Adapting to Climate Change through IWRM · Adapting to Climate Change through IWRM ... CHAPTER-4...

National Environment Commission & Department of Agriculture, Royal Government of Bhutan

AdaptingtoClimateChangethroughIWRM

ContractN°CDTABHU-8623

IRRIGATIONENGINEERINGMANUALMarch2016

█EgisEau&RoyalSocietyforProtectionofNature&

BhutanWaterPartnership

█ Egis Eau & RSPN/ BhWP Irrigation Engineering Manual

i

Documentqualityinformation

Generalinformation

Author(s) BasisthaAdhikari

Projecttitle AdaptingtoclimatechangethroughIWRM

Documenttitle IrrigationEngineeringManual

Date November2015

Reference

Addressee(s)

Sentto:

Name Organization Senton:

LanceGORE AsianDevelopmentBank,Manila

Copyto:

Name Organization Senton:

TenzinWangmo Chief,WaterResourcesCoordinationDivision,

NationalEnvironmentCommissionSecretariat,

RoyalGovernmentofBhutan

KarmaTshethar Chief,EngineeringDivision,Departmentof

Agriculture,MinistryofAgricultureandForests,

RoyalGovernmentofBhutan

JigmeNidup DeputyChief,WaterResourcesCoordination

Division,NationalEnvironmentCommission

Secretariat,RoyalGovernmentofBhutan

Historyofmodifications

Version Date Preparedby: Reviewed/validatedby:

1

2 26/08/2015 BasisthaRajAdhikari Dr.LamDorji(TeamLeader)

3 03/11/2015 BasisthaRajAdhikariDr.LamDorji(TeamLeader)andThierry

Delobel(ProjectDirectorEgis)


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Acronyms

ADB AsianDevelopmentBank

AWDO AsianWaterDevelopmentOrganisations

BhWP BhutanWaterPartnership

BSR BhutanScheduleofRates

CA Commandarea

CC ClimateChange

DG DirectorGeneral

DOA DepartmentofAgriculture

DWS DrinkingWaterSupply

FAO FoodandAgricultureOrganization

FGD FocusGroupDiscussion

FMIS FarmerManagedIrrigationSystem

GIS GeographicInformationSystem

GPS GeographicPositioningSystem

GW Groundwater

HP Hydropower

IFAD InternationalFundforAgricultureDevelopment

IWRM IntegratedWaterResourcesManagement

JICA JapanInternationalCooperationAgency

MOAF MinistryofAgricultureandForest

MOWHS MinistryofWorksandHumanSettlement

MOIC MinistryofInformationandCommunication

NEC NationalEnvironmentCommission

NECS NationalEnvironmentCommissionSecretariat

NIIS NationalIrrigationInformationSystem

NIMP NationalIrrigationMasterPlan

RBMP RiverBasinManagementPlan

RGOB RoyalGovernmentofBhutan

RSPN RoyalSocietyforProtectionofNature(Bhutan)

TA TechnicalAssistance

TAC TechnicalAdvisoryCommittee

UNCDF UnitedNationCapitalDevelopmentFund

USBR UnitedStatesBureauofReclamation

USDA UnitedStatesDepartmentofAgriculture

WB WorldBank

WRCD WaterResourcesCoordinationDivision

WUA WaterUsersAssociation


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TABLEOFCONTENTS

EXECUTIVESUMMARY.........................................................................................................................................X

CHAPTER-1...........................................................................................................................................................1

1. INTRODUCTION............................................................................................................................................1

1.1 BACKGROUND..................................................................................................................................................1

1.2 SCOPEOFTHEMANUAL......................................................................................................................................1

1.3 IRRIGATIONINBHUTAN......................................................................................................................................1

1.4 METHODOLOGYADOPTED..................................................................................................................................2

1.5 CONTENTS OF THE MANUAL........................................................................................................................3

CHAPTER-2...........................................................................................................................................................4

2. PROJECTSTUDYANDSURVEY......................................................................................................................4

2.1 NAMINGANDCODINGOFIRRIGATIONSCHEME......................................................................................................4

2.2 PROJECTDEVELOPMENTCYCLE............................................................................................................................4

2.3 IRRIGATIONPLANNING.......................................................................................................................................5

2.4 IDENTIFICATIONSTUDY......................................................................................................................................6

2.5 FEASIBILITYSTUDY.............................................................................................................................................8

2.6 ENVIRONMENTALIMPACTSTUDY.......................................................................................................................13

2.7 CLIMATECHANGEIMPACTSTUDY......................................................................................................................15

2.8 REPORTPREPARATION.....................................................................................................................................21

2.9 PROJECTSELECTIONANDPRIORITYRANKING........................................................................................................22

CHAPTER-3.........................................................................................................................................................24

3. HYDROLOGYANDAGRO-METEOROLOGY...................................................................................................24

3.1 HYDROLOGYOFBHUTAN..................................................................................................................................24

3.2 WATERAVAILABILITYFORIRRIGATION................................................................................................................24

3.3 ESTIMATIONOFDESIGNFLOODDISCHARGE.........................................................................................................28

3.4 WATERREQUIREMENTASSESSMENT..................................................................................................................37

3.5 WATERBALANCEASSESSMENT..........................................................................................................................46

3.6 DISCHARGEMEASUREMENT..............................................................................................................................46

CHAPTER-4.........................................................................................................................................................49

4. INTAKESANDHEADWORKS........................................................................................................................49

4.1 INTRODUCTION...............................................................................................................................................49

4.2 TYPESOFINTAKES...........................................................................................................................................49

4.3 SELECTIONOFSITEFORINTAKESANDHEADWORKS................................................................................................51

4.4 SIDEINTAKE...................................................................................................................................................52

4.5 DOUBLEORIFICEINTAKE..................................................................................................................................56

4.6 BOTTOMINTAKEORTRENCHINTAKE..................................................................................................................60

4.7 DIVERSIONHEADWORK....................................................................................................................................64

4.8 ENERGYDISSIPATIONINHYDRAULICSTRUCTURE...................................................................................................81


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4.9 PUMPEDIRRIGATIONSYSTEM............................................................................................................................84

CHAPTER-5.........................................................................................................................................................93

5. CANALSYSTEMDESIGN..............................................................................................................................93

5.1 INTRODUCTION...............................................................................................................................................93

5.2 TYPEOFCANALS.............................................................................................................................................93

5.3 LAYOUTPLANNING..........................................................................................................................................93

5.4 DESIGNCONCEPTOFIRRIGATIONCANALS............................................................................................................95

5.5 DESIGNOFUNLINEDCANALS............................................................................................................................96

5.6 LININGOFCANALS........................................................................................................................................102

5.7 COVEREDCANALS.........................................................................................................................................106

5.8 DESIGNOFPIPELINES.....................................................................................................................................114

5.9 CANALSALONGUNSTABLEAREAS.....................................................................................................................128

CHAPTER-6.......................................................................................................................................................132

6. CANALSTRUCTURES.................................................................................................................................132

6.1 INTRODUCTION.............................................................................................................................................132

6.2 REGULATINGSTRUCTURES..............................................................................................................................132

6.3 DROPSTRUCTURES.......................................................................................................................................145

6.4 CROSSDRAINAGESTRUCTURES.......................................................................................................................164

6.5 SEDIMENTCONTROLSTRUCTURES....................................................................................................................183

6.6 RETAININGWALLS........................................................................................................................................190

6.7 WATERCONTROLGATES................................................................................................................................195

6.8 STRUCTURALDESIGNOFCANALSTRUCTURES.....................................................................................................197

CHAPTER-7.......................................................................................................................................................204

7. MICROIRRIGATION..................................................................................................................................204

7.1 INTRODUCTION.............................................................................................................................................204

7.2 SPRINKLERIRRIGATIONSYSTEM.......................................................................................................................204

7.3 DRIPIRRIGATION..........................................................................................................................................214

7.4 LOWCOSTSIMPLEDRIPSYSTEM.....................................................................................................................222

7.5 WATERHARVESTING.....................................................................................................................................224

CHAPTER-8.......................................................................................................................................................230

8. RIVERTRAININGANDFLOODCONTROLWORKS.......................................................................................230

8.1 RIVERSOFBHUTAN.......................................................................................................................................230

8.2 FLOODS......................................................................................................................................................230

8.3 RIVERTRAININGWORKS................................................................................................................................233

8.4 BANKPROTECTIONWORKS............................................................................................................................242

8.5 MATHEMATICALMODELSFORRIVERSTUDIES.....................................................................................................246

CHAPTER9.......................................................................................................................................................248

9. PROJECTEVALUATION.............................................................................................................................248

9.1 INTRODUCTION.............................................................................................................................................248


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9.2 AGRICULTUREBENEFITASSESSMENT................................................................................................................248

9.3 PROJECTCOSTESTIMATE...............................................................................................................................251

9.4 ECONOMICANALYSIS.....................................................................................................................................253

10. BASISTERMSUSEDINTHEMANUAL....................................................................................................257

11. REFERENCES.........................................................................................................................................259


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LISTOFTABLES

Table 2-1 Project development cycle ........................................................................................................... 4Table 2-2 Topographical survey standards ............................................................................................... 10Table 2-3 Potential impacts of climate change on irrigation ....................................................................... 15Table 2-4 Components of the baseline assessment .................................................................................. 17Table 2-5 Example of climate change adaptation planning ........................................................................ 20Table 2-6 Evaluation criteria for scheme priority ranking ........................................................................... 23Table 3-1 80% Reliable Flow of Thimchhu at Lungtenphug (m3/s) ........................................................... 25Table 3-2 Worked out example of 80 % reliable flow ................................................................................. 26Table 3-3 Values of coefficients for WECS method ................................................................................... 27Table 3-4 Example of mean monthly flow .................................................................................................. 27Table 3-5 Suggested Return Period ........................................................................................................... 28Table 3-6 Annual Maximum Flow of Thimpu Chhu (m3/s) ......................................................................... 29Table 3-7 Standard Normal Variate ............................................................................................................ 32Table 3-8 Hydrological Soil Groups and CN Values .................................................................................. 34Table 3-9 Slope Categories ........................................................................................................................ 34Table3-10Growthfactorsformaximumrainfall .............................................................................................. 35Table 3-11 Areal Reduction Factor for 24 Hours Point Rainfall ................................................................. 35Table 3-12 Possible Cropping Patterns of Bhutan ..................................................................................... 37Table 3-13 Worked out example of Cropwat-8 ........................................................................................... 39Table 3-14 Crop Coefficients of Selected Crops ........................................................................................ 40Table 3-15 Land Preparation Requirements .............................................................................................. 40Table 3-16 Estimated Deep Percolation Losses (mm/day) ........................................................................ 41Table 3-17 Worked out example of reliable and effective rainfall (mm) ..................................................... 43Table 3-19 Irrigation water duty .................................................................................................................. 44Table 3-18 Worked out example of crop water requirement ...................................................................... 45Table 3-20 Discharge Measurement with Bucket-Watch Method .............................................................. 48Table 4-1 Velocity and head loss across gravel trap .................................................................................. 53Table 4-2 Design of undersluice ................................................................................................................. 72Table 4-3 Design example of weir .............................................................................................................. 77Table 4-4 Selection of energy dissipaters .................................................................................................. 82Table 5-1 Recommended side slopes of canal .......................................................................................... 98Table 5-2 Maximum permissible velocities and tractive forces .................................................................. 98Table 5-3 Values of roughness coefficient ............................................................................................... 101Table 5-4 Recommended roughness coefficients for lined canal ............................................................. 105Table 5-5 Coefficients for inlet and outlet of different transitions ............................................................. 108Table 5-6 Hydraulic properties of part full flowing pipe ............................................................................ 108Table 5-7 Water pressure according to the head ..................................................................................... 114Table 5-8 Length to diameter ratio of different fittings .............................................................................. 115Table 5-9 Hazen-William's flow coefficients ............................................................................................. 120Table 5-10 Values of pipe roughness ....................................................................................................... 121Table 5-11 Head loss coefficients at pipe inlet ......................................................................................... 123Table 5-12 Head loss coefficients for sudden pipe expansion ................................................................. 124Table 5-13 Head loss coefficient for sudden contraction of pipe .............................................................. 124Table 5-14 Head loss coefficient for gradual expansion .......................................................................... 124Table 5-15 Head loss coefficients for elbows of pipeline ......................................................................... 125Table 5-16 Head loss coefficients for orifice ............................................................................................ 126Table 5-17 Head loss coefficients for valves ............................................................................................ 127Table 5-18 Sample design sheet of pipeline works .................................................................................. 131Table 6-1 Hydraulic jump calculations ...................................................................................................... 135Table 6-2 Widths of Outlet Openings ....................................................................................................... 142Table 6-3 Discharge through Pipe Outlets (Pipe length < 6 m) ................................................................ 143Table 6-4 Bhutan Road Standards ........................................................................................................... 161


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Table 6-5 Coefficients for friction loss in siphon barrel ............................................................................. 178Table 6-6 Flow velocity through gravel trap ............................................................................................. 183Table 6-7 Design Calculations of Retaining Wall ..................................................................................... 192Table 6-8 Standardized size of gates and their weight ............................................................................ 197Table 6-9 Self-weight of materials ............................................................................................................ 198Table 6-10 Allowable bearing pressure of the soil ................................................................................... 201Table6-11Lanesweightedcreepratio .......................................................................................................... 202

Table6-12ConcreteGradesandUses ........................................................................................................... 202

Table6-13ReinforcementBars ..................................................................................................................... 203Table 7-1 Infiltration rate of different soils ................................................................................................ 209Table 7-2 F-factor value for head loss in pipes ........................................................................................ 209Table 7-3 Reference crop evapo-transpiration and crop coefficients ....................................................... 210Table 7-4 Calculation of reservoir capacity .............................................................................................. 211Table 7-5 Design of pipelines for sprinkler irrigation system .................................................................... 212Table 7-6 Reference crop evapo-transpiration and crop coefficients ....................................................... 218Table 7-7 Head loss calculations of laterals ............................................................................................. 220Table 7-8 Head loss calculations in sub-main line ................................................................................... 221Table 7-9 Head loss calculation in main line ............................................................................................ 221Table 7-10 Typical models of simple drip irrigation system being used in India ...................................... 224Table 7-11 Pond lining materials .............................................................................................................. 228Table 9-1 Sample agricultural benefit assessment matrix ........................................................................ 250Table 9-2 Sample quantity estimate form ................................................................................................. 252Table 9-3 Sample cost estimate form ....................................................................................................... 253Table 9-4 Typical Example of Economic Analysis .................................................................................... 256

LISTOFFIGURES

Figure 2-1 Parameters and issues of vulnerability assessment ................................................................. 18Figure 3-1 Worked out Example of Reliable Rainfall .................................................................................. 42Figure 3-2 Discharge measurement with float area method ...................................................................... 47Figure 4-1 Layout plans of side intakes ...................................................................................................... 51Figure 4-2 Idle Locations for Side Intakes .................................................................................................. 52Figure 4-3 Free flow and submersed flow orifice ....................................................................................... 53Figure 4-4 Side intake design ..................................................................................................................... 56Figure 4-5 Typical sketch of a double orifice intake ................................................................................... 57Figure 4-6 Double orifice intake ................................................................................................................. 58Figure 4-7 Bottom intake design ................................................................................................................ 62Figure 4-8 Definition sketch of weir and its components ............................................................................ 65Figure 4-9 Vertical drop weir ...................................................................................................................... 65Figure 4-10 Sketch of rockfill weir .............................................................................................................. 66Figure 4-11 Sketch of concrete weir ........................................................................................................... 66Figure 4-12 Typical weirs ........................................................................................................................... 67Figure 4-13 Design sketch of undersluice .................................................................................................. 74Figure 4-14 Design sketch of weir .............................................................................................................. 79Figure 4-15 Sketches of different types of energy dissipaters ................................................................... 83Figure 4-16 Sketches of Pumps ................................................................................................................. 84Figure 4-17 Sketch of total dynamic head .................................................................................................. 85Figure 4-18 Sketches of pump characteristic curves ................................................................................. 86Figure 5-1 Canal Layouts ........................................................................................................................... 94Figure 5-2 Small Scale Canal Layouts ....................................................................................................... 94Figure 5-3 Alternative Layout for Small Scale Canals ................................................................................ 95


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Figure 5-4 Schematic Diagram of Canal System ....................................................................................... 97Figure 5-5 Typical section of trapezoidal canal .......................................................................................... 99Figure 5-6 Typical longitudinal profile of canal ......................................................................................... 100Figure 5-7 Typical stone masonry and concrete lining ............................................................................. 103Figure 5-8 Ferro cement and soil-cement linings ..................................................................................... 104Figure 5-9 Typical covered canals ........................................................................................................... 107Figure 5-10 Design chart for concrete pipes ............................................................................................ 112Figure 5-11 Design chart for HDPE pipes ................................................................................................ 113Figure 5-12 Gate and globe valves .......................................................................................................... 115Figure 5-13 Typical layout of pipeline ....................................................................................................... 117Figure 5-14 Typical air release valves ...................................................................................................... 117Figure 5-15 Typical plan of break pressure tank ...................................................................................... 118Figure 5-16 Typical valve chamber .......................................................................................................... 119Figure 5-17 Moody's diagram for friction loss ........................................................................................... 122Figure 5-18 Sudden expansion and contraction of pipes ......................................................................... 124Figure 5-19 Typical gradual expansion of pipeline ................................................................................... 125Figure 5-20 Sketch of flow from straight pipe to tee and tee to straight pipe .......................................... 126Figure 5-21 Typical sketch of trash screen bars ...................................................................................... 127Figure 5-22 Three types of canal instability .............................................................................................. 129Figure 5-23 Canal instability problems and their solutions ....................................................................... 130Figure 6-1 Alignment of Head Regulator .................................................................................................. 132Figure 6-2 Flow conditions of an orifice .................................................................................................... 133Figure 6-3 Design Sketch of head regulator ............................................................................................. 136Figure 6-4 Proportional Dividers ............................................................................................................... 139Figure 6-5 Typical outlet structure ............................................................................................................ 141Figure 6-6 Typical outlet of Tsirang Irrigation Schemes ........................................................................... 142Figure 6-7 Pipe Outlets ............................................................................................................................ 144Figure 6-8 Definition sketch of drop structure .......................................................................................... 145Figure 6-9 Design sketch of vertical drop ................................................................................................. 146Figure 6-10 Nomogram for the design of Vertical Drop Structure ............................................................ 147Figure 6-11 Relation between Froude number and length of jump .......................................................... 148Figure 6-12 Design sketch of chute drop ................................................................................................. 149Figure 6-13 Energy Loss in Hydraulic Jump ............................................................................................ 150Figure 6-14 Design sketch of pipe prop ................................................................................................... 152Figure 6-15 Design graph of outlet basin (USBR) .................................................................................... 153Figure 6-16 Design sketch of cascade drop ............................................................................................. 155Figure 6-17 Sketch of stop-log escape ..................................................................................................... 159Figure 6-18 Typical road bridge ............................................................................................................... 161Figure 6-19 Tributary River Crossing Options .......................................................................................... 166Figure 6-20 Typical aqueducts ................................................................................................................. 168Figure 6-21 Design sketch of aqueduct .................................................................................................... 170Figure 6-22 Typical super passages ........................................................................................................ 173Figure 6-23 Design sketch of super passage ........................................................................................... 175Figure 6-24 Typical shallow and deep siphons ........................................................................................ 176Figure 6-25 Design sketch of trash rack ................................................................................................... 177Figure 6-26 Typical deep siphon .............................................................................................................. 180Figure 6-27 Typical photos of HDPE pipe crossing .................................................................................. 181Figure 6-28 Typical gravel trap structure .................................................................................................. 184Figure 6-29 Typical configuration of settling basin ................................................................................... 185Figure 6-30 Settling basin design curves ................................................................................................. 188Figure 6-31 Scouring velocities for sand trap ........................................................................................... 189Figure 6-32 Design load diagram of retaining wall ................................................................................... 192Figure 6-33 Possible retaining walls in small canals ................................................................................ 194Figure 6-34 Typical vertical lift gate and stop-logs ................................................................................... 196Figure6-35Schematicpressurediagramofwalls ........................................................................................... 199

Figure6-36Sketchofseepagepathunderneaththestructure ......................................................................... 201


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Figure 7-1 Type of sprinkler irrigation system .......................................................................................... 206Figure 7-2 Components of sprinkler irrigation system .............................................................................. 207Figure 7-3 Typical sprinkler irrigations ..................................................................................................... 208Figure 7-4 Layout Plan of Sprinkler Irrigation System .............................................................................. 213Figure 7-5 Examples of drip irrigation method ......................................................................................... 214Figure7-6Schematicdiagramofwaterdistributioncomponentofdripirrigationsystem .................................. 216Figure 7-7 Layout plan of designed drip irrigation system ........................................................................ 222Figure 7-8 Schematic diagram of simple drip irrigation system ................................................................ 223Figure 7-9 Typical examples of rainwater harvesting from rooftops ......................................................... 225Figure7-10Typicalcrosssectionofexcavatedponds ...................................................................................... 226Figure 7-11 Standards of water harvesting ponds ................................................................................... 227Figure 8-1 Examples of river training works in Bhutan ............................................................................. 233Figure 8-2 Length and Plan shape of Guide Banks ................................................................................. 235Figure8-3Embankmentsectionofguidebank ............................................................................................... 236Figure 8-4 Spur types in relation to flow direction .................................................................................... 237Figure 8-5 Spur angle (φ) to flow direction ............................................................................................... 238Figure 8-6 Definition sketch of cut-off ....................................................................................................... 238Figure8-7Typicalbankfailure ...................................................................................................................... 242

Figure8-8Rip-rapforbankprotection .......................................................................................................... 243

Figure8-9Basictypesoftoeprotection ......................................................................................................... 244


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CHAPTER-6

6. CanalStructures

6.1 Introduction

Irrigationschemesinthehillshavesmallcanalstructuressuitableformountainenvironments.The

major canal structures are flow regulating structures or water division structures, conveyance

structures, cross drainage structures, and protection structures. Head regulator, flow dividers,

divisionboxesandofftakesbelong to the regulating structures.Theconveyancestructuresmainly

consist of drop structures, canal crossing structures such as foot bridges and culverts. The cross

drainagestructuresmainlyconsistoffourtypesofstructuresaqueduct,superpassage,siphonand

levelcrossing.Theprotectionstructuresaremainlyretainingwalls,andbankprotectionwalls.This

chapterdealswiththedesignconceptandproceduresofstandardcanalstructures.

6.2 RegulatingStructures

6.2.1 HeadRegulator

Introduction

A canal head regulator controls the amount of water flowing into the off-taking canal. A head

regulator is structured at the head of each canal taking water from themain canal or from the

headwork.Theheadregulatorisequippedwithverticalslidinggates,gateoperatingplatform,road-

bridge,andflowmeasuringdevisedependinguponthesizeofthecanalandschemetype.Insmall

scaleirrigationschemes,theheadregulatormaynothavearoad-bridgeorflowmeasuringdevice.

Designconcept

Theheadregulatorisnormallyalignedbetween90oto120

owithrespecttotheweiraxis(Figure6-1)

andisequippedwithslidinggatestoregulatetheflowofwatertothecanal.Theheightofgatesis

keptuptothepondlevel.Tocheckthefloodwaterenteringintothecanal,abreastwallisprovided

abovetheheightofthegateuptothedeckoftheregulator.Thewidthoftheregulatorcrestiskept

consideringthewidthoftheofftakingcanal.Theflowoftheheadregulariscalculatedaccordingto

its downstream flow conditionswhether it is submergedor free flow.A stillingbasin and flexible

protection is provided downstream of the regulator to dissipate energy. The head regulator is

provided with a deck slab for gate operation andmovement across the structure. The hydraulic

designofheadregulatorconsistsofthefixationofthecrestwidth,crestlevel,gateopeningheight,

scourdepths,anddownstreamenergydissipationdevices.

Figure6-1AlignmentofHeadRegulator


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Designprocedure

i. Fix the pond level of the head regulator at its upstream side considering full supply level

(FSL)ofcanalandheadlossintheregulator,

ii. Calculatethecrestlevelofregulator,itswidthandshapeofthesillfromtheconsideration

offreefloworsubmergedflowatitsdownstream(Figure6-2),

Figure6-2Flowconditionsofanorifice

Freeflow Submersedflow

Formulaforfreeflowcase,

⎟⎠

⎞⎜⎝

⎛−×××= 63.02

12/3

a

hgabCCQ vd

Formulaforsubmergedflowcase,

( )21

2/32 hhgabCCQ vd −×××=

Where,Cdisthedischargecoefficient=0.60,

Cvisthevelocitycoefficient=1,

bisthewidthoftheorifice,

aistheheightoftheorifice,

h1isthedepthofwaterupstreamoforifice,and

h2isthedepthofwateratd/soforifice,

iii. Determinethetotalwaterwayofregulatorconsideringpiersandabutments,

( ) WHKNKLL eapet +++= 2

WhereLtisthetotalwaterway,

Nisthenumberofpiers,

Kpisthepiercoefficientdependsuponitsshape,

Kaistheabutmentcoefficientdependsuponitsshape,

Heisthetotalheadovercrest,and

Wisthewidthofallpiers

iv. Calculate the hydraulic jump formations at the downstream for different flow conditions

suchas10%,50%and100%.

v. Fixthecisternlengthandlevelswithconsiderationofjumpprofile,


134

vi. Checktheheadregulatorfloorforsub-surfaceflowconditions,

vii. Calculateupstreamanddownstreamcutoffdepths,

viii. Fixtheupstreamanddownstreamprotectionworks

DesignExampleofHeadRegulator

Designdata:

Fullsupplydischargeofcanal,Qc 0.44m3/s

Fullsupplylevelofcanal(FSL) 398.47 m

Designbedlevelofcanal(CBL) 398.04 m

PermissibleAfflux 0.50m

PondLevel 398.62 m

Riverbedlevel 396.56m

Highfloodlevel(HFL) 399.12m

Siltfactor 1

Safeexitgradient 0.20(1/5)

Fixationofcrestlevelandwaterway

Crestlevelofheadregulatoristakenas10cmhigherthancanalbedlevel,

=398.04+0.10=398.14m

Usingdrownedweirformula,

( ) ( )1121

223

2ghhLCghhLCQ ××+×××=

Where,C1=0.577andC2=0.80

h=pondlevel–FSL=398.62-398.47=0.15m

h1=FSL–crestlevel=398.47–398.14=0.33m

Then,L=Qc/(((2/3)*0.577*(sqrt(2*9.81)*0.15^(3/2))+(0.80*0.33*sqrt(2*9.81*0.33))))

L=0.57m,provideL=1.00m

Discharge passing through head regulator, Q = 2/3*0.577*1.00*0.15 *(sqrt(2*9.81*0.15))

+0.80*1.00*0.33*(sqrt(2*9.81*0.33))=0.55m3/s

Calculategateopeningforthecaseofhighflooddischarge,

ghACQ 2××=

Where,Ciscoefficient=0.62

Aistheareaoforifice,A=L*a,whereaistheheightoforifice,

histheheadcausingflow,h=u/sHFL–d/sFSL=399.12–398.47=0.65m

Then,a=Q/(0.62*L*sqrt(2*9.81*h)=0.44/(0.62*1*sqrt(19.62*0.65))=0.20m

Calculatevelocitythroughorifice,V1=Q/L*a=0.44/1*0.20=2.20m/s

Calculateheadlossatentry,he=0.5V12/2g=0.50*2.2^2/2*9.81=0.12m

TELu/soforificegate=HFL=399.12m

TELd/soforificegate=TELu/sofgate–entryheadloss=399.12-0.12=399.00m


135

Calculatetotalheadlossavailable=TELd/sgate–d/sFSL=399.00–398.47=0.53m

Dischargeintensity,q=Q/L=0.44/1.0=0.44m3/s/m

CalculateheadlossatFSLpassingatpondlevel,he=Pondlevel–FSL

he=398.62–398.47=0.15m

Dischargeintensity,q=Q/L=0.44/1.0=0.44m3/s/m

Themainhydraulicjumpcalculationsarecarriedoutintabularform(Table6-1).

Table6-1Hydraulicjumpcalculations

Descriptions Formaximumflood

flow

Forpondlevelflow

Dischargeintensity,q=1.84xH3/2

0.44 0.44

u/swaterlevel,m 399.12 398.62

d/swaterlevel,m 398.47 398.47

d/sTEL(d/sFSL) 398.47 398.47

u/sTEL(d/soforificegate) 399.00 398.62

Headloss 0.53 0.15

Postjumpdepth,D2-trialvalue 0.59 0.48

Pre-jumpdepth,

⎟⎟⎠

⎞⎜⎜⎝

⎛+

×+−= 2

2

2

2

21

4

1/2

2

1D

D

qgDD

0.10 0.14

d/sspecificenergy,

( )222

222/ DgqDEf ×+=

0.62 0.52

u/sspecificenergy,

( )122

112/ DgqDEf ×+=

1.14 0.67

Ef1-Ef2=Headloss(shouldbeequal

toabovevalue)

0.53 0.15

Levelofjump=d/sTEL-Ef2 397.85 397.95

Lengthofconcretefloor,

L=5(D2-D1)

2.46 1.70

FroudenoF=q/(g*D31)0.5

4.63 2.80

d/sfloorlevelisprovidedat(minimumofjumplevel) 397.85 m

d/sfloorlength(maximumoflengthofconcretefloor) 2.46mroundupto3.0m

DepthofCut-offfromscourconsideration

Dischargeintensityofheadregulator=0.44m3/s/m

Scourdepth,R=1.35x(q2/f)1/3=1.35*(0.44^2/1)^1/3=0.78m

Ond/ssideallowcut-offdepthas1.75R=1.75*0.78=1.37m


136

RLofbottomofscourhole=d/sFSL–cut-offdepth=398.47–1.37=397.10m

Adoptd/scutofflevelas396.50m

Depthofd/scut-offfromcanalbed=Designbedlevelofcanal–bottomlevelofcut-off

d=398.04–396.50=1.54m

Providethicknessoflaunchingapronas0.75m

Lengthoflaunchingapron=3m

Totalfloorlengthandexitgradient

Theexitgradientshouldbecheckedfortheconditionthatthecanaliscompletelyclosedwhenhigh

floodispassingintheriver;thisprovidesworststaticcondition.

