Adapting to Climate Change through IWRM - adb.org · Project title Adapting to climate change...

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)


ii

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


iii

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


iv

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


v

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

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

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

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

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


vi

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


vii

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


viii

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


ix

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


x

EXECUTIVESUMMARYThe updating of the existing irrigation engineering manual 1998 covers the planning, survey, design and evaluation of an irrigation scheme. The manual mainly focuses on the hydrology, hydraulics, planning and design of hydraulic structures, canal layout and design headwork and intakes, micro irrigation and river training and flood control works. In addition, the manual covers crop planning, water demand assessment, and water balance assessment. The limitation of the manual is that it does not cover institutional and construction management aspects of irrigation project preparation. The target group of the manual is the engineering professionals of the Department of Agriculture. However, the manual will also benefit all irrigation practitioners from both the public and private sectors. The Irrigation Engineering Manual is organized into nine chapters. The introductory chapter (Chapter 1) presents irrigation in Bhutan, the methodology adopted and the contents of the manual. Chapter two (2) of the manual describes project study and survey procedures, highlighting identification and feasibility studies, including environment assessment.

In the planning of an irrigation scheme, the water requirement and its availability at the particular location need to be assessed first in addition to the availability of suitable land for agricultural production. The water availability and requirement together decide the area to be irrigated, and the size and capacity of the conveyance system. The planning and design of an irrigation scheme must consider whether the land is suitable for irrigated agriculture, climate conditions are favorable, and water is available for the proposed cropping pattern. The identification of the scheme is the very first step of the project development process. The Dzongkhag/Gewog level office should ask the farmers through their leaders about the necessity of the rehabilitation of an existing scheme or the construction of a new scheme. In addition, local level officers should disseminate information about the plans and program of the Department of Agriculture (DOA) on irrigation development and management.

The feasibility study is the basis for project implementation and is carried out by a team of experts with engineering, agriculture, and socio-environmental backgrounds. The feasibility study covers topographical, hydrological, agricultural, geological and socio-economic surveys. In addition, the study assesses the technical feasibility, economic viability and institutional suitability of project implementation. The priority ranking of the scheme is carried out based on techno-economic, social and environmental criteria.

Chapter three of the manual addresses hydrology and agro-meteorology questions. This chapter highlights the procedures followed to assess the availability of water for irrigation, flood flows, and crop and irrigation water requirements, including water balances. According to the Food and Agriculture Organization (FAO), 80% dependable river flow is used to estimate the water available for irrigation diversion. Given the large spatial variation in run-off in Bhutan and the significant number of small catchments, it is unlikely that gauging stations will be found at the point of irrigation diversion. For gauged catchments, water availability is assessed with the help of frequency analysis; for ungauged catchments, other empirical methods need to be adopted.

The discharge adopted for the design of a hydraulic structure is referred to as design flood discharge. The estimation of design flood is essential for the design of all hydraulic structures. The return period (T) of the flood flow is the indicator which determines the relative importance of the various hydraulic structures. For gauged catchments, design flood is assessed with the help of statistical analysis; for ungauged catchments, other empirical methods are used which are briefly described.

The crop water requirement is estimated with the evapo-transpiration (ETo) of a reference crop and multiplied by the crop coefficients of proposed crops. The ETo is calculated using Cropwat-8, software developed by the FAO. The total crop water requirement is assessed with the addition of land preparation and deep percolation losses during cultivation practices. The net crop water requirement is assessed by deducting effective rainfall from the total crop water requirement. The effective rainfall is calculated from the 80% reliable rainfall of the area. The intake water requirement is calculated with consideration of the irrigation system’s efficiency. The water balance at the diversion point of the source river is calculated by considering downstream environmental flow and upstream abstractions.


xi

Chapterfour(4)providesbasicconceptsofintakesanddiversionheadworksalongwiththeirdesign

proceduresandworkedoutexamples.This chapteralsoaddressespump irrigationwith itsdesign

proceduresandaworkedoutexample.Intakestructuresarehydraulicdevicesbuiltattheheadof

an irrigation canal, whereas the structure supplying water to the off-taking canal is called a

headwork. Headworks may be a storage headwork or a diversion headwork. According to the

methodofdivertingwaterfromthesource,intakesarebankintakes,sideintakeswithcrossweirs,

bottomintakes,frontalintakes,submergedintake,andtowerintakes.

A diversionheadworkmaybe aweir or a barragedependingupon theobjective of the irrigation

scheme. Weirs are solid walls across the river to raise the water level and to divert it into the

irrigation canal.Weirsmay be providedwith small shutters on the top. During flooding, there is

considerableaffluxontheupstreamsideofaweirbecausetheentiredischargeoftheriverhasto

passovertheweir’screst.

Abarrageissimilartoaweirbutisequippedwithgatestoheadupthewateronitsupstreamside.

The barrage crest level is on the average river bed level and has minimum afflux. From an

operationalpointofview,abarrage isbetterthanaweir;however, thecostofabarrage ismuch

higherthanthatofaweir.Themaincomponentsofaheadworkareweirs,dividewalls,fishladders,

under sluices, head regulators, abutments, guide bunds and canals. Based on the flow

characteristics,weirs are categorizedas sharp crestedweirs andbroad crestedweirs. In addition,

weirsarealsocategorizedaccordingtothematerialsusedintheirconstruction.Thedesignofaweir

involveshydraulicdesignandstructuraldesign.Thehydraulicdesigninvolvestheassessmentofthe

lengthofthewaterway,crestlevelsoftheweirandundersluice,depthofcutoffsinrelationtoboth

scourandexitgradients,level,lengthandthicknessoffloors,andlengthofprotectionworks.

This chapter also provides design concepts on energy dissipation devices to be used in hydraulic

structures which are mostly based on the hydraulic jump formation. In addition, this chapter

presents thedesignconcept,proceduresandworkedoutexampleofapumped irrigationsystem,

includingbriefnotesonpumpcharacteristics,theselectionandsitingofpumps,andtheselectionof

pipesandfittings.

