RecommendationsLowcostwaterharvestingstructures

Lowcostwaterharvestingstructures

LDPEFilmLinedTank

Toincreasetheirrigationpotentialinhills,itisnecessarytodevelopthewaterresourcesbymeansofsmalltanks(capacityupto100m3).Thecriterionforthedesignoftanksshouldbesuchthat itnotonlyreducestheseepage lossesbut it iscosteffectiveaswell.Thecementtankforinstanceisabletochecktheseepagelossesbutthecostbecomesthelimitingfactorineconomicexploitationofwaterresources.Ontheotherhandunlinedtankspronetoheavyseepage losses (300400 litre per day per m2) and may not be able to supply water forirrigation during stress. Thus, for the effective water storage, the tank should be withminimumseepagelossandcosteffective.

Lowdensitypolyethelene (LDPE) filmhasbeen successfullyused toeffectively control theseepagelossatmuchlowercostthanthecementtanks.Thefilmusedshouldhaveminimumthicknessof200,otherwiseitbecomespronetophysicaldamage.Enhancementofeffectivelifeofplasticfilmandtoavoidthephysicaldamagetheprotectionofthefilmisnecessitatedwhichcanberealizedthroughpitching.

Benefitsofpondliningwithplasticfilms

Reductioninseepagelossestothemaximumextent(95%)

Harvestingandstoringofrainwaterfromearlymonsoons.

Utilizationofharvestedrainwaterforshortduringcropsaswellasduringoffseason.

Lining of ponds and reservoirs with plastic film improve water availability over alongerperiodoftime.

Itishighlyusefulinporoussoilswhereretentioninpondsandwaterharvestingtanksisminimal.

Economicalandeffectivemethodofstoringwater.

Eliminateswaterloggingandpreventsupwardintrusionofsaltsintostoredwater.

Usefulforthepurposeofstorageofdrinkingwater,forpiscicultureandforprovidingsupplementaryirrigation.

Preventssoilerosion.

Techniqueisalsosuitableforliningofeffluentpondsandchannelstoreducesoilandgroundwatercontamination.

It can also be used in the lining of saltpans for improving productivity aswell asqualityofsalt.

Thedetailsofsubsidyareasunder:

Item Maximum permissiblecost*

PatternofAssistance*

(a) Communitytanks/on farmponds/onfarmwaterreservoirswithuse ofplastic/RCClining.

Rs. 15 lakh/unit inplain areas, Rs. 17.25lakh/unitinhillyareas.

100%of cost for 10haof command area,withpondsizeof100mx100mx3moranyothersmaller size on pro rata depending upon thecommand area, owned & managed by acommunity/farmer group. Cost for nonlinedponds/tanks (only in black cotton soils)will be33% less. Assistance under NHM will berestricted to the cost of plastic/RCC lining.However, for non MNREGS beneficiaries,assistance on entire cost including constructionof pond/tank as well as lining can be availedunderNHM.

(a) Waterharvestingsystem forindividuals for storage ofwater in20mx20mx3mponds/dugwells @ Rs.100/cum

Rs. 1.20 lakh/unit inplain areas, Rs. 1.38lakh/unit inhillyareasfor maximumcommandareaat2ha

50%ofcostincludinglining(inplains)and75%ofcost i.e. Rs. 1.03 lakhs per beneficiary in hillyareas. For smaller size of the ponds/dugwells,cost will be admissible on pro rata basis.Maintenancetobeensuredbythebeneficiary.

*Subsidies are liable to be changed from time to time and please consult your local horticultureofficersforlatestpatterns.

Layout,Shape,sizeandstoragecapacity

Shapedependsonthesoiltype,whichdictatesthemaximumpossibleslopethatwillstayinplacewithoutfalling in.normally,theshape iskeptastrapezoidalwithsideslopesof1:1to1:2andeffectivedepth1.5to5m.Thesideslopesshouldhavearatioof1:1and1:2forstableandunstablesoils,respectively.Thepondshouldbeconstructedintheformoffrustumofan

invertedpyramid(Fig.1&2).Thesizeofwaterharvestingstructuresisimportant,butmoreimportant is its sustainability, efficiency and appropriateness. The demand dictates therequiredstoragecapacity.Thecatchmentareaandrainfalldeterminethesupplyandstoragecapacityarenecessaryforachievinghigherwaterproductivity.Thebasicprincipleforsizinganyrainwatersystem isthatvolumeofwaterthatcanbecapturedandstored(thesupply)mustequalorexceedthevolumeofwaterrequired(thedemand).Thewaterstoragecapacityofpond(V,volume)having1:1sideslopescanbecalculatedwithfollowingformula:

V=H/2[(W2F)(L2F)+(L2H2F)(W2H2F)

Where,L=Toplength(m);W=Topwidth(m);H=Depthoftank(m);F=Freeboard(m)

Note:LandHcanbedecideddependinguponsiteconditions.However,Hshouldnotbelessthan1.5mandmore than5m.normally, freeboard (F) for the smallpondsmaybekeptbetween0.1to0.3m.unitsofmeasurementscanbetakeninfeetormeters.