Maximumstatichead,H=u/sHFL–d/sfloorlevel=399.12–397.85=1.27m

Depthofd/scutoff=d/sfloorlevel–cut-offlevel=397.85–396.50=1.35m

Exitgradient,H

dGE

=λπ

1

Hence,λπ

1 =1/5*1.35/1.27= 0.212

FromKhosla'sexitgradientcurve,α(alfa)=2.2

Requiredtotalfloorlength,b=αxd=2.20*1.54= 3.39m

Adopttotalfloorlength=7.00m

Thefloorlengthshallbeprovidedasperfollowingproportion:

Crestwidthofheadregulator=1.00m

d/shorizontalfloorlength,=3.24m

d/sglacislengthwith3:1slope=3*u/sfloor–d/sfloor=3(398.14-397.85)=0.87m

Balanceshouldbeprovidedasupstreamfloor=2.00m

Totalfloorlength=2.00+1.00+0.87+3.24=7.11m

Figure6-3DesignSketchofheadregulator


137

Pressurecalculations

Letthefloorthicknessintheu/sbe0.50mandnearthed/scut-offbe0.50m

Determinetheupliftpressureactingonthefloorand%pressuresatu/sandd/ssheetpiles:

%pressureatu/ssheetpileline

Upstreamcutoff,d1=398.14–396.56=1.58m

Lengthoffloor,b=7.11mandu/scutoffdepth,d1=1.58m

Then,1/α=d1/b=1.58/7.11=0.22,α=4.61

( ) 2/112αλ ++= =(1+sqrt(1+4.61^2))/2=2.86

FromKhosla’scurves,φE=40.30%

φD=27.47%

φC1=100-φE=100-40.30=59.70%

φD1=100–φD=100-27.47=72.53%

Applycorrectiontotheabovevalues,

Thicknessofu/sfloor,t1=0.50m

Cf1correctionfordepth=t1/d1(φD1–φC1)=0.50/1.58*(72.53-59.70)=4.06(+ve)

Cf2correctionforinterferenceofd/scutoff

Khosla’sformula,⎟⎠

⎞⎜⎝

⎛ +×⎟⎟

⎠

⎞⎜⎜⎝

⎛=

b

dD

b

DC

1

50.0

1

19

d1=397.85-0.50-396.50=0.85m

D=398.14-0.5-396.50=1.14m

b1=6.61m

b= 7.11m

C=19*(1.14/6.61)^0.5*(1.14+0.85)/7.11=2.25%(+ve)

φCcorrected =59.70+4.06+2.25= 66.01%say66.00%

Downstreamcutoff,d2=397.85-396.50=1.35m

Lengthoffloor,b=7.11m andd/scutoffdepth,d1=1.35m

Then,1/α=d2/b=1.35/7.11=0.19

FromKhosla’scurves,φE1=38%

φD1= 26%

φE1-φD1=38–26=12.00%

φE1correctionfordepth=0.50*12/1.35=4.44(-ve)

φE1correctionforinterferenceduetou/scutoff

d=397.85-0.50-396.50=0.85m

D=398.14-0.50-396.50=1.14m

⎟⎠

⎞⎜⎝

⎛ +×⎟⎟

⎠

⎞⎜⎜⎝

⎛=

b

dD

b

DC

1

50.0

1

19 =19(1.14/6.11)^0.5*(0.85+1.14/7.11)=2.29%(-ve)

φEcorrected =12.00-4.44-2.29=5.27%say5.30%


138

φC=66.00% andφE=5.30%

Floorthickness

Themaximumstaticheadwilloccuronthefloorwhenthereisnoflowinthecanalbutmaximum

floodispassingdownriver.Inthiscasethemaximumstatichead=1.27m

Downstreamfloor

At1mfromd/send

Unbalancedhead=(66.00-5.30)/100*1.27*1.00/7.11+5.30*1.27/100=0.18m

Floorthickness=0.18 /(2.24-1)=0.15m

Providefloorthickness=0.50throughoutthebasin

Protectionworksbeyondimperviousfloor

i. Upstreamblockprotection

Scourdepth,R=1.35(q2/f)

1/3=1.35(0.44^2/1)^1/3=0.78m

Anticipatedscour=1.50*R=1.50*0.79=1.17m

Upstreamscourlevel=u/sHFL-anticipatedscourlevel=398.14–1.17=396.97m

Scourdepth,Dbelowu/sfloor=crestlevelofundersluice–scourlevel=1.58m

Volumeofblockprotectiontobegiven(D)=1.58m3/m

Provide1.5mx1.5mx1.0mconcreteblockover0.50mthickgravel

Thelengthrequired= 1.58/(1.0+0.50)=1.05m

Providetworowsoftheaboveblockinalengthof2.00m

ii. U/sLaunchingapron

Quantityoflaunchingapronshouldbe2.25Dm3/m

Thicknessoflaunchingapron=1.00m

Thelengthrequired=2.25*1.58/1.00=3.55m

Providelaunchingapronas4.00mlength

iii. Downstreamprotection

Scourdepth,R=0.78m

Anticipatedscour=2*R=1.56m

Downstreamscourlevel=FSL–anticipatedscour=398.47–1.56=396.91m

Scourdepth,Dbelowd/sfloor=d/sfloorlevel–scourlevel=397.85–396.91

D=0.94mSay1.00m

iv. Invertedfilter

Thelengthofinvertedfilter=Dmandthicknessequaltothed/slaunchingapron Providerowof

1.50mx1.50mx1mconcreteblockwith10cmgapfilledwithpeagravelover0.5 mthickgraded

filter

Thelengthrequired=1.00/(1.0+0.50)=0.66m=1m

v. D/sLaunchingapron

Quantityoflaunchingapronshouldbe2.25Dm3/m

Thicknessoflaunchingapron=1.00m


139

Thelengthrequired=2.25*1/1.00=2.25m

Providelaunchingapronas3.00mlength

6.2.2 FlowDivider/ProportionalDivider

Concept

A flowdividerdivides the incoming flowof thecanal into twoonemorebranchingcanals.A flow

dividermaybeaproportionaldivideroradivisionbox.Aflowdividermaybeequippedwithgatesor

not.

The proportional distribution of irrigation water means that flow in a canal is divided equally

betweentwoormoresmallercanals. Ingeneral, the flows in thesecanalsareproportional to the

areastobeirrigatedbyeachofthem.Thedivisionofwaterisperformedwiththestructuresnamed

proportional dividers, which divide the flow with equal crest heights and proportional widths.

Alternatively, the allocation of irrigation water is based on the set norms and practices of the

community (Figure 6-4). This practice leads to the rigid systemofwater distribution and farmers

havenoopportunitiestotakeextrawater.Thesystemrunsautomaticallywithnoneedtoopenor

closeoradjusttheflows.Proportionaldividershavethefollowingsocio-technicalcharacteristics:

• Visiblewaterdistribution,

• Simplestructure,

• Setrulesandregulations,

• Reducewaterrelateddisputes

Thewidthofthestructureisproportionaltotheallocateddischargeforeachoff-takingcanalwhich

isclearlyvisibletofarmers.Thedownstreamconditionoftheproportionaldividershouldbeoffree

flow condition in all off-taking canals. In other words, the downstream water level of the canal

should not influence the flowing pattern of the proportional weir. In addition, the upstream

approachvelocityshouldbeuniformlydistributedacrossthesection.

Figure6-4ProportionalDividers

DesignProcedures

Thedesignofaproportionaldivider isconcernedwith the fixingof thewidthof theweirandthe

provisionofenergydissipatingarrangementsdownstreamoftheoff-takingcanals.Thewidthofthe

weiriscalculatedconsideringthebroadcrestedweir:

2/371.1 HBQ=


140

Where,Bisthewidthoftheweir,andHistheheightofthecrest,

The width of the weir is proportional to the discharge in free flow conditions. The downstream

energydissipationiscalculatedasperthehydraulicjumpcalculations.Forsmallscalestructures,the

lengthofthebasinis5to6timestheheightofthedrop(differenceinwaterlevelsofupstreamand

downstreamcanals).

DesignExampleofProportionalDivider

Data

Parentcanaldischarge,Qp 220l/s

Dischargebranchcanal-1,Q1 132l/s

Dischargeofbranchcanal-2,Q2 88l/s

Waterdepthinparentcanal,Dp 0.40m

Waterdepthinbranchcanal-1,D1 0.35m

Waterdepthinbranchcanal-2,D2 0.30m

Bedwidthofparentcanal ,Bp 0.40m

Bedwidthofbranchcanal-1,B1 0.35m

Bedwidthofbranchcanal-2,B2 0.30m

Calculations

Providewidthofweirslightlywiderthanparentcanalwidth,W=0.50m

Calculatethewidthofweirforcanal-1 W1=W*Q1/Qp=0.50*0.132/0.220=0.30m

Calculatethewidthofweirforcanal-1 W2=W*Q2/Qp=0.50*0.88/0.220=0.20m

Checkthetotalwidthoftheweir,W=W1+W2=0.30+0.20=0.50m,OK

Hence,thedesignisacceptable.

6.2.3 FieldOutlets/Offtakes

Field outlets or offtakes are small structure used to supply water from canals to the field.

Dependinguponthetype,sizeand importanceofthescheme,outletsareofvarioustypes.The

maindesignobjectiveoftheseoutletsistosupplyirrigationwaterequitablyuptothetailendof

the scheme. In large irrigation systems, outlets are both gated and ungated. For small scale

irrigation,outletsareeithersimplepipeoutlets fixedat theheadof theprimarycanal/ tertiary

canalsoropencutopenings.Thecontrolofwater inpipeoutletscouldbemanagedbymanual

inspection, closingwith localmaterial or letting to flow continuously. Inopen cutoutlets, stop

log grooves exist for the insertion of wooden planks. In hill schemes, these outlets provide

sufficientdroptotheirrigatedareawhichisconsiderablylowerthantheparentcanal.

DesignConcept:

Thedesignofoutletsshouldmeetthefollowingrequirements:

§ Allowfarmerstoallocatetheavailablewateramongtheusersintheagreedmanner,

§ Allowfarmersmaximumflexibilityofoutletoperationduringnormalandlow-flowperiodsin

thesupplycanal,


141

Theallocationanddistributionofavailablewatermayleadtoconflictamongusers,especiallyduring

periodsoflowwateravailability.Inaddition,duringperiodsoflowflowinthecanal,moreeffective

waterdistributioncanbeachievedbyapplyingrotationof flowamongtheusers.Thiswill require

everyoutlettobedesignedinsuchamannerthat:

§ on/offflowcontrolthroughtheoutletispossible,

§ efficient flow rates are maintained through the outlet even when the water level in the

supplycanalislow

Simplewoodenplanksarethemostappropriateforon/offflowcontrolinsmallirrigationschemes

whether theoutlets arepipedoutletsoropen cut in lined canals.Whenwater level is low in the

parent canal during a low flow period, the flow in the outlet is also low. To divert the high flow

towardstheoutletacheckstructureisessentialontheparentcanal.

Insmallirrigationschemes,threetypesoffieldoutletsorofftakesareusedindevelopingcountries:

• Simpleopening,

• Simpleopeningwithdownstreamdrop,and

• Pipeoutlet

A simple opening type offtake is provided in small irrigation schemeswith earthen sections. The

openings can easily be opened or closed by farmers using locally availablemud clods, stones or

inserting wooden plans on the grooves. The maximum size of the opening should be 300mm.

However,smalleropeningscouldalsobemadeupto100mminsize.Asketchofatypicalstructure

isshownhere(Figure6-5).

Figure6-5Typicaloutletstructure


142

Asimpleopeningwithadownstreamdropisusedwhentheparentcanalliesatahigherelevation

thanthecommandarea.Thedropoftheofftakeshouldbelimitedto1.5m.Thisofftakestructureis

constructed as lined sectionwith stonemasonry or concretewithminimum length and the ends

roundedontheearthbanks.Thesizingoftheopeningissimilartothatofasimpleopeningofftake.

Atthedownstreamendoftheofftake,adequatepitchingorastillingbasinneedtobeconstructed

todissipatetheenergy.Alternatively,naturallargestonesorfirmgroundneedtobeatthebottom

of theofftaketopreventscouringanderosionof thestructure.Thesetypesofoff takestructures

existintheTsirangHillIrrigationProject(Figure6-6).

Figure6-6TypicaloutletofTsirangIrrigationSchemes

Thewidthsoftheoff-takeopeningfordifferentdischargesareprovidedhere(Table6-2).

Table6-2WidthsofOutletOpenings

Depthofwateroverthesillatintake(mm) 100 150 200 250

Dischargeper100mmwidthofsill(l/s) 5 10 15 20

DesignofPipeOutlet:

Submergedoutlet:

Thedesignformulaforasubmergedorificeis:

HACQ Δ=

Where, Qisoutletdischargeinlps,

Aistheareaofthepipeinm2

∆Histheheaddifferencebetweenparentcanalwater levelandwater levelof

outletcanal

Ciscoefficientandequalsto3,300forpipeshorterthan6mand2,800forpipe

longerthan6m

FreeFlowPipeoutlet:

Thedesignformulaforafreefloworificeis:


143

HACQ Δ=

Where, Qisoutletdischargeinlps,

Aistheareaofthepipeinm2

∆Histheheaddifferencebetweenparentcanalwater levelandwater levelof

outletcanal

Ciscoefficientandequalsto2760

TypicalsketchesofsubmergedandfreeflowoutletsarepresentedinFigure 6-7.

DesignExampleofPipeOutlet

Insubmergedpipeoutlethavinga100mmdiameterHDPpipewitha0.25mheaddifference,the

dischargeis:

Q=3300x0.102x0.25

0.5=13l/s

In free flowpipe outlet having a 100mmdiameterHDPpipewith a 0.25mheaddifference, the

dischargeis:

Q=2760x0.102x0.25

0.5=11l/s

The detailed worked out examples of pipe outlets for both submersed and free flow cases are

presentedhere(Table6-3).

Table6-3DischargethroughPipeOutlets(Pipelength<6m)

Headloss

(m)

PipeDiameter(mm)

100 150 200 250 300

Submergedflow(l/s)

0.05 6 13 23 36 52

0.10 8 18 33 51 74

0.15 10 23 40 63 90

0.20 12 26 46 72 104

0.25 13 29 52 81 117

0.30 14 32 57 89 128

Freeflow(l/s)

0.15 8 19 34 52 76

0.20 10 22 39 61 87

0.25 11 24 43 68 98

0.30 12 27 47 74 107

0.35 13 29 51 80 115

0.40 14 31 55 86 123

0.45 15 33 58 91 131

0.50 16 34 61 96 138


144

Figure6-7PipeOutlets


145

6.3 DropStructures

6.3.1 GeneralConcept

Dropstructuresarerequired inacanalwhenthetopography issteeperthanthecanalgradient. If

drops are not provided, erosion will occur in the canal. A drop structure may be provided at a

location where the canal water level outstrips the ground level but before the bed of the canal

comesintofillings(Figure6-8).Inthehills,ifthecanalisinrock,quitesteeplygradedcanalscanbe

madewithnodropstructures.Four typesofdropstructuresaresuitable for small scale irrigation

schemes:

• Verticalorstraightdrop,

• Inclineddroporchutedrop,

• Pipedrop,and

• Cascadedrop

Figure6-8Definitionsketchofdropstructure

Inallthesedropstructures,strongerwallsandfloorsareneededtoresisterosion,impactforcesof

water and abrasion. In addition, complex stilling basins are required to control turbulence

associatedwithlargedropheights.Inremotehillyareas,theconstructionoflargedropstructuresis

difficultandundesirable.Hence, it issuggestedto limitdropheightsto2mforverticalandchute

drops and 4m for pipe and cascade drops. For small irrigation schemes, drop structures can be

combinedwithcontrolorcheckstructures.

Thebasichydraulicprincipleforallofthesedropstructuresisthedissipationofexcessenergyasthe

water drops fromahigh level to a low level. All of thewater’s excess energymust bedissipated

insidethedropstructurebeforethewaterisallowedintothedownstreamcanal.Energyofflowing

watercanbedissipatedby:

• Hydraulicjumps,

• Allowingtheflowtostriketobaffles,baffleblocks,andraisedsills,and

• Usingwatercushion

A simple hydraulic jump, or a combination of a hydraulic jumpwith a raised sill, standingwater

pools,orcushionsarethemostcosteffectivemeasuresinhillirrigationschemes(Figure6-9).


146

Figure6-9Designsketchofverticaldrop

6.3.2 VerticalDropStructure

DesignProcedure

i. Fixthecrestwidthandlevel:Thecrestwidthofthedropshouldbemadeapproximatelyequal

totheupstreamcanalbedwidth.Thecrestlevelshouldbesetsothatthereisnodrawdownin

thechannelupstream.Theformulaforflowoverthebroadcrestedweiris:

Q= 1.7Wh3/2

Where,Q= theflowacrossthecrest(m3/s)

W= thecrestwidth(m)

h= thewaterdepthacrossthecrest(m)

ii. Fixthewidthofstillingbasinwhichshallbeslightlywiderthanu/screstwidth;

iii. The length and depth of stilling basin is fixed with the consideration of hydraulic jump

formation and energy dissipation therein. The calculation is carried out by trial and error

methodassumingdepthofstillingbasin;

iv. Calculatedropheight,dischargeintensityanddropnumberwithfollowingformula:

Heightofdrop(Y)=drop+basindepth+u/swaterdepth+u/svelocityhead-headoverthe

crest,

Dischargeintensity(q)=discharge/crestwidth

Dropnumber,D=q2/gY

3

v. Based on drop number and drop height calculate pre jump and post jump depths using

formula:

d2/Y=1.66*D^0.27andd1/Y=0.54*D^0.425

vi. Calculatedroplengthusingformula:Ld/Y=4.3*D^0.27

vii. Theverticaldropcanbedesignedusingnomograms(Figure6-10).

viii. CalculateFroudenumberFr=q/d1/ (gd1)0.5 andbasedonFroudenumber calculate lengthof

jump,


147

ix. Lengthofbasin=lengthofdrop+lengthofjump

x. Checkassumeddepthofbasin:

Downstreamwaterdepth+basindepth>postjumpdepth

Large vertical drop structures should be checked for safety against uplift and piping failure

downstream.Downstream,acutoffwallisprovidedtoincreasetheunderflowpathlengthandthus

reduceexitgradient. Insomecases, liningneeds tobeprovidedat thedownstreamof thestilling

basintopreventerosionofcanalbedandbanks.Inaddition,basinsizeanddepthshouldbechecked

for100%,50%and10%ofthedesigndischargeforlargedropstructures.

Figure6-10NomogramforthedesignofVerticalDropStructure

DesignExampleofverticaldrop

DesignData

Discharge = 100lps

Drop = 1.0m

Canalbedwidth= 0.30m

Fullsupplydepth= 0.30m

Bedslope = 1/500

Velocity = 0.60m/s

Sideslope = 1:1(V:H)

Roughnesscoefficient=0.025


148

Calculations

Crestwidthandlevel

Crestwidth=slightlymorethancanalbedwidth=0.4m

Headofwaterovercrest,hwisgivenbytheexpression,

Q=1.7whw3/2

givinghw=0.28m

Upstreamflowdepthinthecanal=0.3m.Thereforetopreventdrawdowneffectthecrestmustbe

setat0.30–0.28=0.02mabovethebedoftheupstreamcanal;forpracticalreasons,allow0.05m.

ThusP=0.05m,

Widthofstillingbasin=U/Screstwidth+0.1m=0.40+0.10=0.50m

Thedesigncalculationfromthispointonwardsisofatrialanderrornature.Themainhydraulic

calculationsfordeterminingthelengthofdrop,lengthofjumpandthesillheightrequire

assumptionofstillingbasindepth,

Assumestillingbasindepth(d4)=0.20m

Calculatevelocityhead=v2/2g=0.30^2/19.62=0.016m

Then,heightofdrop(Y)=drop+basindepth+waterdepth+velocityhead-headoverthecrest

Y=1.0+0.20+0.30+0.016-0.28=1.24m

qisthedischargeperunitwidthinthestillingbasin=0.1/0.5=0.2m3persec

Dropnumber,D=q2/gY

3=0.0011

d2/Y=1.66*D^0.27=0.26thenpostjumpdepth(d2)=1.24*0.26=0.32m

d1/Y=0.54*D^0.425=0.029thenprejumpdepth(d1)=1.24*0.029=0.04m

Ld/Y=4.3*D^0.27=0.86thenlengthofdrop(Ld)=0.68*1.24=0.84m

CalculateFroudenumberFr=q/d1/(gd1)0.5=0.2/0.04/(9.81*0.04)^0.5=7.98

ForFr=7.98Lj/d2=6.25()thenlengthofjump(Lj)=6.25*0.32=2.03m

Totallengthofbasin=lengthofdrop+lengthofjump=0.84+2.03=2.87msay3.00m

Checktheassumeddepthofbasin


0.30+0.20=0.50m>0.32m,hencebasindepthof0.20misOK

Figure6-11RelationbetweenFroudenumberandlengthofjump


149

6.3.3 ChuteDropStructure

DesignConcept

Inhillyregions,canalalignmenthastopassthroughsteepslopesandachutedropisappropriatefor

slopesupto1:2.Thechuteflumeandstillingbasinshouldbemadein1:3slopeswithstonemasonry

orconcreterectangularsection(Figure6-12).Forlongchutes,stillingbasinsareprovidedforevery

10mdropinheight.

Figure6-12Designsketchofchutedrop

DesignProcedure

i. Theflumewidth(W)shouldbemadethesameastheupstreambedwidthofthecanalor

slightlynarrower,

ii. Thecrestheightiscalculatedusingbroadcrestedweirformulaas:

Q=1.71Wh3/2

Where,Qistheflowacrossthecrest(m3/s),

Wisthecrestwidth(m),and

histheheadoverthecrest(m)

iii. Calculatedischargeintensity,q=Q/W

iv. Calculatecriticaldepthofflow3

2

g

qdc =

v. CalculatedroptocriticaldepthratioH/dc,Histhedifferenceinu/sandd/senergylevel,

vi. Fromgraphortablefindd1/dcandcalculateprejumpdepthd1,

vii. CalculateFroudenumber

1

1

gd

VF = whereV1=q/d1

viii. Calculatepostjumpdepthd2from ( )1812

1 2

1

2 −+= Fd

d

ix. FindfromgraphlengthofjumpL/d2

x. Calculatetotallengthofdrop


150

Figure6-13EnergyLossinHydraulicJump


151

DesignExampleofChuteDrop

Data

Discharge=220l/s

U/sandd/scanalwaterdepths=0.50m

Canalbedwidth=0.75m

Dropheight=2m

Calculations

Fixthewidthofchutesameaswidthofu/scanalbed=0.75m

CalculateheadoverthecrestfromQ= 1.71Wh3/2

H=(Q/1.71W)^2/3=(0.22/1.71*0.4)^2/3=0.309m

Sincewaterdepthis0.5mputcrestheightof0.50-0.31=0.19m

Calculatedischargeintensityq=Q/W=0.22/0.75=0.293m3/s/m

Calculatecriticaldepth 3

2

g

qdc = =(0.293^2/9.81)^1/3=0.206m

Differenceinu/sandd/senergylevelisequaltodropheight=2m

ThenH/dc=2/0.206=9.70

Fromthegraphofenergylossinhydraulicjump,d1/dc=0.208(Figure 6-13)

Then,d1=d1/dc*dc=0.208*0.206=0.043m

CalclatevelocityatthestartofjumpV1=q/d1=0.293/0.043=6.84m/s

CalculateFroudenumber1

1

gd

VF = =6.84/(9.81*0.043)^1/2=10.54

Calculate ( )1812

1 2

1

2 −+= Fd

d=½((1+8*10.54^2)^0.5–1)=14.41

Then,d2=14.41*0.043=0.62m

FindfromgraphLj/d2=6.25,thenLj=6.25*0.62=3.86m(Figure 6-11)

Providelengthofbasin=4.00m

Anddepthofbasin=d2-d/swaterdepth=0.62-0.50=0.12m

Providedepthofbasin=0.15m

Hence,designisacceptable.

6.3.4 PipeDropStructure

DesignConcept

Pipedropsmaybeusedforfallsupto4mormoreandareanobviouschoicewherearoadcrossing

isalsorequired.Energydissipationinpipedropsisachievedbyeitherbaffledoutletsorstillingwells.

Thepipesneedtobewellconstructedandwatertight.However,pipesaresusceptibletoblockage

bydebris. Themaximumvelocity in thepipedependsupon theenergydissipationarrangements.

Forsmallirrigationschemes,HDPpipeswithamaximumvelocityofupto3m/sareconsideredand

anoutletboxwithasmallbafflewallissuitablefordownstreamenergydissipation(Figure6-14).To

checkwater hammer, adequate submergence is necessary at the entry and exit of the pipe. The

minimumsubmergenceshouldbehalfofthesectiondepthabovethesoffit.


152

Figure6-14Designsketchofpipeprop

DesignProcedure

Thedesignprocedureforapipedropinvolvestheselectionoftherequiredpipesizebasedonthe

dischargeandassessmentoftheheadlosstocheckavailableheadtherein.

i. Assumeflowvelocityinthepipe,

ii. Calculatetheareaofflowanddiameterofthepipe,

A=Q/Vandπ

Ad

×=4

Where,Aistheareaofthepipe,

Qisthedischargeofthepipedrop,

Visthevelocityinthepipe,

Disthediameterofthepipe

iii. Matchthecalculateddiameterofthepipewiththeavailablestandarddiameterofthepipe

andselectappropriatesizeofthepipe,

iv. Calculatethelossesinthepipe:

a. FrictionlossfromDarcy-WeisbachequationgD

fLVh f

2

42

=

Frictionlosscanbealsoassessedfromthenomogram(Figure 5-11).

b. Entryandexitloss( )

g

VVhe

25.1

2

1

2

2−

=

c. Jointlossistakenas0.25mextralengthforonejoint,

v. Checkthetotallosswithavailabledropinthepipe,

vi. FixthedimensionoftheoutletbasinbytheassessmentofFroudenumber

gd

VF =


153

Where,Vistheincomingvelocityofpipeflowinm/s,gistheaccelerationduetogravity

inm/sec2,disthedepthofflowenteringthebasinandisthesquarerootoftheflow

area,

AccordingtoFroudenumberoftheincomingflowthewidthtodepth(W/D)ratioiscalculatedfrom

USBRgraph(Figure6-15).Theotherdimensionoftheoutletbasindependsuponthewidthofthe

basin.

Depthofbasinatu/s(H)=¾widthofbasin(W),

Lengthofbasin(L)=4/3W,

Lengthoffirstcompartment(a)=1/2W,

Heightofbafflewall(b)=3/8W,

Depthofstillingbasinbelowpipeinvert(d)=1/6W,

Figure6-15Designgraphofoutletbasin(USBR)

Forsmallirrigationschemesbasindimensionsarefixedtentatively.


154

DesignExampleofPipeDrop

Data

Designdischarge(Q) 132l/s

Canalwaterdepth 0.35m

Canalbedwidth 0.35m

Velocityofflow 0.54m/s

Availabledrop 3.00m

Calculations

Assumevelocityinpipe(Vp)=2.5m/s

Calculaterequiredpipeflowarea(A) =Q/Vp=0.132/2.5=0.053m2

Finddiameterofpipe(D)=sqrt(4*A/π)=(4*0.053/3.14)^0.5=0.259m

Thenearestavailablepipesize(D)=280mm(outerdiameter)

Findinnerdiameterofpipe(d)=D-2*thicknessofpipe=280-2*9=262mm>259mm

Calculateheadlossinpipe,

Frictionheadlossfromtablefor100mlength(hf)=1.28mandfor15mhf=0.192m

Velocityofflowinpipeaspertable(Vp)=2.30m/s

Calculateentryandexitloss(he)=( )

g

VVhe

25.1

2

1

2

2−

=

he=1.5(2.3^2-0.54^2)/2*9.81=0.38m

Calculatetrashrackloss(ht)=0.10m(assumed)

Calculatejointandbendloss(hj)[email protected]

hj=1.28/100*0.50*100=0.064m

Totalheadloss=hf+He+ht+hj=0.192+0.38+0.10+0.06=0.74m

Thetotalheadlossislessthanavailableheadatsite(0.74m<3m)andhence,OK

Calculatebasindimensions

Flowareainpipe(A)=πD^2/4=3.14*0.262^2/4=0.054m2

Calculatethedepthofflowenteringthebasin(d)=sqrt(A)=(0.054)^0.5=0.23m

CalculateFroudenumber

gd

VF =

=2.3/(9.81*0.23)^0.5=1.53

Findwidthtodepthrationfromgraph(USBR)forF=1.53,W/D=3.55

Hence,widthofbasin=3.55*0.23=0.82m

Calculateotherdimensionsofthebasin:

Lengthofbasin(L)=4/3W=4/3*0.82=1.10m

Heightofbasin(H)=3/4W=¾*0.82=0.62m

Lengthoffirstcompartment(a)=1/2W=1/2*0.82=0.41m

Depthofbasinbelowpipeinvert(d)=W/6=0.82/6=0.14m

Heightofbafflewall(b)=3/8W=3/8*0.82=0.31m



155

6.3.5 CascadeDropStructure

DesignConcept

Cascadesarebasicallyamasonrystaircasedownaslope.Thedropsofeachstepshouldnotexceed

1 m and should match the step length. A control weir is provided at the top of the cascade to

prevent drawdown in the canal. The stilling basin at the bottom of the cascade is designed as a

straightdrop fallwithadropof1m.Theoriginalground level shouldbeabove thecascade floor

level. Cascade drops are suitable in hills with steep and long drops due to their effectiveness in

dissipatingenergy. Inaddition,cascadedropscanbeconstructed incurvedalignmentusingstone

masonryorconcrete(Figure6-16).

Figure6-16Designsketchofcascadedrop

DesignProcedure

Thedesignprocedurerelateswiththefixationofdropdimensions..

i. Fixthecascadewidth,0.8B<W<1.25B

WhereBisthebedwidthofthecanal,and

Wisthewidthofcascadecrest

Thewidthofthecascadeisgenerallyequaltothewidthoftheupstreamcanalbed,

ii. Calculatedischargeintensityofcascadeweir,

iii. Selectthestepheightbasedontotalheightandshouldbelessthan1m,

iv. CalculatedropnumberfromformulagZ

qD

2

=

Whereqisthedischargeintensityinm3/s/m,


156

Gistheaccelerationduetogravity9.81m/s,

Zisthestepheightinm

v. Calculatehydraulicdroplengthfromformula 27.03.4 DZL

d××=

vi. CalculatethelengthofstepasL>Ld+0.10andcomparewithavailablegroundslope.The

step length and ground slope shouldmatch otherwise re-calculate the step length by

increasingordecreasingwidthofthecascade,

vii. Calculatethecrestheightatthecontrolweiratthetopofthecascadefromthesharp

crestedweirformula6/1

2/384.1 ⎟⎟

⎠

⎞⎜⎜⎝

⎛×××=

t

tB

DDLQ

WhereLtisthelengthofcrestinm,

Distheheadoverthecrestinm,and

Btisthewidthofcrestinmisequalto0.30mforstonemasonrywall,

viii. Calculatesillheightandwidthsofallsteps,

ix. Calculate the lengthofstillingbasinat the laststepbasedonhydrauliccalculationsof

verticaldropstructure,

Design Example of Cascade Drop

Data

Designdischarge(Q) 132l/s

Canalwaterdepth(d) 0.35m

Canalbedwidth(B) 0.35m

Velocityofflow(V) 0.54m/s

Availabledrop(Dr) 4.00m

Calculations

Fixthewidthofcascadeasper0.80B<W<1.25B,whereBisthebedwidthofupstreamcanal,W=B=

0.35m

Calculatedischargeintensity(q)=Q/W=0.132/0.35=0.38m3/s/m

Selectstepheightofthecascade(Z)=0.90mshouldbe<1.00m,

Calculatedropfactor(D)=q2/gZ=0.38^2/9.81*0.90=0.016

Calculatehydraulicdroplength27.0

3.4 DZLd

××= =4.3*0.90*0.016^0.27=1.27m

CalculatesteplengthL=Ld+0.1+sillthickness=1.27+0.1+0.30=1.67m

CalculateslopeandcomparewithgroundslopeS=L/Z=1.67/0.90=1.87which isalmostequal to

thenaturalgroundslopeof1:2andhence,stepdesignisOK.

Calculatetheheadoverthecrestatcontrolsectionfromsharpcrestedweirformula:

WhereLtisthecrestwidthandsameasofcascadewidth=0.35m

Btisthewidthofsillformasonrystructureitistakenas=0.30m

Qisthedesigndischarge=0.132m3/s

6/1

2/3

84.1 ⎟⎟⎠

⎞⎜⎜⎝

⎛×××=

t

wwt

B

hhLQ


157

Puttingthesevalues,5/3

615.0 ⎟⎟⎠

⎞⎜⎜⎝

⎛=

t

wL

Qh thenhwcomesas0.34m

Heightofthecrest=u/swaterdepth–headoverthecrest=0.35–0.34=0.01m

Adaptsillheightas0.05mforallsteps

Designoflaststillingbasinasverticaldropstructure

Assumestillingbasindepth(d4)=0.20m

Calculateheightofdrop(Y)=drop+basindepth+waterdepth+velocityhead–headoverthecrest

Takingdropas0.90m,Y=0.90+0.20+0.35+0.01–0.34=1.12m

qisthedischargeperunitwidthinthestillingbasin=0.132/0.35=0.38m3/sec/m

Calculatedropnumber,D=q2/gY

3=0.38^2/(9.81*1.12^3)=0.01

d2/Y=1.66*D^0.27=0.48thenpostjumpdepth(d2)=1.12*0.48=0.54m

d1/Y=0.54*D^0.425=0.08thenprejumpdepth(d1)=1.12*0.08=0.09m

Ld/Y=4.3*D^0.27=1.2thenlengthofdrop(Ld)=1.25*1.12=1.40m

CalculateFroudenumberFr=q/d1/(gd1)0.5=0.38/0.09/(9.81*0.09)^0.5=4.73

ForFr=4.66Lj/d2=6.00thenlengthofjump(Lj)=6.00*0.54=3.25m

Totallengthofbasin=lengthofdrop+lengthofjump=1.40+3.25=4.65msay5.0m

Checktheassumeddepthofbasin


0.35+0.20=0.55m>0.54m,hencebasindepthof0.20misOK

Hence,designisacceptable.

6.3.6 EscapeStructure

DesignConcept

Escape structures are safety structures which release the excess water of the canal. Escape

structuresarealsoused for theemergency releaseof canalwater in caseof canalbreach.Excess

water can enter into the canal both in the dry and rainy seasons. Choking of canals by minor

landslipscanoccurwithoutanywarningandcancausecanalwatertoovertop.Escapestructuresare

essential primarily along the main canals in order to facilitate the safe disposal of excess canal

water.Theseescapestructuresarepositioned:

• Neartheintake,

• Inthemaincanalatanintervalof1kminthehills,

• Atthetailofmainandsecondarycanals,

• Attheupstreamofthemaincrossdrainagestructuressuchassiphons,longcoveredcanalin

landslidezone,aqueductsetc.