Chapter five (5) presents the basic concept of canal design in both erodible and non-erodible beds, along with design procedures and worked out examples of a canal system. This chapter also contains concept notes on the types and layout of canals, lined and unlined canals, and piped canals. Design parameters such as canal side slope, permissible velocities, bed width to depth ratio, canal longitudinal slope, Manning’s roughness coefficient, freeboard, and minimum radius of curvature are briefly presented. With regard to lining types, this chapter includes brief notes on stone masonry lining, concrete lining, soil cement lining, Ferro cement lining, plastic lining and slate lining, followed by design procedures and worked out examples. The design concept and worked out examples of pipes flowing full and pipes flowing partially full are provided in the covered canal section. In addition, this chapter covers the design of pipelines, including concept notes on hydraulic grade lines, control valves, frictional head loss pressure limits, air blocks and washouts, control valves, and break pressure tanks. It also provides formulae for the assessment of head losses in pipes and fittings. Furthermore, this chapter covers issues and concerns of hill irrigation canals in unstable areas such as those prone to landslides, and mass slips. Chapter six (6) presents the design concept, procedures and worked out examples of canal structures, highlighting various types of structures. The canal structures are grouped into regulating structures, drop structures, cross drainage structures, and sediment control structures. For regulating structures, the manual provides the design concept, procedures and worked out examples of the head regulator, flow divider or proportional divider, and field outlets or offtakes. The drop structures


xii

section covers the design concept, procedures and worked out examples of vertical drops, chute drops, pipe drops, cascade drops, escape structure, bridges and culverts. Cross drainage structures are the main structures in an irrigation scheme, and are grouped into aqueduct, super passage, siphon, pipe crossings and level crossings. The manual provides selection criteria for cross drainage structures, design considerations, merits and demerits, design procedures and worked out examples of these structures. For the sediment control structures, the manual presents the design of gravel traps and settling basins along with worked out examples. The section on retailing walls describes common types of retaining walls based on the materials used, the design concept, the procedures for design and a worked out example. Chapter six also provides the general design concept for water control gates, including types of gates and the standard size for small scale irrigation schemes. In addition, this chapter provides the structural design concept of hydraulic structures including loadings and structural analysis, factor of safety for stability, and bearing pressure. This section also includes the concept of seepage and uplift design, and concrete design for hydraulic structures. Chapter seven (7) covers micro irrigation, with a focus on the design concept, procedures and worked out examples of sprinkler irrigation system, drip irrigation system and water harvesting. In addition, this chapter briefly describes the advantages and constraints of sprinkler and drip systems, types of sprinkler systems and their components. This chapter also includes the design concept and typical models of a simple drip irrigation system. The water harvesting section provides information on types of water harvesting, the design concept of ponds and tanks, design standards used elsewhere in the region, methods of sealing water harvesting ponds or tanks, and a worked out example of pond design. Chapter eight (8) contains the design concept and design procedures of river training and flood control works. This chapter opens with general information on the rivers of Bhutan, types of floods, and flood prone areas in the country. This chapter also provides brief concept notes on types of river training works, which include embankment bunds and guide bunds, spurs and groynes, cutoffs and pitching of banks along with their design parameters and standards. This chapter illustrates the detailed design procedures of river training works. It also includes the design concept and methods of bank protection works along with the design procedure of revetment works. At the end of this chapter, brief notes on the mathematical models used for river studies -- HEC HMS, HEC RAS, MIKE BASIN, MIKE 11 and MIKE FLOOD—are provided. The Irrigation Engineering Manual concludes with chapter nine (9), which covers project evaluation, providing the concept and procedures of agricultural benefit assessment, estimate of project costs, and economic analysis. Agricultural benefits are assessed on the basis of per unit benefits of crop yields and the agricultural inputs involved in growing particular crops. The main agricultural inputs are seeds, fertilizer, pesticide, labor, and animal power. The project cost estimate includes unit rates of the items of work, detailed quantity of the works and cost of the works. The derivation of unit rates is based on the Bhutan Schedule of Rates (BSR), which is published each year by the Ministry of Works and Human Settlement (MOWHS). The quantity of works is assessed according to the dimensions of the works as prescribed in the approved drawings. The cost of civil works comes from the multiplication of quantity and the unit rates. The total project cost may include other costs such as mobilization costs, cost for consulting services, supervision costs, cost for bank commissions etc.


1

CHAPTER-1

1. Introduction

1.1 Background

Development of irrigation will benefit the agriculture sector by facilitating adaptation to climate

change, increasingagricultureproductivityand incomes,and improvingrural livelihoods(Severaju,

2014). Although Bhutan has a long history of irrigation scheme planning, design and

implementation,itwasfeltthattheexistingirrigationengineeringmanualneededtobeupdated.In

thiscontext,AsianDevelopmentBankTechnicalAssistance(ADB-TA-8623)onAdaptationtoClimate

Change through Integrated Water Resources Management (IWRM) aims to update the existing

IrrigationEngineeringManualof theDepartmentofAgriculture (DOA)aspartof its strengthening

governance and capacity development. This manual will benefit those involved in the planning,

design,construction,operationandmaintenanceofirrigationschemes.

1.2 Scopeofthemanual

Themainobjective is toupdatetheexisting IrrigationEngineeringManual1998.Thescopeof the

manualcovers irrigationprojectpreparation, fromplanningandsurveys todesignandevaluation.

The manual mainly focuses on the hydrology, hydraulics, planning and design of hydraulic

structures, canal layout, conceptof structuraldesignof irrigation infrastructure,and river training

andfloodcontrolworks. Inaddition,themanualcoverscropplanning,waterdemandassessment,

andwater balance assessment. The limitation of themanual is that it does not cover the socio-

institutionalandconstructionmanagementaspectsofirrigationprojectpreparation.Thetargetsof

the manual are the engineering professionals of the Department of Agriculture. However, the

manual will also benefit all irrigation practitioners from both the public and private sectors. The

manual also includes a separate volume for the typical drawings planned for small irrigation

schemes.

1.3 IrrigationinBhutan

IrrigationinBhutanhasbeenpracticedsincetimeimmemorial.FarmersofBhutanhaveconstructed

smallscaleirrigationschemeswiththeirownresourcesandskills.Thesefarmer-managedirrigation

systems(FMIS)wereconstructedtoprovideassuredirrigationtopaddyfields(Chuzhing).Withthe

introduction of planned development in Bhutan, these FMIS started to receive support from the

Government (Pradhan,1989).TheGovernmentbegantodevelop irrigationwith the first fiveyear

plan(1961-66).TheIrrigationDivisionwasestablishedin1967undertheDepartmentofAgriculture

totakeresponsibilityfortheplanning,designandimplementationofirrigationfacilities.

WiththerequestoftheRoyalGovernmentofBhutan,theinvolvementofbilateralandmultilateral

donoragenciesbeganinthe1980s.Majordonorsinvolvedintheirrigationdevelopmentsectorare

Indian Aid, United Nation Capital Development Fund (UNCDF), International Fund for Agriculture

Development (IFAD), Asian Development Bank (ADB), and the World Bank (WB). Major projects

developed with financial assistance from donor agencies are the Punakha-Wangdi Valley

DevelopmentProject(IFAD),TaklaiIrrigationProject(UNCDF),GelephuLiftIrrigationProject(Indian

Aid),TsirangIrrigationProject(ADB),andDecentralizedRuralDevelopmentProject(WB).TheTaklai

irrigationscheme(1,350ha),thelargestschemeinBhutan,wasrecentlyrehabilitatedwithtechnical


2

and financial support from Japan International Cooperation Agency (JICA). The Gelephu Lift

IrrigationProjectisdefunct.

