canals on alluvial soils The principle of design of a canal on alluvial soil is totally different from that of a canal on nonalluvial soil. Canals on alluvial soil carry appreciable silt and sand load. Silt concentration in the canal water affects the velocity of flow considerably. Hence the Manning's formula cannot be used here to determine the velocity of flow. When the canal water has excess silt load silting occurs in the canal. On the contrary when the water is silt free it picks up the silt from the canal bed and sides. It results in erosion of the canal section. Manning's equation or Chezy's equation do not consider this aspect. Taking the problem of silt transportation into account it was necessary to evolve some basis for the design of a stable section with critical velocity. There are two important and most commonly used theories. 5.6.1 Kennedy's theory R.G Kennedy, an executive engineer of Punjab P.W.D, carried out extensive investigations on some of the canal reaches in the upper Bali Doab canal system. He selected some straight reaches of the canal section, which had not posed any silting and scouring problems during the previous 30 years or so. 77 Irrigation System Layout and Canal Design From the observations, he concluded that the silt supporting power in a channel cross-section was mainly dependent upon the generation of eddies, rising to the surface. These eddies are generated due to the friction of the flowing water with the channel surface. The vertical component of these eddies try to move the sediment up, while the weight of the sediment tries to bring it down, thus keeping the sediment in suspension. So if the velocity is sufficient to generate these eddies, so as to keep the sediment just in suspension, silitig will be avoided. Based upon this concept, he defined the critical velocity (V 0 ) in a channel as a mean velocity (across the section) which will just keep the channel free from silting or scouring, and related it to the depth of flow by the equation 𝑉𝑜 = 𝐶1 ∗ 𝑦 𝐶2 Where C1 and C2 are constants depending upon silt charge. C1 and C2 were found to be 0.55 and 0.64 respectively. Kennedy plotted various graphs between Vo and depth of flow and finally gave a formula to calculate Va. The formula is: Vo= CDn where Vo is critical velocity in m/sec D is full supply depth in m C is a constant. n is some index. It also depends on the type of silt. It depends on the character of silt. Coarser the material greater is the value of ‘C’. Table 23 Values of C for various materials Type of load Alluvium in sand Light sandy silt Coarse sandy silt Sandy loam Coarse silt C 0.63 0.82 0.90 0.99 1.07 Type of silt n fine silt sandy silt sandy silt 0.53 0.64 0.69 78 Irrigation System Layout and Canal Design After recognizing the fact that silt grade also plays an important part, he modified the formula. The new form is V = 0.546 m.D0.64 where m is Table 24 Recommended values of m S.N Type of silt Value of m 1 Silt of River Indus (Pakistan) 0.7 2 Light sandy silt in North Indian Rivers 1.0 3 Light sandy, loamy silt 1.1 4 Sandy, loamy silt 1.2 5 Debris of hard soil 1.3 Design procedure using Kennedys theorey When an irrigation canal is to be designed by the Kennedy theory it is essential to know, F.S.D.(Q), coefficient of rugosity (N), C. V.R. (m), and longitudinal slope of channel (S) beforehand. By making use of the following three equations a canal section can be designed by trials: i. V = 0.546 m.DO.64 ii. Q =A. V and iii. 𝑉 = 𝐶√𝑅𝑆 The procedure of design may be outlined in the following steps: 1. Assume a reasonable full supply depth D. 2. Using equation (i) find out value of V. 3. With this value of V, using equation (ii) find out A. 4. Assuming side slopes and from the knowledge of A and D find out bed width B. 5. Calculate R, which is ratio of area and wetted perimeter. 