Escapestructuresarebothgatedandungated.Inaddition,anovertoppingsectioninthelinedcanal

or in aqueducts ismade in small scale irrigation schemes in order to escape excesswater.Gated

escapestructuresareprovided in large irrigationschemesandneedhuman interventiontoadjust

andcontrolflow;theyarehencenotrecommendedinremotelocations.


158

DesignProcedureofSimpleEscape

i. Fixthedesigndischargeoftheescapetobeescapedthroughthestructure.Generally100%

of the canal discharge is estimated as excess dischargewhenweir crest is set at the bed

levelofthecanal.Forautomaticspillwaysection50%ofcanaldischargeisestimated,

ii. Fixthetopofthestoplogsas0.10mabovethewaterlevelofthecanal(Figure6-17),

iii. Calculatethewidthoftheescapechannelfromtheformula:

Q=1.71Wh3/2

Where, Qistheflowacrossthecrest(m3/s),

Wisthecrestwidth(m),and

histhewaterdepthacrossthecrest(m)

iv. Forsafetyconsiderationthecalculatedwidthoftheescapeismultipliedby1.2,

v. Forsmallhillirrigationschemesthewidthofescapeshallbe1mwide,

vi. Providedownstreamenergydissipationarrangementbasedonvelocityofflowandnatural

topography,

OvertoppingSections

Over topping sections are provided in the primary canals where appropriate topography allows

eithertochannelizetheescapedwatertodissipatetheenergyof flowingwater.Theovertopping

sectionisusuallysetslightlyabovethecanalfullsupplylevel.Spillingstartsautomaticallywhenthe

water level rises above the sill level. In remoteareas,overflow sectionsandescape sectionswith

stoplogsarecombinedtogethertoeasetheoperation.Inthiscase,spillcapacitycanbeincreased

byremovingthewoodenstoplogs.


159

Figure6-17Sketchofstop-logescape


160

6.3.7 BridgesandCulverts

General

Many irrigation structures incorporate bridges and culverts in order to facilitate vehicular and

animal traffic across the canals and natural drainages. In medium irrigation schemes, head

regulatorsmayhaveapipeculvert totakeoffwater fromtheparentcanal,and in large irrigation

schemesabridgedeckmaybeincorporatedintheheadregulator.Whenaroadcrossesacanalat

somedistance fromanyother structure itwill be necessary to provide an independent bridgeor

culvert. The choice of culvert or bridge depends on the nature of the crossings and economic

grounds.Forsmall structures,pipeculvertsaresuitableandconcreteboxculvertsaresuitable for

largecanals.

Designconceptofbridge

The design of bridges involves hydrological assessment, hydraulic design, and structural design.

Under hydrological assessment, it involves the determination of the design discharge and design

waterlevel.Thedesigndischargeofthebridgeisequaltothedesigndischargeofthecanalincase

of canal. For a natural drainage channel, the design discharge is equal to 1.50 times the design

discharge of the drainage channel. The design discharge of the natural drainage channel shall be

assessed using various formulae as described in Chapter-3 Hydrology and Agro-meteorology. The

designwaterlevelofthebridgeshallbedeterminedaccordingtothedesigndischargeasmentioned

above.

Thehydraulicdesignofthebridgeinvolvesthecalculationofthewaterway,scourdepth,afflux,and

velocity due to obstruction. For canals, the waterway shall pass the design discharge at normal

velocitywhile for natural drainage channels, thewaterway shall be determined based on Lacey’s

formula and/or the topographical conditions of the site. Scour depth is determined considering

regimewaterwayforthedesignoffoundationdepthandprotectionworks.Theaffluxandvelocity

due to obstruction is determined for the assessment of the clear height of the bridge above the

designwaterlevel.

Thestructuraldesignofthebridgeiscarriedoutassimplysupportedreinforcedconcretestructure

dependinguponthetypeofthebridgeandcodeofpracticeofthecountry.Amultiplespanbridgeis

accommodatedforlargespans.Thegeneraldesignfeaturesofthebridgearewidthofcarriageway,

widthofsidewalkandcurb,freebroad,expansionjoints,bearings,andloadingconditions.TheRoad

ActofRoyalGovernmentofBhutan,2004 includesfifteenroadtypeswhichareclassified intofive

categories:

i. Nationalhighways,

ii. Dzongkhagroads,

iii. Farmroads,

iv. Thromderoads,and

v. Accessroads

Thebasicdesign featuresof thebridgedependupon the roadclassificationswhicharepresented

hereunder(Table6-4).


161

Table6-4BhutanRoadStandards

Road

classification

Primary national

highways

Secondary

nationalhighways

Dzongkhagroads Farmroads

Carriageway

width(m)

6.50 3.50 3.50 3.50

Shoulder width

(m)

1.50x2 1.50x2 0.50x2 0.50x2

Structure loading

(minimum)

HS20-44 As per DOR

standard

As per DOR

standard

As per DOR

standard

Source:RoadGuidelines,RGOB,2004

Normally 100mm pipes are provided at mid-span of bridge deck for drainage. Bridge decks are

supportedwithcontinuous rubber stripbearingateitherendwithoneend fixedwithdowelbars

andoneendfree.TrafficloadingsaretakenaccordingtotheDORstandard(Figure6-18).

Figure6-18Typicalroadbridge

Designprocedureofbridge

i. Determinethedesigndischargeofthebridge,incaseofcanalitisthefullsupplydischarge.

Fornaturaldrainageitisdeterminedonthebasisofreturnperiodflood,

ii. Determinethemaximumwaterlevelbasedonthedesigndischarge,

iii. Calculatethelineareffectivewaterwayofthebridgefromformula,

QCL =

WhereListhelinearregimewaterwayinm,

Cisthecoefficientdependsonlocalconditionsandvariesfrom4.50to6.30,and

Qisthedesigndischargeofthebridgeinm3/s,

iv. Calculatetheeffectivespanandwidthofthebridge,

v. Determine the loading conditions and calculate loads dead load, live load,wind load and

earthpressure,

vi. Designofconcreteslabofbridgedeck,

vii. Designofconcretebeamsandgirders,


162

viii. Carryoutfoundationdesignofthebridge,

Designconceptofculvert

Aculvertisdefinedasstructuresizedhydraulicallytoconveysurfacewaterunderahighway,farm

roads,andotherembankments.Thedesignofaculvertinvolveshydrologicalassessment,hydraulic

design,andstructuraldesign.Underhydrologicalassessment, it involves thedeterminationof the

design discharge and the designwater level. The design discharge of the culvert forwhichwater

levelshallbedesigned isequal tothedesigndischargeof thecanal incaseofcanal.Foranatural

drainagechannel thedesigndischarge isequal to1.50 times thedesigndischargeof thedrainage

channel. The design discharge of the natural drainage channel shall be assessed using various

formulaeasdescribedinChapter-3HydrologyandAgro-meteorology.

Thehydraulicdesignofaculvertdependsonitsflowconditionswhetheritisopenchannelflowor

conduitflow.Theflowthroughaculvertiscalculatedas:

hgCbdQ Δ= 2 incaseofopenchannelflow,

hgCAQ Δ= 2 incaseofconduitflow

Where∆histheheadlosscrosstheculvert,

banddarethewidthanddepthofculvertforopenchannelcondition,and

Aistheareaofculvertincaseofconduitflow,

Thewaterwaywidth of the culvert is determined based on the type of culvert andwidth of the

channel. In a trapezoidal channel, thewidth is kept similar or slightly less than thewidth of the

channel. For culverts flowing full, the head loss through the culvert shall be limited to 15 cm for

determining the sizeof theboxorpipe. For small culverts,head loss isoften takenas0.10m. In

ideal conditions, pipes should not run more than 2/3 of their diameter. Scour depths at the

upstream and downstream of the culvert is considered as 1.25 and 1.50 times the regime scour

depth.

Thebasicdesignfeaturesoftheculvertdependonwhetherit isaboxorpipeculvert.Thesizeof

thepipeculvertdependsuponthestandardconcretepipediametersofBhutanwhichare300mm,

450mm,600mm,900mmand1200mm.Theminimumcovertothepipesshouldbe0.60mand

theroadshouldbeprovidingwithahardcoresurfacing.InBhutantwocategoriesofconcretepipes

areavailable for thepurposepipe culverts:NP2andNP3.Generally,NP2pipesareused for road

works while NP3 pipes are used for irrigation works. The detailed characteristics of standard

concretepipesisfoundinISCode458:2003.

Designprocedureofculvert

i. Determinethedesigndischargeoftheculvert,incaseofcanalitisthefullsupplydischarge.

Fornaturaldrainageitisdeterminedonthebasisofdesignflooddischarge,

ii. Determine the maximum water level and velocity of flow based on the design flood

discharge,

iii. Calculate thedesignvelocity into the culvert,dcVV 5.1= whereVc is thevelocity in culvert

andVdisthevelocityindrain,

iv. Calculatetheflowareaoftheculvert,cdP VQA /= whereQdisthedesignflooddischargeof

theculvert,


163

v. Selectthepipediameterofthepipebasedonavailablestandardofpipes,twopipesormore

maybeselectedifnecessary,

vi. Calculate the pipe base level at its upstream side,pdp DDElEl ×−+= 80.0 where Eld is

upstreambedlevelofdrain,DisthedepthofwaterofdrainandDpisthediameterofpipe,

vii. Checkthesufficiencyofthepipecoverbasedonthetoplevelofpipeandcanalbedlevel,

viii. Determinetheheightofparapetwalls,

ix. Calculatethelengthofpipebasedontheroadcarriagewaywidth,shouldersandcoverover

thepipe,

x. Determinethenumberofconcretepipesbasedontheavailablelengthofthepipe,

xi. Determinetheheadlossthepipeandscourdepthforu/sandd/scutoffwalls,

xii. Calculatetheu/sandd/sapronlengths,

Designexampleofpipeculvert

Data

Designfloodflow(Qd) =0.67m3/s

Velocityatfloodflow(V) =1.5m/s

Waterdepthofthedrain(D) =0.40m

Bedlevelofdrain(u/s)(Eld) =550.00m

Roadtoplevel(Elc) =553.00m

Roadbaselevel(Elb) =552.00m

Calculations

Velocityofflowinculvert,Vc=1.5*V=1.50*1.50=2.25m/s

Calculateflowareainthepipe,Ap=Qd/Vc=0.67/2.25=0.30m2

Select the pipe diameter from the available nearest pipe size and for water depth of 0.80 times

diameterofpipe,Ap=0.67d2,thend=sqrt(Ap/0.67)=0.67m

Takepipediameterof900mmwithareaof0.545m2,

Calculatethepipebaselevel,pdp DDElEl ×−+= 80.0

Elp=550.00+0.40–0.80*0.90=549.68m

Calculatethepipetoplevelconsideringthicknessofpipe,=549.68+0.90+0.10=550.68m

Checkthecanalover=552.00–550.68=1.32mwhichisgreaterthan0.60mandacceptable

Provideheightofparapetwallas0.30mandcalculatetoplevelofparapet,=550.68+0.30

Elpar=550.98m

Calculatetotallengthofpipe,Lp=carriageway+2*slopelength+2*transition

Lp=3.50+2*(553.00-550.98)*1.5+2*1.5=12.56m

Provide5pipeswith2.50meach,totallengthofpipe,Lp=12.50m

Provideu/sandd/scutoffdepthof1mand2mlengthapron

Hence, the design is acceptable.


164

Designexampleofboxculvert

Data

Designfloodflow(Qd) =1.96m3/s

Velocityatfloodflow(V) =1.5m/s

Waterdepthofthedrain(D) =0.78m

Bedlevelofdrain(u/s)(Eld) =550.00m

Naturalwaterwayofdrain(L) =3.00m

Depthofdrainfromgroundlevel =4.00m

Siltfactor(f) =1.25m

Calculations

CalculateLacey’swaterway,db QL 75.4= =4.75*(1.96)^0.50=6.65m

NaturalwaterwayissmallerthanLacey’swaterway.

Calculatethewidthofboxculvertfrombroadcrestedweirflow, 2/371.1 HBQ ××=

Then, ( )2/371.1/ HQL db ×= =1.96/(1.71*0.78^3/2)=1.66m

Providewidthofrectangularboxculvert=2.00mandheightofculvert=2.00m

Calculatescourdepth,3/1

2

35.1 ⎟⎟⎠

⎞⎜⎜⎝

⎛×=

f

qR ,q=Qd/b=1.96/2=0.98

3m/s/m

R=1.35*(0.98^2/1.25)^1/3=1.24m

Calculateanticipatedscoursatu/sandd/softheculvert,

u/sscour,Ru/s=1.25*R=1.25*1.24=1.55m

d/sscour.Rd/s=1.5*1.24=1.50*1.24=1.85m

Calculatethedepthofscourbelowbedlevel,scourdepth–waterlevel

u/sdepth=1.55–0.78=0.77m

d/sdepth=1.85–0.78=1.07m

Provideu/scutoffdepthof1.00mandd/sdepthof1.50m

Checkthedischargingcapacityofboxculvert, 2/371.1 HBQ ××=

Q=1.71*2.00*0.78^1.5=2.35m3/s>1.96m

3/sandhencedesignisacceptable.


6.4 CrossDrainageStructures

6.4.1 Introduction

Themaincanalsofhill irrigationschemesrunalongthecontours.Thesecanalsoftenhavetocross

severaldrainagechannelsorgullies.Specialstructureshavetobeconstructedatdrainagecrossings

topassthecanalover,under,ordirectlyacrossthedrainagestreambed.Thesespecialstructures

areknownascrossdrainagestructures.Inaddition,therearelevelcrossingsusedtopassthecanal

wateracrossthedrainage.Insummary,therearefivetypesofcrossdrainagestructuresinirrigation

schemes.

• Aqueduct-whencanalpassesoverthedrain,

• Superpassage-whencanalpassesunderthedrain,


165

• Siphon-shallowordeep-whencanalpassesunderthedrain,

• Drainculvert-whendrainpassesunderthecanal,and

• Levelcrossing-whencanalcrossesthedrainatthesamelevel

Traditional community-managed irrigation schemes have either level crossings or wooden log

aqueductstocrossthecanalthroughnaturaldrainage.InGovernment-assistedirrigationschemes,

aqueducts,superpassagesandsiphonsarecommonlyseeninBhutan.Inhillirrigationschemes,the

canalcanalsobepassedthroughtheHDPpipecrossing,whichislikeanaqueduct.

6.4.2 SelectionofCrossDrainageStructure

Theselectionofthetypeofthecrossdrainagestructureisthemostimportantaspectofthedesign.

Fivefactorsneedtobeconsideredinselectingthecrossdrainagestructureatagivenlocation:

• Topography,

• Structuralstability,

• Hydrauliccompatibility,

• Stabilityofdrainagestream/gully,and

• Cost

Topographymeanstheshapeofthegullyincrosssectionandinplan.Theshapeofthegullycross

sectionwilllargelydeterminethetypeofcrossdrainagestructureattheparticularlocation.

Differentcrossdrainagestructureshavedifferentstabilityrequirementsandtheselectiondepends

uponthestabilityconditionsofthatlocation.Forexample,deepsiphonsandaqueductsneedstable

banksforstructuralstability.Similarly,asuperpassagerequiresanon-erodingstreambed.

Hydrauliccompatibilitymeansavailabilityofadequatehydraulicheadtodrivethecanalflowacross

the cross drainage structure. In comparison to aqueducts and super passages, inverted siphons

requiresignificantheaddifference inthe inletandoutletofthestructure.Similarly,relativewater

levels of the canal and drainage channel also affect the selection of the type of cross drainage

structure.

Thestabilityofthedrainagestreamorgullyensuressafetyofthecrossdrainagestructure.Widening

gullies candestroycrossdrainagestructure suchasaqueducts, siphons, superpassages,and level

crossings.Deepeninggulliescanalsodestroystructuressuchassiphonsandsuperpassages.

When two ormore technically sound alternatives are available for crossing a drainage gully, the

selectioncanbebasedonthecostsofthedifferentalternatives.Thecostsofallappertainingworks

such as increased length of canal, downstream protection, covered etc must be included in

calculatingthecostsofdifferentalternatives.

These five factors must be jointly considered when selecting the type of cross-drainage works

suitableforanygivenlocation. Atypical layoutfortheselectionofthecrossdrainagestructureis

illustratedhere(Figure6-19).


166

Figure6-19TributaryRiverCrossingOptions


167

6.4.3 DesignConsiderations

DesignFlood

The design flood discharge for the design of a cross drainage structure is assessed following

ungaugedcatchmentfloodflowassessmentdescribedinChapter-3ofthismanual. Insmalldrains,

thefloodreturnperiodmaybeconsideredas5to10years.Foraqueducts,a10yearreturnperiodis

essential while for super passages, siphons, and culverts, a 5 year return period is adequate.

PunakhaValleyDevelopmentProjectManual 1992 suggests a 10 year returnperiod flood for the

designfloodofcrossdrainagestructures.

WaterwayoftheDrain

Thewaterwayof thedrain isessential tocalculate inorder toavoidconstrictionof the floodflow

duetotheconstructionofthestructure.Inhillsandmountains,thewaterwayisprovidedwithinthe

existingdefinedbanksoftheriverordrain. Inplainareas,Lacey’swaterwaymaybeappropriate;

QP 75.4= where,Qisthedesignflooddischargeofthedrainorriver.

ContractionandExpansionofWaterway

Thecontractionandexpansionofthewaterwayofthedrainagechannelshouldbesmoothinorder

tominimizethelossofheadtherein.Contractionshouldnotexceed20%fordrains.Thetransitions

oftheaqueduct,siphon,andculvertbarrelshouldnotbesteeperthan2:1incontractionand3:1in

expansion.ThetransitionsaredesignedonthebasisofMitra’shyperbolictransitionequation:

( )fnnf

ffn

xBBXBL

LBBB

−−×

××=

Where,Bxisthebedwidthatanydistancexfromtheflumedsection,

Bnisthebedwidthofnormalcanalsection,

Bfisthebedwidthofflumedcanalsection,

Lfisthelengthoftransition,

HeadLossthroughStructure

The lossofheadacrossthestructurealsogovernsthechoiceofthestructure.Adequatehydraulic

headdrives thecanal flowacross thedrainagestructure.Aqueductsandsuperpassageshave less

headlosswhilesiphonsrequirehigherheaddifferencebetweentheirinletandoutlet.Thevelocity

throughthebarrelofasiphonor intheflumesectionofanaqueduct is limitedto2to3m/s.The

lossofheadatcontractionandexpansionoftheaqueductisassessedasfollows:

Lossofheadincontraction,⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

g

vvhc

220.0

2

122

Lossofheadinexpansion,⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −=

g

vvhe

230.0

2

432

Where,V1,andV4arevelocitiesattheentryandexitcanalsections,V2,andV3arethevelocitiesat

thecontractionandexpansionflumesectionsrespectively.Forsmallcanals,theentryandexitloss

ofanaqueductisalsocalculatedas:


168

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −=

g

vvh cf

f2

50.1

22

Where,VfandVcarevelocitiesofflumeandcanalsectionsrespectively.

ThelossofleadthroughsiphonbarrelisassessedbyUnwin’sformula:

g

V

g

V

R

Lffh

a

221

22

21−⎥

⎦

⎤⎢⎣

⎡×++=

Where,f1isthecoefficientofheadlossatentryanddependsupontheshapeofentry.Forunshaped

mouthf1=0.505andforbellmouthentryf1=0.08.

f2isthecoefficientofheadlossthroughthebarrelduetosurfacefriction, ⎟⎠

⎞⎜⎝

⎛+=R

baf 1

2

aandbarethecoefficientsdependinguponthematerialofbarrelconstruction;arangesfrom

0.032to0.010andbrangesfrom0.025to0.25.

Listhelengthofbarrelinm,

Visthevelocityofflowthroughbarrelinm,

Vaisthevelocityofapproachandisoftenneglectedinm/s,and

Risthehydraulicradiusofthebarrel

6.4.4 DesignofAqueduct

DesignConcept

Aqueducts areusedwhere canal crossesoverdeeply incised streamsordrains. Thereare various

typesof aqueductsdependingupon the constructionmaterial. Stonemasonrywalls and concrete

flumeaqueductsarethemostcommonlyusedaqueducts(Figure 6-20).Inaddition,HDPEpipeson

woodenbeamsorgalvanizedcablesarealsousedinhillsandmountains.Thebasicconsiderationsin

thedesignofanaqueductare:

• HFLofdrainorstreamshouldbesufficientlybelowthebedlevelofthecanal;

• Banksofthedrainshouldbestable;

• Forsmallirrigationschemes,thespanoftheaqueductshouldbelimitedto10m;

• Watervelocitiesintheflumeshouldbefrom1.0to1.5m/s

Figure6-20Typicalaqueducts


169

AdvantagesofAqueduct

• Minimumheadloss,

• Safeagainsthydrologicaluncertainties,

• Noerosionproblemifbuiltonstablefoundations,and

• Tobeusedforpedestrianwalkway

DisadvantagesofAqueduct

• Difficultconstructionincaseofdeepgullies,

• Foundationerosionincaseofshallowandwidegullies,

DesignProcedures

i. CalculatedesignflooddischargeofthedrainanditsHFL,

ii. Establishlevelsanddimensionsofthecanals,

iii. Establishlevelsandwaterwayofdrain,

iv. Fixthespanofaqueduct,

v. Designflumesection,

vi. Fixdimensionsofflumeandtransitions,

vii. Calculatethelossesintheflumeandtransitions

viii. Checkavailableheadatupstreamanddownstreamofthestructure,and

ix. Carryoutstructuraldesignwithsizingofductsection,piersandabutments

The design sketch of an aqueduct is presented in Figure 6-21 .

DesignExampleofRCCAqueduct

Canaldata:

Discharge(Q)=0.05m3/s

Bedwidth(B)=0.30m

Sideslope(m)=1:1

Waterdepth(d)=0.25m

Freeboard(Fb)=0.15

Draindata:

Catchmentarea(CA)=4sqkm

Flooddischarge(Qd)=40m3/s

Bankheight(hb)=5m

Highflooddepth(HFL)=3m

Naturalwaterway(L)=8m

Calculations:

i. Designflumesection

Providewaterdepthofflumesameasofcanaldepth(Hf)=0.25m

Assumevelocityofflowatflume(Vf)be0.75m/s


170

Thenwidthofflumesection=Q/Hf*Vf=0.267m

Adoptbedwidthofflume(Bf)=0.30m

TakeManning’sroughnesscoefficient(n)=0.02

Adoptslopeofflume(S)=1/200

Areaofflowatflume(Af)=Bf*Hf=0.30*0.25=0.075m2

Wetterperimeter(Pf)=Bf+2Hf=0.80m

Hydraulicradius(Rf)=Af/Pf=0.075/0.80=0.094m

Velocityofflow(Vf)=1/n*Rf^2/3*Sf^1/2=0.094^0.0667*(1/200)^0.50/0.02=0.73m/s

Discharge through flume (Qf) = Vf * Af = 0.73 * 0.075 = 0.055m3/swhich is greater than design

discharge(0.05m3/s),

Froudenumber(Fr)=Vf/(g*hf)1/2=0.466whichislessthan0.60andmaintainssub-criticalflowand

hencedesignisacceptable.

Figure6-21Designsketchofaqueduct

ii. Fixflumedimensions

Forsmalldischargeprovidelengthofcontractionandexpansiontransitions=1m

Lengthofflume=clearspanofaqueduct=8m

Takefreeboardforflumesection(Fb)=0.15m

Heightofductwall(Hf)=0.25+0.15=0.40m

Widthofbeam(Bq)=½ofdepth=0.20m,takewidthas0.25m

Assumethicknessofslab(ts)=100mm

Theoveralldepthofaqueductbeam(Dq)=Hf+Fb+ts=0.25+0.15+0.10=0.50m


171

iii. Calculationoflosses

Frictionlossattheflume(hf)=slopeofflumexlengthofflume=1/200*8=0.04m

Entryandexitloss⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −=

g

vvh cf

e2

50.1

22

FlumevelocityVf=0.73m/s

Canalvelocity=Q/A=0.05/(0.30*0.25+1*0.25^2)=0.364m/s

he=1.5(0.73^2-0.364^2)/2*9.81=0.0306m

Totalloss=hf+he=0.04+0.0306=0.070m

Thewaterlevelatthedownstreamendofaqueductshouldbeabout10cmbelowthewaterlevelof

theupstreamcanalwaterlevel.

iv. Structuraldesignofaqueduct

Thestructuraldesignofaqueductinvolvesthedesignofbaseslab,sidebeams,andabutmentwalls.

Designdata:

Spanofaqueduct=8m

Widthofcanal=0.30m

Widthofductsection=0.30m

Widthofsidebeam(bs)=0.25m

Depthofwater=Hf=0.25m

Freeboard=0.15m

Concretetype=M20

Densityofconcrete=24kN/mm2

Permissiblereinforcementstressincompression(pbc)=7.0N/mm2

Permissiblereinforcementstressintension(pst)=230N/mm2

Maximumshearstress(qcmax)=230N/mm2

TensileforceforReinforcementFe415(fst)=230N/mm2

Allowablebondstress(qbs)=1.12N/mm2

Densityofwater(gw)=10.0kN/mm3

Designofbaseslab:

Thicknessofslab=assumeas100mm

Deadloadofslab(qd)=24=0.10*1=2.4kN/mfor1mwidesection

Imposedloadduetowater(qw)=0.40*10.0=4.00kN/m

Totalloadonslab(q)=2.40+4.00=6.40kN/m

Effectivespanofslab(L)=0.30+0.25/2+0.25/2=0.55m

Maxbendingmomentonslab(Ms)=qL2/8=6.40*0.55^2/8=0.242kNm

Maxreactionatthebeam(Rs)=qL/2=6.40*0.55/2=1.76kN

Maxshearforce(F)=Rs=1.76kN

Calculateslabthickness(d)Ms=bd2;d=(Mr/Rb)^1/2=(0.242*1000/0.91)^0.5=16.31mm

Assumeslabcoveras=50mm


172

Totalslabthicknesswith10mmallowance(t)=16.31+50+10=76.31mm

Actualeffectivethicknessofslab(teff)=100-60=40mm

Steelreinforcementforslab(Ast)=Ms/pst*jd=(0.242*1000*1000)/(230*40*0.91)=29.09mm2

Minreinforcementrequired=0.25%ofareaofconcrete=250mm2

Provide12mmdiabars@200mmc/c=Ast’=339mm2

Check for shear stress (qv) = F/bd = (1.738*1000)/(1000*40)= 0.043N/mm2< allowable value of

1.80N/mm2

Distributionsteelontop=0.10%ofareaofconcreteor20%ofAst=100mm2

Provide10mmdiabars@150mmc/c=237mm2

Designofbeam:

Beandeadload(qb)=24*0.25*0.5=3.0kN/m

Deadloadofslab(qs)=0.50*0.30*100*24/1000=0.36kN/m

Weightofwater(qw)=0.50*0.30*0.25*10=0.375kN/m

Totalloadonbeam(q)=6.0+0.36+0.368=3.735kN/m

Effectivespanofbeam(Leff)=8.0+2*0.30=8.60m

Maxbendingmomentofbeam(Mb)=qL2/8=3.735*8.60^2/8=34.53kNm

Maxreactionatthebeam(Rs)=qL/2=3.728*8.60/2=16.06kN

Maxshearforce(F)=Rs=16.06kN

Calculateslabthickness(d)Mb=bd2;d=(Mb/Rb)^1/2=(34.53*1000/0.91)^0.5=194.80mm

Assumeslabcoveras=50mm

Totalslabthicknesswith10mmallowance(t)=194.80+50+10=254.80mm

Actualeffectivethicknessofslab(teff)=500-60=440mm

Steelreinforcementforslab(Ast)=Ms/pst*jd=(34.53*1000*1000)/(230*440*0.91)=377.44mm2

Provide2-20mmdiabars=Ast’=628mm2

Checkforbarspacing@minbardiameterorsizeofcoarseaggregateplus5mm=20mmor25mm

Actualbarspacing=beamwidth–twicecover–stirrups–noofbars=250-2*50-10-2*20=90mm>

25mm,henceOK

Checkminreinforcement=0.25%ofconcretearea=0.25*0.25*0.50*1000*1000=312.50mm2,OK

Check for shear stress (qv) = F/bd = (16.06*1000)/(250*440)= 0.146N/mm2 < allowable value of

1.80N/mm2

Distributionsteelontop=0.15%ofareaofconcreteor20%ofAst=187.50mm2

Provide2-12mmdiabars=226mm2

Sidereinforcementonbeam=0.15%ofareaofconcrete=187.50mm2

Provide2-12mmdiabars=226mm2

Thedesignofabutmentwallsiscarriedoutsimilartothedesignofretainingwall.

Hencedesigniscomplete.


173

6.4.5 DesignofSuperPassage

Introduction

Asuperpassageisconstructedwherethecanalcrossesrelativelysteepstreamsorsmallrivers.The

streamisusuallykeptonthesamegradientandthecanalcutintothegroundbelowthebed.Inthe

hills,superpassagesareacommonstructureforsmalldrainagechannels.Asuperpassagetakesthe

irrigation canal under the drainage stream at the same natural gradient as the canal. The canal

waterdoesnotcomeintocontactwiththedrainagewater.

Thecanalisflumedandthestreamiscrossedinalinedchannelovertheflume.Thestreamchannel

is usually guided upstream and downstream of the crossing. Sidewallswith sufficient height are

provided to prevent overtopping during floods, which are sufficiently embedded upstream to

prevent outflanking to the super passage. The crossing should bemade steep enough to prevent

siltation,andchannelprotectionwillbemadetopreventscourdownstream.Typicalphotosofthe

superpassagearepresentedbelow(Figure6-22).

Figure6-22Typicalsuperpassages

AdvantagesofSuperPassage

• Saferthanlevelcrossingsbecausedrainflowspassoverthecanalwithoutanyrestriction,

• Sedimentproblemislargelyavoided,

• Minimumheadloss,

• Nofoundationproblems

DisadvantagesofSuperPassage

• Notsuitablefordeepgullies,

• Needtodesignwithhighmarginofsafetyduetouncertaintyofdrainagecharacteristics,

• Conduitsmaychokebydebris,

• Mayinvolvelongdiversionofthecanaltofindsuitablecrossingpointclosetothehillside

DesignConcept

Thegradientofthedrainagechannelshouldbekeptonthesamegradeasfaraspossibleinorderto

pass safely the flood flow. The stream width should not be restricted as far as possible. The

downstreamof the superpassage shouldbeprotected in adequate length tominimize scour and

erosion. The canal is excavated below the level of the drainage channel and covered masonry

conduitsorconcretepipesmaybeused.Pipesandconduitsshouldbeoversizedtoallowaccessfor


174

cleaning out debris and sediment. The sidewalls of the super passage should be high enough to

allowfreeboardtohighfloodlevelofthedrainagechannel.

DesignProcedure

Thedesignprocedureof the super passage involves the assessmentof thedesign flood flowand

calculationofthesizeofdrainagechanneloverthecanal.

i. Assessdesignflooddischargeofthedrainbasedoncatchmentcharacteristics,

ii. Designlinedcanalsectionorflumesection,

iii. DesigndrainsectionusingManning’sequationtomatchwithdesignfloodflow,

2/13/21SR

nV =

WhereVisthevelocityofdrainageflow,

nistheroughnesscoefficientdependsuponthetypedrainagechannel,

Risthehydraulicradiusofthedrainagesection,

Sisthewatersurfaceslopeofthedrainageflow

iv. CalculateFroudenumber

gD

VF =

v. Assesstheenergydissipationrequiredatthedownstreamofthesuperpassage(Figure6-23)

DesignExampleofSuperPassage

CanalData:

Designdischarge(Qd): 50lps

Bedwidth(B): 0.30m

Waterdepth(d): 0.25m

Freeboard(Fb): 0.15m

Bedslope(S): 1/500

Canalsideslope(m): 1:1

Roughnesscoefficient(n): 0.025

DrainData:

Designflooddischargeat5yearsreturnperiod(Qf):5m3/s

Widthofdrain(w): 5m

Slopeofdrain(Sd): 1/20




175

Figure6-23Designsketchofsuperpassage

Designoflinedcanalsection:

AdoptB/Dratioas: 1

Bedwidth(B): 0.35m



Bedslope(S): 1/500

Canalsideslope(m): 0(rectangularsection)


Areaofflow(A)=B*D 0.12m2

Wettedperimeter(P)=B+2*D 1.05m

Hydraulicradius(R)=A/P 0.12m

Velocityofwater(v)=1/n*R^2/3*S^1/2 0.53m/s

Discharge(Q)=A*V 0.07m3/s;hence>0.05m

3/s,OK

Designofsuperpassage(linedsection):

Adoptwidthofpassage(W): 5m(sameasdrainwidth)

Providedepthofwater(d): 0.40m


Areaofflow(A)=B*D 2.0m2

Wettedperimeter(P)=B+2*D 5.80m

Hydraulicradius(R)=A/P 0.34m

Velocityofwater(v)=1/n*R^2/3*S^1/2 3.48m/s

Discharge(Q)=A*V=2*3.48= 6.95m3/shence>5m

3/s,OK

ThevelocityissupercriticalasFroudenumber

gD

VF =

=1.76


176

AsFroudenumberis1.76andflowissupercritical,itisessentialtodissipateenergydownstreamof

the super passage by providing protection works. In hilly areas where hard rock may exist

downstreamofthesuperpassage,protectionworksmaynotbeneeded.