According to the National Irrigation Information System (NIIS), there are about 1,100 irrigation

schemesinBhutan,ofwhich962schemeshaveacommandarealargerthan15acres.111ofthese

962 schemes are not functioning for different reasons. In addition, there are about 140 schemes

with a command area less than 15 acres providing irrigation to about 2,500 acres of land. These

small,mediumandlargeirrigationschemesprovidesomekindofirrigationtoabout64,246acresof

land.TraditionallytwotypesofcultivatedlandsexistinBhutan:wetlands(Chuzhing)anddrylands

(Kamzhing).ChuzhingsarerecognizedasirrigatedlandsingeneralwhileKamzhingsareun-irrigated

cultivatedlands.However,notallChuzhingshaveirrigationfacilities.

1.4 Methodologyadopted

ThemethodologyadoptedtoupdatetheexistingIrrigationEngineeringManualinvolvedthe

followingapproaches:

• Reviewofexistingengineeringmanuals,

• Reviewofirrigationdesignmanualsofsimilarregions,

• Identificationofgapsintheexistingmanuals,

• InteractionwithrelatedstakeholderssuchasstaffoftheEngineeringDivisioninDOAand

collectionofexpectations,and

• PreparationoftheIrrigationEngineeringManual

1.4.1 Reviewofexistingmanuals

Thereviewofexistingmanualsstartedwiththecollectionofthemanuals.Theavailablemanualsat

theDOAare:

• IrrigationEngineeringManual1990,

• Punakha-WandiValleyDevelopmentProjectIrrigationManual-1992,and

• FeederRoadManual

The existing Irrigation EngineeringManual 1990 was prepared by the Department of Agriculture

(DOA)asadraftversion in1990and revisedsubsequently in1994and1998. Themanualhas11

chaptersand10appendicesfocusingmainlyonthesurvey,designandconstructionsupervisionof

irrigation schemes. The Punakha-Wangdi Valley Development Project Irrigation Manual was

prepared by Hunting Services andMottMacDonald Consultants in 1992 as part of the Punakha-

Wangdi Valley Development Project under IFAD assistance. The manual has 6 chapters and 5

appendices.TheFeederRoadManualhas threevolumescoveringhydrology, irrigationdesignand

feederroadrespectively.Theirrigationdesignmanualhas12chaptersand7appendices.

1.4.2 Interactionwithstakeholders

TheinteractionwithengineersofEngineeringDivisioninDOAwascarriedoutonanindividualbasis

andtheirexpectationsregardingtheupdatingoftheIrrigationEngineeringManualwerecollected.

The main staff consulted included the chief engineer, executive engineers, deputy executive

engineersandengineersworkingintheEngineeringDivisionofDAO.

AfterreviewingtheexistingmanualsandinteractionwiththeconcernedDOAstaff,theConsultant

prepared a draft table of contents for the Manual. The proposed draft table of contents was


3

presented intheTechnicalAdvisoryCommittee(TAC)meetingon15thDecember2014alongwith

the basic format of the manual and was endorsed. Similarly, the draft table of contents was

presentedtotheEngineeringDivisionon19thDecember2014.

1.4.3 Conceptualframeworkofthemanual

Thebasic formatofthemanual includesthe introduction,designconcepts ,designprocedureand

worked out examples. The design of irrigation structures follows a farmer friendly design with

simple and understandable procedures and illustrations accompanied by photos and sketches

whereverpossible.

1.5 Contents of the Manual

The Irrigation Engineering Manual is structured in nine chapters. The manual starts with this

introductory chapter which presents irrigation in Bhutan, the methodology adopted and the

contentsofthemanual.Chaptertwodescribestheprojectstudyandsurveyprocedureshighlighting

identificationandfeasibilitystudies.Thischapteralsodealswiththenamingandcodingofirrigation

schemes and scheme prioritization criteria. Chapter three deals with hydrology and agro-

meteorology in irrigationprojectdesign.Thischapterhighlightstheproceduresofassessingwater

availability for irrigation, flood flow assessment and assessment of crop and irrigation water

requirementsincludingwaterbalanceassessment.Chapterfourprovidesbasicconceptsofdiversion

headworksandintakesalongwiththeirdesignproceduresandworkedoutexamples.Thischapter

alsocovers theconceptofpump irrigationwith itsdesignproceduresandaworkedoutexample.

Chapterfiveprovidestheconcept,designproceduresandworkedoutexamplesofthecanalsystem.

Thischapteralsoexplainsthetypesandlayoutofthecanals,linedandunlinedcanals,pipedcanals

andcanals inunstableareas.Chaptersixpresentsthedesignconcept,proceduresandworkedout

examples of the canal structures, and highlights various types of flow regulating structures,

conveyance structures, cross-drainage structures, and sediment control structures. Chapter seven

highlights the design concept and procedures of river training and flood control structures. This

chapter also presents the rivers and flood prone areas of Bhutan. Chapter eight presents the

concept of micro irrigation, its application and design procedures. This chapter also describes

differenttypesofmicroirrigationandtheiradvantagesanddisadvantagesinthecontextofBhutan.

Chapternine,whichconcludes themanual, addressesprojectevaluationandprovidesprocedures

for cost estimates, benefit assessments from irrigation projects and the basic economic analysis

conceptsinvolved.


4

CHAPTER-2

2. ProjectStudyandSurvey

2.1 NamingandCodingofIrrigationScheme

MostirrigationschemesinBhutanarenamedaccordingtothelocationoftheircommandarea.For

rehabilitation schemes, the existing namesof the schemes should be adopted. For new irrigation

schemes, thenames shouldbebasedon the locationof the commandareaorbedecidedby the

community.Forthescientificidentificationoftheirrigationschemes,itisessentialtoprovidecodes

foreach schemewhich shouldbebasedon thenational identification system.The coding system

adopted in the National Irrigation Information System (NIIS) seems appropriate and is based on

geocoding standards published by the Ministry of Information and Communication (MOIC). The

same coding hence should be adopted for future irrigation development and management. An

exampleofthecodingsystemisasfollows:

01002

The first three digits represent theGewog codewhile the last two digits represent the irrigation

schemecode.01002isthePhusairrigationschemeinGetanaGewogofChhukhaDzongkhag.

2.2 ProjectDevelopmentCycle

Allirrigationschemesencompassseveraldevelopmentcyclestages,startingwithplanning,followed

byimplementation,monitoringandevaluation.Eachstagehasmanyactivitiestobecarriedoutto

fulfill the specified purpose of that stage. The details of the stages and associated activities are

presentedinthefollowingtable(Table2-1).

