Irrigation Engineer Lesson Canal Systems for Major and Medium Irrigation Instructional objectives On completion of this lesson, the student shall learn about the following: The canal system of irrigation under major and minor irrigation schemes Components of a canal irrigation system Layout of a canal irrigation system Lining methods in canal irrigation Negative impact of canal irrigation S":".°°!\." 3.6.0 introduction As noted in earlier chapters, major irrigation schemes are those which have culturable command area (C.C.A) more than 10,000 ha. Irrigation schemes having C.C.A between 2000 and 10000 ha are called medium irrigation schemes. All these schemes are flow types of irrigation systems and water from a river is diverted to flow through a canal by constructing diversion structure across the river. The main canal further divides into branches and distributaries water from the distributaries is let off through gated outlets into the fields with the help of water courses. It is important to note that the canal method of water conveyance and distribution is a dynamic system with variation in demand occurring according to the crops planted in the command area. Also, the source of water, usually a river, may not be able to supply sufficient amount of water all times. Nevertheless, the canal system has to be planned and designed for the maximum expected demand. The layout of the canal is also important as it should ensure smooth flow by gravity in each channel. Wrong alignment may lead to possible stagnation of water at some places or too fast moving that may damage the canal itself. This lesson discusses about various aspects of irrigation canal system layout and design. Other canals like those for navigation and power generation are discussed separately. 3.6.1 Canal Components At the diversion structure, a headwork regulates the flow into a canal. This canal, which takes its supplies directly from the river, is called the main canal and usually direct irrigation from the waters of this canal is not carried out. This acts as a feeder channel to the branch canals, or branches. Branch canals generally carry a discharge higher than5 m3/sandactsas feederchannelfor majordistributaries which,in turn carry0.25 to 5 m3/sof discharge.The majordistributarieseitherfeed the water coursesor the minordistributaries, whichgenerallycarrydischargelessthan0.25m3/s. A typical layout of an irrigation canal system may be seen in Figure 1. Version 2 CE IIT, Kharagpur . w,_.STF¬UC1TUi%EFvf3l.3t W cmm. HEABWGR mime wares: t:a.lvEiR.s.iol~a ERANCH swung: 3: vlS'TRlBU?TiN95 REr3Ul_.ATll~tG srnuctum W3UH;:E 1 : T§p§}lE§E1ll«E§}!4lZ}Ut=Clf3§'l irrigatiioracanal system It may be observed that whenever a channel offtakes from the corresponding main branch there has to be gated structures just downstream of the bifurcation, for controlling the water level upstream of the point as well as to control the amount of water going into the off take and the main branches. 3.6.2 Irrigation canal layout Usually it is desirable that a canal off taking from a river should be able to irrigate as much an area as possible. The general layout concept can be explained when studied in respect of the off take point of the canal and the surrounding contours, as shown in Figure 2. Version 2 CE IIT, Kharagpur lzm W_,,~r"f ,!_,,. 3;; ix PRQPGSE3 .-V",.g,ee- I3 3 tme J 2 ff f , / *--W. ,»» 9 "m»'°_"""'=-v..,_\__m_vM_M__.»~" 1 .-»"/ff 125m V-t~*" lg if oz mm .7 -x"-%_ &""~*«w_, J}! gr; FlG|..lRE 2 : lt_m:atEcm of an a takingpointfor an lirrtgatt-on yetem rehowlnga river and %ay%pothetic;al eleovatizczrt conltaurs Now, if a canal were to be laid from the off taking point, then it should lie between the two extreme lines, as explained below. Version 2 CE IIT, Kharagpur _,»=* M;-fa: rlm ~--*~"" 3 m'jK 1, =-~».._, ,,,.~% t Falwnnisut awn fl 5 g,/9 NQTTE : l.ALlGNEvtENTS. inRffwt LEJ HAVE ZERQsome 2. Ai_ft3N mats R ~ R AME Ll_*Have Mmimlum SLDPE.