6.4.6 DesignofCanalSiphon

Introduction

Invertedsiphonsareusedtocrossthedrainagechannelbyacanal.Therearetwotypesofsiphons:

• Shallowsiphontocrossshallowandwideriverwithlittlepressurehead,and

• Deepsiphontocrossdeepriverwithhighpressurehead

Shallowsiphon:Shallowsiphonsaredesignedsothattheriverordrainiskeptonthesamegradient

and the canal flows in a conduit under the river bed. Small siphons are often designed using

concretepipeswhilelargesiphonsaredesignedusingconcretebarrel.Adequatevelocityshouldbe

providedtopreventsiltationat lowflows.Forsmall irrigationcanals,adroptypeinletcanalsobe

provided.Topreventchokingfromfloatingdebris,atrashrackisprovidedupstreamoftheinverted

siphon.

Figure6-24Typicalshallowanddeepsiphons

DeepSiphon:Deepsiphonsareusedinhillschemestocrosstheriverindeepvalleys.Thesiphons

are takendownthevalley side,anchoredbyblocks,and thencrossed the riveron theshort span

bridge.Duetohighpressuresthesiphonsareusuallymadeofsteelpipes(Figure6-24).

Advantagesofsiphon

• Preferredinwidestreams,

• Nofoundationproblems,

• Drainageflowdoesnotaffectstructuralsafety

Disadvantagesofsiphon

• Significantheadloss,

• Pipemaychokewithsedimentanddebris

DesignConceptofShallowSiphon

Atrashrackscreenisrequiredattheupstreamentryofthesiphon.Someformofprotectionisalso

required above the pipe line in the river bed. Adequate submergence should be provided to the


177

pipes below the canalwater levels at the inlet and outlet (0.25m or half of the pipe diameter).

Sallowsiphonshouldbecheckedforflotation,whenemptyinhighgroundwaterareas.

Designprocedure

The design procedure of the shallow siphon includes the calculation of barrel size to meet the

requirementofflowandcalculationofheadlossthroughthebarrel.

i. Assessfloodflowinthedrainagechannel,

ii. Calculatewaterwayofdrainagechannel, QPw 74.4=

iii. Assumevelocityofflowinthebarreltopreventsiltinginsidethebarrel,

iv. Calculatethesizeofbarrel(pipeorconduit),

v. Calculateheadlossintheconduitbarrelusingfollowingformula:

FrictionheadlossgD

fLvh f

2

2

= orfromnomogramofconcretepipe(Figure5-10),

Where,hfistheheadlossinm,

fisthefrictionalcoefficientofthepipe,whichiscalculatedbyMoodyequation,

Listhelengthofthepipeinm,

Visthevelocityofwaterinthepipeinm/s,V=Q/A

gistheaccelerationduetogravityinm/sec2,

Disthediameterofpipeinm,

Qisthedischargeofthepipeinm3/s,

Aisthecrosssectionalareaofthepipeinm2,

Entryandexitheadloss⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −=

g

vvh c

e2

50.1

22

Bendlossg

Vkhb2

2

=

Wherekisthecoefficientdependingupontheangleofbendsandranges0.10to0.20

Thetrashrack(Figure6-25)lossattheinletofthesiphon(hr)=3Sinδ(s/b)4/3*V

2/2g

Where,Visthevelocityinsiphonbarrel,Sisthetrashrackbarthicknessinmmandbisthespacing

ofrackbarinmm

Figure6-25Designsketchoftrashrack

Forsmallsiphonconduittrashrackloss(ht)istakenas0.10m.


178

vi. ForbarrelconduitheadlossmaybecalculatedusingUnwin’sformula:

g

V

g

V

R

Lffh

a

f22

1

22

21−⎟

⎠

⎞⎜⎝

⎛++=

Where,f1isthecoefficientofheadlossatentryanddependsupontheshapeofentrybarrel,for

unshapedmouthf1is0.505andforbellmouthentryf1is0.08,

f2isthecoefficientoffrictionlossthroughthebarreland ⎟⎠

⎞⎜⎝

⎛+×=R

baf 1

2

Listhelengthofconduitinm,

Risthehydraulicradiusoftheconduitsectioninm,

Visthevelocityofflowinthebarrelinm/s

Vaisthevelocityofapproachinm

aandbarecoefficientsdependinguponthematerialofconduitbarrelandareprovidedbelow

(Table 6-5).

Table6-5Coefficientsforfrictionlossinsiphonbarrel

S.N Natureofconduitsurface a b

1 Smoothironpipe 0.00497 0.025

2 Encrustedpipe 0.00996 0.025

3 Smoothenedcementplaster 0.00316 0.030

4 Brickworkorashlar 0.00401 0.070

5 Stonemasonry 0.00507 0.25

vii. Calculate total head loss and comparewith available head for the siphon structure: head

lossshouldbelessthanavailablehead

viii. Assesstheprotectionrequirementoverthesiphonontheriverbed

DesignExampleofShallowSiphon

Data:

Spanofriver= 60m

Noofbends= 2

Canaldischarge=0.30m3/s

Upstreamwatervelocity=0.50m/s

Upstreamanddownstreamleveldifference=1.50m

Calculations:

Letpipevelocitybe2m/stopreventsilting

Calculateareaofpipe,A=Q/V=0.30/2=0.15m2

Pipearea,A=πd2/4;or0.15=πd

2/4

Requireddiameterofpipe,d=0.437m


179

This diameter is not available commercially and need to select 600mm diameter pipe with A =

0.283m2

The velocity ofwater at pipe, V =Q/A = 0.15/0.283 = 1.06m/swhich is insufficient to flush the

sedimentinsidethepipe,henceselectpipediameterof450mm,

Areaofpipe,A=0.158m2andvelocityinpipeV=Q/A=0.30/0.158=1.90m/swhichisacceptable.

Calculatefrictionlossinpipe;forconcretepipewithK=0.3mmheadlossfromchart=0.33m/100m,

Lossofheadfor60mlengthhf=0.33/100*60=0.55m

Calculateentryandexitloss⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −=

g

vvh cf

e2

50.1

22

he=1.5(1.9^2-0.5^2)/2*9.81=0.26m

Bendlosshb=kV2/2g=2*0.10*1.90^2/19.62=0.036m

Taketrashrackloss=0.10m

Totalloss=0.55+0.26+0.04+0.10=0.95m

Theheadavailableis1.5mandrequiredheadlossisonly0.95m,hencethedesignisacceptable.

DesignConceptofDeepSiphon

Deepsiphonsareusuallyconstructedofsteelpipesduetohighpressure.Velocitiesandheadlosses

arealsohigh.Thebridgeisoftenasuspensiontypewithapipeheldinwireslingsbelowthecable.

ThebridgecanalsobeofsimplysupportedRCCpierandabutments.Pipesiphonsofupto40mlong

withmaximumpressureheadof50mandasuspendedspanofupto40mhavebeenconstructed.

Atthe inletchamber,atrashscreen isrequiredtoreduceblockageanddroptype inletandoutlet

boxesareadequateforsmallirrigationcanals.Thedesignvelocityinthepipeshouldbeintherange

of1.5mto3m/stopreventsiltation.Adequatesubmergenceshouldalsobeprovidedattheinlet

andoutletanddrainvalveshouldalsobeprovidedatthelowestpointofthesiphontowashout.In

addition, adequate anchorage should be provided to support pipeline across the valley. to resist

trustofthepipeflowattheslopes.

Thedesignprocedureofdeepsiphonissameasforshallowsiphons.


180

Figure6-26Typicaldeepsiphon


181

6.4.7 HighDensityPolythene(HDP)PipeCrossing

DesignConcept

HDP pipes are extensively used in small hill irrigation schemes both for crossing the drainage

channel and as a conveyance canal where an open canal section is not suitable due to difficult

terrain. MajorbenefitsofHDPpipesarethattheyareeasytotransport,haveaflexiblestructure,

andaresuitable inremoteareas.However,skilledmanpower isnecessarytoerect thepipe joints

andsuspendedcables.ThedesignofHDPpipesconsistsoftwocomponents:

• HydraulicdesignofHDPpipe;and

• Structuraldesignofcablesuspension

ThehydraulicdesignoftheHDPpipecrossingconsistsofthecalculationofthesizeofthepipe,its

length,andassessmentofheadlossinthestructure.Inadditiontothefrictionlossofthepipeentry

andexitloss,bendlossandjointlossshouldbeassessed.Thelengthofthepipedependsuponthe

topographyofthedrainageuptostablebanks.

The structural design of a HDP crossing consists of the calculation of saddle difference, cable

inclination,tension incables,and loads.The loads incableareofcable itself,pipewithfullwater,

andwindload.

Figure6-27TypicalphotosofHDPEpipecrossing

Designprocedure

i. CalculatethelengthoftheHDPpipebasedonthetopographyofthesite,

ii. Calculatethesizeofthepipebasedontherequireddischargetopassandlimitingvelocityin

thepipe,

iii. Calculatetheheadlossinpipefromformula:

Headlossfromnomogramorfromtableprovidedbymanufacturer,

Entryandexitlossisassessedas,He=1.5*(V2pipe-V

2canal)/2g

Where,Vpipeisvelocityofwaterinpipeinm/s

Vcanalisvelocityofcanalattheinletinm/s

Jointlossisassessedas25%offrictionlossforeachjoint.

ThebendlossiscalculatedasHb=k*Vpipe2/2g,

Where,kisbendcoefficientandrangesfrom0.10to0.20

Vpipeisvelocityofwaterinpipeinm/s

Thetrashracklossistakenas0.10forsmallirrigationschemes.


182

iv. Calculatethetotalheadlossandcomparewithavailableheadofthesite,

v. Structuraldesignofsuspensioncable

vi. Selectthesizeofcablebasedonstandardspecifications,

vii. Calculatecablelengthfromformula:

Cablelength=1.1xspan+backstaylength+6m

Wherebackstaylengthisthecablelengthbetweensaddlecenterandcenterofanchorage

blockasperfoundationconsideration,

viii. Calculatehostingsag

Thecrosssectionalareaofthecableistheactualareaofthewirewithoutcountingthevoidspace

betweenwires.Thevaluefortheareacanbeestimatedfromtheinformationonthecableweight

and specific gravity (7.843 t/m3) for steel in the wire. The designer should obtain values of E

(modulusofelasticity)andcross-sectionalareafrommanufacturerordesigner’sbestjudgmenthas

tobeused.

DesignExampleofHDPPipeCrossing

Designdata

Dischargetobepassedthroughthepipe=0.05m3/s

Spanofcrossing=20.0m

Leveldifferencebetweenthepipeinletandoutlet=2.0m

TotallengthofHDPpiperequiredfrominletpointtooutletpoint=50.0m

VelocityofflowinopenchannelneartheinletpointofHDPpipe=0.75m/s

Maximumallowablevelocityinpipe<3m/s

Calculations:

Weknow,Q=V.A,A=Q/V,

From standard nomogram (Figure 5-11) for Q=0.05 m3/s, head loss = HL (m/100m) =

0.88/100=0.0088m,andV=1.59m/s

Flowareaofpipe,A=Q/V=0.05/1.59=0.03145m2

Internaldiameterofpipe,Ø=(4*A/3.14)1/2=(4*0.03145/3.14)1/2=0.200m

Chose225mmouterdiameterHDPpipefromseriesIII(datafromHDPpipemanufacturer)

Availableinternaldiameterofpipe=225-2*12.2=200.6mm,whichisclosetochosenØ=200mm

Hence,OK

Calculationoflosses

Headlossduetofrictionin50.0mlongpipe(hf)=50*HL=50*0.0088=0.44m

Entryandexitloss(he)=1.5*(V2pipe-V2canal)/2g

he=1.5*(1.592-0.752)/2*9.81=0.15m

Trashrackloss(ht)=0.10m

Jointlossisestimatedasthefrictionlossinpipelengthequivalentto0.25mperjoint

hj=0.25*(50m/5-1)=3.125m(consideringlengthofonepipetobe5m)

Therefore,jointloss=0.0088*3.125=0.0275m

Bendloss(hb)=k*V2pipe/2g,where(k=0.10)


183

hb=0.10*1.592/2*9.81=0.0123mperbend

Assumingmaximum4bendsinthiscaseweget,bendloss=0.0123*4=0.052m

Totalloss=0.44+0.15+0.10+0.0275+0.052=0.77m

Leveldifferencebetweeninletandoutletis2m

Hence,thedesignofthepipeisOK

Structural design of cable suspension is carried out on standard manufacturers design and

specifications.

6.5 SedimentControlStructures

6.5.1 Introduction

Most rivers and streams of Bhutan carry large quantities of sediment, especially during the

monsoon.Deforestation,soilerosion,landslidesandnaturalgeologicalprocessescontributetothe

high sediment load of mountain rivers. These rivers carry sediment particles ranging from large

boulderstofineclaydependinguponthestagesoftherivers.Duringthemonsoon,floodriversmay

carrylargeboulderstofineclay,andduringnormalflowtheycarrygraveltofineclay.Riverborne

sedimentaffectirrigationcanalsbyreducingcapacity,blockingcanals,andnegativelyaffectingcrop

yields. Hence, sediment entry into irrigation canals can be minimized with the construction of

precautionary structures. The sediment removal structures appropriate for hilly terrain are gravel

trapsandsettlingbasins.

Thechoiceofgraveltraporsettlingbasindependsonvariousfactorssuchasavailabilityofsuitable

location, economyof the structure, remoteness of the location, and farmer’s capacity to operate

andmaintainthestructure.

6.5.2 GravelTraps

DesignConcept

Agraveltrapbasicallyconsistsofadeepwellbuiltinstonemasonryorconcreteimmediatelyafter

theintake. It isdesignedtotrapsedimentconsistingofparticlesgreaterthanthesizeofgravel(2

mm).Thedesignprincipleistoreducetheflowvelocityinthewelltoallowthesedimenttosettleon

the floor. In small irrigation schemes in the hills, a gravel trap behind the intake is provided to

exclude the gravel from entering into the canal. The design of gravel traps differs from that of

settlingbasinsbecausetheformerhandlescoarsematerialwhichentersnearthebed,ratherthan

suspendedmaterialwhichhas tobesettledthroughthedepth.The flowvelocities insidethewell

mayneedtobeloweredto0.30meterpersecondtotrap2mmsizeparticles.Table6-6indicates

thenecessaryvelocitiesinthegraveltrapwhichwillpermittrappingofparticlesvaryinginsizefrom

100mmto2mm.

Table6-6Flowvelocitythroughgraveltrap

ParticleSize

mm

Nominalvelocity(m/s)for

depthsof

Designvelocity(m/s)

fordepthsof

3m 1.5m 3m 1.5m

100 4.0 3.5 2.0 1.7


184

ParticleSize

mm

Nominalvelocity(m/s)for

depthsof

Designvelocity(m/s)

fordepthsof

60 3.4 3.0 1.7 1.5

40 3.0 2.6 1.5 1.3

20 2.3 2.1 1.2 1.1

10 1.8 1.6 0.9 0.8

5 1.8 1.6 0.7 0.6

2 0.8 0.7 0.4 0.3

Graveltrapsmaybeemptiedbyhandorbyflushingandhencethewidthofagraveltrapmustbe

onemeter to allow formanual removalof sediment. The typical sketchof a gravel trap for small

irrigation schemes ispresented inFigure6-28. Theoverall sizeof thegravel trapdependson the

rateofaccumulationofgravelsediment,theintervalbetweenmanualcleaning,availablespaceand

costofthestructure.

Figure6-28Typicalgraveltrapstructure

DesignProcedure

i. Fixthesizeofsedimentparticlestoberemoved,

ii. Decidethesedimentconcentrationduringnormalflowoftheriver,

iii. Assessthelocationofthegraveltrapandavailableheadtoflushthesediment,

iv. Determinethestoragevolumeingraveltrapfromtheformula:


185

6

max

10

243600

×

×××××=

BD

FIEXQV

f

s

Where,Vsisthestoragevolumegraveltrapinm3,

Qistheflowrateinm3/sec,

Xmaxisthenominalsedimentconcentrationofwaterinmgperlitter,

Efisthetrapefficiencyinfraction,

FIistheflushingintervalindays,andBDisthebulkdensityofsedimentintonperm3,

v. Determinethesizeofthegraveltrap,

6.5.3 SettlingBasin

DesignConcept

Asettlingbasinisamodifiedgraveltrapinthatitallowslargeparticlesandalimitedrangeofsmall

particles tobe trapped. Settlingbasinshave threeparts: inlet zone, settling zoneandoutlet zone

(Figure6-29).Forsmallirrigationschemes,thedesignprinciplesofthesettlingbasinareasfollows:

• Lowerthevelocityofflowinthebasintoassistsmallparticlestosettletothefloorrapidly,

• Create hydraulic conditions in the basin to prevent rescouring of the particles during normal

operation,

• Createhydraulicconditionsforautomaticscouringofcertainsmallparticleswhenflushgateis

opened,

• Provideadequatesedimentstoragevolumetakingallfactorsintoaccount,

• Provideanautomaticspillwaytoallowfloodwaterstospilloverthestructure,

• Provide a head regulator gate to control the flow of water into the canal during normal

operation,

Insmallirrigationschemes,itisgenerallyfeasibletoachieveabetterlengthtowidthratio,say8to

10.Basin shapecanbe improvedby subdivisionwith longitudinaldividewalls,whichmayalsobe

desirableforoperationalreasons.

Figure6-29Typicalconfigurationofsettlingbasin


186

DesignProceduresofSettlingBasin

i. Fix thesmallestparticle thatwillbetrapped insidethestillingbasin (recommendedsize is

0.50mm),

ii. Fix the smallest particle that will be flushed automatically when flush gate is opened

(recommendedsizeis0.80mm),

iii. Usethenomogramtodeterminetheminimumsizeofthesettlingbasinrequired,

iv. Use nomogram to determine the minimum flow depth required in the settling basin to

preventrescourofthesmallestparticles,

v. Usenomogramtofixthescourvelocitynecessarytoflushsolidparticlesofsizefixedin(ii)

above,

vi. Determine the required scour slope necessary to generate velocity calculated in (v) from

Manning’sformula,

vii. Assumingstandardgatewidthdeterminethelevelofthesandsluicefloorbelowthelevelof

settlingbasinfloor,

viii. Computetheavailablesedimentstoragevolumeandadjustdimensionsofthesettlingbasin,

ix. Determinenecessarysilllevelattheheadregulatortocontroltheflowintothemaincanal

duringnormaloperation,

x. Determine the length and crest level of automatic side spill assuming flush gate is fully

closed,

xi. Determinetheexpectedflowintothemaincanalduringflood,

xii. Calculatethelengthofsidespillway,

xiii. Designtheflushoutcanal

DesignExampleofSettlingBasin

Data

Designdischarge(Q)=200l/s

Sizeofsmallestparticletobetrappedinsidethestillingbasin=0.50mm

Sizeofsmallestparticletobeflushedautomaticallywhengateisopened=0.80mm

Calculations

Usingnomogram(Figure 6-30)forparticlesizeof0.50mm,ratioofdischargetosurfacearea,Q/As=

0.017

ThensurfaceareaofbasinAs=Q/0.017=0.20/0.017=11.76m2

Takewidthofthebasin(B)=1.25m(recommendedas1mto1.5m)andlengthofthebasin(L)

L=10mwithbasinareaof12.50m2

Checkthelengthtowidthratio(L/W)=10/1.25=8whichisOK

Determineminimumflowdepth(dmin)topreventrescourofsmallestparticles=Q/(V*B)

Velocityofflowfor0.50mmparticlesfromnomogram(Figure 6-30)V=0.24m/s

Thenflowdepth(dmin)=0.20/(0.24*1.25)=0.67m

Calculatestoragevolume

6

max

10

243600

×

×××××=

BD

FIXQVs

η

Where, Xmax =900mg/l

η =0.90


187

FI =7daysandBD=2.0t/m3

Vs=0.20x900x0.90x7x3600x24/2x10^6=49m3

Storagedepth=Vs/areaofbasin=49/(10x1.25)=3.92mfor7daysflushing

Asthestoragedepthfor7daysflushingishigh,itisnecessarytoincreasethesizeofthebasin,

Letlengthofbasinbe=30mandwidthofbasinbe=2m

Minimumflowdepthrequiredtopreventrescourofsmallparticles=Q/(VxB)

Dmin=0.20/(0.24*2)=0.42m

ThestoragedepthDs=Vs/LB=49/(30x2)=0.82mfor7daysflushing

For particle size of 0.80 mm to be automatically flushed scour velocity from nomogram (Figure

6-31),Vsc=1.83m/s

Allowingscouringflow=1.25Qdesign,Qs=1.25x0.2=0.25m3/s

Scourflow/unitwidthqs=Qs/B=0.25/2=0.125m3/sperm

Scourdepthofflow,Ys=qs/Vsc=0.125/1.83=0.07m

MinimumbedsloperequiredtoflushthesedimentSc=(nxVsc)^2x(1/Ys)^4/3

Sc=(0.025x1.83)^2/(1/0.07)^4/3=0.075

Assuminga0.60mwidescourgatedischargeintensity,qg=Qs/Wg=0.25/0.60=0.42m3/sperm

Scourdepthatgate(Yg)=(qs/1.71)^2/3=(0.125/1.71)^(2/3)=0.39m

Depthofsluicebelowbasinfloor(dls)=Yg+0.05–Ys=0.39+0.05-0.07=0.37m

Adoptdepthofsluiceas0.40m

Designofsidespillway

Addextra50%dischargeindesigndischargeforflood(Qf)=1.5Qd=1.5x0.20=0.30m3/s

Dischargetobespilled(Qsp)=Qf-Qd=0.30-0.20=0.10m3/s

Take width of spillway as 2m and calculate head over the crest by sharp crested weir formula,

hsp=(Qsp/1.84xLsp)^3/2=(0.10/1.84*2)^3/2=0.09m

Assumefreeboardas0.11m

Duringfloodflowwaterdepthwillincreaseincanalandinbasinby:

(Qf/Qd)^0.60=dflood/ddesignthenfloodwaterdepth(df)=(0.30/0.20)^0.60x0.35=0.446m

Thedepthofwaterincanalwillriseby0.446-0.35=0.090m

Hence,spillweirheight=increaseddepthofwater+Fb=0.09+0.11=0.20m

Designofflushoutcanal

ScouringvelocityVsc=1.83m/sandQf=0.30m3/s

Theareaofflow,A=Qf/Vsc=0.30/1.83=0.164m2,

AdoptbedwidthofcanalaswidthofgateB=0.60m

Waterdepthincanal,D=A/B=0.164/0.60=0.27m

Takefreeboardas0.08mtotaldepthofcanalbe0.35m

Requiredslopeoftheflushingcanalisgivenby

Sf=(Vsc^2*n^2)(1/D)^4/3=(1.83x0.025)^2(1/0.27)^4/3=0.0065

Theslopeisabout1:155

Designisacceptable.


188

Figure6-30Settlingbasindesigncurves


189

Figure6-31Scouringvelocitiesforsandtrap


190

6.6 RetainingWalls

6.6.1 Introduction

Retainingwalls are the structuresnecessary toprotect the canal fromslippingor slidingalong its

alignment.Inhillirrigationschemes,retainingwallsareessentialwherecanalalignmentisnotstable

enoughduetouphill slidingordownhill slipping.Froma loadingpointofview,retainingwallsare

gravity,cantileverandcounter-fortretainingwalls.Ingravityretainingwalls,pressureactingonthe

wallisresistedwithitsweight,whilecantileverandcounter-fortretainingwallsresistpressurewith

bending actions. The most commonly used retaining walls are gravity walls which are classified

basedonthematerialsusedasfollows:

a) Masonryretainingwalls

Masonryretainingwallsarewidelyusedfornewconstructionofirrigationschemes.Theseretaining

wallsareeitherrandomrubblemasonryordressedstonemasonry.Sufficientweepholesshouldbe

providedthroughthewalltoensurethereisnobuilduppressurebehindthewallwhichmaycause

failureofthewall.

b) Halfdrymasonrywalls

Walls with dry stone panels can be provided on the uphill side of the canal, especially where

seepageorslumpingisaproblem.Themainadvantageofthistypeofretainingwallisitssuitability

in very wet conditions as it allows free passage of seepage through the wall. Half dry masonry

retainingwallsaresuitableforwallheightsupto4m.

c) Drystonewalls

Drystonemasonrywallsareoftenusedonexistingirrigationschemesbuiltbythefarmers.Theyare

relatively cheap but canal water is not retained and seepage can occur. The walls often have

foundationproblems.Theycanbeusedforuphillbankprotectionworks inopencanaltostabilize

the slope. In addition, dry stonewalls areused in steep slopesbelowpipepines. These retaining

wallsaresuitableforwallsfrom1mto3minheight.

d) Concreteretainingwalls

Massorreinforcedconcretewallsarelikelytobemoreexpensivethanmasonrywallsandhavefew

advantages.Thestrengthof theconcreteretainingwalls ishigher incomparisontoothertypesof

walls.Thesewallsareusedtoprotectuphillslopewhereotherretainingwallsareweaker.

e) Gabionwalls

Gabionwallsaremorecommonduetotheirgreaterstrengthandflexibility.Thesewallsaremoreor

lessexpensivethandrystonewalls.Gabionwallsshouldbebuiltasdrystonewallswithstonesbeing

carefully chosen and packed. The crates should be braced with partitions and tied at frequent

intervalstoadjacentcratesusingbindingwires.Sometimesaretainingwallwithgabionfoundation

anddrystoneuppersectionmaybeappropriate.


191

6.6.2 DesignConcept

A gravity retaining wall is one which resists the lateral pressure by its weight. The design of a

retainingwallinvolvesthefixationofbottomwidth,topwidth,heightandmaterialtobeused.The

sizeoftheretainingwallformasonry,concreteandRCCisfixedonthebasisofstabilitycalculation

which is checked against overturning and sliding. The base width of the wall shall be such that

maximumpressureexertedonthefoundationsoildoesnotexceedthesafebearingcapacityofthe

soil.Inaddition,notensionshallbedevelopedanywhereinthewallandwallsmustbesafeagainst

slidingandoverturning.Thefactorofsafetyagainstoverturningistakenas2.0fornormalloading

case,and1.50forextremeloadingcase.Thefactorofsafetyagainstslidingistakenas1.5fornormal

loadingcase,and1.2forextremeloadingcase.

6.6.3 DesignProcedure

i. Calculatetheloadsactingonthewall:

- activesoilpressureduetohorizontalground,

- surchargeloadofthesoilbehindthewall,

- hydrostaticpressurebehindthewall,

- self-weightofthewall,

ii. Assess theallowablestress (compressiveand tensile)of thewallbasedon thematerialof

thewall,

iii. Assessthefiltermaterialstobeusedbehindthewallsection,

iv. Drawtheloaddiagram,

v. Calculatetheforcesandmomentsonthetoeofthewall,

vi. Checkthefactorofsafety,

vii. Calculatethebearingpressureandcheckagainstallowablepressure,

viii. Checkthestressatthejunctionofthewall

Thedesignofaretainingwalliscarriedoutontabularformwiththeassessmentofload,pressure,

forceandmomentsactingonthewall.

6.6.4 DesignExampleofRetainingWall

Data

Walltopwidth=0.50m

Wallbasewidth=3.60m

Heightofwall=3.0m

Foundationdepth=1m

Batterslope=1:5

Angleofreposeofsoil(Ф)=33o

Coefficientof(Ka)=0.295

Coefficientof(Kp)=1(max)

Densityofwater(δ)=9.8kN/m3

Densityofsaturatedsoil=20.0kN/m3

Densityofsubmergedsoil=10.2kN/m3

Densityofmassconcrete= 24.0kN/m3


192

Atrafficsurchargeequivalentto0.6mdepthofsaturatedsoil is tobe included(0.6x20KN/m3=

12KN/m2).Thegroundwatertablebehindthewallis1.5mbelowgroundlevel.

Calculations

Fordesign,thewallisconsideredtobe1mlong.

Drawuptheloaddiagramonthewall(Figure 6-32),

Figure6-32Designloaddiagramofretainingwall

Calculatetheforcesandmoments(aboutA)onthewall(Table 6-7)

Table6-7DesignCalculationsofRetainingWall

S.N. LoadType Pressure(KN/m) Force(KN) Levelarm

(m)

Moment

aboutA

(KNm)

(1) TrafficSurcharge 0.6x0.295x20.0=3.54 3.54x4.0=14.16 2(4/2) -28.28

(2a) SaturatedEarth 1.5x0.295x20.0=8.85 8.85x1.5/2=6.7 32.5+1.5/3) -19.91

(2b) SaturatedEarth 8.85 8.85x2.5=22.125 1.25(2.5/2) -27.66

(3) SubmergedEarth 2.5x0.295x10.2=7.52 7.52x2.5/2=9.4 2.5/3 -7.83

(4) Water 2.5x9.8=24.5 24.5x2.5/2=30.6 2.5/3 -25.5

(5) Water 1.0x9.8=9.8 9.8x1.0/2=4.9 1.0/3 +1.63

(6) SubmergedEarth 1.0x1.0x10.2=10.2 10.2x1.0/2=5.1 1.0/3 +1.70

(7a) Self-Weight - 3.0x0.5x24=36.0 1.25(1+0.5/2) +45.00

(7b) Self-Weight - 3.0x0.3x24=21.6 1.7 +36.72


193

S.N. LoadType Pressure(KN/m) Force(KN) Levelarm

(m)

Moment

aboutA

(KNm)

(1.5+0.6/3)

(7c) Self-Weight - 3.6x1.0x24=86.4 1.8(3.6/2) +155.52

(8) Weightof

saturatedearth

- 1.95x1.5x20=58.5 2.62 +153.27

(8a) - 1.8x1.5x20=54.0 (2.70) (145.80)

(8b) - 0.3/2x1.5x20=4.5 (1.90) (7.60)

(9) Weightof

submergedearth

- 1.65x1.5x10.2=25.2 2.77 +69.8

(9a) - 1.5x1.5x10.2\23.0 (2.85) (65.60)

(9b) - 0.3/2x1.5x10.2=2.2 (1.90) (4.20)

(10) WeightofWater - 1.65x1.5x9.8=24.3 2.77 +67.3

(10a) - 1.05x1.5x9.8=22.1 (2.85) (63.00)

(10b) - 0.3/2x1.5x9.8=2.2 (1.90) (4.30)

(11) Uplift:Totalor (24.5+9.8)/2=17.15 17.15x3.6=61.7 2.06 +127.1

(11a) Intwoparts (9.8)=9.8 9.8x3.6=35.3 (1.80) (-63.55)

(11b) (24.5-9.8)/2=14.7 (14.7/2x3.6)=26.4 (2.40) (-63.55)

(12) BaseFriction=

(downwardloads

–uplift)tanPhi

(252.0-61.7)xtanФ - -

=123.6

Checkfactorsofsafety(FOS)againstslidingandoverturning

Summingallhorizontalforces:

Disturbingforces(1)to(4)=14.16+6.7+22.12+9.4+30.6=82.95kN

Restoringforces(5),(6)and(12)=4.9+5.1+123.6=133.6kN

Factorofsafety(FOS)againstsliding=133.6/82.95=1.61

Minimumfactorofsafety=1.5andhenceacceptable.

Summingmomentsabouttoe:

Overturningmoments(1)to(4)and(11)=28.28+19.91+27.66+7.63+25.50=236.2kNm

Restoringmoments(5)to(10)=1.63+1.70+45.00+36.72+155.52+153.27+69.80+67.30=

530.94kNm

FOSagainstoverturning=530.94/236.20=2.25

Minimumfactorofsafety=2.0andhencedesignisacceptable.

Calculatebearingpressures

Thepositionofthereaction,x,iscalculatedfromV

Mx

Σ

Σ=


194

X=530.94–236.20/(252.0-61.7)=1.55mfromtoe,whichlieswithinthemiddlethirdofthebase

Where,ΣM=sumofthemoments,andΣV=sumoftheverticalforces

Theeccentricityofthereactionaboutthecenterlineofthebase:e=L/2-x

e=3.60/2-1.55=0.25m

Thebearingpressuresarecalculatedfrom ∑ ⎟⎟⎠

⎞⎜⎜⎝

⎛ ±=

bL

eVBP

61

WhereListhebaselengthandbisthewidth(1.0minthiscase)

BP=190.3*(1±6*0.25)/3.6*1=52.90(1±0.42)=75.1or30.7kN/m2

Allowablebearingpressure=100kN/m2andhencedesignissatisfactory.