Table2-1Projectdevelopmentcycle

Projectstage Mainactivities Purpose

Irrigation

planning

• Assessbroadelementsofirrigation

• Appraisaloftheirrigation

development

• Toestablishtheobjectiveand

scopeofirrigationdevelopment

Identification

survey

• Facilitatefarmersawareness

• Perceiveneedsbyfarmers

• Farmersrequestforassistance

• Ensuredevelopmentisdemand

driven

Pre-feasibility

study

• InitialfieldvisitsandPRAs

• Collectexistingphysicalandsocio-

economicdata

• Stakeholdersanalysis

• Firstapprovalorrejectionofpre-

feasibilitybystakeholders

• Firsthandassessmentof

irrigationpotential

• Identifyfarmersobjectives,

requirements,andcapabilities

Feasibilitystudy • Detailedphysicaldatacollection

andinvestigations

• Topographicalsurvey

• Hydrologicalsurvey

• Socio-environmentalsurvey

• Agriculturalsurvey

• Studyonclimatechangeimpacts

• Financialandinstitutionalreview

• Ensureadequateresourcesto

meetfarmersobjectives

• Ensureresourcesavailablefor

proposeddevelopment

• Provideopportunitiestomodify

design

• Providebasisforloans,

managementandO&M


5

Projectstage Mainactivities Purpose

• Preliminarydesignandcosts

• Participationoffarmersindesign

choices

• Initiationofappropriatefarmers

organization

• Prepareprojectfeasibilityreport

includingeconomicappraisal

Detaileddesign • Finaldataassessed

• Detaileddesign,quantities,and

contractdocumentsprepared

• Fundingarrangementsorganized

• Farmerscontributionsclearly

determined

• Finalizedetailsandcosts

• Matchdesignswithfarmers

capability

• Enablefarmerstotake

responsibility

Implementation • Projectapproval

• Tenderreceived

• Selectionofcontractorand

contractagreement

• Farmersparticipationin

construction

• Completionoftheconstruction

• Hand-overtofarmers

• Trainingtofarmersoncultivation,

on-farmwatermanagement,and

O&M

• Enablecost-effectivechoice

• Assurepaymentforworksand

materials

• Promotesenseofownership

• Promoteeffectiveuseofwater

• Farmersassumeresponsibility

Monitoringand

evaluation

• Regularreviewofperformance

• On-goingtrainingdextension

• Ensuretargetsareachievedand

sustained

• Encouragecontinued

improvement

2.3 IrrigationPlanning

Irrigation is the process of applyingwater to soil for plant growth, and involves the acquisition,

conveyance, distribution and application of this water. There are significant spatial as well as

temporalvariationsofrainfallandirrigationaimstosupplementnaturalrainfall.Whenplanningan

irrigation scheme, thewater requirement and its availability at theparticular locationneed tobe

assessed first in addition to theavailabilityof suitable land for agricultural production. Thewater

availabilityandrequirementtogetherdecidetheareatobeirrigatedandthesizeandcapacityofthe

conveyance system. An irrigation scheme can be planned and designed in an area when the

followingpointsaremet:

• Suitabilityoflandforagriculturalproduction,

• Favorableclimateforthegrowthofcrops,

• Adequateavailabilityofwater.


6

In addition to these three points, farmers’ willingness also has to be considered in planning the

scheme.Smallirrigationschemescanbedevelopedrelativelyquicklywhilelargeirrigationschemes

maytaketimeforproperplanning,designandconstruction.Dependinguponthesizeoftheproject,

theplanningstageitselfconsistsofthreephases:preliminaryplanning,whichincludespre-feasibility

and feasibility studies, detailed planning of land andwater use, and detailed design of irrigation

structuresandcanals.

Survey and studyworks are basic requirements of any irrigation project planning. The success or

failureoftheimplementationofaprojectdependsontheaccuracyofitsstudyandsurvey.Survey

worksformthebasisforthestudyoftheprojectinwhichdataandinformationarecollectedboth

fromprimaryandsecondarysources.Thetypeandnatureofthestudyandsurveydependonthe

stage,sizeandimportanceoftheproject.Forsmallscaleirrigationschemes,itmaybesufficientto

carryouttwolevelsofstudy:theidentificationstudyandthefeasibilitystudy.Inaddition,astudyof

climate change impacts and possible adaptation measures needs to be carried out during the

planning and study stagesof irrigationproject development. The following sectionsdealwith the

majorsurveyandstudyworksofanirrigationproject.

2.4 IdentificationStudy

2.4.1 Introduction

The identification of the scheme is the very first step in the project development process. The

Dzongkhag/Gewog level office should ask farmers through their leaders about the necessity of

rehabilitating the existing scheme or constructing a new scheme. In addition, local level officers

should disseminate information about the plans and program of the Department of Agriculture

(DOA).Thisinformationdisseminationshouldbecarriedoutthroughmeetingswithfarmerleaders

and Gewog heads (Gups). The main objective of information dissemination is to seek genuine

demandoftheprojectfromthefarmers.Astandardprojectdemandformshouldbedevelopedand

distributedtothefarmers.Examplesofgenuinedemandmaybe:

• Therequestfordevelopmentmustbegenuinefromthemajorityofthefarmers,

• Farmersarewillingtocontributetocovertheirpartofdevelopmentcosts,

• Farmersarewillingtoorganizeawaterusersassociation,enterintonecessaryagreements

withtheofficeandundertakeoperationandmaintenanceofthesystemaftercompletion,

• Soilsofthecommandareaaresuitableforirrigation(onlyfornewschemes)

Prior to starting the identification study, the request form of the farmers should be reviewed to

establishwhetheritfulfillstheminimumrequirementsthathavebeenset.Thisprocessistheinitial

screeningwhichaimstoidentifygenuinerequests,prioritizeprojectswithhighfarmercommitment,

identify thetypeofproject (new/rehabilitation)andensurethat thesetprinciplesandcriteriaare

met.

CriteriaforselectionshouldbepreparedbytheEngineeringDivisionofDOAbasedontheprinciples

and objectives of the particular program. The criteria for the selection of the project at the

identificationstageshouldincludebutnotbelimitedtothefollowing:

• Atleasteightypercentofthebeneficiaryhouseholdsshouldsigntheprojectrequestform;

• Therearenopotentialwaterusedisputesinthestreamorriver;

• ThebeneficiariescommittotakefutureO&Mresponsibilities;

• Sufficientwaterisavailable.