6. Using equation (iii) find out the value of actual velocity V. 79 Irrigation System Layout and Canal Design When the assumed value of D is correct, the value of V in step (6) will be same as V calculated in step (2), if not assume another suitable value of D and repeat the procedure till both values of velocity come out to be the same. It may be recognized here that for same values of Q, N and m but with different values of S various channel sections may be designed. It is needless to mention that all of them would not be equally satisfactory. To give some guidance for fixing particular slope (S), Woods has given a table on the basis of experience in which he gives suitable B/D ratios for various values of Q, S, N, m. Another advantage of taking a suitable B/D ratio is that it reduces the labour of making trails. Thus when Q, N, m and B/D ratio is known using equation (i) and (ii) two unknowns V and D can be accurately and uniquely determined. Then using equation (iii) required value of S can be calculated. Example :→ Design an irrigation channel to carry a discharge of 45 m3. Assume N = 0.0225 and m = 1. The channel has bed slope of 0.16 metre per kilometer. Z = 0.5:1 Solution : 1. Assume a trial depth D equal to 1.8 m 2. V = 0.55 mD 0.64 = 0.55 * 1 * 1.8 0.64 = 0.8 m/s 3. A = Q/V = 45 / 0.8 = 56.2 m2 4. A = BD + D2 /2 = 56.2 = B(1.8) + (1.8)2/2 From which B = 30.3 m p = B + D√5 = 30.3 + 1.8 √5 = 34.32 5. Perimeter R = A/P = 56.2/34.32 = 1.64 m 6. V = C√𝑅𝑆 S = 0.16 /1000 𝐶= V = 49 √ 1 0.00155 + ∗1000 0.0225 0.16 0.0225 0.00155 ∗1000)∗ 1+ (23+ 0.16 √1.64 23+ 1.64∗0.16 1000 = 49 = 0.793 80 Irrigation System Layout and Canal Design 7. Ratio of velocity found in steps (6) and (2) = 0.793 0.8 = 0.991 = 1 , hence the assumed D is satisfactory 5.6.2 Lacey's theory It is already mentioned in the definition that a channel is in regime when: (i) It flows in "incoherent unlimited alluvium" of same character as that transported; (ii) Silt grade and silt charge is constant; and (iii) Discharge is constant. These conditions are very rarely achieved and are very difficult to maintain in practice. Hence according to Lacey's conception regime conditions may be subdivided as initial and final. Lacey also states that the silt is kept in suspension solely by the force of eddies, But Lacey adds that eddies are not only generated on the bed but at all points on wetted perimeter. Obviously the vertical components of the forces due to eddies are responsible for keeping the silt in suspension. Unlike Kennedy, Lacey takes hydraulic mean radius (R) as a variable rather than depth (D). For wide channel there is hardly any difference between R and D, when the channel section is semi-circular there is no base width and sides actually and hence assumption of R as a variable seems to be more logical. From this point of view velocity is no more dependent on D but it depends on R. Consequently amount of silt transported is not dependent on the base width of the channel only. On the basis of the above mentioned arguments Lacey plotted a graph between mean velocity (V) and hydraulic mean radius (R) and gave a relationship: V = K.R1/2 Where K is a constant. It can be seen here that power of R is a fixed number and needs no alteration to suit different conditions. Lacey recognized the importance of silt grade in the problem and introduced a function 'f' known as silt factor. He adjusted the value of 'f' such that it also comes under square root sign. Thus it gives scalar conception. Equation (i) is thus modified as 𝑉 = 𝐾. √𝑓. 𝑅 (i) 81 Irrigation System Layout and Canal Design Kennedy's general equation is V = c.m. Dn ... (iii) Comparing equations (ii) and (iii) f=m2 Lacey's standard silt has a silt factor unity. He further states that standard silt is sandy silt in a regime channel with hydraulic mean radius equal to one meter. Lacey gave various equations for designing an irrigation canal. 𝑉 O= 𝑂. 639√𝑓. 𝑅 where Vo is regime velocity in m/sec S= [ 𝑓5/3 3340𝑄1/6 ] 𝐴. 𝑓 2 𝐴 = 𝜋𝑟 2 2= 141.24𝑉 O5 Based on these he derived following important equations V = 10.8 R2/3 S1/2 Where V is mean velocity R= 2.46 V2 𝑓 Pw = 4.75 Q1/2 It is known as lacey’s regime perimeter formula N = 0.0225 f1/2 Where N is coefficient of rugosity 𝑆= [ 𝑓5/3 3340∗𝑄1/6 ] f = 1.76 √mr Where mr is average particle size in mm V = 0.4382(Q f2 )1/6 A = 2.28 Q5/6/f 1/3 R1 = 1.35 (q2/f)1/3 Where R is depth of scour in m and q is discharge per unit length. 82 Irrigation System Layout and Canal Design Design of irrigation canal making use of the lacey theory Full supply discharge for any canal is always fixed beforehand. The value of 'f' for a particular site may be calculated using equation (9) or if C.V.R. is given then f=m2 Thus when Q and f are known design can be done in the following steps: 𝑉=[ 1. Find out V using equation, 𝑄∗𝑓2 1/6 ] 140 5 𝑉2 2. Calculate value of R using equation , R = 2 ∗ 𝑓 3. Calculate wetted perimeter P using Lacey's regime perimeter equation, P = 4.75 √𝑄 4. Calculate cross- sectional area A, from equation Q = A V. 5. Assuming side slopes, calculate the full supply depth from A, P and R. 6. Calculate longitudinal slope of the canal using equation above. 