Analternativeandsimplifiedformulaforcomputingthevolume(V)oftrapezoidshapepondswith1:1slope(Fig.2)is:

V=[L1XW1+L2XW2+(L1XW2+L2XW1)/2]H/3

Where,L1=Toplength,W1=Topwidth,L2=Bottomlength,W2=Bottomwidth,H=Effectivedepth

Thecostmayvaryfromsitetositeasperheadload,sizeofpondetc.andsiteconditions.Acontingency,excavationprovisionextrarateconsideredforestimationarecurrentpertainingtomarketrate.Thedetailsofcalculatingpondsof100x100x3mand20x20x3misgivenattheendofthischapter.

Computationof requirementofmaterial:Thematerial for trapezoidalshaped farmpondwithsideslopesof1:1canbecalculatedasunder:

Areaoftheliningsheet =[22(H+F)+1.0+(W2H2F)]x[(22(H+F)+1.0+(L2H2F)]

Areaofbrick/stonepitching =[22(H+F)x(W+L2F2H)]+[(L2H2F)x(W2H2F)]

(Excludingperipheralpath)

Thumbruleforcomputingareaoftheliningsheet:

Lengthofsheet:[2xPathwidth(m)+2xSlanteddepth(m)atcorner+Bottom(floor)length(m)+2(0.5m)forsheetanchoringintheperipheraltrench]

Widthofsheet:[2xPathwidth(m)+2xslanteddepthatcorner(m)+Bottom(floor)width(m)+2(0.5m)forsheetanchoringintheperipheraltrench]

Areaofsheet=Lengthofsheetxwidthofsheet

Approximately4.5m2LDPEsheet(250micron)weighsabout,1.0kg

Weight(kg)ofblackLDPEsheet= Areaofthesheet

4.5

Weight(kg)ofcrosslaminated= Areaofthesheet

Silpaulinsheet(200gsm) 5

ThethicknessofLDPEsheetshouldbe200250microns(8001000gauge)andshouldconformtoBISstandard2508/1977.For largesizefarmpondsofcapacitymorethan1000m3,1000gaugesheetshouldbeused.

Constructionoffarmpond

Theconstructionprocess includes (i)Selectionofsite (iiExcavation (iii)Liningofpondwithsealants.

Selectionofsite:Thesiteshouldbeselectedkeepinginviewthefollowingissues:

Thepondshouldreceivesufficientrunoff.

Thesiteshouldbesuchthathigherstorageexcavationratiocanbeachieved.

The site should not have hard rocks which would require blasting consequentlyincreasingthecostofconstruction.

Thepondshouldbeclosetothecommandareatoensureeaseofirrigationpreferablythroughgravityflow.

Thesiteshouldnotbeveryclosetotheexistingsprings,ifany;otherwiseexcavationmaydisruptthecapillarylinesofthespringresultinginreduceddischarge.

Thepondshouldnotbeconstructedoverthenaturalwatersourceiffoundduringtheexcavationprocess.

Thereshouldnotbeanylargetreenearthesite.

Excavation :Firstofall,theponddimensions(L,W,Dandsideslope) identifyandthenmarkthe top level (outer rectangle/trapezium) as well as the bottom level (innerrectangle/trapezium)lengthandwidthclearlyontheground(Fig.3).Putpegsatthecornersofboththerectanglesandtiearopetodemarcateponddimensions.Startdiggingtheareaofthe innerrectangletilldesireddepth isachieved.Excavatedsoilcanbekeptnearthepondaroundtheperipheryandusedlaterfollowingtheprincipleofcutandfillratio.Give1:1slopeby digging the side walls of outer rectangle for better stability of walls and pitching of

stones/bricks. This is readily ascertainedby joining the top cornersof theouter rectanglewiththeadjacentandnearestbottomcornersofinnerrectanglewithawoodenframe.