7

Aftertheveryfirstinitialscreening,athree-stepprojectidentificationstudyneedstobeconducted

including: (a) Desk study, (b) Field visit, and (c) Analysis and reporting. The identification study

shouldbecarriedoutbyateamcomprisingatleastanirrigationengineerandanagricultureofficer.

2.4.2 Deskstudy

Atthissteptheprojectlocationshouldbeidentifiedon1:50,000scaletopographicmaps.Theteam

shouldcollectandreviewprevious reports ifanyare related to theproject.The teamshouldalso

studythedemandformsubmittedbythefarmers.Inthecaseofarehabilitationproject,theteam

shouldverify the information inthedemandformwiththeNational Irrigation InformationSystem

(NIIS)ofDOA.

2.4.3 Fieldvisit

Afterthedeskstudy,amessageshouldbesenttothefarmersinformingthemaboutthetentative

date of the field visit. The tools and equipment to be taken on the field visit are: a copy of the

projectrequestform,theprojectidentificationquestionnaire,topographicalmaps,notebooksand

necessarystationary,GPS,stopwatch,calculator,camera,anda3mlongmeasuringtape.Thefield

visitshouldconsistofatleastthreeactivities:initialmeeting,walkthrough,andconcludingmeeting.

a) Initialmeeting:localfarmersshouldbemet,discussionsshouldbeheldandarrangementmade

fortheschemeinspection.Duringthismeetinganoverviewoftheprojectshouldbemade:

• Istheschemeaneworimprovementproject?

• Whyhavethefarmersrequestedtheproject?

• Whereistheintakesite,maincanalandcommandarea?

b) FieldInspection:visittheintakesite,maincanalalignmentandcommandarea.Theteamneeds

toassessthefollowingmainpointsinadditiontofillinginthequestionnaire:

• Historyoftheproject;

• Conditionsofintakesite;

• Wateravailabilityandissueofwateruse;

• Suitabilityoflandarea;

• Presenceofcross-drainageworksandtechnicalcomplexities;

• Lengthofmaincanalandlandslidezonesifany;

• Majorcropsgrownandcroppingpattern;

• Farmersattitudetowardsprojectimplementation

c) Concluding meeting: After the field inspection and before leaving the site, the study team

shouldholdaconcludingmeetingwiththefarmers.Theteamshouldsharetheirexperienceof

theschemeinspection,majorfindingsandthepossibilityforfurtherstudy.

2.4.4 AnalysisandReporting

Basedon thedataand informationcollectedduring the fieldvisit, the teamneeds toanalyze the

project findings and finalize the identification study. The analysis should be based on technical,

environmentalandsocialaspectsofproject identification.Basedon thisanalysis, the teamhas to

prepareareportstatingtheprojectrecommendationforfurtheractions.Thereportshallcomprise


8

mainlytheintroductiontotheproject,theprojectsummarysheet,thecompletedquestionnaire,a

topographical map with the project location, area and water resources calculation, and the

recommendation.

2.5 Feasibilitystudy

2.5.1 Introduction

Thefeasibilitystudyisthebasisforprojectimplementationandiscarriedoutbyateamofexperts

with backgrounds in engineering, agriculture, and socio-environmental fields. This study normally

formsthebasisoffinancingbyexternalfundingagenciesortheGovernment.Thestudyassessesthe

technicalfeasibility,economicviabilityandinstitutionalsuitabilityoftheproject’s implementation.

Thefeasibilitystudyiscarriedoutinthefollowingsteps:

i. Deskstudy

ii. Fieldsurveywork

iii. Fielddataanalysisandfeasibilitydesign

iv. Costestimate

v. Economicanalysis

vi. Projectreportingandrecommendation

2.5.2 Deskstudy

The desk study is a review of thework carried out at the project identification stage. This study

generallyneeds toexamine twoaspects.The first is the listofoutstandingmatters tobestudied,

andanypossibleschemealternatives. Theremaybeseveraloutstandingissuesnottouchedduring

previousstudieswhichshouldbestressedduringthefeasibilitystudy.Schemealternativesneedto

bereviewedinthefeasibilitystudy,whichmayincludethefollowing:

Fornewschemes:

• Alternativewatersourcesfromdifferentrivers,pumpedsupply,groundwater,

supplementaryrivers,etc

• Alternativeintakesiteontheriver,and

• Alternativecanalalignment

Forrehabilitationschemes:

• Extensionofcommandarea,

• Combiningseveralschemes,

• Revisedcanalalignment,and

• Revisedintakesite

2.5.3 FieldSurveyWork

Thefieldsurveyworkmaydifferslightlybasedonthetypeofthescheme(neworrehabilitated).The

followingarethemainactivitiestobecarriedoutduringthefieldsurveywork:

i. Listthenames,commandareaandwaterwithdrawalamounts(indifferentseasons)of

existingirrigationsystemsandotheruses(drinkingwatersupply)locatedinthevicinity

oftherivers/streams.Assesstheimpactofthesystem’sdiversionondownstreamwater


9

users, particularlywhere additional irrigationwater is needed for the extension area.

Verifytheriverwaterflowsandintakewithdrawalamountthroughflowmeasurements.

ii. Locate theschemeon1:5,000mapsbasedonavailableortho-photosand/orcadastral

maps.GoogleEarthmayalsobeagoodsourceforsuitablesystemmaps.Thelocationof

themainstructuresshouldalsobemarked.

iii. Collect data regarding the command area, irrigation source, water rights, catchment

condition,soils,landuse,inadesignatedformat.

iv. DiscusswiththefarmersgroupandconfirmtheprojectrequestandthelistoftheWUA

members with individual landholding distribution within the existing and proposed

extensionarea.

v. Basic demographic parameters will be collected such as number of households,

household size, and life expectancy. Other social parameters such as education level,

majorsourceofincome,andfoodsecuritywillalsoberecorded.

vi. Together with the social mobilizer undertake a "walk through" with representatives

fromthefarmersgrouptodeterminethepresentandfutureirrigatedareasandidentify

necessaryworks.

vii. Perform engineering surveys such as river long sections and cross sections. Similarly,

undertakelongitudinalandcrosssectionssurveysofthemainandbranchcanals.

viii. Conductsoilsurveysandbasicsoilstestswithinthecommandareaatadensityofabout

onepitsiteforevery10haofcommandareatoassesssoilsuitabilityforagriculture.

ix. Performflowmeasurement toverifyexistingcanalcarryingcapacities, seepage losses,

andverificationofwaterrequirementsandotherhydraulicparameters.

x. Inconsultationwithfarmergroups,determinethelocationoftherequiredmajorworks

inthecanalalignmentwithappropriatedigitalphotos,whichwillbe incorporated into

theinfrastructureimprovementplan.

xi. Collect and confirm data on the existing irrigated area; cropping pattern; input use

including fertilizer, seeds, and pesticides; yields, in a systematic manner based on

landholdingsizes.