𝑆= [ Example : 𝑓5/3 3340∗𝑄1/6 ] A channel section has to be designed for the following data : Discharge Q = 30 m3 , silt factor f = 1.00 Side slope = ½ :1 Find also the longitudinal slope: Solution 1. The value of f is given as 1.00 2. Velocity 𝑉=( 30∗1 140 )1/6 = 0.773 m/s 3. A = 30/0.773 = 38.8 m2 4. 𝑃 = 4.75√30 = 26 m 5. D = 𝑃− √𝑃2 −6.994𝐴 3.472 = 1.67 m , B = P – 2.36 D = 26 – 2.236*1.67 = 22.6 m 5 1 6. Hydrualic mean radius R = ∗ 2 Also R= 1 𝐵𝐷+ .𝐷 2 /2 𝐵+𝐷√5 0.7732 = 9 1.49 m 83 Irrigation System Layout and Canal Design 7. Slope S = 1/3340* 30 1/6 = 1 5900 Hence the channel has a bed width B = 22.26 m and a depth of 1.67 m. The longitudinal slope S= 1 5900 Comparison of Kennedy and lacey’s theories and improvements over Laceys theory. 1. The concept of silt transportation is the same in both the cases. Both the theories agree that the silt is carried by the vertical edidies generated by the friction of the flowing water against the channel surface. The difference is that kennedy considered a trapiziodal channel section and, therefore, he neglected the eddies generated from the sides, on the presumption that these eddies has horizontal movement for greater part and, therefore, did not have silt supporting power. For this reason, Kennedy’s critical velocity formula was derived only in terms of the depth of flow (y). On the other hand, Lacey considered that an iririgation channel achieves a cup-shaped section (semiellipse) and that the entire wetted perimeter (p) of the channel contributes to the generation of silt supporting eddies. He, therefore, used hydraulic mean radius (R = A/P) as a variable in his regime velocity formula instead of depth (y). 2. Kennedy stated all the channels to be in state of regime provided they did not silt or scour. But, Lacey differentiated between the two regime conditions, i.e initial regime and final regime 3. According to Lacey, the grain size of the material forming the channel is an important factor and should need much more rational attention than what was given to it by Kennedy. Kennedy has simply stated that critical velocity ratio (v/Vo = m) varies according to silt conditions ( i.e silt grade and silt charge). Lacey, however, connected the grain size (d) with his silt factor (f) by the equation f = 1.76 √𝑑. The silt factor occurs in all these Lacey’s equations, which are used to determine channel dimensions. 4. Kennedy has used Kutters formula for determining actual channel velocity. The value of Kutters rugosity coefficient (n) is again a guess work. Lacey, on the other hand, after analyzing huge data on regime channels, has produced a general regime flow equation, stating that V = 10.8 𝑅2/3 𝑆1/3 5. Kennedy has not given any importance to bed width and depth ratio. Lacey has connected wetted perimeter (P) as well as area (A) of the channel with discharge, thus establishing a fixed relationship between bed width and depth. 84 Irrigation System Layout and Canal Design 6. Kennedy did not fix regime slopes for his channels, although his diagrams indicated that steeper slopes are required for smaller channels and flatter sopes are required for larger channels. Lacey, on the other hand, has fixed regime slope, connecting it with discharge by the formula S= 𝑓5/3 3340 𝑄1/6 85 Irrigation System Layout and Canal Design REFERENCES Davis, C.V and K.E. Sorensen, 1969. Handbook of applied hydraulics. McGraw-Hill Book Company, New York, N.Y. Labye, Y., M.A, Olsen, A. Galand, and N.Tsiourtis. 1988. Design and optimization of irrigation distribution networks. FAO Irrigation and drainage Paper 44, Rome, Italy. 247 pp Carte, A.C. 1953. Critical tracctive force on channel side slopes. Hydrualic laboratory Report No HYD-366. U.S. Bureau of Reclamation, Denver, CO. Garg, Santosh Kumera, 1976. Irrigation Engineering and Hydrualic Structures. Kahanna Publishers, Delhi. Lane, E.W, 1950. Critical forces on channel side slopes. Hydrualic Laboratory Report No. HYD295. U.S. Bureau of Reclamation Denver, Co. Lane. E.W. 1952, Progress No. HYD-352. U.S. Bureau of Reclamation, Denver, CO. Lane. E.W. 1952, Progress No. HYD-352. U.S. Bureau of Reclamation, Denver, CO. Smerdon, E.T, and R.P, Beasley. 1961. Critical tractive forces in cohesive soils. J. of Agric. Enging. American Soc of Agric Engineers. Pp. 26-29. USBR. 1974. Design of small canal structures. U.S Government Printing Office, Washington, D.C 435 pp. 86