£tLGt*»lG RWER DiR.ECl'tUl~l L 3. image. (WATERSHED Bl«.!lBE~31i_ilt~l§ES ARE mn.H.l:EcA.a ~-«--l-»--~ F§Eil_lRE. .3 Possible alignments based on which aspractical alignment of canal has to. be chosen, Consider the possible alignments of canals as shown in Figure 3. A shown in the figure, the right bank canal (generally termed RBC) and off taking from a point R on the river bank may be aligned somewhere in the region bounded by R-R, the contour line at the elevation of R or the right riverbank R-R. It is not possible to align the canal along R-R, as there would not be any slope, whereas an alignment along R-R would mean zero command area for the canal. Hence, a suitable slope of the canal that is neither too flat nor too steep (as discussed later in this lesson) would be the most appropriate. Based on the above logic, possible alignments of the right and left bank canals (RBC and LBC) have been shown in Figure 4. Version 2 CE IIT, Kharagpur WE V A, _, .. H _ 125m mm 1;_ Auewmfgamrr mLLow.s H--r ,. r WDQE Ll§'~lE* MGHT BANK }l:,lI«F£FiI.'L {nan} ,AL§Nf.fE_N FULLUWS r__REGE,LlME IL Fianna é ;:Practical: alignments of canals taking off from right and Ee banks of the printer at lrmaon L--R In order to demonstrate the effect of the adjacent valley slopes on the canal layout, the right and left bank contours have been chosen in such a way that the slope of the right bank valley is flatter than that of the left bank. Hence, as may be observed from Figure 5, for the same canal slope on both the banks, the right bank canal covers a larger command area (the area between the canal and the river). Rw:ER_A_,, b g 150m ENTER,- BASJIVE Rlf.-EELENEQR W?-\TEF~'tSl~£Ei3 jI2ilV'lE'fE CUMMRMD AREA DEF Frame 5 : Ciommancl area tor a typical canal system The alignment of a canal can be done in such a way that it is laid up to a ridge line between two valleys, which would allow a larger command area for the same canal, as shown in the figure, which shows possible contour lines between two rivers that the canal off taking from one river is able to irrigate areas between the river of the adjacent valley, too. There is one more advantage of leading a canal up to the watershed divide line: the number of small streams to be crossed by the canal would be a minimum, though once a canal may be aligned along the watershed divide line, generally, it may be necessary to provide a shorter path it the divide line is tortuous. A few more points may be noted on the layout of a canal may be noted, as mentioned below. c As far as possible, curves should be avoided in the alignment of canals because the curves lead to disturbance of flow and a tendency to silt on the inner bend and scour the toe of the outer (concave) bend. However, if curves have to be provided; they should be as gentle as possible. Further, the permissible minimum radius of curvature for a channel curve is shorter for lined canals than unlined Version 2 CE IIT, Kharagpur ones and is shorter for small cross sections than for large cross sections of canals. According to the Bureau of Indian Standard code IS: 5968-1970 Guide for planning and layout of canal system for irrigation, the radii of curves should usually be 3 to 7 times the water surface width subject to the minimum values as given in the following table. Typeof canal Unlined canals Capacityof canal(m3/s) Minimumradius(m) 80 and above 30 to 80 15 to 30 3 to 15 0.3 to 3 Less than Lined canals 3 280 and above 200 to 280 140 to 200 70 to 140 40 to 70 10 to 40 3 to 10 0.3 to 3 Less than 0.3 o The alignment should be such that the cutting and filling of earth or rock should be balanced, as far as possible. o The alignment should be such that the canal crosses the natural stream at its narrowest point in the vicinity. In order to finalize the layout of canal network for an irrigation project, the alignment of channels should be marked on topo-sheets, until an optimum is reached. This alignment is then transferred to the field by fixing marking posts along the centerline of the canal. Formal guidelines for canal layout may be had from Bureau of Indian Standard IS: 5968-1987 Guide for planning and layout of canal system for irrigation. Version 2 CE IIT, Kharagpur 3.6.3 Lining of Irrigation canals Though irrigationcanals may be constructedin naturalor compactedearth, these suffer from certaindisadvantages,like the following Maximumvelocitylimitedto preventerosion Seepage of water intothe ground Possibilityof vegetationgrowthin banks, leadingto increasedfriction Possibilityof bank failure,either due to erosionor activitiesof