2.5.4 Fielddataanalysisanddesign

The collected data and information has to be analyzed, on return to the office, before a final

decision can be taken on the project. The major activities of the analysis are related to the

estimationofthenetcommandareaandwaterbalanceatthesourceriver.Beforestartingthecanal

design,itisimportanttocheckthewaterbalancetoconfirmwhethervariablewaterissufficientto

theproposedcommandareainallseasons.Afteranalyzingthedata,feasibilitydesignshallstartand

comprisesthefollowing:

i. Systemlayout:Usingtopographicmapsaplanof thesystemshouldbemadeshowing the

source,primary canalalignment,major structuresandmainpartof thecommandarea.A

schematiclinediagramofthesystemlayoutneedstobeprepared;

ii. Canaldesign:Forfeasibilityleveldesign,typicalcanalcrosssectionscanberelatedtocanal

discharge and ground conditions. For rehabilitation schemes canal designs, are only

requiredforreacheswherecompleteremodelingisneeded.

iii. Structure design: It should be prepared for intake andmajor structures. Typical standard

drawingsmaybeusedforminorstructures.


10

iv. Standardsforfeasibilityleveldesign

• HeadWorks

• Drawings(plan,typicalsection)showinglocationandlevelsanddimensions

shouldbepreparedandprincipalquantitiesestimated,

• Layout

• A layout plan of the scheme and command area showing primary and

secondarycanalalignmentshouldbeprepared,

• Canals

• Canalroutes,capacities,slope,levelsanddimensionsshouldbeestablished,

• Alongitudinalsectionofthecanalshouldbeprepared,

• Typicalcrosssectionsshouldbedrawn,

• Earthworkquantitiesshouldbeworkedout,

• Structures

• Drawingsofthetypicalstructuresshouldbeprepared,

• Eachsizeandtypeofstructureshouldbeassessed,

• Drawingsofmajorstructuresshowinglocation,levelsanddimensionshould

beprepared,

• Principalquantityshouldbecalculated

2.5.5 TopographicalSurvey

The topographical survey is theprocessofdetermining thepositionboth inplanandelevationof

natural featuresof thearea.Theextentof the topographical surveydependsupon the size, type,

andleveloftheprojectstudy.Forsmallscaleirrigationschemes,thetopographicalsurveyincludes:

• Benchmarksurvey,

• Traversesurvey,

• Alignmentsurvey,and

• Longitudinalandcrosssectionalsurvey

Thetopographicalsurveystandardforsmallscaleirrigationschemesmaybeasfollows(Table2-2).

Table2-2Topographicalsurveystandards

S.N Principaluse Details

1 Scheme

identification

• Useexistingmapsbutsupplementwithsiteinspection

2 Feasibilitystudy • Useexistingmaps,sitesketches

• Carryouttraversesurvey(GPS)

• Benchmarksurveyattherateof0.50to1.0kmintervals

• Carryoutdetailedsitesurveyforstructures-intake/headwork,

majorcrossdrainagestructures

• Forcommandareatopographicalsurveytake100minterval

gridforcontourintervalof0.50to1.0m.


11

S.N Principaluse Details

• CarryoutcanalalignmentsurveywithL-sectionandX-

sections

3 Construction

stage

• Usedesigndrawings

• Checkandagreelevelsanddimensions

• Carryoutadditionalsurveyfordetailsofthesiteifrequired

2.5.6 Hydrologicalsurvey

Hydrological survey is carried out to provide data and information for the assessment of water

availabilityforirrigation,andassessmentoffloodflowsfortheintake/headworkandcrossdrainage

works.Thedataandinformationtobecollectedduringthehydrologicalsurveyare:

• Catchmentareaofthesourceriver/stream,

• Catchmentconditionssuchaslanduselandcover,slopeandlengthoflongestchannel,

• Longtermrainfallandclimaticdatawithinthecatchmentarea;

• Waterflowrecordsofsourceriver/stream(ifavailable),

• Flowmeasurementoftheriver/stream,

• Highfloodlevelorthesourceriveratthepointofintakeordiversionweir(trashmarksif

any),

• Informationonriverbedmaterial

2.5.7 Socio-economicsurvey

Thesocio-environmental survey iscarriedout todetermine thesocial structureof thecommunity

anditslivelihoods.Thesurveyincludesthecollectionofquantitativeaswellasqualitativedataand

informationonsocialstructure,socio-culturalinstitutions,andeconomicactivitiesofthefarmersof

the scheme command area. Some of social and economic indicators of the community are as

follows:

Socialindicators:

• Willingness/commitment- verbal request, formation of committee, submission of

requestform,

• Education- literacy, school and college, awareness about irrigated agriculture, prior

experiencewithirrigation,

• Ruralorganizations,

• Familysize-male/female,economicallyactivemembers,

• Migration-temporary,permanent,foreign/urbanareas

Economicindicators:

• Landholdingsize-landless,marginalland(<1acre),largelandowners>10acre),

• Mainoccupation-agriculture,service,labor,foreignservice,business,

• Sourceofincome-agriculture,service,remittanceetc,

• Expenditure-food,cloth,schooling,festivals,livestock,agriculture


12

2.5.8 AgriculturalSurvey

Forsmallirrigationschemes,theagriculturalsurveymayincludedataandinformationregardingthe

soiltype,landuseandagriculturepracticesofthecommandarea.Thesoilsurveymayincludethe

assessmentofthetypeofsoil inthecommandareaanditssuitabilityforirrigatedagriculture.The

soilsmaybealluvial,sandy,gravelandbouldermixed.

The landusesurveymay includethegeneralassessmentof landuse in thecommandarea,which

mayclassifythepercentofagriculturalland,forestland,grazingland,wetlands,NationalParksand

reserveforestarea.

Theagriculturesurveyincludesthecollectionofdataandinformationon:

• Existingcroppingpattern,

• Existingcropyields,

• Inputsusedanditsavailability,

• Marketingfacilityandlaborsituation

2.5.9 GPSSurveyMethodology

The Global Positioning System (GPS) aided walk through method uses relatively recent

improvementsinsurveyaids.GPSequipmentstoresanddownloadstracksandwaypoints,digitized

ordigitalcartography,highresolutionsatelliteimagery,andelaboration/presentationofthesurvey

resultsinGISsoftware.