Thewall sections are adequate. The typical examples of retainingwalls are given in following

figures(Figure6-33).

Figure6-33Possibleretainingwallsinsmallcanals


195

6.7 WaterControlGates

6.7.1 General

Agateisoneofthemostimportantcomponentsofthecanalstructure,andisusedtocontrolthe

water and floods. The gate should bewater tight to ensure stablewater intake and firm enough

againstthedynamicforcesofwater.Themainfunctionsofthegateare:

• Tomaintherequiredwaterleveltopassdesigndischargethroughintake,

• Tooperatequicklyandsafelytoreleasefloodwaterincaseoffloods,and

• Tomaintainwatertightness

The gate structure consists of a leaf plate, guide frame,bonnet andhoisting equipment. The leaf

platereceivesthehydraulicloadofwaterandconveysittothebodyandbonnet.Theguideframeis

theembeddedpart in theconcreteandadjacentto thesealingpartof the leaf tomaintainwater

tightness.Inaverticalliftgate,theguideframetransmitstheexternalloadtotheconcreteormain

bodyofthestructure.Thehoististhemechanismtooperatethegate.

6.7.2 TypeofGates

Therearemainlytwotypesofgate:averticalliftgateandahingedtypegate.

Theverticalliftgatemaybe:

• Fixedwheeltypegate,

• Slidegate,

• Doubleleafgate,and

• Stoplog

Thehingedtypegatemayalsobe:

• Radialgate,

• Sectorgate,

• Mitregate,

• Flapgate,and

• Swinggate

Inafixedwheeltypegate, loadistransmittedtothehorizontalmaingirderthroughtheskinplate

and the supporting girders at both sides of the gate. The supporting girders are equipped with

wheelsateachsidewhichmakesoperationeasier.Thegateisusuallyraisedverticallybywirerope

ora spindlehoist.This typeofgate ismechanicallyandstructurally simpleand isused forawide

rangeofapplications,fromsmallspancanalstructurestolargebarrages.

Slide gates are suitable for small spans and water level gaps. This type of gate is simple and a

metallicplatecanbeusedfortheguideframe.Sincetheleafplatehastoslideontheguideframe,a

heavyhydraulic loadis imposedonthehoistingduringoperation.Aspindleisfixedtoliftthegate

withorwithoutgearboxes.

A double leaf gate can control discharges and suspended load can be passed easily. Discharge

controland reducedheightof thepiersare thesignificant featuresof this typeofgate.However,

duetoitscomplexmechanismforsealing,guideframeandoperation,itsusesarelimited.

Stop-log is a kind of gate that is used for discrete operations to facilitate inspection and

maintenance of the structure. In addition,wooden stop-logs are used extensively for small canal

structurestocontrolwater.For largeregulatororbarragegates,stop-logsaremadeofskinplate,


196

andguideframeandplacedingrooveswiththehelpofwireropesorothermechanicaldevices.For

small scale canals stop-logsaremadeupof100mmthickwooden logswith100mmor150mm

heightofeachlog.TypicalverticalgateandstoplogsarepresentedinFigure 6-34.

Figure6-34Typicalverticalliftgateandstop-logs

Verticalliftgateinsmallcanal Stop-logsinsmallcanal

Radialgatehasacirculararcshapedskinplatebracedonthesupportbeamwithatrunnionpinat

thecenterofthecirculararc.Thehydraulicloadistransmittedtothepedestalthroughthesupport

beam, main beam, gate arm and trunnion. It rotates around the trunnion pins and operates

hydraulicallyandmechanicallywithasmallhoisting load.Radialgatesareusedfor largespillways,

sluicewaysandwaterways.Aradialgatesystemconsistsofthefollowingparts:

• Thegate leafassemblywhich includescurvedskinplate, crossbracing, supportingbeamsand

girdersandrubberseals,

• Thearmassemblywhichincludesarmcolumns,bracing,andhub,

• Thetrunnionassemblywhichincludesapin,bushing,thrustwashers,lubricationprovisions,and

yoke,

• Thetrunnionsupportassemblywhichincludestrunnionbeamandanchoragesystem,and

• Thewireropehoistsystem

Otherhingedtypegatesarenotusedincanalstructuresandhencearenotbriefedhere.

6.7.3 Designconceptofgates

Thedesignofgates incanal structuresmustconsider theresistance toanticipated load, sufficient

water tightness, ease and stability of operation, durability and free from vibration and ease of

maintenance.Eachpartof thegate isdesigned tominimize thesecondarystressescauseddue to

change in stiffness, self-deflection and eccentricity of the part. To ensurewater tightness, elastic

materialssuchasrubbersealsareuseddependingonthewaterpressures.Allofthemetallicparts

ofthegateshouldbesufficientlysound,durableandresistcorrosion.Thedesignofagateinvolves

theassessmentoftheloadsandtheircombinationfordifferentconditions,calculationofallowable

stressesinstructuralpartsofthegate,thicknessoftheleafwiththeconsiderationofitsdeflection,

size of guide frames and their material, selection of sealingmaterial and assessment of hoisting

equipment. The loads to be considered are self-weight, hydrostatic pressure, earth pressure,

dynamicwaterpressure,seismicload,windload,andvibrationeffects.


197

Water control gates shall be designed taking into consideration the recommendeddesign criteria

andthematerialsusedfordifferentcomponentsshallconformtotherelevantISCodes.Smallcanal

gatesshallbesquareorrectangular inshape.Thesizeofasquareopeningshallbe300,450,600,

750,900,1050and1,200mmandthatofarectangularopeningshallbe1,200mminwidthand900

mminheight.

The hoisting capacity and diameter of the lifting rod shall be determined by taking into

considerationtheoperatingheadunderwhichthegateistobeoperated.Thetypeandcapacityof

theheadstockanddiameterofthehandwheelshouldbesufficienttoenableoperationofthegate

underthemaximumoperatingheadbyasinglepersonwithaneffortontherimofthehand-wheel

notexceeding136Newton.

Thegatesshallbeshoptested.Theleakage,ifany,shallnotexceed1.5literperminutepermeter

lengthof thesealingperimeterwhile testingwithouthydrostaticpressure.Approximateweightof

thecanalgatesasperstandardizedsizeandspindlelengtharepresentedbelow(Table 6-8).

Table6-8Standardizedsizeofgatesandtheirweight

S.N Gatesize(mm) Spindle size (mm) with

1.50mlength

Approximate weight

(kg)

1 300mmx300mm 40mm 150

2 450mmx600mm 48mm 240

3 600mmx600mm 55mm 320

4 900mmx900mm 60mm 500

Basicdesignparametersoftheverticalslidegateareasfollows:

• Clearopeningofthegate(W)whichisthewidthofthebayroundedupto10cm,

• Height of the gate (H) which is assessed as FSL- crest level +10 cm, the additional height is

providedtoensurethatthewaterneverspilloverthegate,eveninthepresenceofwavesand

surgescausedbywind,

• Centertocenterdistanceofsideseal(b)whichisequaltotheclearopeningofthegate(W),

• Centertocenterdistance(a)oftrackwhichisequaltoclearwidth(W)plus50mmto100mm,

• Designhead(h)isequaltotheheightofthegate

6.8 StructuralDesignofCanalStructures

6.8.1 Introduction

Inaddition to thehydraulicdesign, canal structureshave tobedesigned for structural soundness

and stability. Thisheadingprovides theguidelines for the structural and stabilitydesignsadopted

during the feasibility design of the irrigation project. The structural design provides appropriate

thicknessforstructuralmemberstoresisttheforces,bendingmoment,andshearstressesimposed

byloadsonthestructure.Thestabilitydesignprovidesadequatestructuraldimensionssothatthe

structure will resist sliding and overturning, and limits foundation pressures to less than the

allowable bearing pressure. Both structural and stability design involve the consideration of the

loadswhichthestructuremaycarryfromtimetotime,howtheseloadsarecombinedtogetherand

interact,and thematerialsofwhich thestructure ismade,and their strengthandbehaviorunder

loads.


198

6.8.2 LoadingsandStructuralAnalysis

a) General

Loads are normally grouped into two categories: dead loads and live loads. Dead loads may be

describedasthosewhicharealwayspresent,whereasliveloadsaretransient.Themainloadswhich

canalstructuresmustbecapableoftakingare:

• Self-weight;

• Earthpressure;

• Waterpressureincludinguplift;

• Imposedliveloadssuchastrafficloads,

• Imposeddeadloadssuchasspanloadsetc.

• Earthquakeloading

Windloadingisignoredinthedesignasallowablelateralloadswilldominate.

b) WeightofMaterials

Theweightsofmaterialusedtoderivetheself-weightofstructuresarepresented intabular form

(Table6-9).

Table6-9Self-weightofmaterials

S.N Material Unit weight

(kN/m3)

S.N Material Unit weight

(kN/m3)

1 Steel 78.5 6 Earthdry 16.0

2 Reinforcedconcrete 24.0 7 Earthwet 20.0

3 Plainconcrete 23.0 8 Gravel 18.0

4 Stonemasonry 21.0 9 Water 9.80

5 Gabions 14.0 10 Timber 6.00

c) LateralPressure

Lateralpressuresneedtobeconsideredinthedesignofthestructurewallsagainstoverturning,and

aremainlycausedbywater,earthandtrafficloadings.Earthquakesalsowill inducelateralthrusts.

Thefollowingsectionpresentstheformulasforcalculatingtheselateralpressurestobeincurredon

thestructures.

Waterpressure

Waterpressureactingonthestructureiscalculatedas:

Pw=½Ƴh2

Where,Ƴistheunitweightofwater=9.80kN/m3,

histhedepthofwaterabovebaseinm,

Water pressure (Pw) acts at 1/3 of its height.Water pressure canbe reducedby providingweep

holesontheretainingwalls.Afilterbacking isalsonecessarytopreventwashoutofsoilparticles

withwater.

Earthpressure

Twotypesofearthpressuresactontheretainingwalls.Activeearthpressureisthepressureofthe

earthwhichiscausingthewalltomoveawayfromtheearth.Passivepressureisthepressureofthe

earthwhichisresistingmovement.TheactiveearthpressureiscalculatedbyRankinsformula:


199

Pa=0.50*Ka*W*h2andKa=(1+sinΦ)/(1-sinΦ)

Where,Ka–activeearthpressurecoefficient,

Φ–angleofinternalfrictionofthesoilanddependsuponthetypeofsoil,

W–unitweightofsoilinkN/m3

h–heightofbackfilledmaterial

Theschematicpressurediagramoftheretainingwallwithsurchargeloadactingbehindthewall is

presentedin(Figure6-35).

Figure6-35Schematicpressurediagramofwalls

Where,H1-surchargeload,

H2–drybackfillload,

H3-drybackfillload,

H4–saturatedbackfill,

H5–waterpressure

W1-unitweightofdrysoil,

W2-unitweightofsaturatedsoil,

W3–unitweightofwater,

S–surcharge

Thepassiveearthpressureiscalculatedas:

Pp=0.50*Kp*W*h2andKp=(1-sinΦ)/(1+sinΦ)

Where,Kp–passiveearthpressurecoefficient,

Φ–angleofinternalfrictionofthesoil,

W–unitweightofsoilinkN/m3

h–heightofbackfilledmaterial


200

Surcharge

Traffic loads includingconstructionandoperatingequipmentandpeople/animalscauseadditional

lateralpressureonthewallsandabutments.Anassumedvalueofsurcharge isconsideredfor the

designofwallsandabutments.

Earthquake

Earthquakeloadingsarecalculatedusinganaccelerationcoefficient,whichismultipliedbythedead

loadofthestructuretoproduceadditionalhorizontalloadactingatthecentreofthegravityofthe

structure.Forsmallcanalstructuresearthquakeloadcanbeignored.

Self-Weight

Self-weight is the weight of the structure itself. In stability calculations the self weight resists

overturningandslidingforces.

UpliftPressure

Uplift isthepressureunderneaththestructurecausedbywaterpressureoneitherside.Theuplift

counteractstheself-weightinresistingslidingandoverturning.

BaseFriction

Basefrictionistheforcealongthebaseofthestructurewhichresistssliding.Itiscalculatedas:

BF=(W-U)tanΦ

Where,W-self-weight,

U–uplift,and

Φ–angleofinternalfriction

6.8.3 FactorsofSafetyforStability

Stabilityanalysisofcanalstructuresnormallyinvolves:

• Assessmentofresistanceagainstoverturning;

• Assessmentofresistancetosliding;and

• Assessmentofpositionofthebasereaction

Thefactorsofsafetyadoptedforthedesignofcanalstructuresagainstoverturningare:

• 2.00fornormalloadingcases,

• 1.50forextremeloadingcases

Thefactorsofsafetyadoptedforthedesignofcanalstructurestoslidingare:

• 1.50fornormalloadingcases,

• 1.20forextremeloadingcases

Thereactionmustbewithinthemiddlethirdofthestructurei.e.notensionbetweenbaseandsoil

isallowed.

6.8.4 BearingPressures

Thefoundationscanfailintwoways:byshearfailureorbysettlement.Thefoundationsshouldbe

safeagainstshearandthelimingfactorwillbethesettlement.Themaximumallowablesettlement

offoundationsforcanalstructuresshouldbetakenas25mm.Theallowablebearingpressureofthe

soil is calculated from theDirect Cone Penetrometer (DCP) test carried out along themain canal

alignment. The allowable bearing pressure of various soils is also estimated from standard civil

engineeringbooks(Table6-10).


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Table6-10Allowablebearingpressureofthesoil

S.N Soiltype Allowable bearing pressure

(kN/m2)

1 Softclaysandsilts <80

2 Firmclaysandfirmsandyclays 100

3 Stiffclaysandstiffsandyclays 200

4 Verystiffboulderclays 350

5 Compactwellgradedsandsandgravel/sandmixtures 200

6 Looseuniformsand 150

6.8.5 SeepageandUpliftDesign

All hydraulic structures built on pervious foundations exhibit seepage flow underneath the

structure. Thewater seepingbelow thebodyof thehydraulic structureendangers the stabilityof

thestructureandmaycauseitsfailureintwoways:

• Bypiping,or

• Bydirectuplift

Failurebypipingoccurswhenseepagewaterretainssufficientresidualforceatdownstreamendsof

thestructure.Theseepingwatermayliftsoilparticlesandmayleadtotheprogressiveremovalof

the soil underneath the foundation. The structuremayultimately subside intoahollow, resulting

intoitsfailure.

Failurebyupliftoccurswhenseepingwaterbelowthestructureexertsupliftpressureonthefloorof

thestructure. Ifthispressureisnotcounterbalancedbytheweightofthefloor,thestructurewill

failbyaruptureofpartofthefloor.Theseepagepathofthesub-surfaceflowisillustratedinsketch

(Figure6-36).

Figure6-36Sketchofseepagepathunderneaththestructure

Bligh’screeptheory,Lane’sweightedcreeptheoryandKhosla’stheoryarethemostcommonlyused

methodstoassessseepageandupliftforhydraulicstructuresbuiltonperviouslayers.Smallcanal

structuresaredesignedbasedonLane’sweightedcreeptheoryorBligh’screeptheorywhile large

structures are designed based on Khosla’s theory of independent variables. Head regulators and


202

small canal structuresaredesignedusing Lane’smethodwhile cross regulators aredesignedwith

Khosla’smethod.ThelengthoftheweightedcreepasperLane’smethodisasfollows:

LW=H/3+VwhichmustnotbelessthanCW*HL

Where,LWistheweightedcreeplengthinm,

Histhesumofhorizontalseepagepercolationpathlinesinm,

Visthesumofverticalpercolationpathlinesinm,

HListhemaximumheadofwateracrossthestructureinm,and

CWistheweightedcreeprationwhichisbasedonthetypeofsoilasgiven(Table6-11).

Table6-11Lanesweightedcreepratio

S.N Typeofsoil Weighted creep

ratio(CW)

1 Veryfinesandandsilt 8.50

2 Finesandandalluvialsoil 7.00

3 Coarsesand 5.00

4 Finegravel 4.00

5 Coarsegravel 3.00

6 Softclay 3.00

7 Hardclay 1.80

8 Veryhardclayandimperviouslayer 1.60

6.8.6 ConcreteDesign

General

The concrete design is carried out with permissible limit state method using Indian Standard

IS456:2000 Plain and Reinforced Cement Concrete-Code of Practice. Irrigation structures are

designed to be built with mass and reinforced concrete complying IS456:200. The density of

reinforcedconcreteistakenas24kN/m3.

ConcreteGrades

Gradingof concrete is basedon the characteristic compressive strengthof the concrete,which is

definedasthe28daystrengthbelowwhichnotmorethan5percentofthetestedspecimensfail.

The specified characteristic compressive strength is that of a 150mm cube at days, expressed in

N/mm2. The following concrete grades are used in the design of the irrigation structures (Table

6-12).

Table6-12ConcreteGradesandUses

Concrete

Grade

Typical Mix

(cement: sand:

aggregate)

Specified

characteristic

compressive strength

at28days(N/mm2)

Useofconcretegrades

M10 1:3:6 10 Blinding concrete/lean

concrete


203

Concrete

Grade

Typical Mix

(cement: sand:

aggregate)

Specified

characteristic

compressive strength

at28days(N/mm2)

Useofconcretegrades

M15 1:2:4 15 Mass concrete, cast in situ

concreteslabs

M20 1:1.5:3 20 Reinforced

concrete/structuralconcrete

M25 1:1:2 25 Water retaining structures,

reinforcementconcrete

Source:PWVDP,IrrigationManual,1992

ReinforcementSteel

Thereinforcementshouldcomplywith IS1786, ISO4066:2000,AASTHO/2007,allofwhichspecify

thetestsforcompliancetoobtainthecharacteristicstrength(Table6-13).

Table6-13ReinforcementBars

Grade Nominalsizeofbars

(mm)

Minimumyield

stress(Mpa)

Type

500 8mmto32mm 500 TMTbars(Thermo

mechanicallytreated)

The cover of concrete to reinforcement depends on the dimensions of structural elements,

maximumsizeofaggregateandconditionsofcontacttowaterandsoil.Theminimumcovershallbe

notlessthanthatthespecifiedvaluesorgreaterthannominalsizeofaggregate.


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CHAPTER-7

7. MicroIrrigation

7.1 Introduction

Micro irrigation refers to the alternativemethods of irrigationwater acquisition, conveyance and

application as an efficient way to providewater to plants. The concept and technology ofmicro

irrigationwas firstdeveloped in Israel in the1960s. Sprinkler irrigationanddrip irrigationare the

mainmicro irrigationmethodsofwaterapplicationbeingadoptedworldwide. Thesetechnologies

are especially suitable for smallholder farmers of arid and semiarid regions of Asia and Africa.

Despite the abundance of water in Bhutan, its availability in time and space limits irrigation for

smallholderfarmers,whomostlydependonerraticrainfallpatterns.Numeroussmallstreamsand

springsscatteredaroundthehillsofBhutancanbeeffectivelytappedformicroirrigation.Likewise,

inthesmallplotsof landwith limitedwatersources,micro irrigationcanbe introducedefficiently

fortheproductionofmorevaluablecrops.Inacontextwheresmallwatersourcesaredryingupdue

to climate change, water harvesting technologies and efficient water application methods are

climateresilienttechnologiesforirrigatedagriculture.

Hence,micro irrigation is related to thealternativemethodsofbothwater applicationandwater

acquisitionforirrigation.Thischapterdealswiththeconceptanddesignofsprinklerirrigation,drip

irrigationandwaterharvestingmethodsofmicroirrigationtechnologiesfeasibleforBhutan.

7.2 SprinklerIrrigationSystem

7.2.1 General

A sprinkler systemapplieswater for irrigation in the formof a spraywhich somewhat resembles

rainfall.Thesprayisdevelopedbytheflowofwaterunderpressurethroughsmallorificesornozzles

referred to as sprinklers. The water pressure in the sprinkler pipes can be developed either by

pumpingfromlowerelevationsorbyflowingfromhigherelevationsbygravity.Inaddition,sprinkler

irrigationisparticularlysuitableforwaterscarceareas,hillslopesandhigherodiblesoils.Toprevent

water pooling and surface run-off, sprinklers are designed to applywater at a rate that does not

exceedtheinfiltrationcapacityofthesoil.

7.2.2 AdvantagesofSprinklerIrrigation

Sprinkle irrigation has several advantages over conventional surface irrigation methods. The

versatility of use in different land, soil, and crop conditions are the key features of the sprinkle

irrigationmethod.Theadvantagesofasprinkleirrigationsystemaredescribedhereunder:

i. Controloverwater

Sprinklerirrigationoffersahighdegreeofcontrolledwaterapplication.Theamountandfrequency

ofwatercanbecontrolledveryefficiently.Inaddition,overirrigation,pounding,andrunoffcanbe

eliminated through the control ofwater application. Thiswill help tomaintain optimum growing

conditionsofthecropsandoptimumwaterutilization.

ii. Savingofwater

In sprinkler irrigation, water is supplied through the pipes and relatively high degree of water

distributionuniformitycanbeattained.Thuswithagivenquantityofwater, thesprinklermethod

canirrigateamuchlargerareathansurfaceirrigation.Inotherwords,irrigationrequireslesswater


205

perunitarea. Inawell-managedsystem,theapplicationefficiencyofasprinklersystemcanbeas

high as 75%, against 50% in flood irrigation. Furthermore, small water sources can be utilized

effectively.

iii. SavingofLabor

Sprinkler irrigation requires less labor for theoperationandmanagementof the irrigation system

becauseitdoesnotrequirethesameattentionassurfaceirrigation.Soildoesnottendtoclodunder

sprinkler irrigationandaswater isuniformlydistributedover theentire landarea, land leveling is

alsonotrequired.Thiswillsavethecostoflandpreparationandtopsoilwithhigherfertilitywillnot

bedisturbed.

iv. Protectionofthesoilandcrops

Damageofsoilsandcropsduetofrostandwindcanbeminimizedwithsprinkler irrigation.This is

moreimportantforcropsatgerminationandearlystages.Thecontrolledapplicationpreventssoil

erosionandalsohelpskeepgoodsoilstructure.

v. Adaptationtomarginalsoils

Asprinkler irrigationsystemcanbedesigned tooperateefficientlyonalmostall topography.This

methodmaybeaneffectivetechniquetoirrigateintheslopingterrain,irregularplotsandmarginal

soilconditions.

vi. Efficientuseoftheland

Sinceconveyancechannelsandbundsarenotrequiredinsprinklerirrigation,almosttheentirearea

canbeusedforcropproduction.Bunds,levees,andthewatercoursesoftenoccupyanappreciable

amountoflandareawhichcannotbeputundercultivation.

7.2.3 ConstraintsofSprinklerIrrigation

Despitetheseveraladvantagesofsprinkler irrigation, therearesomeconstraints in itsapplication

whicharepresentedbelow.

i. Highinvestmentcost

Theinitialinvestmentcostofsprinklerirrigationisrelativelyhigh.Inaddition,runningcostsarealso

highbecausesprinklersrequireenergytooperate.

ii. EffectofthePressure

Water distribution of the sprinkler is greatly affected by the pressure ofwater. If the pressure is

aboveor below theoptimum,waterwill not be uniformly distributed.At lowpressure,water jet

does not break up andmost of thewater tends to fall at some distance from the sprinkler. The

largerdropswillalsodamagethesoilstructureandthecrop.Ontheotherhand,ifthepressureis

toohigh,afinespraywilldevelopintheformofmistthatfallsclosetothesprinkler.

iii. Effectofthewind

Waterdistributionremainspoorifthesprinklersareoperatedinwindyconditions.Thefinesprays

fromsprinklerscanbeeasilyblownawaybythewind,whichwilldistortthewettingpatterns.Thisin

turnwillupsetirrigationuniformity.


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iv. Cloggingofthenozzle

Iftheirrigationwatercontainssuspendedmatterssuchassilt,inorganicimpurities,thenozzleofthe

sprinklerscanbeblocked.Smallsizesprinklersaremorevulnerabletocloggingproblems.

7.2.4 TypesofSprinklerSystem

Sprinklersystemsaregroupedintotwotypesbasedonthearrangementofthespray:

i. Rotatingheadsystem,and

ii. Perforatedpipesystem

The rotating head system consists of small sized nozzles placed on riser pipes fixed at uniform

intervals along the length of lateral pipes. The lateral pipes are laid on the ground surface. The

nozzleof thesprinkler rotatesduetoasmallmechanicalarrangementwhichutilizes the thrustof

flowingwater (Figure 7-1). The riser pipes andnozzlesmay also bemountedonposts above the

cropheight.

Theperforatedpipesystemconsistsofholesperforatedinthelateral irrigationpipesinaspecially

designed pattern to distribute water uniformly. The spay emanating from the perforations are

directedonbothsidesofthepipeandcancoverastripofland6mto15mwide.

Figure7-1Typeofsprinklerirrigationsystem

Rotatingheadsystem Perforatedpipesystem

Based on the installations of the system, sprinklers are grouped into portable, semi-portable and

permanent types. The portable sprinkler system consists of a portable main line, laterals and

pumpingplantwhileapermanentsprinklersystemconsistsofapermanentlylaidmainline,laterals,

and a stationarywater source andpumpingplant. In a semiportable sprinkler system, thewater

sourceandpumpingplantarefixedandotherpartsareportable.

7.2.5 ComponentsofSprinklerSystem

The complete sprinkler system can be broadly divided into two components: awater acquisition

component and a water distribution component. The water acquisition component involves

acquiring water from the source and delivering it to the field while the water distribution


207

componentinvolvesthecontrolofwaterwithinthefieldanditsapplicationtothecrop.Thewater

acquisitioncomponentisfurtherdividedintofivesub-components:

• Watersource,

• Transmissionpipe,

• Storagetank,

• Distributionpipeand

• Maincontrolvalve

Thewateracquisitioncomponentsshallnotonlysupplytherequiredamountofwaterforirrigation

butalsomaintainsufficientpressuretooperate thesprinklersystem.Forsmallandmediumsized

sprinklers,theoperatingheadvariesbetween10mto30m.Dependingonthewatersourcevarious

methodsareusedtoobtainthedesiredpressureofwateratthesprinklerhead.Themostcommon

methods of obtaining pressure are the installation of a pumping unit, gravitational energy, and a

directconnectionfromthesupplyline.

Figure7-2Componentsofsprinklerirrigationsystem

WaterAcquisition WaterDistribution

Thewaterdistributioncomponentisalsofurtherdividedintofivesub-components:

• Fieldcontrolvalve,

• Connectionpipe,

• Mainlinepipe,

• Riserpipeand

• Sprinklerhead

Thefieldcontrolvalveisconnectedwiththemaincontrolvalveofthedistributionpipeandsupplies

watertotheconnectionpipeordirectlytothemainline.Themainlinesuppliesthesub-mainand

laterals. Inconfigurationswithoutasub-main, themain linedeliverswaterdirectly tothe laterals.

Themainsmaybeeitherburiedorportabletypes.Portabletypemainsaremadeof lightmaterial

such as aluminum or plastic. Laterals are laid along the contour of the land. The vertical pipe


208

connectinglateralswiththesprinklerheadiscalledariserpipe.Theschematicdiagramofsprinkler

systemcomponentsispresentedinFigure7-2.

7.2.6 DesignConceptofSprinklerSystem

Thedesignofasprinklersysteminvolvestheassessmentofthewatersource,shapeandsizeofthe

fieldtobeirrigated,cropstobeplanted,topographyofthearea,andirrigationrequirements.The

watersourceshouldbenearesttotheirrigationcommandareainordertoavoidthecostofwater

conveyance.

Asprinklersystemmustbedesignedtoapplywateruniformlywithoutsurfacerunofforsoilerosion.

Theapplicationratemustmatchtheinfiltrationrateofthesoil.Thecapacityofthesprinklersystem

depends on the peak water requirement during the crop growing season, effective crop rooting

depth,typeofsoil,andavailablewaterorpumpingrate.

The sprinkler irrigation system shall be selectedwith the thorough evaluation of the specific site

conditionsandthecomputationoftheavailableflow.Oncethedesignofthesystemiscompleted,a

detailedlistofalltheequipmentnecessaryfortheinstallationofthesystemmustbepreparedwith

fulldescriptions,standards,andspecificationsofeveryitem.Typicalsprinklersarepresentedbelow

(Figure 7-3)

Figure7-3Typicalsprinklerirrigations

7.2.7 DesignProcedures

i. Layoutplanoftheplottobeirrigated

- Dividetheareasintoblocks,

- Decidethetypeofsprinklersystem:permanentorportable,

- Decidewaterrequirementbasedoncropstobeplanted,

- Decideoperatingpressure

ii. Calculatenumberofsprinklersandselectthesizeofthenozzle

- Basedon the soil infiltration rate (Table7-1), slope,andavailablepressure selectappropriate

sprinkler and recommended spacing between sprinklers. This information is available on the

productcatalogueofthemanufacturer,

- Calculatethewaterapplicationrate,


209

Applicationrate=flowrateofonesprinkler(lpm)*60/(areabetweensprinklers(sqm))

- Calculatenumberofsprinklersandtheirspacing

Table7-1Infiltrationrateofdifferentsoils

SoiltypeClay Cayloam Siltloam Sandyloam Sand

Water intake rate

(mm/h)

1to5 6to8 7to10 8to12 10to25

iii. Designoflaterals

- Determinethelength,directionandnumberoflaterals,

- Determineflowrateoflateral=numberofemitterperlateralxemitterflowrate

- Numberoflateralsoperatingsimultaneously=systemflow/flowrateoflateral

- Numberofshiftstocompleteoneirrigation=totalnumberoflaterallines/numberoflaterals

operatingsimultaneously

- Durationofapplication=irrigationrequirementinmm/applicationrateinmm

- Orirrigationrequirementinm3perhour/systemflowinm3perhour

iv. Determinationofthesizesofthepipeline

- Designoflateralpipeinvolves:

- Assumediameteroflateralforvaryinglateralflow,

- Computeheadlossinlateralandcheckavailableatitstwocorners,

- Adoptassumeddiameterifheadlossisacceptable

- Designofsub-mainline,

- Similartodesignoflateralsanddischargedecreasesfromheadtotail,

- Computeheadlossandcheckitwithavailablehead,

- Designofmainline,

- Dischargeinmainlineisconstantthroughthelength,

- Computetheheadlossandcheckitwithavailablehead,

HeadlossduetofrictioninlateralsoftheplasticpipeisgivenbyHazenWilliams’sequation:

FLD

QHf ××=

872.4

852.1

27.15

Where,Hfisheadlossduetofrictioninm,

Qistheincomingdischargeinthelateral(lps)

Distheinternaldiameterofthelateral(plasticpipe)incm

Listhetotallengthofthelateralinm

F is the reduction factoror correction factor forhead lossdue to friction inpipewith

multipleoutlets.ThevalueofFdependsonthenumberofoutletsinonelateral(Table

7-2).

Table7-2F-factorvalueforheadlossinpipes

Numberofoutlets F-value Numberofoutlets F-value

1 1.00 12 0.376

2 0.62 15 0.367

3 0.52 20 0.36


210

Numberofoutlets F-value Numberofoutlets F-value

4 0.47 24 0.355

5 0.44 28 0.351

6 0.42 30 0.35

7 0.41 40 0.345

8 0.40 50 0.343

9 0.39 100 0.338

10 0.385 >100 0.333

Source:FAO,2000

v. Designofstoragetank

- Designofstoragetankisnecessarytogetheadforsprinklers,

- Forhillyregionorterracedlandatleast25mofheadisavailableforsprinklers,

- Thecapacityofstoragetankshallbefixedforonedayirrigation,

- Thecapacityisbasedonthenumberoflateralsanditsdischargingcapacity

7.2.8 DesignExampleofSprinklerSystem

Data

OnevillageinTsirangnearDamphutownhas22households.Eachhouseholdhasabout0.25acres

ofuplandnearthehomesteadandaspringwatersourceisalsolocatedatadistanceofabout1km

fromthevillage.Theverticalheightdifferencebetweenthesourceandthevillageisapproximately

115m.Themeasureddischargeof thewaterat the spring source inMay is2.2 litersper second.

Eachhouseholdinthecommunityiswillingtogrowvegetables.

Layoutofthesystem

The layout of the sprinkler system consists of intake at the source andmain pipe line up to the

storage reservoir. From the reservoir, water will be distributed to the six outlets or taps. It is

assumedthat4to5householdswilluseoneoutletandirrigatetheirfields.Thelayoutofthesystem

ispresentedinFigure7-4.

Wateravailabilityassessment

Itislearntthatthesourceofwaterforthesprinklerirrigationsystemisperennialandthemeasured

flow inMay is 2.2 lps. Considering the reliability of themeasured flow the available flow at the

sourceisassessedas1.80lps.