Finishingofpondbedandsidewalls:Afterexcavation,thesideslopesofpondaremadesmoothandcompactedbyremovingangularprojections,protrudingstones,pebbles,rootsetc.Before layingthesheet,apply thin layerofslurryofclayandcowdungorplacea thin layerofusedgunnybags/softgrassbetween the sheet and thewallsof thepond to avoiddamageby sharpprotrudingobjects.SprayofAtrazine@0.4g/m2 isgivenonthesidewallsafterapplyingslurryforcontrollingweeds.Ifneeded,simplepipe(5cmdiameter)spillwaymaybeprovidedtoregulatetheoutflow.

Liningofpondwithsealants:Thepondsarelocatedonavarietyofsoiltypeswhichexhibitwide range of seepage characteristics. The pondmust be linedwith a suitable sealant tominimize seepage losses and to improve storage efficiency. Different sealants have beenevaluatedbydifferent institutes inthestate(Table1).BothblackLDPEandSilpaulinsheetswere found equally good. The black LDPE sheet has been proved to bemore appropriateowing to its low costand reliability.The Silpaulin sheet is costlier than theblack LDPE. Inaddition,Tarfeltandothersheetsarealsoavailablewhichcanbeusedforlining.

JoiningofLDPEsheet:Generallyplasticsheetsareavailableupto14mwidth.Ifpossible,joiningofsheetsshouldbeavoidedbyprocuringsheetofrequiredwidth.Ifjoiningrequired,bitumenheatedto1000Cisspreadover30cmwidestripononesheetandthispaintedportionofthesheetisputoverthecounterpartandispressedbyasmoothobject.Itisthenallowedtocool.Sheetcanalsobejoined/repairedwithsyntheticrubberadhesive.

LayingofLDPEsheetandfixingoutletpipe:Theplasticsheetisspreadoverthedugoutareaafterensuring that thereareno sharp stones,pebbles,etc.on the side slopes.Anchor theliningsheetedgesinasmalltrench(0.25x0.25m) dugaroundtheexternalperipheryofthepondtoensurethatsheet liessmoothlyonthesidewallsofthepond andfirmlyheld.Thetrenchcanbeapproximately0.5mawayfromtheexternalperipheryofthepond.Theliningsheetshouldfit inthepondarea looselysothat itdoesnotbreakwhen it isanchored.Thesheetshouldnotbeplacedonaveryhotandwindyday.Forfixingtheoutletpipethroughthe sheet,aholeof5 cmdiameter, slightly larger than thatofoutletpipe ismade in thesheet. The sheet is then embedded with pipe in the embankment by pouring additionalcementconcrete(1:3:6)overthesurfaceadjacenttohole.

Stone/brickpitching:Thestone/brickpitchingimpartslongevitytothesheetaswellaspondandbothare importantconsiderationsoffarmers.BothblackLDPEandSilpaulinsheetsareUV resistantbut thesearedamagedby sunlight ifexposed for longerduration.Theplasticsheetmustbeprotectedbycoveringwithinertmaterialsuchassoil,bricks,stones,etcwhichis not damaged byUV rays. Pitchingwith bricks/stones also protects the sheet from anyaccidentalphysicaldamagebysharpobjects.Thebrickpitchingofbottomofthepondcanbeavoidedandsheetiscoveredwithabout15cmsoillayer.Ifsoilisnottobeused,atoewallisessentialtosupportthebricksinthesideslopes.Forthisatrench(about5x5cm)istobedugalong the bottom periphery of the pond.Once the toewall is constructed then the brickpitching of the sides should be done from bottom to top. In areas,where bricks are not

readilyavailable,flatstoneswithbluntedgescanbeused.Stonepitchingisstartedbyplacingbiggerdiameterstones(about20cm)atthebottomoftheslope.Thediameterofthestonesshouldbegraduallyreduced(about10cm)asthepitchingworkproceedstowardsthetopofthesideslopes.Thebricks/stonesarearrangedinamannerthatprovidesgriptoeachother.The gap between the bricks/stones may be then filled with cement mortar (1:8) forstrengthening the pond. The top 30 cmwidth of stone pitching is plasteredwith cementmortar(1:4).Ifdepthofpondismorethan2.0m,30cmwideverticalbondbycementmortarmaybeprovidedatanintervalof0.6m.

Siltretentiontrench:Constructsiltretentiontrench(2x1m)alongtheinletchannelnearthepondtopreventsiltgettingintothepondincaseofsurfacerunoffharvesting.Thetrenchisdividedinto23segmentstolowerdownthevelocityofrunoffandtoallowmoretimeforthesilttosettledown.ThesiltretentiontrenchshouldalsohaveproperLDPEsheetliningandbrickpitchingtoavoidseepage.Therunoffwaterisfirstdirectedtosiltretentiontrenchandtheinletchannelfromthistrenchisconnectedthereaftertothestoragepond.