burrowinganimals All these reasons lead to adoption of lining of canals, though the cost may be prohibitive.Hence, before suggestinga possible lining for a canal, it is necessary to evaluatethe cost vis-a-visthe savingsdue to reductionin water lossthroughseepage. Apart from avoidingall the disadvantagesof an unlinedcanal, a linedcanal also has the advantageof giving low resistanceand thus reducingthe frictionalloss and maintaining the energy and water surface slopes for the canal as less as possible. This is advantageousas it means that the canal slope may also be smaller, to maintainthe same dischargethan for a canal with higherfrictionloss. A smallercanal slope means a largercommandarea. 3.6.4 Types of canal lining Differenttypes of canal liningsare possible,and the bureau of Indian standardscode IS: 5331-1969 Guide for selectionof type of liningsfor canals may be consultedfor details. In general, the followingtypes of liningsare generallyused. 3.6.4.1 Concrete lining Cement concrete lining made from selected aggregate gives very satisfactoryservice. Despite the fact that they are frequently high in their initial cost, their long life and minimummaintenancemake them economical.Cement concrete liningare best suited for main canals which carry large quantitiesof water at high velocities.However, a firm foundation is essential for avoiding any possibilityof cracking due to foundation settlement. Expansive clay Soils should be avoided and proper moisture and density control of the sub grade soil should be maintained while lining. In areas where the groundwater table is likelyto rise above the invert level of the liningand cause undue uplift pressure, drains are laid below the lining to release the water and relieve the pressure, generally, a thickness of about 5 to 12 cm is generally adopted for larger canals and stable side slopes are consideredto be between 1.5H: 1V to 1.25H: 1V. Reinforcementto the extent of 0.1 to 0.4 percentof the area in longitudinaldirectionand 0.1 to 0.2 percentof the area in the transversedirectionreduceswidth of the shrinkage cracks, thereby reducing seepage. Further details regarding cement concrete linings may be had from Bureau of Indian Standardscode IS: 3873-1987 Code of practicefor laying in-situ cement concrete lining on canals, since there would be Version 2 CE IIT, Kharagpur construction/contraction joints in the lining,it is essentialto plug the joints,for which the followingcode may be referred IS: 13143-1991 Joints in concrete lining of canalssealingcompound. 3.6.4.2 Shotcrete lining Shotcrete, that is, cement mortar in the ratio of 1 cement to 4 sand proportionsis through a pump-pipe-nozzle system on the surface of the channel. Wire mesh reinforcementis generally, though not necessarily,is clamped to the channel surface (as for a rocky excavation)before applyingshot Crete. Equipmentunits used for shot Crete constructionare relativelysmall and easily moved. They are convenientfor lining small sections, for repair of old linings, and for placing linings around curves or structures.Shot Crete liningsare generallylaid in a thicknessof about 3.5cm, but many standard code IS: 9012-1978 recommended practice for shotcreting (Reaffirmed in 1992) may be consultedfor details. 3.6.4.3 rick or burnt clay tile lining precast concrete tile lini lg This type of lining is popular because of certain advantages like non-requirementof skilled mason or rigid quality control. Further, since it is more labour intensive, it generates employmentpotential. Brick tiles can be plastered to increase the carrying capacity of canal with same section and help in increasingthe life span of the lining. Sometimes a layer of tiles is laid over a layer of brick masonry. The top layer is generally laid in 1:3 cement mortal over 15mm thick layer of plaster in 1:3 cement plaster. The size of tiles is generally restrictedto 30mm x 150mm x 53m. This type of liningis stable even if there is settlementof foundation,since the mortarjoint between bricksor tiles providesfor numerouscracksso fine that seepage is insignificant.Further details may be had from the followingBureauof IndianStandardcodes: o IS: 3860-1966 Specificationsfor precastcement concretecanal Iinings. o IS: 3872-1966 Code of practicefor