Theavailableflowperday=1.80x3600x24=155,520liter==155.50m3perday

Calculationofcropwaterrequirement

The crop water requirement is calculated based on potential evapo-transpiration (ETo) using

CropWat-8.ForDamphutownEToispresentedinTable7-3.

Table7-3Referencecropevapo-transpirationandcropcoefficients

Month ETo(mm/day) Crop Maximumcrop

coefficientvalues

January 1.81 Onion 1.15

February 2.26 Cauliflower 1.05

March 2.99 Cabbage 1.05

April 3.45 Cucumber 1.15


211


coefficientvalues

May 3.56 Eggplant 1.05

Peakwaterrequirement=EToxKcxA/Ep

Where,EToisthereferencecropevapotranspirationinmm/day

KcisthecropcoefficienttobetakenfromFAOmanuals,

Epistheefficiencyofthesprinklersystem=0.70,

Aistheareairrigatedinonedecimal=405sqm,

Peakwaterrequirement=3.56x1.15x405.0.70=2368liter/day

Add10%forlosses =1.1x2368=2605liter/day/decimal

Capacityofwaterstoragereservoir

Numberofhouseholdsinthevillage=22HH

[email protected]/HH=0.25x22=5.50acres

Peakwaterdemandperdecimalofland=2,605liter/day

Totalwaterdemandforthecommandarea=2,605x55=143,275liter/day

Minimumflowrequiredtomeetwaterdemand=143,275/(24*3600)=1.65lps

Adjustedflowfromthesource =1.71lps

Numberofoutlets/taps =6

Totalwatervolumepertrapperday=1.71x3600x24/6=24,624liter

Operatinghours =14hours

Dischargepertrap =24,624/(14x3600)=0.48lps

Adjusteddischargeoftap =0.45lps

The size of the reservoir is calculated based on the balance of the inflow (supply) and outflow

(demand)ofthesystemandispresentedintabularform(Table7-4).

Table7-4Calculationofreservoircapacity

Numberoftaps 6 Nos

Dischargeofeachtap 0.45 lps

Reservoiroperation 14 hrs

Reservoiroperation

schedule

Duration,

hrs

Flow,lps Inflow

(supply),

lit

Outflow

(demand),

lit

Balance,lit

7PM-4AM 9.0 1.71 55,404 - 55,404

4AM-12Noon 8.0 1.71 49,248 77,760 26,892

12Noon-1PM 1.0 1.71 6,156 - 33,048

1PM-7PM 6.0 1.71 36,936 58,320 11,664

Fromthecalculationitisassessedthattherequiredcapacityofreservoir=55,404lit

Takethecapacityofreservoiras56,000lit=56.00m3

Assumethedepthofreservoiras1.4mtheareaofreservoir=56/1.4=40m2

Takesizeofrectangularreservoiras8mx5mwithverticalwalls

Designofpipelines

Thedesignofpipelinesiscarriedoutintabularform(Table7-5).


212

Table7-5Designofpipelinesforsprinklerirrigationsystem

S

No

PipeLine Length(m) Flow ReducedLevel(m) Level Static HGLUp Total

avail.

Pipe(mm) Head Residual HGLDn

From To Measure

d

Design lps From To Diff

(m)

Head

(m)

m Head(m) OD ID % Loss(m) Head(m) m

1 S M

1

245 269.5 1.71 1150.00 1135.35 14.65 14.65 1150.00 14.65 63(6) 53.4 1.31 3.530 11.12 1146.47

2 M1 M

2

340 374.0 1.71 1135.35 1133.64 1.71 16.36 1147.71 14.07 63(6) 53.4 1.31 4.899 9.17 1142.81

3 M2 R 376 413.6 1.71 1133.64 1125.22 8.42 24.78 1144.53 19.31 50(6) 42.2 3.95 16.337 2.97 1128.19

4 R D

1

187 205.7 2.70 1125.22 1110.16 15.06 15.06 1125.22 15.06 63(6) 53.4 2.90 5.965 9.09 1119.25

5 D1 D

2

432 475.2 1.35 1110.16 1095.09 15.07 30.13 1119.25 24.16 50(6) 42.2 2.60 12.355 11.80 1106.89

6 D2 T1 256 281.6 0.45 1095.09 1056.48 38.61 68.74 1106.90 50.42 32

(10)

24.0 5.70 16.051 34.37 1090.85

7 D2 D

3

205 225.5 0.90 1095.09 1088.67 6.42 36.55 1106.90 18.23 63(6) 53.4 0.40 0.902 17.33 1106.00

8 D3 T2 189 207.9 0.45 1088.67 1042.48 46.19 82.74 1106.00 63.52 25

(10)

18.9 17.60 36.590 26.93 1069.41

9 D3 T3 212 233.2 0.45 1088.67 1040.48 48.19 84.74 1106.00 65.52 25

(10)

18.9 17.60 41.043 24.48 1064.96

10 D1 T4 310 341.0 0.45 1110.16 1064.44 45.72 60.78 1119.25 54.81 32

(10)

24.0 5.70 19.437 35.37 1099.81

11 D1 D

4

498 547.8 0.90 1110.16 1088.84 21.32 36.38 1119.25 30.41 63(6) 53.4 0.40 2.191 28.22 1117.06

12 D4 T5 128 140.8 0.45 1088.84 1076.64 12.20 48.58 1117.06 40.42 32(6) 27.0 3.10 4.365 36.06 1112.70

13 D4 T6 318 349.8 0.45 1088.84 1062.87 25.97 62.35 1117.06 54.19 32

(10)

24.0 5.70 19.939 34.25 1097.12


213

Figure7-4LayoutPlanofSprinklerIrrigationSystem


214

7.3 DripIrrigation

7.3.1 General

Drip irrigation isawaterapplicationmethodwhich irrigatesonlypartof thesoilat theplant root

zone.Dripirrigationisalsoreferredtoastrickleirrigationandinvolvesdrippingwaterintothesoilat

averylowrate(2-20literperhour).Thismethodconsistsofapipednetworkoflateralslaiddown

onthegroundorburiedatashallowdepthwithlowflowoutletscalledemittersordrippers.These

emittersaredesignedtoemitatrickleratherthanajetofwaterandareplacedsoastoproducea

wetstripalongtheplantroworawettedbulbofsoilateveryplant(Figure7-5).

Thistypeofirrigationisusuallypracticedinwaterscarceareasforspacingcropslikefruits,flowers

and vegetables. It is considered to be one of themost efficientmethods ofwater application tomeetcrop waterrequirements,andneedssixtoeighttimes lesswatercomparedtoconventional

irrigationsystems. InternationalDevelopmentEnterprise (IDE),an internationalnon-governmental

organization (INGO) has developed a simple drip irrigation technology being used in developing

countriesincludingNepalandBhutan.Duetoitssimplifiedtechnologyandfarmers’engagementin

high-valueagriculture,dripirrigationisnowshowinggreatpromiseforraisingthelandproductivity,

waterefficiencyandincomesofpoorsmallholders(Polak,2001).Moreover,theapplicationofdrip

irrigation ispopular inseveraldevelopingaswellasdevelopedcountriesduetohighercropyields

andhigherwateruseefficiency.

Figure7-5Examplesofdripirrigationmethod

Spacingofcropsindripirrigation Largescaledripirrigation

7.3.2 AdvantagesofDripIrrigation

Adripirrigationsystemoffersspecialagronomical,agro-technicalandeconomicadvantagesforthe

efficient use of water and labor. Furthermore, it provides an effective means for utilizing small

streamsofwater.Theadvantagesofdripirrigationsystemaredescribedhereunder:

i. Controloverwater

Dripirrigationoffersahighdegreeofcontrolledwaterapplication.Theamountandthefrequency

of water application can be controlled very finely. This will help to maintain optimum growing

conditionsofthecropswithoptimumutilizationofwater.


215

ii. SavingofWater

Due to thecontrolledand localizedapplicationofwater, losses fromdirectevaporationanddeep

percolationaresignificantlyminimized.Waterapplicationefficiencycanreachupto90%,compared

to70%forsprinklerand40%forsurfaceandfurrowirrigationmethods,whichinturnsaveswater

significantly.

iii. SavingofEnergyandLabor

Drip irrigationrequires less labor, lessenergyand lesspreparatorywork thanothermethodsas it

requiresrelativelylowheadtooperate,doesnotrequirelandleveling,andtherearelessweeds.In

addition,dripirrigationprovideseasieraccesstocropfieldsandthepossibilityoffertilization.

iv. Greaterandhigherqualityyield

Duetotheoptimumapplicationofwatertoeachplant,favorableagronomicconditionsarecreated

inthesoil,waterandair interfaceunderdrip irrigation,enhancingplantgrowthandyields.Asthe

crop foliage under a drip system remains dry, the incubation of the many plant pathogens is

retarded.Thesespecific featuresofdrip irrigationcontribute to less infestationresulting inhigher

andbetterqualityyield.

v. Possibilityofapplicationofnutriments,pesticides

Withdripirrigation,itispossibletoapplysolublefertilizersandchemicalsalongwiththeirrigation

water in a precise manner. This type of efficient application of fertilizer and pesticides is the

inherentadvantageofdripirrigation.

vi. Adaptationtopoorsoils1

Drip irrigation systems can be designed to operate efficiently on almost all topography including

sloping terrain, irregular plots and poor soil conditions. This is the most environment friendly

methodofirrigationthatdoesnotcauseanyerosionandlandslide.

7.3.3 LimitationsofDripIrrigationSystem

Despitethenumerousadvantagesofdripirrigation,thereareafewlimitationswhicharedescribed

hereunder.

i. Cloggingofdrippers/emitters

Clogging isoneof themajorproblemsassociatedwithdrip irrigation.Because theemitteroutlets

areverysmall,theycaneasilybecomecloggedbyparticlesofmineralororganicmattermixedwith

water.Cloggingreducesemissionratesandtheuniformityofwaterdistribution,whichcanleadto

crop loss or failure. A good filtration is the most important prerequisite for minimizing clogging

problems.

ii. Saltaccumulationinsoils

Whenbrackishwaterisusedindripirrigation,saltstendtoconcentrateatthesoilsurfaceandthe

entire wetting front. This can become a potential hazard for both the standing crop and the

subsequentcrop.Thisisbecauseofthelightrainsthatcanmovetheaccumulatedsaltsintotheroot

zone.Henceduringlightrains,supplementaryirrigationmaybenecessarytoleachexcesssalts.

iii. Distributionun-uniformityinthesteepland

Ifpoorlydesignedandimproperlymaintained,thewaterdistributionuniformityofadripsystemcan

be low in the long, sloping, or undulating fields. Furthermore, the lines will drain through lower

emittersafterthewaterisshutoffandhencesomeplantsreceivetoomuchwaterandotherstoo

little.

iv. Systemcostandcomplexityofoperation

The initial investmentcostofdrip irrigation ishighcomparedtoconventionalsurface irrigation. In

addition, drip irrigation requires trained personnel to operate and manage the system. The

1 Poor soils refer here as high porosity soils


216

constraint of investment and complexity, however, does not apply in the caseof simple low cost

dripirrigationsystems.

7.3.4 ComponentsofDripSystem

Likesprinklerirrigation,adripirrigationsystemcanalsobebroadlydividedintotwocomponents:a

water acquisition component and a water distribution component. The water acquisition

componentinvolvesacquiringwaterfromthesourceanddeliveringittothefield,whilethewater

distributioncomponentinvolvesthecontrolofwaterwithinthefieldanditsapplicationtothecrop.

Thewateracquisitioncomponentisfurtherdividedintofivesub-components:

o Watersource,

o Transmissionpipe,

o Storagetank,

o Distributionpipeand

o Maincontrolvalve

The sub-components of the water acquisition part of drip irrigation are similar to that of the

sprinkler irrigation system presented earlier. The water distribution component is also further

dividedintofivesub-components:

o Fieldcontrolvalve,

o Connectionpipe,

o Mainlinepipe,

o Lateralsand

o Emittersordrippers

Figure7-6Schematicdiagramofwaterdistributioncomponentofdripirrigationsystem

The water distribution sub-components are also similar to that of the sprinkler irrigation system

with theexceptionof the lateral pipes andemitters. Laterals are laid along the crop rowson the

groundandholdemittersordrippersatadefinitespacing.Thediameterofpipesusedforlateralsis

usually9,12and15mm.Emittersaresmalldispensingdevicesandareaffixedtothelaterals.The


217

mainfunctionoftheemitteristodischargeaconsistentamountofwaterneartheplantintheform

ofdrops. Theemitterburnsoffpressureenergy to theatmosphericpressureor to createa great

head loss. Sometimesmore thanoneemitter isprovided foronecrop,especially formature fruit

trees.Forvegetableswithsmallspacing,emittersmaybecloselyspacedtogetacontinuouswetting

pattern.Atpresentvarioustypesofdrippersarecommerciallyavailableonthemarket.Basedonthe

mechanismofdissipatingpressure,thefollowingfourtypesofemittersareusedinthedripsystem.

• Longpath:capillarysizedtubeorchannel

• Orificeemitters:nozzlewithbaffles

• Vortexemitters:employvortexeffect

• Compensatingemitters.

Inlongpathemitters,headlossesarecausedbylongitudinalfrictionofwaterflowalongthenarrow

tube thus covering a relatively long path before exit. The Orifice emitters on the other hand

dissipateenergyduetotheflowofwaterthroughaconstrictedcrosssection.TheVortextypesof

emitters compel water to whirl in a circular chamber where centrifugal opposition develops

sufficientheadlossesdespitethelargecrosssection.Compensatingdrippersfunctioninsuchaway

thatthesizeoftheorificeislargeatlowpressureandbecomessmallathighpressure.Inthisway,

theyalwaysmaintainaconstantflowoverawiderangeofpressurevariations.

7.3.5 DesignExampleofDripSystem

Data

Plotsize=12mx10m=0.012acres

Soiltype=Siltyloam

Groundslope=mildslope

Elevationofthetankabovethefarmplot=10m

Proposedcrop=vegetablesuchonion,tomato,cabbage,eggplant,caulifloweretc.,

Proposedirrigationmethod=dripirrigation

i. Layoutoftheplot

Farmplotarea=12x10m=120sqm=0.012acre

Fixthelateralalongthewidthoftheplotandlengthoflateral=10m

Spacingbetweenlaterals=1m

Providesixlateralsononesidewithaccesspathatthecenter,

Totalnumberoflaterals=2x6=12

Thespacingofemittersshallbe30cmcentretocentreandhencefor10mlateraltherewillbe30

emitters,

Assumingthesoleofthegroundislessthan3%,thedirectionofthelateralsshallbealongtheslope,

andotherwisethedirectionofthelateralsshallbealongthecontour.

ii. Calculationofcropwaterrequirement

Crop water requirement for drip irrigation is assessed for peak water requirement of daily

applicationfor8to12hours.Foraperiodoflesswaterdemand,waterapplicationhoursneedtobe

reduced.Table7-6presentsreferencecropevapotranspirationoftheParoareaandcropcoefficient

factorsforproposedvegetablecrops.


218

Table7-6Referencecropevapo-transpirationandcropcoefficients


coefficientvalues

January 1.46 Tomato 1.2

February 1.96 Cauliflower 1.05

March 2.53 Cabbages 1.05

April 3.02 Cucumber 1.15

May 3.34 Eggplant 1.05

June 3.20 Watermelon 1.0

July 3.14

August 3.14

September 2.83

October 2.54

November 1.84

December 1.37

Peakwaterrequirement=EToxKcxCpxA

Where,EToisthereferencecropevapotranspirationinmm/day

KcisthecropcoefficienttobetakenfromFAOmanuals,

Cpisthecanopyfactor(Canopyfactorchangeswiththedevelopmentstageofcrop.Ifnovaluesare

available,Cpcanbetakenas1.0),

Aistheareairrigatedbyonelateral=1x10m=10sqm,

iii. ConsideringthecropofTomato

Thepeakwaterrequirement

(For an area irrigated by one

lateral)

=3.34/(10*100)x1.2x1x10

=0.04008m3/day

=40.08litresperdayperlateral

Add20%forlosses =40.08x1.2=48.09litres/day/lateral

Tomatoisplantedat60cmspacingatalternativedripper,

Noofplants=15plants

Waterrequirementperplant=48.09/15=3.20litresperplant

iv. Consideringthecropofeggplant

Thepeakwaterrequirement

(For an area irrigated by one

lateral)

=3.34/(10*100)x1.05x1x10

=0.03507m3/day

=35.07litresperdayperlateral

Add20%forlosses =42.08litresperdayperlateral


219

Eggplantisplantedat30cmspacing,

Noofplantsinonelateral=30plants

Waterrequirementperplant=42.08/30=1.40litresperplant

Considerdesigndischargeofthelateralas48litres/day

v. Selectionofemitters

Forsimplicityofoperation, it isproposedtousebuilt inemitters.Built-indrippersaretheholesin

thelateralpipeswhichgiveacontinuouswettingstrip.Eachholeisequippedwitharotatingtape,

whichcanadjustthewaterreleasethroughit.Dischargingcapacityofbuilt-inemittervariesfrom1

to 5 litres per hour. The higher the discharging capacity of the emitter, the higher will be the

capacityofalateralandthefasterwillbetheirrigation.

Select twomedium sized emitterswith 1.5 litres and 2 litres per hour capacity and calculate the

dischargingcapacityofthelateral.

Capacityoflateral,Ql=qexNe

Where,qeisthedischargingcapacityofemitter=1.5l/hrand2.0l/hr

Neisthenofoemittersinalateral=30

Ql1=1.5x30/(60x60)=0.013lps

Ql2=2.0x30/(60x60)=0.017lps

Calculatetimerequiredfordelivering48litresofwaterofalateral,T=48/Qlx(60x60)

T1=48/(0.013x60x60)=1.07hour

T2=48/(0.017x60x60)=0.80hour

vi. Designoflateral

Theflowinalateraldecreasesgraduallyandisminimalattheend.Thus,headlossduetofrictionin

alateralwithseriesofemittersisgivenbyHazenWilliams’sformula:

Hf=15.27(Q1.852/D4.872)*L*F

Where,Hfisheadlossduetofrictioninm,

Qistheincomingdischargeinthelateral(lps)

Distheinternaldiameterofthelateral(plasticpipe)incm

Listhetotallengthofthelateralinm

Fisthereductionfactor(correctionfactorforheadlossduetofrictioninpipewithmultipleoutlets),

whichdependsonthenumberofdripperinonelateral

vii. Computingthelateraldiameter

Laterallength=10m,and

F=0.35for30numbersofdripperinalateral(Table7-2)

Thehead loss (Hf) calculations foravailable lateralpipesare carriedout in tabular formwith two

emitterssize1.5lit/hrand2lit/hrandassumingtheheadlossattheendofeachlateraltobe0.50

m.Theavailablesizesofpipesforlateralsare10mm,12mm,14mmand16mm,thecalculations

arepresentedinTable7-7.


220

Table7-7Headlosscalculationsoflaterals

The head loss due to friction along the lateralsmust stay within the allowable limit tomaintain

allowableflowvariations.Thisconditionlimitsthelengthoflaterals.Thedesirablelimitforemitters

flowvariationislessthan10%,whichlimitsvariationintheavailableheadtoabout20%.

FromTable7-7itisclearthatpipesandemittersarewithintheallowablelimitsofflowvariation.For

practical purposes, 12 mm diameter pipe with 1.50 litres per hour of emitter is suitable. The

variationinhydraulicheadinthiscasewillbelessthat2%whichisverysafe.

viii. Designofsub-mainline

Proposednumberoflateral=12(6oneachside)()

Lengthofsubmainline=6m

Thedesignof the sub-main line is very similar to thatof the lateral line. Thedischargedecreases

graduallytowardstheendofthesub-mainline.So,theHazen-Williamsequationusedforthedesign

oflateralsisalsousedhere.ThecalculationsarepresentedinTable7-8.

ix. Designofmainline

Considerlengthofthemainline=5m

Unlikethelateralsandsub-mains,dischargeinthemainlineremainsconstantthroughoutitslength.

The calculation of head loss does not need to consider the F-factor to account for decreasing

discharge.Table7-9presentsfrictionlossthroughthemainlineundervaryingconditions.

Dripper

flow rate

(lph)

Discharge of

lateral, Q

(lps)

Pipe

diameter, D

(cm) (inside)

Head

loss Hf

(m)

Add 30 % for

other losses

(bends, fittings

etc) Total loss =

1.3*Hf

Head at

the tail end

of lateral

(m)

Head at u/s

end of lateral

(m)

Variation in

hydraulic

(pressure)

head (%)

Acceptability

10 m

0.35

2.00 0.017 1.00 0.030 0.040 0.50 0.540 7.3 OK

2.00 0.017 1.20 0.013 0.016 0.50 0.516 3.1 OK

2.00 0.017 1.40 0.006 0.008 0.50 0.508 1.5 OK

2.00 0.017 1.60 0.003 0.004 0.50 0.504 0.8 OK

1.50 0.013 1.00 0.018 0.023 0.50 0.523 4.5 OK

1.50 0.013 1.20 0.007 0.010 0.50 0.510 1.9 OK

1.50 0.013 1.40 0.003 0.005 0.50 0.505 0.9 OK

1.50 0.013 1.60 0.002 0.002 0.50 0.502 0.5 OK

Head loss (Hf) = 15.27 x F x (Q^1.825 / D^4.871) x L

Length of laterals (L)

Reduction factor (F)


221

Table7-8Headlosscalculationsinsub-mainline

Table7-9Headlosscalculationinmainline

Designrecommendations

Basedonthecalculationsperformedintabularformsforlaterals,sub-mainsandmainlinepipes,the

followingrecommendationsaremade.

Usebuilt-indripper/emitterwithdischargingcapacityof2litresperhour,

Use12mmdiameter(internal)lateralswithdripperspacingof30cmcentertocenter,

Use18mmdiameter(internal)pipesforboththesub-mainandmainlines,

Totalheadrequiredis0.85m(Table7-9)installtankatabout1.5mabovethegroundlevel

Dripper

flow rate

(lph)

Discharge of

lateral, Q,

(lateral) (lps)

Sub-main

flow rate

(lps)

Pipe

diameter, D

(cm)

(inside)

Head

loss Hf

(m)

Add 30 % for

other losses

(bends, fittings

etc) Total loss =

1.3*Hf

Head at

the tail end

of sub-

main (m)

Head at u/s

end of sub-

main (m)

Variation in

hydraulic

(pressure)

head (%)

Acceptability

Total number of laterals on each side 6

6 m

0.369

2.00 0.017 0.100 2.00 0.017 0.022 0.52 0.538 4.2 OK

2.00 0.017 0.100 1.80 0.029 0.038 0.52 0.554 6.8 OK

2.00 0.017 0.100 1.60 0.051 0.067 0.52 0.583 11.4 OK

2.00 0.017 0.100 1.40 0.098 0.128 0.52 0.644 19.8 Margin

1.50 0.013 0.075 2.00 0.010 0.013 0.51 0.523 2.5 OK

1.50 0.013 0.075 1.80 0.017 0.022 0.51 0.532 4.2 OK

1.50 0.013 0.075 1.60 0.030 0.039 0.51 0.549 7.2 OK

1.50 0.013 0.075 1.40 0.058 0.076 0.51 0.586 12.9 OK

Reduction factor (F)

Length of each sub-main (L)

Head loss (Hf) = 15.27 x F x (Q^1.825 / D^4.871) x L

Design flow

Q (lps)

Size

Diameter

(mm)

Design flow

Q (lps)

Size

Diameter

(mm)

Design flow

Q (lps)

Size

Diameter

(cm)

Head loss

Hf (m)

5 m

2.00 0.017 10.00 0.100 20.00 0.20 2.00 0.138 0.180 0.54 0.718 OK

2.00 0.017 12.00 0.100 18.00 0.20 1.80 0.231 0.300 0.55 0.854 OK

2.00 0.017 14.00 0.100 16.00 0.20 1.60 0.410 0.533 0.58 1.116 OK

2.00 0.017 16.00 0.100 14.00 0.20 1.40 0.786 1.022 0.64 1.666 Check

1.5 0.013 10.00 0.075 20.00 0.15 2.00 0.082 0.106 0.52 0.629 OK

1.5 0.013 12.00 0.075 18.00 0.15 1.80 0.137 0.178 0.53 0.710 OK

1.5 0.013 14.00 0.075 16.00 0.15 1.60 0.243 0.315 0.55 0.864 OK

1.5 0.013 16.00 0.075 14.00 0.15 1.40 0.465 0.604 0.59 1.190 OK

Lateral Submain Main Add 30 % for other

losses Total loss =

1.3*Hf

Head at

the tail

end of

sub-main

(m)

Head at u/s

end of sub-

main (m)

Head loss (Hf) = 15.27 x (Q 1̂.825 / D 4̂.871) x L

Length of main line (L)

AcceptabilityDripper

flow rate

(lph)


222

Designofstoragetank

Thestoragetankfordripirrigationsystemisdesignedtostorewaterrequiredforonedayirrigation.

Designdischargeononelateral=48litresperday

Designdischargeof12laterals=12*48=576litresperday

Thus,use1000litresoverheadtankwhichislocatedat10mabovethefarmplot.

7.4 LowCostSimpleDripSystem

Thesimpledripsystemisoneofthelowcostmicroirrigationtechnologiesfocusingonsmallholder

farmersindevelopingcountries.Thislowcostdripsystemissuitableforporoussoilwhereseepage

loss is veryhighand flood irrigation is very inefficient.A simpledrip systemenables smallholders

Figure7-7Layoutplanofdesigneddripirrigationsystem


223

andmarginal farmers togrowoff-seasonvegetableswithvery littlewater, enabling them toearn

incomeandalsoincreasetheirnutrientintakethroughincreasedvegetableconsumption.

The main simplifications made from the drip irrigation system are low operating head, simple

drippersandfittings,andsmallsizesystem.Astheoperatingheadcanbejust1meter,thetankcan

alsobefilledmanually.Thewaterqualityhastobefreeofanappreciableamountofsedimentand

organicmatter.Pre-filtrationisrequiredifwaterissedimentladen.Drippersaresimpleholesspaced

60 cmcenter to center ina8mmdiameterPVC flexiblepipes. Thedischargeof adripper ranges

from2.1literperhourto2.5literperhour.Thewatertankcanbeplacedonalocallymadeelevated

platformorpedestalmadeofbambooandwooden logs.Anoutletpipeconnects this tankwitha

finefilteratthebottomofthepipetoavoidblockageinthedrippipe.Outsidethetankthereisan

outletset,alevelpipetoindicatethelevelofwaterandvalvetoopenandclosethewaterflow.The

outlet is connectedwith a 14mmdiametermain linepipe,which is joined to the drip pipewith

fittingsandanadjustmentpipe.AtypicalsimpledripirrigationmodeldevelopedinNepalandIndia

ispresentedhere(Figure7-8).

Figure7-8Schematicdiagramofsimpledripirrigationsystem

IDEhasdevelopedseveralmodelsofsimpledripirrigationwithallaccessorieswhicharemarketed

atlostcostandreadyforuse(Table7-10).ThemainaccessoriesusedinSDIaregroupedintothree

parts.Thefirstgroupincludesthetankandfilterswhilethesecondgroupcomprisespipes.Thepipes

arethemainpipeline,sub-mainpipelineandlaterals.Thethirdpartcomprisesfittings,andthese

fittingsaremadeofplasticmaterial.


224

Table7-10TypicalmodelsofsimpledripirrigationsystembeingusedinIndia

Description Verysmall Small Medium Large Verylarge

Irrigationarea 96sqm 125sqm 250sqm 500sqm 1000sqm

Noofdippers 80 120 240 480 960

Nooflaterals 4 6 12 24 48

Tanksize 50liter 50liter 50liter 100liter 200liter

Dailywater

requirement

Winter

Spring

150liter

250liter

200liter

300liter

400liter

600liter

800liter

1,200liter

1,600liter

2,400liter

Heightofplat

form

75cmto1m 75cmto1m 1mto1.5m 1.5mto2m 2mto3m

Tentativecost US$12 US$15 US$25 US$40 US$70

7.5 WaterHarvesting

7.5.1 Introduction

Waterharvestingistheoldesttechnologyofcapturingavailablewaterandtransferringitforhuman

uses.Initiallythistechnologywasadoptedespeciallyinaridandsemi-aridregionsofAsiaandAfrica.

Historicalevidenceshowsthatavarietyofwaterharvestingtechniquesweredevelopedinancient

timestocapture,transportanduserainwater.Thebasicprinciplesofwaterharvestingtechnology

involvesthecapturingorcollectionofrainfallfallinginonepartoftheland,transferringittoother

partsassurfacerunoff,andtoaddtotherainthatfallsdirectlyontothecrops.Moreover,harvested

waterisappliedtoirrigatecrops,tosupplywaterfordrinkingandforlivestockconsumption.

7.5.2 Componentsofwaterharvesting

Water harvesting techniques are categorized according to the source of water they harvest. The

main categories are runoff harvesting, groundwater harvesting, and floodwater harvesting. The

runoff harvesting is also referred as rainfall harvesting. In general, a water harvesting system

consistsoffourcomponents:(a)catchment,(b)conveyance,(c)storagefacility,and(d)target.

Thecatchmentistheareafromwhichthewateriscollected.Itmayberoofs,roads,pavedareaslike

court yards, farm lands, andhill slopes. The suitable catchments are oneswhere surface and soil

characteristicsallowrunofftobegeneratedregularly.Thecatchmentscanbeartificiallyconstructed

byinstallingbundsorexcavatingapitoratrench.Inaddition,roofsofthehouses,roadsandpaved

areasarealsocatchmentsfromwherewatercanbecollectedafterchannelingwithdrainpipes.

Theconveyancedevicesconcentrateandchannelthecollectedrunoff fromthecatchmentstothe

storagefacility.Theconveyanceconsistsofbunds,canals,pipesandmaybeequippedwithcontrol

devicessuchasgatesandwaterdistributionsystems.Theconveyancedevicesareofteninstalledin

larger catchmentswhere runoffwould otherwise be lost due to infiltration orwhere the storage

facilities are located at a large distance from the catchment. In small cultivated catchments,


225

conveyance devices may not be necessary due to the proximity of the storage facility with the

catchment.

The storage facilities can be ofmany types including natural depressions, ponds, small reservoirs

formedeitherbyexcavatingsoilsordammingwithearthenembankment,andconcreteormasonry

walls. The storage facilities function as a buffer between a short rainfall and runoff eventswhen

naturalwaterisprovidedandlongdryperiodswhenwaterisrequired.Hence,thestoragecapacity

hastomeetthewaterdemandsduringdryperiods.

Thetarget isreferredtohereastheuseroftheharvestedwater.Dependinguponthequalityand

quantity of the available water, the targets differ from irrigation to drinking water supply and

livestockconsumption.

7.5.3 Rainwaterharvesting

Rainwater harvesting comprises the collection and storage of rainwater runoff from a variety of

surfacesfordomesticandagriculturaluses.Thetermrainwaterisoftenusedinterchangeablewith

runoff water. In general, two types of runoff harvesting are distinguished by the size of the

harvestedcatchment:microcatchmentandmacrocatchmentrunoffharvesting.

Microcatchmentreferstorooftopsystems,roadsandpavedcourtyards,whicharesuitableforthe

collectionofrunoff.Thecollectedrunoff isusuallyconveyedbyaguttersystemtotanksorponds

and used for domestic uses and for irrigation of kitchen gardens.Water can also be collected in

damsfromrainfallingonthegroundandproducingrunoff.Excavatedpondsaresimplesttobuildin

relativelyflatterrain.Thesepondsarebestsuitedtothe locationswherethedemandforwater is

small. From the point of source of feeding excavated ponds are of two types: one is fed from

surfacerunoffandtheotherisfedbygroundwateraquifers.Forthcomingheadingprovidesconcept

onthedesignofwaterstoragefacilitiessuchasponds,tanksandreservoirs.

Figure7-9Typicalexamplesofrainwaterharvestingfromrooftops

7.5.4 DesignConceptofPonds/Tanks

Themost important aspect of water harvesting ponds is to store asmuch rainwater as possible

whereitfallsduringthemonsoonandsaveitforthedryseasonfor irrigationandotheruses.The


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locationoftheexcavatedponddependsuponthetopographyofthesiteandgenerallyflatterrainis

recommendedforpondexcavation.Tobalancethecuttingandfillingoftheexcavatedmaterial,half

cuttingandhalffillingisdesirable.Apondcanbelocatedinabroadnaturaldrainagewayortoone

side of the drainageway if the runoff can be diverted into the pond. The low point of a natural

depressionisoftenagoodlocationforapond.

Iftheexcavatedpondistobefedbysurfacerunoff,enoughimpervioussoilatthesiteisessentialto

avoid excess seepage losses. The most desirable sites are where fine-textured clay and silt clay

extend well below the proposed pond depth. In sand and gravel mixed soil, artificial lining is

essentialtopreventseepage.