DuringGPSwalkthroughsurveys, tracksofthemainandbranchcanalalignmentswillberecorded

andconveyanceorstructuralproblemsinthesecanalswillbemarkedonthetracks.Inadditionto

thecanalalignmentsurveys,thecommandareaboundariesofthesystemwillberecordedwiththe

GPSequipment.ThecanaltrackandmarkedwaypointswillbedownloadedandprocessedinArcGIS

or compatible GIS software, using Garmin DNR software or other GPS equipment software to

converttherecordedGPSfileformattotheGISshapefileformat.IntheGISsoftware,therecorded

canal tracks and command area boundaries will be overlaid on the newly available digital

topographicalmaps from theSurveyDepartmentand if availablehigh resolution satellite imagery

thatcanbedownloadedcommerciallythrougha licensedversionofGoogleEarthortheaccessto

theDigitalGlobearchivesthroughtheGlobalmappersoftware.Highresolutionimagescanalsobe

composedfromthefreeversionofGoogleEarthbydownloadingtherequiredcoveragetilebytile.

There is also software available atminimal cost that processes the tile by tile downloading from

GoogleEarthautomatically.

The results of the surveys will permit the preparation of accurately geo-referenced mapping of

systemlayouts,commandareaboundariesandsystemdeficiencies.Theaccurategeo-referencingof

the surveys and resultingmapswill allow verification of the accuracy of the surveys through the

overlay on the digital topographicalmaps and especially on the high resolution satellite imagery.

ConversionoftheShapeformattotheGoogleKMZ(KML)proprietyformatwillfacilitatethedisplay

ofthelayoutmapsdirectlyinGoogleEarthwhichallowsa3Drenderingofthecanalsystemlayout

andcommandarea(ShapefilescanbedirectlyconvertedtoKMZ/KMLformatinArcGIS.Verification

ofthesurveyandmappingresultsinGoogleEarth3Dfurtheraidsinassessingthecorrectnessofthe

surveys and resulting system layout and facilitates thepresentation to awideaudienceof survey

resultsinaneasyandcosteffectivemanner.


13

2.6 EnvironmentalImpactStudy

2.6.1 LegalProvisions

AccordingtotheEnvironmentAssessmentAct2000,EnvironmentalClearance(EC)ismandatoryfor

anyproject/activity thatmayhaveadverse impact(s)on theenvironment. TheRegulation for the

Environmental Clearance of the Projects 2002 defines responsibilities and procedures for the

implementation of the Act concerning the issuance and enforcement of the environmental

clearance. In addition, the Regulation under its Annex-2 mentions that all irrigation channels

(irrigationschemes)tobedevelopedneedtoobtainenvironmentalclearancefromthecompetent

authority. TheNational Environment Commission (NEC) has revised, updated and published eight

environmental assessment guidelines for the forestry, general, highway and roads, hydropower,

industry,mining,tourismandtransmissionsectors.However,theirrigatedagriculturesectorhasnot

yetbeenincludedinthesesectoralguidelinesandthegeneralenvironmentalassessmentguideline

appliestoirrigationprojectpreparationandimplementation.

Theenvironmentalassessmentprocessendeavorstomitigateandpreventundesirableimpactsfrom

the development of the irrigation project. It must include all of the procedures required under

Bhutanese law intended to identify the means to ensure that a project is managed in an

environmentallysoundandsustainableway.

2.6.2 Potentialsocio-economicandenvironmentalimpacts

Themajorsourceofenvironmentaleffects isfrominfrastructureworks,the impactsofwhichvary

according to the type, size, and location. Highly significant and/or irreversible adverse

environmental impacts are not expected from the implementation of the irrigation project. In

addition, the rehabilitation of existing age-old community managed irrigation schemes will have

negligibleornoimpactsontheenvironment.Theenvironmentalissuesrelatedtotherehabilitation

ofexistingschemesandconstructionofnewirrigationschemesare:

• Landslideandsoilerosion,

• Lossand/ordegradationofforestandvegetationcover,

• Healthandsafety,andsanitationissues,and

• Constructionperioddisturbances

However, these impacts are readilymanageable asmitigationprocedures for such impacts are in

practice in Bhutan. In addition, the irrigation development activities will have socio-economic

impacts on the project site. Potential socio-economic impacts incurred from irrigation activities

include:

• Livelihoodimpactsasaresultoflandacquisitionforcanalalignment,

• Impactsonstructuresrequiringrelocation,

• Impactsonlocalirrigationfacilities,

• Lossofcropsandtrees,

• Lossorreplacementofruralstructures,

• Construction-relatedimpacts,suchaspublichealth,dustandsafety,

• Impactsonvulnerableanddisadvantagedgroups


14

2.6.3 ProceduresforEnvironmentalImpactAssessment

The Environmental Clearance (EC) application needs to be backed up with information on no

objection certificates (NOC) andEnvironmental Information (EI). TheEI need to include: potential

adverseenvironmenteffects, complianceplan,amanagementplan,andenvironmentalandother

benefitsoftheproject.ThecompetentauthoritycheckstheEI,socialclearanceandNOCs,aspartof

environmental screening and will lead to either issuance of an EC, instructions for further study

(Environmental Impact Assessment) or rejection of the application based on the situation.

Consultationwithaffectedcommunities (forsocialclearance) isexpected to takeplaceduring the

NOCandEAprocess.TheECissuingagencyisresponsibleformonitoringcompliance.TheNational

EnvironmentCommissionand/orthecompetentauthorityaremandatedtomonitorcompliance.

The environmental impact assessment (EIA) is a tool to identify the potential environmental and

social impacts arising from the implementation of the project. According to the Environmental

assessmentgeneralguidelineprovidessixgenericstepsintheEIAprocess:

§ Screening,

§ Scoping,

§ Baselinedatageneration,

§ Impactassessment,

§ Mitigationofimpact,and

§ Environmentalmanagementplan

Screening is the first stepanddetermineswhetheranEIA isnecessary for theprojectornot.The

second step is scoping, which aims to establish environmental and social priorities, set the

boundaries for the study and define Terms of References (TOR). The third step is to generate

baseline data which provides information on the existing status of the environmental and social

componentsofthestudyarea.Theimpactassessmentisthefourthsteponwhichcharacteristicsof

thepotentialimpacts,evaluatedandpredictedusingbaselinedata.Impactpredictionsarenormally

made using commonmethodologies andmodels. The fifth step is the identification ofmitigation

measuresofpotentialimpacts.Thesemayeitherbepreventiveorremedialmeasures.Thelaststep

is the preparation of environmental management plan which should include recommended

mitigationmeasuresinspecificactionplansthatwillbeimplementedbytheproponent.

2.6.4 IntegrationofEIAintotheprojectcycle

An irrigation project is accomplished in seven stages; the activities of each stage were briefly

presentedinsection2.2ProjectDevelopmentCycle.EIAplaysanimportantroleineachstageofthis

projectcycle.MostoftheactivitiesofEIAtakeplaceduringthepre-feasibilityandfeasibilitystages

of the project. Between irrigation planning and pre-feasibility, the EIA process involves site

selection, screening, initial assessmentand scopingof significant issues. ThedetailedEIA starts in

thefeasibilitystage,whichevaluatessignificantimpacts,collectionofbaselinedata,predictionand

quantificationofimpacts,andreviewofEIAbytheregulatoryagency.