liningof canals with burntclay ti|es. - IS: 10646-1991 Canal Iiningscementconcreteti|es 3.6.4.4 oulder Lining Also called dry stone liningor stone pitching,is used for liningthe earthen canal cross section,by proper placementand packingof stones, either after layinga filter layer over the soil surface or without any such filter, depending upon the site requirement.To reduce the resistance to flow, a 20 to 25mm thick cement plaster is provided as a finishingsurface. Stones are generallyplaced on leveled sub-grade,and hand packed. This type of lining is of course suitable where stones of required specificationare available in abundance locally. For details of this type of lining, one may refer to the Bureau of Indian standard code IS: 4515-1976 Code of practice for boulder lining of canals. One advantage of this type of lining is allowing free flow of water from the submerged or saturated sub-grade into the canal. Hence, this type of lining does not need any drainage arrangementor pressure relief values, etc. which may be required for concreteor bricklining. 3.6.4.5 Low density polyethylene lining (from IS: 9698-1980/1991) Version 2 CE IIT, Kharagpur 3.6.4.6 Hot bitumen/Bituminous felt lining (from lS:90971979) (Reaffirmed1990) 3.6.4.7 Earth linings The differenttypes of earth liningsthat are used in canals includethe following: i. Stabilized earth linings: Here, the sub-grade is stabilized using either clay for granularsub-gradeor by addingchemicalsthat compactthe soil. ii. Loose earth blankets:Fine grained soil is laid on the sub-gradeand evenly spread. However, this type of liningis prone to erosion,and requiresa flatter side slopes of canal. iii. Compacted earth linings:Here the graded soil containingabout 15 percent clay is spread over the sub-gradeand compacted. iv. Buried bentonite membrames: Bentonite is a special type of clay soil, found naturally,which swell considerablywhen wetted. Buried bentoniteliningsfor canals are constructed by spreading soil-bentonite mixtures over the sub-grade and coveringit with gravel or compactedearth. v. SoiIcement lining:Here, cement and sandy soil are mixed and then compactedat optimummoisturecontentor cement and soil is machine mixedwith water and then laid. The Bureau of Indian Standards code IS: 7113-1973 Code of Practice for soil cement lining for canals (Reaffirmed in 1990) may be controlled for details regardingthis type of lining. 3.6.5 IIleffects of canal irrigation:Water logging Canal is an artificial channel for conveying water through lands that was perhaps naturallydevoid of sustainedwater flow. Hence, water seepingfrom canals down to the soil below may, at times, raise the ground water very close to the ground level. This may result in blockingall the voids in the soil and obstructingthe plant rootsto breathe. It has been observedthat water loggingconditionsadverselyaffectscrop productionas it is reduceddrastically.Apart from seepage water of canals, excessiveand unplanned irrigationalso caused water loggingconditions.This happens because the farmers at the head reaches of canals draw undue share of canal water in the false hope of producinglargeragriculturaloutputs. Apart from ill aerationof plants,other problemscreated by water loggingare as follows: o Normal cultivationoperations, such as filling, ploughing,etc. cannot be easily carried out in wet soils. In extreme cases, the free water may rise above the groundlevel makingagriculturaloperationsimpossible. Version 2 CE IIT, Kharagpur o Certain water loving plants like grasses, weeds, etc. grow profuselyand Iuxuriantlyin water-logged lands, thus affecting and interferingwith the growthof the crops. o Water logging also leads to a conditioncalled salinity, which is caused when the capillaryfringe of the elevated water table rises within the root zone of plants. Since the rootsof the plantscontinuouslydraw water from this zone, there is a steady upward movementof water which causes rise of salts, especially alkali salts, to come up to the ground surface. This situationis termed as salinity. In order to avoid water-loggingconditionto occur for canal irrigationsystem, certain steps may be taken as follows: o Canals and water courses may be lined. Also if possible,the full supply level of canal may be reduced. o Intensityof irrigationmay be reducedand farmers advisedto applywater judiciouslyto their fieldsand not over-irrigate. 