Thedesignofawaterharvestingpondinvolvestheassessmentofthefollowingcomponents:

• Wateravailabilityassessmentbasedonthecatchmentareaortypeofwatersource,

• Waterdemandperdayfortheproposedcrops;

• Design of intake and conveyance canal or piped system such as collection chamber,washout

chamber,flowregulatingchamber,

• Designofpondcapacityorwaterstoragereservoir;and

• Water application methods such as drip irrigation, sprinkle irrigation and or free flooding

method

Thedepthofthepondshouldnotexceed2mtoreducetherisksofchildrenfallingintothepond.

Thedepthofwatershouldbe1.50mandthefreeboard0.50m.Thesideslopeofearthenponds

dependsuponthetypeofsoilandranges from1:1to1:2 (vertical tohorizontal).For linedponds,

the side slopes depend on the liningmaterials and itsworkability. For stonemasonry lining, side

slopesshouldbe1:1.50.Thesidesalsoshouldbemadeverticalwithawallthicknessofatleast50

cmattop.

Theinlettotheponddependsuponthetypeofwatersourcetobecollectedinthepond.Generally

aHDPEpipe isusedforthe inletofthepondeither fromtherooftopsor fromthespringsource.

Alternatively,astonemasonryorconcretelinedchannelisalsosuitablefortheconveyanceinletof

awaterharvestingpond.

Overflow spill is an important component of thewater harvesting pond as it reduces the risks of

spilling water and damaging the ponds. A stonemasonry spillway is constructed for large ponds

whileasmallHDPEpipeisinstalledforsmallponds.

Forexcavatedponds,thesoilbanksareformedfromtheexcavatedsoils.Thetopwidthofthesoil

bankshouldbeat least2mwithaninsideandoutsideslopeof1:1.5m.Theinsidetoeofthesoil

bankiskept1mawayfromtheedgeofthepondinordertocreatetheberm.Atypicalcrosssection

ofexcavatedpondispresentedhere(Figure7-10).

Figure7-10Typicalcrosssectionofexcavatedponds


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7.5.5 DesignStandards

The Local Infrastructure for Livelihood Improvement (LILI) Project under HELVETAS/Swiss

IntercooperationinNepalhasdevelopeddesignstandardsofwaterharvestingpondsforsmallscale

hillirrigationschemes(Figure7-11).Themainfeaturesofthestandardsareasfollows:

• Minimumwateravailabilityatsource:2430literperacreperday,

• Averageirrigationwaterdemand:4050literperacreperday,

• Peakdemand:threetimestheaveragedemand,

• Irrigatedareatobecoveredbyasinglepond:minimum1.2acreandmaximum5acres,

• Waterreleaseintervalfromthepond:12hours,24hours,36hoursand48hours

• Capacityofthepond:15m3,30m

3,45m

3,and60m

3

Figure7-11Standardsofwaterharvestingponds

7.5.6 Sealingthepond

Excessive seepage in ponds is generally due to a poor site where the impounding area is too

permeable to hold harvested water. A poor site is often selected due to inadequate study and

investigations.Inareaswhereasatisfactorysiteisunavailableanddemandforwaterissufficiently

high, the design of the pondmust include plans for reducing seepage by sealing the impounding

area.Therearevarioustechniquestosealthepondsandafewofthemarepresentedhereunder.

Compaction

Compactionof soils isoneof thesimplesealing techniquesof thepond if thematerials containa

widerangeofsoilparticles,includingclayandsilttoeffectaseal.Thismethodiscosteffectiveand

applicable for shallowponds. The recommended thicknessof the compacted seal is 30 cmwhere

thedepthofwaterinthepondislessthan3m.

Clayblankets

Blanketingwithclayisoneofthemethodsofreducingexcessiveseepagefromthepondswherea

high percentage of coarse-grained soil exists. The entire upstream area where water is to be


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impoundedshouldbeblanketedwithaclay layer.Theblanketshouldconsistofawell-gradedsoil

material containing at least 20% clay. The thickness of the blanket depends upon the depth of

impoundingwater in the pond and a thickness of 30 cm is sufficient for less than 3m of water

depth.Itisessentialtoprotecttheclayblanketfromcrackingwitha30cmlayerofgraveloverthe

entiresurface.

Bentonite

Bentoniteisfine-texturedcolloidalclay.Addingbentoniteisanothermethodofreducingexcessive

seepageinsoilscontainingahighpercentageofcoarse-grainedparticlesandnotenoughclay.When

wetbentoniteabsorbsalargeamountofwaterandatcompletesaturation,itswellsasmuchas8to

20 times its original volume. It is applied with a mixture of coarse-grained material in correct

proportionsandisrecommendedtoapplyitonlesswaterfluctuatingponds.

Waterprooflinings

Usingwaterproofliningisanothermethodofreducingexcessiveseepagefrombothcoarse-grained

and fine-grained soils. Polyethylene, vinyl, butyl-rubber membrane, silpaulin, and asphalt sealed

fabric lines are gaining wide acceptance as linings for ponds. These materials virtually eliminate

seepageifproperlyinstalled.

Thinfilmsofthesematerialsarestructurallyweak,butifnotbrokenorpuncturedtheyarealmost

completelywatertight.Inaddition,asoilcementliningisalsousedinsmallscalepondliningworks.

Allplasticmembranesshouldhaveacoverofearthorearthandgravelnotlessthan15cmthickto

protect against punctures. Typical thickness of waterproof linings is presented hereunder (Table

7-11).

Table7-11Pondliningmaterials

S.N Materials Ratiobyvolume Thickness(mm)

1 Cement-termitemound 1:8 50-60

2 Cement-soil 1:8 50

3 Cement-sandmortar 1:3to1:4 50

4 Polythenesheet/Silpaulin 5

7.5.7 DesignExamplesofPond/Tank

Designexampleofsmalltank

CalculatethesizeofanexcavatedpondortankinThimpuareatosupplywaterfromrooftops.

Calculations:

• Takeannualaveragerainfallas700mm.

• Thesizeofroofforwaterharvesting =10x10=100m2,

• Annualrunoff=100x700/1000=70m3,

• Assumethereliabilityofgetting700mmisonly60%,


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• Thevolumeofthetank=70x0.60=42m3,

• Assumethatthedepthofthetankbelowthegroundis1.2m,

• Thesurfacearearequiredfortankis42m3/1.2m=35m

2,

• Thesizeofrectangulartankbe7mx5m,


230

CHAPTER-8

8. RiverTrainingandFloodControlWorks

8.1 RiversofBhutan

Byvirtueofitshighelevationandgeographicallocation,Bhutanpossessesenormouswaterresource

potentialflowingthroughnumerouslarge,mediumandsmallrivers.Onthebasisofthenatureof

theirsourceanddischarge,theriversofBhutanmaybegroupedintothreecategories:

• Largeperennial riversoriginating fromHigherHimalayasandcarryingsignificantdischarge

throughout theyear. These large rivers includeAmochhu,WangChhu,Punatshang chhu,

Mangdechhu,andDrangmechhu.These riversarealso termedasnorth-south riversand

havemanyeast-westtributaries,

• Mediumriversoriginatingfromlowermountainsandfedbyprecipitationandgroundwater

regeneration. Although these rivers are also perennial, they are characterized by a wide

seasonalfluctuationofdischarge.TheseriversincludeJaldhaka,Maochhu,Nyera-ama,and

Dhansari,and

• Smallriversoriginatingfromlowermountainfoothillsandfedbymonsoonrainfall.Alarge

number of such small rivers are seasonal with little or no flow during the non-monsoon

season.Theseriversare locatedmostly in threesoutherndistrictsofSamtse,Sarpangand

SamdrupJonkhar.

Thelargeriversprovidewaterforhydropowergeneration,wasteassimilation,tourismandecology

andnotmuchfor irrigation.2Mediumriversandtributariesof largeriversare themost important

riversfromawaterutilizationpointofview,whichhaveplentyofcultivatedlands.Smallriversthat

originatefromsouthernfoothillsarealmostdrymostoftheyearbutcharacterizedbyflashfloods

duringthemonsoonseason.

Due to the steep topographic and fragile geological conditions, river banks are eroding and their

coursesarechangingfromtimetotimealongwiththeirflowpattern.Everyyear,significantdamage

occursanda lotofprivateandpublicpropertyandagricultural landarebeingwashedaway,with

thelossofhumanlives,alongtheseriverbanks.Hence,itisimportanttocontrolthefloodandtrain

theseriverstoprotectproperty,landandhumanlivesfrombeingfurtherdamagedandultimatelyto

supportthenationaleconomicgrowthofthecountry.

8.2 Floods

8.2.1 Typesoffloods

A flood is a condition of toomuch water and results when a river overflows its banks. In other

words,afloodistheresultofanunusuallyhighstageofariverwhenthestreamchannelsbecome

filledandwaterspillsoverthebanks.InSouthAsia,floodsoccurduetohighintensityrainfallduring

themonsoon.Amaximumfloodofariverisoneofthemostimportantaspectsofaflowhydrograph

concernedwiththedesignofhydraulicstructures,includingrivertrainingworks.Therearevarious

typesoffloods,themostcommontypesarepresentedhere:

2ParochhuisbeingusedforirrigationinParovalley


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i. Normalflood:Thesetypesoffloodsoccuralmosteveryyear.Normalfloodmagnitudeis

alsocalledthedominantdischargeandismainlyresponsiblefordeterminingtheplan-

formandmorphologicalcharacteristicsofariver,

ii. Flashfloodsoccurafterheavyrainfall inshortduration.Cloudoutburstmaybeoneof

thecausesofaflashflood,

iii. Manmadefloodsmayoccurduetoencroachmentorothersimilarhumanactivities,

iv. Landslide induced flood: such floods occur due to blockage of river flow due to a

landslidefollowedbyitssuddenbreak,

v. GlacierLakeOutburstFlood(GLOF):floodsinducedduetoasuddenbreakofamoraine

damofglacierlakes.

8.2.2 Floodplainmanagement

Asettlementorcultivatedareawhichisinthevicinityofoneormoreriversandreceivesunwanted

water(asperthedefinitionofflood)fromtheseriversiscalledafloodplain.Forlargefloods,aflood

plainactsasaconveyanceandtemporarystorageoffloodflows.Theeliminationoffloodproblems

froma floodplain is a very costly affair and is generallynot attempted. Floodplainmanagement

thusdoesnotmeaneliminatingfloods,butratherreducingthenegativeconsequenceofflooding.A

flood plain, which is frequently affected by flood problems, can be provided with mitigating

measuresbroadlyintheformofbothstructuralandnon-structuralmeasures.

Structuralmeasuresare those inwhich themainobjective is to reducedischarge, velocityof flow

and depth of flow in the area to be protected. Structural measures can be one or more of the

followings:

i) Dams(reservoirs)toabsorborregulatefloodflow,

ii) Diversion structures to divert the flood water to another river, which can carry additional

dischargewithoutcausingdamagealongitscourse,

iii) Detentionbasins,whichcanstorewaterforsometimewithoutcausingdamage,

iv) Floodembankmenttopreventspillingoffloodwaterfromarivertothefloodplain,

v) Channelizationworkstoenhancedischargecarryingcapacityoftheriverorchannel,

vi) Rivertrainingworkssuchasembankments,revetments,spurs,guidebunds,etc,

vii) Plantationofvegetationalongtheriverbanksandwatershed,riverterracing,etc.

Non-structuralmeasuresare those inwhich themainobjective is tominimize theadverseeffects

withoutdirectlyconstructinganyphysicalstructure,butratherwiththehelpof:

i) Enforcementof acts, regulations andadoptionof a flood control policy, suchasdelineatinga

floodhazardzoneandenforcinglawnottoallowbuildcostlyhousesandinfrastructurewithin

thehazardouszones,etc,

ii) Judicioususeofthefloodplain,suchasconstructingaschoolbuildingawayfromthehazardous

area,butaplaygroundofthesameschoolwithinthehazardousarea,etc,

iii) Flood forecasting and warning system which may helps timely evacuation of people and

valuablepropertiesfromafloodhitarea,

iv) Preparedness for flood fighting, this includes responsible agencies’ preparedness for flood

fightingintheeventofadisastrousflood,


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v) Floodproofing-thismeanswaysoflivingwithfloodswithoutallowingmuchdamagetolifeand

property.Storingfoodgrainsinanelevatedstructure,raisingtheplinthheightofabuilding,etc.

aresimpleexamplesoffloodproofing.

Hence, flood plain management considers the integrated approach of all engineering,

bioengineering, and administrative measures for mitigating losses due to flooding in a

comprehensivescale.

8.2.3 FloodproneareasinBhutan

LocatedintheHimalayanregionandinproximitytotheBayofBengal,Bhutanexperiencesahigher

incidence of natural disasters like earthquakes, flash floods,wind storms, fires and landslides. Of

thesenaturaldisasters,floodsareamongthemostprominentandreoccurringdisastersinBhutan.

The main causes of the floods are heavy rainfall, wind storms and Glacier Lake Outburst Flood

(GLOF).AllofthesouthernfoothillsborderingIndiaarehighlypronetoflashfloodswhichcauseloss

ofhumanlives,properties,lossofcultivatedlands,anddamagetonaturalenvironment.Inaddition,

past eventsofGLOFhad caused significantdamages to lives andproperties inPunakha-Wangdue

valley.Bhutanhas677glaciersand2,674glacierlakesoutofwhich25arepotentiallydangerous.

AccordingtothepastrecordsofGLOF,Lunanaisoneofthepotentialglacierlakesthathadcaused

floodinginBhutan.TherewerefourfloodeventsfromLunanaglacierin1957,1960,1968and1994.

Themost dangerous flood eventwas October 1994 floodwhich damaged significant amounts of

property,livesandcultivatedlands.

Thefloodseventsof2000,2004,2009and2011arerecordedfloodeventscausedbyheavyrainfall

andwindstorms.TheareasinandaroundthePasakhaindustrialareainChhukhaDzongkhagwere

severelyaffectedbytheflashfloodsof2000.Theareahadexperiencedheavyrainfallofmagnitude

1,812mminthemonthofAugust.ThehighestamountofrainfallrecordedinadayatTalawas400

mminthemonthofAugust2000.Thishighamountofrainfallexperiencedbythisareaisthemain

factor forthehigh incidenceof flashfloodsanderosion.BarsaandSigyeRivers,whichflowinthe

Pasakhaarea,swell toanunbelievablemagnitudeandhavedamaged lives,propertyand lands. In

addition,thenationalhighwaytoThimpufromPhuentsolingwasclosedforamonthduetoerosion

andlandslides.

In2004,heavyrainfalloccurred inthesixeasternDzongkhags,contributingtoheavyfloodevents.

Themost affectedwere Trashigang, Trashiyangste and Samdrup Jongkhar Dzongkhags, where 91

peoplelosttheirlives,housesandlandswerewashedaway,publicinfrastructureandserviceswere

damaged,andconsequentlythewholesocialandeconomiclifeoftheareawasimpaired.

Similarly, inMay2009heavy floodingoccurred inBhutandue to theunprecedentedCycloneAila

originatingintheBayofBengal.Thisfloodeventhit17Dzongkhagsandcausedsignificantdamage

tolives,property,cultivatedlands,andpublicandcommunityinfrastructure.

Based on past experience, the most flood prone areas are Punakha-Wangue valley from

Punatsangchhuriver,Pashakha-PhuentsolingareafromTorsa,BarsaandSingyerivers,Sarpangarea

fromSarpangriver,GelephuareafromMaoanditstributaryrivers,HaatownfromHaaChhu,and

ParotownfromParoChhu.Inaddition,Daragaon,Hawazhori,LhamoizingkhaBazaarandMajigaon

arefloodproneareasalongtherightbankofSunkoshriverinDaganaDzongkhag.

Examplesof river trainingworks inBhutanare thePochhuriverprotectionworks,PunakhaDzong

protectionworks,Haatownprotectionworks,SetikhareKholachannelization(Figure8-1)etc.


233

Figure8-1ExamplesofrivertrainingworksinBhutan

Haatownprotectionworks SetikhareKhola,Gelephu

8.3 RiverTrainingWorks

8.3.1 Introduction

River training deals with a series of works that include the construction of structures along and

acrossarivertofulfillspecialobjectives.Thesestructuresincludetheconstructionofdikesalongthe

banksoftherivertocontrolfloodsandoftransversalwalls(groynesspurs,studs,spikesetc)tolead

flowinadefinedpath.Likewise,rivertrainingmaybeaccomplishedthoughdredgeoperationsorby

bounding the flowwidth fornavigation, thediversionof flowwater into secondary channels, and

the construction of shortcut channels in themain river path. Hence, themain objectives of river

trainingmeasuresare:

• Topasshighflooddischargesafelyandquicklythroughthereach,

• Totransportsedimentloadefficiently,

• Toallowminimumwaterdepthfornavigation,

• Tofixcertaindirectionofflow,and

• Tomaintaintheriverintherequiredcondition

8.3.2 Typesofrivertrainingworks

Thefollowingarethecommonmethodsofrivertrainingandfloodcontrolmeasures,theapplication

ofwhichisdeterminedthroughaspecificdesignprocess:

• Marginalembankmentsorleveesandguidebunds,

• Permeableandimpermeablespursorgroynes,

• Bankpitchingandrevetmentwithlaunchingaprons,

• Cut-offs


234

8.3.3 Embankmentbundsandguidebunds

An embankment is a man-made bank to raise the natural banks in order to prevent flooding.

Embankmentsareusuallyconstructedusinglocalsoilmaterial.

Guide banks are used to confine the flow to a single channel, to improve the distribution of

discharge across the weir, barrage or bridge, to control the angle of attack, to control meander

patterns, and toprevent erosionof embankments. Two guidebanks are generally requiredwhen

thewaterwayopening is located in themiddleofawide flood-plainorbraidedstreamwhere the

directionofthemainflowcanshiftfromsidetoside. Asingleguidebankmaybesufficientwhen

thestreamisconfinedtoonesideofavalley,orwhereadvantagecanbetakenofanaturalbank,

whichisresistanttoerosion,ononeside.

The minimum width between guide banks is determined with consideration of the required

waterwayopeningareathroughthestructure.Thewaterwayforregimewidthshouldbe:

QdCWs = whereQdisthedesignflooddischargeappropriatetothedesignreturnperiodflood,

Cisthecoefficientthatrangesfrom3.3to4.9

ThechoiceofreturnperiodforQd isrelatedtotheimportanceofthestructuretobeprotectedor

designed,theriskofout-flankingduringriverfloods,andtheavailabilityoffloodways.Thechoiceof

returnperiodisdiscussedinChapter-3HydrologyandAgro-meteorology.

Majordesignfactorsofguidebundsare:

• Orientationtothewaterways,

• Planshape-straight,curvedandhockeyshaped,

• Upstreamanddownstreamlengths:u/s-¾ofwaterwaywidthandd/s¼ofwaterway

width(Figure8-2),

• Embankmentcrosssectionalshape(Figure8-3),

Guide banks should normally extend above the design high-water level, with an appropriate

freeboardallowance:theirmostimportantroleistoguidefloods.Theelevationofthedead-water

pondtrappedbehindtheguidebankswillbehigherthanthefloodstageattheweirbyanamount

approximatelyequal to thenormal fallalongthechannel inside theguidebanks,plus thevelocity

headinthecontractedwaterway.


235

Figure8-2LengthandPlanshapeofGuideBanks


236

Figure8-3Embankmentsectionofguidebank


237

8.3.4 Spursandgroynes

Spursorgroynesarestructuresconstructedwiththeaimofkeepingflowatadistancefromerosive

riverbanks,modificationof river flow,anddeepeningtheriver for floodcontrol.Thesestructures

have some advantages which include making sediment trapping between them and recovery of

marginal river lands. According to their effect against flow, groynes are divided into two groups,

close(impermeable)groynesandopen(permeable)groynes.

Impermeablegroynesarebuiltofmaterialswithlowpermeability,likerock,soilandsoon,whichdo

notallowflowtopassthroughthemandactlikeadamagainststreams.Agroynestructureisoften

builtwith a specific anglewith flow directionswhich are classified into perpendicular or vertical,

repelling and attracting. The inclination angle θ varied from 10 to 30 degrees (Figure 8-4). Spur

orientation refers to thedirectionof themain flow;vertical spursareeconomical at90owith the

bank; upstream directed spurs have higher scour at tips and accumulate debris; 90o spurs can

deflectcurrentsfromconcavebankbetterthanotherspurs.

Figure8-4Spurtypesinrelationtoflowdirection

Designparametersforspurs

Thedesignofgroynesorspursinvolvesthedeterminationofthelength,height,distanceandtypeof

groynes, in suchamanner thatbankerosion is stoppedor reduced toacceptable conditions.The

majordesignparametersofspursareasfollows:

Height of groyne: According to the experimental results, the maximum height of groyne crown

shouldbe0.5to0.6mabovethechannelbank.Laboratorystudieshaveshownthatproperheightof

agroyne'scrownis0.45to0.90mabovethechannelbank.

Lengthofgroyne:Thelengthsofgroynes(Lp)varydependingontheirspacing,localconditions,and

construction cost. The length shouldnotbe less thanone thirdof thenormalwaterdepth in the

endingedgeanditshouldnotbesolongthatitcausesmorethanevercontractioninriverwidth(b).

Groynesthataresmallerthan30mareappliedforlocalprotection.Thelengthofspurneedstobe

adjustedtoprovideevencurvatureof thethalweg.The lengthshouldneverbemorethan20%of

the bank to bank width of the river, as it would create a negative impact on the other bank.

Practically,thespurlengthshouldbe10%to12%oftheriverwidth.

Spacingbetweengroynesusuallyisacoefficientofthelengthofgroyneanddependsupontheflow

pattern,flowvelocity,radiusofcurvatureofpathafterimprovement,theangleofgroyneswiththe

bank, the typeof groyne and the aimof constructing the groyne. There is nounique formula for

groyne distance and researchers according to model experiments have given various

recommendations.


238

Instraightpart3Lp<d<5Lp

Inconvexpartofcurved=Lp

Inconcavepartofcurve2Lp<d<2.5Lp

Figure8-5Spurangle(φ)toflowdirection

8.3.5 Cut-offs

Pilotchannelsorcut-offchannelsareusedinthecaseofsharpbendsinfinesand-bedrivers.Insuch

bends pilot channels are excavated for 15% to 30% of the dominant discharge to straighten the

channel,whichduetotheincreasedbedshearstress(duetosteepslope)willwidennaturallyand

theentirerivertakesanewcourse(Figure8-6).Aftersomefloodevents,thestraightenedchannel

tendstoassumeitsoldcourseagain.Forthat,protectionmeasuresmaybenecessary.Pilotchannels

oftencauseinstabilityintheriverequilibriumandrequirescontinuousprotectionworkstomaintain

thechannelalignment.

Figure8-6Definitionsketchofcut-off


239

8.3.6 Pitchingofbankswithprovisionoflaunchingaprons

In curved channels, the top flow lines,which contain less sediment concentrationbut havemore

velocity,movetowardstheconcavebank,whereasthebottomflowlineswithsedimentladenwater

but lessvelocity separate themselves fromthe toponesandmove towards theconvexbank.The

neteffectremainssuchthatahelicalflowtakesplace(anti-clockwiseiftheconcavebankisonthe

leftsideorviceversa)advancingdownstream.Thisphenomenonoccursduetothecentrifugalforce

associatedwiththeflowalongabend.

Thosesedimentdeficienttopflowlines,whiledivingdownalongtheconcavebank,pickupalarge

amountof sediments. Similarly, thebottomoneswithheavy sediment loadaftermoving towards

theconvexbankreleasetheloadthereandrollupagaintoreachtheconcavebankwithrelatively

sedimentfreewater.Thisphenomenoncausesdegradationofthebanktoe,whichultimatelymakes

the bank slope steeper than the critical one and slope failure takes place preceded by a tension

crackonthelandsurface.

8.3.7 DesignProceduresofRiverTrainingWorks

Step-1Identifytypeofriverproblem

• Bankerosionatonelocation-erosionatouterbendsoftheriver,singlechannel,

irregularchannelpattern,bankslumping,

• Bankerosionatdifferentlocations-downvalleymovementofmeander,erosionofboth

banks,weakbankmaterial,rapidfluctuationinflow,

• Bedloweringanderosion,

• Bedrisinganderosion,

• Inundation

Step-2Classifytypeofriverandassessrivercharacteristics

First it is essential to identify the rivers based on their geomorphic features such as valley, flood

plainandalluvialfans.Riversneedtobefurtherclassifiedinto:

• Straightriver,

• Meanderingriver,

• Braidedriver,

• Singlechannelhighvelocityriver,

• Singlechannelmeanderingriver,

• Multiplechannelmeanderingriver,

• Braidedwithdistributarychannels,

• Braidedwithsplitchannels

Step-3Obtainkeydesigndata

Thecollectionoftheactualfielddataisanessentialstepinthedesignproceduresofrivertraining

works.Theimportantfielddatarequiredfordesigningare:

• Siteplanoftheriverstretchtothedesigned,

• Cross-sectionsoftheriver,


240

• Longitudinalprofileoftheriver,

• Bedandbankmaterialsamples,

• Dischargemeasurementifpossible,

• Highfloodmarks,

• Previousstructuralmeasures

Inadditiontofielddata,secondarydataalsoareessentialforthedesign.Theseareflowrecordsof

the riverandcatchmentdetails. It is alsonecessary toassess thepast interventionsmade in that

stretchoftheriverandtheirperformancewithrespecttorivertraining.

Step-4Assessalternativesforsolvingriverproblems

Basedontheproblemidentificationandassessmentofrivercharacteristics,itisessentialtopropose

alternativesforsolvingtheriverproblems.Thealternativesmaybe:

• Holdtheriverinexistingchannel(bankerosionatonelocation),

• Relocatemainflowchannel(bankerosionatmanylocations,braidedchannel),

• Holdverticalpositionofriverbed(bedlowering,bankslumping)

Step-5Quantifydesignparameters

Therequireddesignparametersare:

• Designdischarge(floodreturnperiods,50yearsreturnperiodmeans(1/50)2%risk),

o Frequencyanalysisforgaugedriver,

o Regionalfloodfrequencyrelationshipsifavailable,

o Highwatermarks,

o Bankfulldischarge-Manning’sequation

• Flowvelocities,

o Averagevelocity,Vm=designdischarge/cross-sectionarea

o LocalvelocitiesatrivertraininglocationsV1=2/3Vm,4/3Vm,Vm

• Scourmagnitude,

Scouristheremovalofsoilparticlesbycurrentorlocalizederosionofbedmaterial.Scourdepthis

themeanhydraulicdepthmultipliedbyascour factor.Thescour factor isdifferent fora rangeof

conditionssuchasbends,abruptobstructions,constrictions,bridgepiers,spurs,etc.

• Stablewaterways,

Lacey’swaterwayiscommonlyusedforestimatingstablewaterwaysattherivertraininglocations.

Step-6Selecttypesofrivertrainingmeasures

• Structuralmeasures:

o Bankrevetments-riprap,gabionrock-filled,bambooandsandbags,

o Spurs-roundnose,hockeytype,t-type,slopingandJ-spur,

o Embankmentwithstuds,

o Canalizationofriveralignment,

o Bioengineeringmeasures

• Non-structuralmeasures:

o Preparedness,


241

o Reliefactivities,

o Landusezoning,

o Floodforecasting,

o Floodwarningandevacuation

Step-7Assessconstructionprocedures

• Gabionstructures,

• Sandbagfillinginnylonnets,

• Layoutofstructures,

• Workingmethodsunderwater,

• Diversionanddewatering,

• Bamboo/timberpiledriving,

• People’sparticipation

Step-8Identifymaintenanceprogram

Amonitoringandmaintenanceprogramisessentialtoensurethattheworksperformasdesigned.

Thecostofeffectivemaintenanceisusuallysmallerthanthecapitalcostofreplacingrivertraining

works that faileddue toa lackofmaintenance.Regularmonitoringandperiodicmaintenanceare

essentialpartsofrivertraining.

Step-9Estimatecostofrivertrainingmeasures

The cost of river trainingmeasures is computed based on an estimation of each identifiedwork

items.TheanalysisofeachitemisderivedfromtheBhutanScheduleRatesupdatedeveryyearby

theMinistryofWorksandHumanSettlement.

Step-10Assesssocio-economicbenefitsoftheproposedmeasures

The river training benefits are the value of losses or damage that would have occurred without

trainingmeasures.Thespecificbenefitsasanticipatedwouldbe:

§ Increaseincropyields,

§ Increaseincroparea,

§ Increaseinpriceofland,

§ Savingintransportationexpenses

Besidesthebenefitstobeassessedasabove,therewouldbemanyotherbenefitsfromrivercontrol

and training measures such as employment opportunities, increased production from reclaimed

land,humansafety,etc.

Step-11Assesseconomicevaluation

Theeconomicevaluationoftherivertrainingmeasuresiscarriedoutonthreeparameters.

§ Benefitcostratio,

§ Netpresentvalue,


242

§ Economicinternalrateofreturn

8.4 BankProtectionWorks

8.4.1 GeneralConcept

Flood problems are mainly of two types: bank erosion and inundation. Bank erosion is more

pronounced in Bhutan and itwill be dealtwith inmoredetail hereunder. Before considering any

bankprotectionmeasure,itisnecessarytounderstandhowbankerosiontakesplaceinanalluvial

river.Bankmaterialsmaybecohesive,non-cohesiveandcomposite.Bankmaterial iscarriedaway

bytheriverandthephenomenonisrepeatedinthenewslopeline.Thus,thebankretreatsaslong

asthehydraulicandgeo-technicalconditionsallowittodoso.Massfailureofbankscanoccurdue

tobankslopegeometry,surfaceandgroundwaterflowregime,surchargeloading,tensioncracking

and vegetation. These failuresmay be shallow failure, planar failure, surface erosion failure, and

deepseatedrotationalfailure.Atypicalbankfailureispresentedhere(Figure8-7).

Figure8-7Typicalbankfailure

8.4.2 Methodsofbankprotection

The type of bank protectionworks or revetmentworks to be usedwill depend upon the cost of

materials,durability,safetyandappearance.Themostcommonlyusedtypesofflexiblerevetment

includestonerip-rap,gabions,wiremeshovera layerofstones,baggedconcrete,andarticulated

concreteslabs.Attentionisgivenfirsttostonerip-rapbecauseofitsconsiderableadvantagesover

othertypes.Riprapisflexibleandisnotimpairedbyslightmovementoftheembankmentresulting

fromsettlement.

Rip-rapProtection

The term rip-rap implies the use of irregular stones provided to meet a fairly close grading

specification.Inmostsituationscoveredinstandardmanualsanddesigns,rip-rapcomesfromthe


243

quarryingofstone,whichproducesangularstoneswithagoodshape,i.e.thelongestdimensionnot

more than 2 times the smallest orthogonal dimension. Although there is no shortage of quarry

rocksinBhutan,acheapersourceofsupplyisfromtheriverbeds.Thisimposestwoproblems:the

size locally available in largequantities isprobablynot largeenough forbankprotection, and the

stonesareextremelywellrounded.Thesepointstothecommonuseofgabions,buthereagainthe

roundness of the stones available makes effective packing difficult, so that they are highly

permeableandapttodeformandsettleafterinitialconstruction.

Rip-rap protection is more likely to be appropriate in plain rivers, while gabions may be more

appropriate in the hill region and in cobble bed rivers. In dealing with rip-rap in what follows,

remember that the roundness of the stones often used in Bhutan increases their vulnerability

comparedwiththemoreeffectivequarriedrock,unlessthestonesatthesurfacearecarefullyhand

packed,preferablywiththelongaxisperpendiculartothebankface.

Figure8-8Rip-rapforbankprotection

FilterLayer

Ifstonebankprotectionweretobeplacedoveranembankmentformedofmuchfinermaterial,the

turbulenceoftheflowworking itswaybetweenthestoneswouldentrainthisunderlyingmaterial

andremoveitthroughtheintersticesintheprotection.Inthecourseoftime,theprotectivelayer

wouldsinkandbecausethiswouldhappenunevenly, itwould inevitablygenerategapsandwould

then fail toachieve itspurpose. Rapiderosionof thecoreof thebankwouldoccur. It is for this

reasonthataninterveninglayer,orseverallayers,ofintermediatesizedmaterialisrequired.

The recommendation in the Design of Small Dams (USBR) is for a filter layer of crushed rock or

naturalgravelgradedfrom50mmto90mm,ofthicknessequaltoone-halfthethicknessoftherip-

raplayer,butinanycasenotlessthan300mm.Otherrecommendationsareformulti-layerfilters,

each being quite closely graded but the successive layers being designed so that the smallest

particles in one grading cannot pass through voids between the largest particles in the overlying

layer. In fact, if 1 m diameter material is required as the main slope protection, and the core


244

material is fine sand or silty soil, amulti-layer filter would be essential. It is normal practice to

replacethefinestlayerinsuchsituationswithageotextile(filterfabric).