Table1.CostofdugoutLDPEfarmpond(20x20x3mwitheffectivecapacityof650m3)withdifferentliningmaterials

Sr.No. Liningmaterial Approx. cost ofconstruction(Rs.inlakh)

Cost (Rs/m3 ofwaterharvested)

Expected life(years)

1. Kaccha pond withoutsealant

0.31 48.00

2. BlackLDPEsheetlining 0.61 78.38 57

3. BlackLDPEsheetlining&pitchingwithbricks

2.34 299.50 20+

4. Silpaulinsheet lining (UVresistant)

0.70 82.30 57

5. Silpaulinsheet lining (UVresistant)&pitchingwithbricks

2.85 351.00 20+

6. Masonry cement/RCCtank

16.00 2462.00 20+

Irrigationpotential

Pondsof50200m3capacitycanbeeasilyconstructedon individualfarmerbasiswhicharesufficient for vegetable cultivation in aboutone kanal area.A 100m3 capacitypondoncefilledcanprovidefiveirrigationsthroughtraditionalsystemseachof5cmtoonekanalarea(400 m2). The irrigation potential can be increased upto 56 times if hightech irrigationsystemslikedriporsprinklerirrigationsystemsaretobeused.Itispertinenttomentionherethat rooftop area of polyhouses can be effectively used for rain water harvesting. Apolyhouseof200m3rooftopareacanyieldupto160m3water intheareahavingaverageannualrainfall100cmwhichcanfullymeetthewaterrequirementofcropsgrowninsidethepolyhouse.

Protectionandmaintenance

Trespassingbyanimalsshouldbeavoided.

In case the sheet ispuncturedordamaged,paste required sizeofLDPE sheetusingheatedbitumenatdamagedportion.

Clean the silt retention trench regularly. Inspect inlet channel and collecting arearegularly.Periodicallycleanthepondandremoveaccumulatedsilt/algae.

Storageofwateroffirst12monsoonshowersshouldbeavoidedforpreventingquicksedimentationofsiltationtank.

Thedos&dontsinLDPEfarmpond

Dos

Ensuregooddressingbeforelayingthesheet.

Jointingofthesheet,ifrequired,isdonebyheatsealing/syntheticrubberadhesiveatthesiteitself.

Directtherainwatertothepondthroughasinglepoint.

Recommendedcement:sandratiois1:4forbricksjoiningatthetop30cm.

Ensure a systematicprocedural followup and theuseof rightqualitymaterialandgoodworkmanship.

Donts

Dontusehooksandalsodontdragthesheet.

Dontwalkonthesheetwhileitisbeinglaid.

Dontuseroughtoolsorequipmentsforcleaning.

Whileconstructingpond,makesuretherearenotreesnearthepond.

Costestim

Topleng

Topwidt

Widthof

Areaofs

No.ofbr

Costofs

Costofb

Costofe

Totalcos

Unitcost

Continge

Unitcost

mateofLDPE

gth=20m

th=20m

Lengthof

fLDPEsheet

sheet=676m

ricksrequire

heet=150x2

bricks=28,80

arthwork5

st=Rs.2,34,

twithbrick

ency,excava

twithoutbr

Elinedfarmp

Bo

Bo

fLDPEsheet

=11+12.72

=Lengtho

m2=150kg

dforlining=

200=30,000

00x6=1,72

0%cutfill@

2

060/

lining =

=

=0

tioncharges

ricklining

= 61,

pondsize20x

ottomLength

ottomwidth

=11+2(6.36)

+2 =25.72

ofsheet=25

(BlackUV20

=24x24x50=

0

2,800

@Rs.80/m3=

2,34,06

7,81.5

Rs.299

0.30Paisape

s10to15%e

,260

x20x3m(Top

h=11m

=11m

+1+1

2m

5.72(say26

0micron)

=28,800

=781.5x80

60

9.5perm3

erlitreorRs

extra(unitc

=Rs.0.078p

pDimension)

mtr)

=Rs.31,

s.300perm

costmayvar

perlitreorR

260.00

m3

yasperhea

Rs.78.38per

adloadatsit

rm3

te)

781.5

Unitcostmayvaryaspersiteconditionasescalationcharges(Therateconsideredarecurrentprevailingtomarketrate)