0 Provide an efficient drainage system to drain away excess irrigation water. - Introducemore tubewellsfor irrigationwhichshall lowerthe water table o Cropping pattern may be suitably modified such that only low water requiringcrops are plantedinsteadof those requiringheavy irrigation o Natural drainage of the soil may be improvedsuch that less of excess surfacewater percolatesand mostlydrainsoff throughnaturaldrains. Since water loss by seepage from canals is an importantfactor through which they pass, it is essential to scientificallymonitor when they pass and devise a suitable seepage preventionstrategy. This may be done by certain methods,which have been elaborated in the Bureau of Indian Standards code IS: 9452 Code of practice for measurementof seepage lossesfrom canals, with the followingparts: - IS: 9452 Part 1-1980 Ponding method (Reaffirmed1991) - IS: 9452 Part 2-1980 |nf|owoutf|ow method (Reaffirmed1990) o IS: 9452 Part 3-1988 Seepage meter method (Reaffirmed1990) Another method, the analytical method, detailed in IS: 9447-1980 Guidelines for assessmentof seepage lossesfrom canals by analyticalmethod to adopt. 3.6.6 Maintenance of Canals It is often seen that the conditionsunder which a canal system is designed is not maintainedduringthe years of its system is operation,Physicaldamage resultingfrom erosion/depositionof sediments, ground water, soil subsidence,human activity,weed growth, etc. cause much changes from the ideal conditions. Hence, it is not only Version 2 CE IIT, Kharagpur essential to design a canal system well; it should also be maintained well. The Bureau of Indian standards has published the code IS: 4839 Code of practice for maintenance of canals under the following volumes for the purpose. o IS: 4839 Part 1-1992 Unlined canals (Second revision) o IS: 4839 Part 2-1992 Lined canals (Second revision) 3.6.7 Drainage of land for canal Irrigation Commands A proper design of canal irrigation system also consist provision of a suitable drainage system for removal of excess water. Of course, this may not be required all over the command area of the canal, but may be necessary in areas of high water table and in river deltas. The drainage system may also help to drain out storm water as well, and thus to prevent its percolation and to ensure easy disposal. There are two types of drainages that may be provided, which are either of surface-type or of sub-surface types. These are briefly described in the following paragraphs. 3.6.7.1 Surface Drainage These constitute open ditches, field drains, proper land grading and related structures. The open drains which are broad and shallow are called shallow surface drains and carry the runoff to the outlet drains. The outlet drains are termed as the deep surface drains. Land grading, or properly sloping the land towards the field drains, is an important method for effecting surface drainage. The Bureau of Indian Standards code IS: 8835-1975 Guidelines for planning and design of surface drains (Reaffirmed in 1990) may be referred to for further details. 3.6.7.2 Sub-Surface Drainage These are installed to lower the water table and consists of underground pipes which collect water and remove it through a network of such pipes. These pipes are usually made of porous earthenware and circular in section and the diameter varies from 10 to 30 cm. for installation of these drains, trenches are dug in the ground and these pipe sections are butted against each other with open joints which help in allowing ground water to enter into the pipes. The trenches are then backfilled with sand and excavated material. The water drained by the tile drain is discharged into a bigger drain or into a deep surface drain. If the tile drain network is buried quite deep into the ground, it may be necessary to discharge the water of the drains into an underground sump and remove the water from the sump by pumping. Generally an area is under laid with a network of tile drains, it is essential to calculate the spacing of these drains based on the depth through which the water table in the region is to be lowered. Version 2 CE IIT, Kharagpur