UseofGabions

Gabionsare themostcommonlyusedbankprotectionmeasures.Theadvantageofgabionsis that

smallerstonescanbeusedwithintheconfinesofawirecrate.Thus,apossibledesignmethodisto

comparetheirstabilityinflowingwaterwiththatofthestonesbythemselves.Theindicationsare

thatthestoneswithinagabiongainthestabilityofstonesofdoubletheirdiameterthatarenotso

encased,providedtheFroudenumberoftheflowis lessthan1.5.ForhigherFroudenumbersthe

advantageisless.

ToeProtection

Thetoeofthebankprotectionmeasuresisessentialtoprotectfromunderminingbyscour,whichis

causedbystreamfloworseepage.Toeprotectioncanbeachievedintwoways:

• Byconstructingacut-offtowithstandscour,

• Byprovidingapronofflexibleprotectionacrossthebedfromtoeofbank(Figure8-9),

Figure8-9Basictypesoftoeprotection

8.4.3 Designprocedureofrevetment

Revetmentandanassociated launchingapron is themost common typeofbankprotectionwork

beingadoptedindevelopingcountries.

Thethicknessofrevetmentisgivenby

Dn=ΦcKTKh0.035u2/(ΔmKsθc2g)

Where,


245

Φc=slopestabilityfactor=0.50forgabionmattress,and

Φc=0.75forstoneriprap(withoutgabion)

KT=turbulencefactor

TakeKT=1fornormalturbulenceasinstraightreachoftheriver

KT = 1.5 for increased turbulence and non-uniform flow as in gentle concave bends, in the

downstreamofstillingbasins,nearthebridgesiteswithpiers,etc.

KT=2.0forhighturbulenceasinsharpconcavebends

KT=3forextremelyhighturbulenceasinjetimpact

Kh=depthandvelocitydistributionfactorgivenby=2[log(12h/Kr)+1]-2

Where, Kr = mean equivalent roughness height (take Kr = thickness of largest particle or the

thicknessofrevetmentitself)

Δm=relativedensityofgabionmattress=(γs-γf)x(1-n)/γf

Where,n=porosityofgabionmattress=0.4

u=averagevelocityofflow

γsandγfareunitweightofstoneandwaterinN/m3

Ks=slopereductionfactorgivenby,

φ

α2

2

1Sin

SinKs=

Whereα=riverbankslopeangleand

φ=angleofreposeofstones(about360)

θc=criticaldimensionlessshearstressandequalsto0.06forgabionmattress,and0.035forstone

riprap

DesignofLaunchingApron

Lacey'ssiltfactorf= )(d1.76 50 mm

Whered50isthemediandiameteroftheriverbedmaterial,whichmeanstheparticlesizeinmm

correspondingto50%intheparticlesizedistributioncurve,

ScourdepthcanbepredictedbyLacey'sformula,whenthereisnoconstriction.Normalscourdepth

measuredfromtheHFLisgivenby,

R=0.47x(Q/f)1/3whereQisthedischargeinm3/s,fisthesiltfactor

AdopttheanticipatedscourdepthbelowHFL=1.25*R,forstraightbend,

1.5*Rformoderatebend,

1.75*Rforseverebend,and

2.0*Rforrightangledbend

Afterknowingthepredictedscourlevelduringhighflood,assumethattheapronwillbelaunchedto

thepredicteddepthbelowthe toeof the revetmentor riverbankata slopeof1:2 (V:H).For that

inclinedslope,itisnecessarytoprovidelaunchingapron.

(i)Incaseofboulderfilledgabionmattresslaunchingapron,alwaysprovide


246

Lengthoftheapron(whichistobeplacedhorizontallyattheriverbedlevel) =

inclinedlengthofthelaunchingslope

=[(riverbedlevel–predictedscourlevel)*√5]

Providethicknessoftheapron=thicknessoftherevetment

Nevershortentheapronlengthbyincreasingthethicknessincaseofgabionapron.

(ii)Incaseoflaunchingapronoflooseboulders,

Compute per unit volume of the boulders required by multiplying the length and thickness

computedasincaseofgabionlaunchingapronabove.

Increase thevolumeby25% toaccount for theuneven thicknesswhile rollingdown theboulders

alongthescourslope

Providetheapronbydumpingbouldersinthetrenchalongthetoeoftheriverbankordikeslopeso

thatthefillvolumeisequaltothevolumecomputedabove.

8.5 MathematicalModelsforriverstudies

Thedesignofrivertrainingandfloodcontrolworksarecarriedoutwiththehelpofmathematical

models which represent hydraulic and hydrologic parameters of the river. The most common

computermodels are HECHMS, HEC RAS,MIKE BASIN,MIKE 11, andMIKE FLOOD. Results from

thesemodelsareappliedforstudyingtheextentoffloodareasandinfluenceofcontractioninthe

river sections resulting from the construction of flood control and river training measures and

extractwaterlevelchanges.

TheHEC-RAS(HydrologicEngineeringCenterRiverAnalysisSystem)modelingsystemispartofthe

U.S.ArmyCorpsofEngineers“NextGeneration”software.DevelopedattheHydrologicEngineering

Center, thesoftwarepackage incorporatesvariousaspectsofhydraulicmodeling, includingsteady

and unsteady flow. HEC-RAS applications include one-dimensional modeling of river channels,

streams, and floodplains, from single reaches to complex stream networks. Bridge and culvert

geometry can be defined and added, as well as inline and lateral structures, such as weirs and

levees.HEC-RAShasroutinesforimportingUNETdataaswellasdatafromothersources.HEC-RAS

operatesintheMicrosoftWindowsenvironmentandusesagraphicaluserinterfacefordataentry,

graphics, and display of program results. It uses implicit finite-difference schemes for solving

unsteadyflowequations.

SWAT(SoilWaterAssessmentTool)isariverbasinscalehydrologicmodeldevelopedtoquantifythe

impactof landmanagementpractices in largeandcomplexwatersheds.Themaincomponentsof

SWAT include weather, surface runoff, return flow, percolation, evapo-transpiration, reservoir

storage,cropgrowthandirrigation,groundwaterflow,andwatertransfer.SWATisapublicdomain

model actively supported by the USDA Agricultural Research Service at the Grassland, Soil and

WaterResearchLaboratoryTemple,Texas.

MIKE BASIN developed by DHIWater and Environment, is a mathematical representation of the

riverbasinencompassingtheconfigurationofthemainriversandtheirtributaries,thehydrologyof

thebasininspaceandtime,existingaswellaspotentialmajorschemes,andtheirvariousdemands

onwater.MIKEBASINisstructuredasanetworkmodelinwhichtheriversandtheirmaintributaries

arerepresentedbyanetworkconsistingofbranchesandnodes.ItisanextensiontoArcViewGIS,

suchthatexistingGISinformationcanbeincludedinthewaterresourcessimulations.

MIKE 11 developed by DHI (Danish Hydraulic Institute) Water and Environment, is a software

package for simulating flows, water quality and sediment transport in rivers, channels and other


247

waterbodies.MIKE11iscarriedoutinMIKEView,whichisWindowsbasedresultpresentationand

reportingtoolforbothMIKE11andMOUSE.

MIKEFLOODisacomprehensivemodelingpackagecoveringallmajoraspectsoffloodmodelingand

a tool for understanding flooding, analyzing scenarios, and testing mitigation measures. MIKE

FLOODintegrates floodplains,streets, riversand littoralandsewer/stormwatersystems intoone

package.


248

CHAPTER9

9. ProjectEvaluation

9.1 Introduction

Project evaluation involves the assessment of the agricultural benefits to be accrued from the

proposed irrigation scheme, the cost of scheme implementation including operation and

maintenance costs, and economic analysis of the proposed scheme. Project evaluation tries to

justify theeconomic viabilityof the schemeafter its technical feasibility. Theagricultural benefits

areassessedwiththecomputationofabenefitmatrixbasedonwithandwithoutprojectsituations.

Thecostestimate ispreparedaccording toGovernmentnorms,BhutanScheduleRates (BSR),and

pastexperienceofthesimilar irrigationschemes.Aftertheassessmentoftheagriculturalbenefits

and the costs of the proposed schemes, an economic analysis of the project is carried out. The

followingsectionsbrieflydescribetheproceduresofprojectevaluation.

9.2 AgricultureBenefitAssessment

9.2.1 BenefitMatrixApproach

Thebenefitmatrix isastandardizedmethodofeconomicanalysisbasedonsimplifiedbenefit-cost

techniqueused in irrigationproject evaluation. Thebenefit analysis involves discounting the cash

flowsofbenefitsoverthelifetimeoftheprojecttoassesstheeconomicviability.Thebenefitmatrix

iscarriedoutseparatelyforthe“withoutproject”situationandthe“withproject”situationandthe

net economic returns from each situation are assessed. The net agricultural benefits are the

differenceofwithprojectbenefitsandwithoutprojectbenefits.

9.2.2 ComponentsofBenefitMatrix

CommandArea

Thenetactuallycultivatedareaisgenerally80%to85%ofthegrossareaoftheschemeinthecase

of a new project. For a rehabilitation scheme, the net areawill remain the same as thewithout

projectsituation.

CropYields

Cropyieldsarecollectedfromtheagriculturalsurvey(AnnualAgriculturalStatisticspublishedbythe

Department of Agriculture,Ministry of Agriculture and Forest) during the feasibility study of the

project.Theexistingcropyieldsshallbetakenduringthefieldsurveyandshallbeconsideredasthe

“withoutproject”situation.Cropyieldsforthe“withproject”situationareconsideredmoreuniform

duetoapplicationofirrigationwater.

AgricultureInputs

The main agricultural inputs are seeds, fertilizer, agro-chemicals, labor and animal power. The

requirementoftheseinputsforunitlandarea(ha)needstobeassessedforboth“withoutproject”

and“withproject”situations.Forthe“withoutproject”situation,itissuggestedtousetheexisting

inputpracticesbasedonthedataand informationcollectedduringthe fieldsurvey.For the“with

project” situation these agricultural inputs are taken from the suggested assumptions of the

DepartmentofAgriculture.


249

EconomicPrices

Allpricesofagriculturalinputsandoutputsneedtobeconvertedtoeconomicpricestoreflectthe

realvalueofthenationaleconomy.Thereisabigvariationinthepricesofagriculturalcommodities

acrossthecountryduetothecostoftransportationandhencethepricesofeachprojectneedtobe

assessed separately. The prices of crop yields need to be analyzed for the economic farm gate

prices.Thepricesofagriculturalinputsaretakenfromthenearestmarketprices.

9.2.3 ProcedureofBenefitAssessment

Thebenefitsofagriculturalproductsareassessedwiththepreparationofthecropbudgetforwith

andwithoutprojectsituations.Theprocedureofbenefitassessmentisasfollows:

• Assessgrossreturnfromeachcropplanted,

o Valueofcropyields,

§ Yieldsofeachcrop,

§ Rateofeachcrop,

§ Valueofeachcrop=yieldxrate

o Valueofby-product,

§ By-products,

§ Rateofeachby-product,

§ Valueofby-product=by-productxrate

o Grossreturn(R)=valueofeachcrop+valueofeachby-product

• Assessinputsforeachcrop(seeds,fertilizer,pesticide,labouranddraftanimal),

o Valueofseeds,

§ Quantityofseedsforeachcrop,

§ Rateofseeds,

§ Valueofseeds=seedsxrate

o Valueoffertilizer,

§ Quantityoffertilizerforeachcrop,

§ Rateoffertilizer,

§ Valueoffertilizer=fertilizerxrate

o Valueofpesticide,

§ Lumpsumamountofpesticideforeachcrop,

o Valueoflabour,

§ Quantityoflabourforeachcrop,

§ Rateoflabour,

§ Valueoflabour=labourxrate

o Valueofdraftanimal,

§ Quantityofanimalforeachcrop,

§ Rateofdraftanimal,

§ Valueofdraftanimal=draftanimalxrate

o Totalinputs(I)=valueofseeds+valueoffertilizer+valueofpesticide+valueoflabour

+valueofdraftanimal,

• Assessnetbenefitfromunitareaofeachcrop,

o Netbenefit(NB)=grossreturns(R)–totalinputs(I)

• Assesscultivateareaofeachcrop(A),

• Assessnetbenefitfromeachcrop(B)=Netbenefit(NB)xcroppedarea(A)


250

• Assesstotalbenefit(TB)=sumofbenefitsfromallcropsplanted

Asmentionedearlier thebenefit assessment is carriedout forboth thewith andwithoutproject

situations. The net incremental benefit from agricultural production is assessed as the difference

betweentotalbenefitsofthewithprojectandwithoutprojectsituations.

Netincrementalbenefit=totalbenefitfromwithproject–totalbenefitfromwithoutproject

Atypicalsampleofbenefitassessmentispresentedhere(Table9-1).

Table9-1Sampleagriculturalbenefitassessmentmatrix

S.N

.

Particulars Units WithoutProject FuturewithProject

Padd

y

Maiz

e

Mill

et

Vegetab

le

Paddy Maiz

e

Mill

et

Vegetab

le

1 Yields

1.1 YieldperCrop Kg

/acre

1,00

0

500 500 600 1,100 525 525 700

RatePerKg Nu/Kg 12 10 10 25 12 10 10 25

Value Nu/

Acre

12,0

00

5,00

0

5,00

0

15,000 13,20

0

5,25

0

5,25

0

17,500

1.2 By-Products kg/

Acre

100 110

Rate Nu/kg 1 1

Value Nu/

Acre

100 110 0 0 0

GrossReturn(R) Nu/

Acre

12,1

00

5,00

0

5,00

0

15,000 13,31

0

5,25

0

5,25

0

17,500

2 Inputs

2.1 Seeds Kg/

Acre

20 8 8 0.5 20 8 8 0.5

Rate Nu/Kg 30 30 30 75 30 30 30 75

Seedvalue Nu/

Acre

600 240 240 37.5 600 240 240 37.5

2.2 Fertlizer Nu/

Acre

500 400 200 800 500 400 200 800

2.3 Pesticides Nu/

Acre

200 0 0 500 200 0 0 500

2.4 Labour pd/

Acre

15 10 10 25 15 10 10 25

Rate Nu/pd 200 200 200 200 200 200 200 200

Laborvalue Nu/

Acre

3,00

0

2,00

0

2,00

0

5,000 3,000 2,00

0

2,00

0

5,000

2.5 DraftAnimal ad/

Acre

4 2 2 0 4 2 2 0

Rate Nu/ad 400 400 400 400 400 400 400

Valueanimalinput Nu/

Acre

1,60

0

800 800 0 1,600 800 800 0

TotalInputs(I) Nu/

Acre

5,90

0

3,44

0

3,24

0

6,338 5,900 3,44

0

3,24

0

6,338


251

S.N

.

Particulars Units WithoutProject FuturewithProject

Padd

y

Maiz

e

Mill

et

Vegetab

le

Paddy Maiz

e

Mill

et

Vegetab

le

3 Benefits

3.1 NetBenefit(NB=R-I) Nu/

Acre

6,20

0

1,56

0

1,76

0

8,663 7,410 1,81

0

2,01

0

11,163

3.2 CultivatedArea(A) Acre 33 22 22 50 52 26 26 88

3.3 BenefitperCrop‘000

(B=AxNB)

Nu 204.

6

34.3

2

38.7

2

433.13 385.3

2

47.0

6

52.2

6

982.3

3.4 CommandArea(CA) Acre 176 176

3.5 CroppingIntensity % 72% 109%

3.6 TotalBenefit(TB=∑B) Nu 710,765 1,466,940

3.7 AverageBenefit

(AB=TB/CA)

Nu/

Acre

4,038 8,335

3.8 NetincrementalBenefit

(NIB=DifferenceofAB)

Nu/

Acre

4,296

TotalNetIncremental

Benefit(=NIBxCA)

Nu 756,175

Note:pd=persondayad=animalday1Acre=4047m21Quintal=100Kg

9.3 ProjectCostEstimate

9.3.1 ComponentsofCostEstimate

Theprojectcostestimatecomprisesmainlythreecomponents:

• Unitratesoftheitems,

• Quantitiesoftheworks,and

• Costoftheworks

TheMinistryofWorksandHumanSettlementprovidestheBhutanScheduleofRates(BSR)whichis

updatedeveryyear tobeconsidered inestimatingthecostofworks,goodsandservices.BSRhas

five sections and section one provides basic rates while section two provides mechanical

transportation rates. Section three provides built-up rates and section four includes cost indices.

BSRconcludeswithsectionfiveprovidinginformationonapprovedmaterialbrands.

9.3.2 DerivationofUnitRates

Theanalysisoftheratesofdifferentitemsofworksiscalculatedinaspecifiedformatasprescribed

in the BSR. The BSR is updated in each year by the Ministry of Works and Human Settlement

(MOWHS)and itsquantity,quality, and scopearegovernedby theSpecifications forBuildingand

RoadWorks (SBRW). The rate analysis is carried out in standard unit of the items of works and

hencenamedasunitrateofitems.Basically,theunitratecoversthreecomponents:

• Costoflabors,

• Costofmaterials,and

• Costofplantsandequipment

TheBSRprovidesbuilt-upratesforfourbasetowns:Phuentsholing,Gelegphu,SamdrupJonkharand

Thimpu.Thesebuilt-upratesareapplicable foronlya10kmradiusaroundthesebasetowns.For

otherplaces,thebuilt-upratesneedtobemultipliedbylocationcostindicesderivedfromspecified

procedures. The built-up rates include the cost of tools, plants andmachinery (5% of the labour


252

cost),thecostofworkcharges(1%of labourcost),andthecostofconstructionoverhead(10%of

the total rate). In addition, the built-up rates are derived from the “Labour and Material

Coefficients”ofthesameyearpublishedbyMOWHS.

Thebasic rates for thespecified locationsareobtainedbymultiplyingwithcost indiceswhichare

calculatedaccordingtothefollowingprocedures:

i. Determinebasicratesofeachitemsofworksfromnearestbasetown,

ii. Determinestheratesofeachitemsofworksoftheprojectlocation,

iii. Calculatetheincreaseinprojectratesinpercentfrombasetownrates,

1001Pr

×⎟⎟⎠

⎞⎜⎜⎝

⎛−=

ratetownBase

rateojectrateinIncrease

iv. Calculateweightageofeachitemoftheworkswhichshallremainfixed,

v. Calculateweightagecostindexofeachitemofworks,

100÷=Weightage

rateinIncreaseindexCost

vi. Calculatethetotalcostindexoftheprojectbysummingthecostindicesofeachitemsofworks,

Thecostindicesareapplicableforallbasicratesandanyotherworkswhereonlylabourisinvolved

suchasdismantlingworks,earthworks,installationandtestingworksetc.Theunitrateoftheitem

thatisnotincludedinBSRneedtobeapprovedbytheChiefEngineerofEngineeringDivision.

9.3.3 CalculationofQuantities

Thequantitiesarecalculatedaccordingtotheexactdimensionsoftheworksasmentionedonthe

projectdrawings.Thesedimensionsarefragmentedintosmallgeometricalshapeandsizeinorder

to ease the calculations. The geometrical shapesmay be rectangle, circle, triangle, parallelogram

etc.Inmostcasesthequantitiesarecalculatedmultiplyingthelength,widthandheightordepthof

theobjecttobeassessed.Inaddition,thequantitiesmaybeinvariousunitssuchasnumber,length,

area and volumebasedon thenatureof theworks. The appropriate formatof quantity estimate

with example is given here (Table 9-2). Furthermore, some of the items of work are difficult to

quantify and these items are expressed in lump sum quantity such as site preparation, site

mobilization,etc.

Table9-2Samplequantityestimateform

Item

no

Description Unit No Length Breadth Height Quantity Remarks

1 Earth work

excavation

m3 2 2.50 2.00 1.00 10.00

2 Siteclearance m2 2 2.50 2.00 - 10.00

3 Barbedwire m 2 90.00 - - 180.00

4 Railing post of

specifiedsize

no 1 - - - 1.00


253

9.3.4 EstimateofCost

Thecostof theworks isderivedfromthemultiplicationof theunit itemratewiththequantityof

eachitem.

Cost=quantityxunitrate

In addition to themain items of work of the project, the cost shall also include the cost of the

followingitemsbasedonthenatureandtypeoftheproject:

• Mobilizationanddemobilizationcost,

• Insurancecost,

• Costofconsultingservice,

• Constructionsupervisioncost,

• Costofprofessionalsecurityandsafety,

• Costforpreparingcompletiondrawing,and

• Costofbankcommissionforperformancedeposits

The preparation of the cost estimate of the project depends upon its nature, size and funding

agency.Theformatoftheestimatingthecostoftheprojectisprescribedhere(Table9-3).

Table9-3Samplecostestimateform

Item

no

Code Description

ofitems

Unit Quantity Unit

rate

Amount Remarks

(a) (b) (c) (d) (e) (f) (g) (h)

1 Item1 (e)x(f)

2 Item2 (e)x(f)

3

Totalbasiccost(i) = ∑g

Contingency@5%ofbasiccost(j) (j)=0.05x(i)

Sub-total(k) (k)=(i)+(j)

VAT@13%(l) (l)=0.13x(k)

Totalconstructioncost(m) (m)=(k)+(l)

9.4 EconomicAnalysis

9.4.1 Introduction

Economicanalysis istheprocessofevaluatingthebestfeasibleprojectsamongseveralalternative

projects. The main objective of the economic analysis is to assess the economic viability of the

project under consideration. For irrigation projects, the analysis is based on the net incremental

benefitsarisingfromtheprojectandtheeconomiccostoftheprojectfor itsassumedlifetime.To

assesstheincrementalbenefits,itisnecessarytoidentifythecostsandbenefitsthatwillarisewith

the implementation of the project and to compare themwith the costs and benefits thatwould

havearisenwithout the implementationof theproject.Mostof the irrigationprojectsare judged


254

with threeeconomic indicators,NetPresentValue (NPV),Economic InternalRateofReturn (EIRR)

andBenefit-costRation(BCR).Thefollowingsectionsbrieflydescribetheseeconomicindicators.

9.4.2 EconomicIndicators

NetPresentValue(NPV)isthesumofpresentvaluesoftheincrementalbenefits,discountedatthe

fixed rate. The project with a positive NPV is said to be economically viable and in comparing

alternatives,theprojectwiththehigherNPVwouldbechosen.

Economic Internal Rate of Return (EIRR) is the discount rate which makes the NPV of the

incrementalbenefitsequaltozero. It isthediscountrate,whichensuresthatthepresentvalueof

theprojectbenefitsisthesameasthepresentvalueofthecosts.TheEIRRoftheprojectshouldbe

more than the opportunity cost of capital or discount rate. The discount rate is usually 10% in

developingcountries.

Benefit-CostRatio(BCR)isthepresentvalueoftheprojectbenefitsdividedbythepresentvalueof

costs.IftheBCRisgreaterthanone,thentheprojectiseconomicallyviable.

Discountingistheprocessofcomparingpresentamountsofmoneywithfutureamountsofmoney.

Inaddition,discountingcalculatestheamountofmoneyrequiredtodaywhich,wheninvestedata

rateofinterestequivalenttothediscountrate,wouldyieldthefutureamount.Discountingisused

to compare the costs andbenefits of theprojectof different times at a singlepointof time. It is

usuallyat the timeofprojectappraisal. If thecapital isRs1000andthe interest rate is8%, then

totalcapitalafteroneyearwouldbeRs1080,orinoneyearRs1080isworthRs1000ifthediscount

rateis8%.

9.4.3 BasicAssumptions

Economicanalysisiscarriedoutbasedonsomebasicassumptionswhichareasfollows:

w Life of the project- depends upon the nature of structure to be constructed. For small

irrigationschemesthelifeoftheprojectmaybe10-20years,

w Construction period- the construction period of the project depends upon its size and

location.Forsmallscaleirrigationschemestheconstructionperiodmaybe1-2years,

w Build up period for full agricultural benefits- with the implementation of the irrigation

projectagriculturalbenefitswouldnotaccrueatonceandwilltake1-3years,

w OpportunitycostofcapitalordiscountrateinBhutantobetakenis10%;

w Standardconversionfactorsfortheconversionoffinancialcosttoeconomiccostsaretaken

as0.95forconstructioncostsand0.90foroperationandmaintenancecosts.

9.4.4 ProceduresofEconomicAnalysis

i. Assesstotalnetagriculturalreturnswithoutproject

§ Preparecropinputandoutputassumptions

§ Calculatefarmgatepricesofcommodity

§ Preparewithoutprojectcropbudgets

§ Calculatewithoutprojectcroppedareasinthecommandarea

§ Calculatetotalnetagriculturalreturnswithoutproject


255

Totalwithoutprojectnetagricultural returnsperannum=CommandareawithoutprojectxCropping

pattern=AreaofeachcropxNeteconomicreturnfromeachcrop

ii. Assesstotalnetagriculturalreturnswithproject

§ Preparecropbudgets

§ Calculatewithprojectcroppedareasinthecommandarea

§ Calculatetotalnetagriculturalreturnswithproject

Total with project net agricultural returns per annum = Command area with project x Cropping

pattern=AreaofeachcropxNeteconomicreturnfromeachcrop

iii. Calculatenetagriculturalbenefitsperannumatfullproduction

Netagriculturalbenefitperannum=Totalwithprojectnetagriculturalreturnsperannum(a)-Total

withoutprojectnetagriculturalreturnsperannum(b)

iv. Estimatethefinancialcostoftheproject

§ Summaryofcostofconstruction

§ Calculateoperationandmaintenancecost

§ Costofland

v. Subtractcostoflandacquisitionfromfinancialcosttheproject,

Financialconstructioncosts–Landacquisitioncost=Netfinancialconstructioncosts

vi. Convertnetfinancialconstructioncoststoeconomiccosts,

§ Allocateconstructioncostintoperiod:

§ Addtotalpresentvalueofoperationandmaintenancecosts“withproject”,

§ Addtotalpresentvalueoflandlevelingcosts“withproject”,

§ Subtracttotalpresentvalueof“withoutproject”operationandmaintenancecosts,

§ Presentvalueoftotalcosts,

Netfinancialo&mcostsxConversionfactor=Economico&mcosts

vii. Assessnetpresentvalue

Thepresentvalueofbenefitsisdividedbythepresentvalueofcoststoproducethebenefit-cost

ratio,whichistakenasthebasicmeasureofthesystem’seconomicviability.

TotalPVofbenefits/TotalPVofcosts=Benefit/costratio

viii. ComputeEconomicInternalRateofReturn(EIRR)

EIRR = %&' +%&' − *&'

(,-'/0%&' − ,-'/0*&'× 1 − ,-'/0%&'

Where,EIRRiseconomicinternalrateofreturn,

HDRishigherdiscountrate,

LDRislowerdiscountrate,

BCRisbenefitcostratio

Atypicalexampleofeconomicanalysisispresentedin.


256

Table9-4TypicalExampleofEconomicAnalysis

NameofProject:

TotalProjectCost(NRs): 850,976.16 Assumptions:

NetIncrementalBenefit(NRs): 383,019.25 Lifeofproject: 10years Coversionfactor:

O&Mcost(NRs): 25,529.28 Benefitstartsfrom2ndyear Forconstructioncost: 0.95

ForO&Mcost: 0.90

at10% at12%

10 12 at10% at12% at10% at12%

1 808,427.35 808,427.35 0.909 0.893 734,860.46 721,925.63 - -

2 22,976.36 22,976.36 191,509.63 0.826 0.797 18,978.47 18,312.16 158,186.95 152,633.17

3 22,976.36 22,976.36 383,019.25 0.751 0.712 17,255.24 16,359.17 287,647.46 272,709.71

4 22,976.36 22,976.36 383,019.25 0.683 0.636 15,692.85 14,612.96 261,602.15 243,600.24

5 22,976.36 22,976.36 383,019.25 0.621 0.567 14,268.32 13,027.59 237,854.95 217,171.91

6 22,976.36 22,976.36 383,019.25 0.564 0.507 12,958.66 11,649.01 216,022.86 194,190.76

7 22,976.36 22,976.36 383,019.25 0.513 0.452 11,786.87 10,385.31 196,488.88 173,124.70

8 22,976.36 22,976.36 383,019.25 0.467 0.404 10,729.96 9,282.45 178,869.99 154,739.78

9 22,976.36 22,976.36 383,019.25 0.424 0.361 9,741.98 8,294.46 162,400.16 138,269.95

10 22,976.36 22,976.36 383,019.25 0.386 0.322 8,868.87 7,398.39 147,845.43 123,332.20

808,427.35 206,787.21 1,015,214.56 3,255,663.63 855,141.69 831,247.13 1,846,918.82 1,669,772.42

B/CRatioat10% 2.16

B/CRatioat12% 2.01

EIRR % 23.36

EconomicAnalysis

Year Construction

cost(NRs)

O&Mcost

(NRs)

Totalcost

(NRs)

Benefit(NRs) Discountfactors Presentvalue(NRs)

Cost Benefit


257

10. BasisTermsUsedintheManual

Afflux : the rise of water level on the u/s of the weir or barrage after its construction,

Alignment : center line of the canal to be followed for excavation or profiling,

Apron : floor protecting the surface from erosion,

Aqueduct : cross drainage structure made over a drainage to carry the canal water,

Barrage : structure across the river with gates which raises the water level in the irrigation canals,

Basin : pool of water - stilling basin, desilting basin, etc.

Bench mark : geodetic reference point taken from a national survey of the country,

Breast wall : vertical wall immediately above the face of the orifice or gate,

Cascade : structure which conveys water over a series of drops,

Catchment area: area from which rainfall flows into a drainage line,

Chute : high velocity conduit for conveying water at the sloping profile,

Command area : area which can be irrigated or cultivated,

Crop coefficient: ratio of reference crop evapotranspiration to crop evapotranspiration,

Cropping intensity: percentage of irrigated area cropped within the irrigation command area,

Cropping pattern: crop planting sequence and crop mix throughout a year,

Cross drainage works: structure needed to carry irrigation canal water across the drainage channel,

Culturalcommandarea(CCA): totalareawhichcanbecultivated,

Culvert : structure allowing water to flow,

Cut-off : wall constructed below the floor of the structure to reduce percolation,

Desilting basin : structure which creates a pool of water to allow suspended sediments to settle,

Drop : structure for dropping the flow of the canal to dissipate its surplus energy,

Embankment : earthwork raised above the natural ground to protect against flood water,

Escape : structure required to enable excess water to escape from the canal,

Flood control : provision to minimize or reduce flood magnitude,

Free board : margin between a canal bank and the full supply level,

Gross command area (GCA): total area which can be economically irrigated by an irrigation system,

Guide bank : training bank constructed at the site of a bridge or barrage to guide the river,

Head regulator : control structure constructed at the head of the off-take canal,

Headwork : combination of structures constructed at the diversion point of the river,

Hydrograph : graph showing the stage, flow of water with respect to time,

Intake : structure to divert water from the source to the canal system,


258

Lining : method of sealing sections of the canal to prevent seepage,

Main canal : canal that conveys water from the headwork or from the intake,

Marginal bund : embankment constructed along the river at a short distance from the margin,

Offtake : structure carrying water from a parent canal to a relatively smaller canal,

Percolation : flow of water in sub-soil due to the force of gravity or pressure head,

Pier : intermediate support of the bridge or culvert,

Pitching : covering of the sloping surface with stone, concrete blocks, bricks, etc,

Regime : state of the river flowing in non-scouring and no-silting condition,

Retaining wall : vertical wall retaining the soil mass behind it,

Return period : an estimate of the probability that an event will occur again,

Runoff : part of rainfall that reaches the stream or drain from the catchment,

Scour : removal of material from the channel bed by flowing water,

Sediment : non-floating material transported by water (sand, silt, gravel and clay),

Seepage : percolation of water into the soil underneath the structure,

Side slope : sloping distance of a canal section, usually measured in a ratio,

Silt : water-borne sediment consisting of fine earth, sand or mud,

Siphon : closed inverted conduit for a cross-drainage structure,

Sluice : conduit for carrying water at high velocity,

Spillway : passage for the flow of surplus water in a weir or conduit,

Spur : structure used to train river flow at the banks,

Stilling basin : structure used to create a pool of water to disseminate the surplus energy of water,

Super-passage : structure over the canal to bypass the drainage flow,

Tail-water : water just downstream of the structure,

Uplift : upward pressure of water on the base of a structure,

Waterway : distance required for flowing water, e.g. Lacey’s waterway,

Weep-holes : horizontal holes on walls to allow the flow of seepage water behind it,

Weir : barrier built across the river to raise the water level upstream,

Wing wall : splayed extension of an abutment wall of the culvert,


259

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