Followingtheseinitialsteps,environmentalprotectionmeasuresareidentified,operatingconditions

aredetermined,andenvironmentalmanagement isestablished. Inthe lastphaseofthefeasibility

study, monitoring needs are identified and an environmental plan and monitoring program are

formulated.Theenvironmentalmanagementplanaimingtoreduceadverseenvironmentalimpacts

isconsideredintheprojectcyclestartingfromtheprojectappraisalstage.

The World Bank funded Remote Rural Communities Development Project (RRCDP) under the

Ministry of Agriculture and Forest developed Environmental and Social Assessment Framework

ImplementationGuidelinesandEnvironmentalandSocialScreeningProcedureGuidelines in2013.


15

Theseguidelinesprovide seven stepsof environmental and social safeguardprocessingwhichare

listedbelow:

Step1:Preliminaryenvironmentalandsocialinformationandanalysis,

Step2:Environmentalandsocialscreeningandassessment,

Step3:Environmentalandsocialrecommendationsandpreparationofsub-projectDetailedProject

Report(DPR),

Step4:Environmentalclearanceandsocialclearance,

Step5:PreparationofSocialActionPlan(SAP),

Step6:Implementerssiteenvironmentalandsocialmanagementplan,

Step7:Complianceandfinalmonitoring

2.7 ClimateChangeImpactStudy

2.7.1 Climatechange

Understanding and copingwith climate change is an important global issue and serious concerns

have arisen in the planning and design of water projects, including irrigation. The increase in

greenhouse gas concentrations in the atmosphere has led to global warming, which causes

unpredictable and extreme climate events. According to the IPCC, climate change projections to

2100forSouthAsiaingeneralandBhutaninparticularconsistofincreasesinaveragetemperatures

with relativelywarmerweather at higher altitudes and during the dry season, increase in annual

average precipitation, and continued spatial variation in temperatures and precipitation due to

complex topography (NEC, 2009). The number of hot days is projected to increase. Increased

temperatureswillbeaccompaniedbychangesincloudcover,precipitation,airhumidity,radiation,

wind, and other meteorological elements (NEV, 2011). These effects will lead to changes in

hydrological cycles with impacts on mass water balance, soil moisture, and evapo-transpiration.

Climate changewill affect agriculture throughhigher temperature andmore variable rainfall. The

followingsectionspresentthemajorimpactsofclimatechangeonirrigatedagricultureandpossible

adaptationmeasurestocopewiththepotentialimpactsofclimatechange.

2.7.2 Potentialimpactsofclimatechangeonirrigation

Irrigated agriculture is planned and designed based on location specific physical conditions,

includingtheclimate.Climatechangeinducedbyglobalwarmingposessignificantriskstoirrigated

agriculture in general and water management in particular. According to the results of various

researchstudiesontheimpactsofclimatechange,climatevariabilityhassignificantimpactsoncrop

areaandcropproduction,especiallyinperiodsofdroughtsandfloods.Inaddition,climatechange

willaffect irrigatedagriculturethroughincreasedcropevapo-transpiration,changesintheamount

ofrainfall,andvariationsinriverflows.Hence,theimpactofclimatechangeonirrigatedagriculture

mustbeconsideredinawidercontextwhichincludeswaterdemand,waterqualityandcompetition

over water at the system as well as basin levels. The potential threats of climate change and

associatedimpactsarepresentedhereintabularform(Table 2-3).

Table2-3Potentialimpactsofclimatechangeonirrigation

Climatechange/threats Impacts

Increaseinrainfall • Higherstreamorriverflows

• Benefittoin-streamecology


16

Climatechange/threats Impacts

• Morewateravailableforirrigation

Decreaseinrainfall • Lowerstreamorriverflows

• Pressureonenvironmentalflow

• Pressureonwaterforirrigation

Changeinrainfallpatterns • Increaseinpressureonwateravailabilitydueto

seasonalvariation

• Reliabilityofirrigationwaterinquestion

Increaseinrisksofdroughts • Verylowflowinstreamsandrivers

• Pressureonenvironmentalflow

• Nowaterforirrigationabstraction

Increaseintemperatureand

increaseinhotdays

• Adverseeffectonstreamecology

• Increaserisksinweedgrowth

• Increaseinvariabilityofwaterquality

• Increaseinirrigationwaterdemand

• Increaseinmaintenancecostofwater

infrastructure

• Increaseinwaterdemandforlivestock

Increasedfrequencyofheavy

rainfallevents

• Increasefrequencyoflargefloods

• Increaseinsedimentyields

• Increaseinerosionincatchment

• Increaseriskofwaterinfrastructuredamage

• Increaseinlandslideevents

• Increaseinsusceptibilityofcanalalignment

Increaseinwindiness • Increaseinerosionoftopsoil

• Increaseinirrigationwaterdemand

Thepotentialthreatsandtheirimpactssuggestthatclimatevariabilityandchangewillbethemain

drivers of change and will necessitate specific climate change-related responses. The impacts of

climatechangewillvary fromonesystemtoanother,buttheadaptationstrategyshouldconsider

the context of each specific system. During the planning and study of the irrigation system, it is

essential toassessthepotential impactsandextentofvulnerabilityofthesysteminordertoplan

themitigationaswellasadaptationmeasures.

2.7.3 Impactandvulnerabilityassessment

During thepre-feasibility and feasibility studyof the irrigation scheme,a vulnerability assessment

needstobecarriedouttoascertaintheextentofclimatechangeimpacts.Vulnerabilityisdefinedas

theextenttowhichclimatechangemaydamageorharmthephysicalinfrastructureingeneraland

theirrigationsysteminparticular.Itdependsnotonlyonthesensitivityoftheirrigationsystem,but

alsoonitsabilitytoadapttonewclimaticconditions.Vulnerabilityassessmentplaysavitalrolein


17

the design of appropriate adaptation measures targeting climate change impacts. Vulnerability

assessmentandadaptationplanninginvolvesseveralstepspresentedbelow.

i. Determiningthescopeanddefiningtheassets

Thescopedescribesthelimitsoftheplanningtasksincludingtime,geographicalarea,assetstobe

covered,andresourceavailabilityforassessment.

ii. Baselineassessment:

Thebaselinedescribesthepastandexistingsituations,trends,anddriversacrosseachofthetarget

systemsandanalyses thechanges to thesesystemsthatwilloccur irrespectiveofclimatechange.

The main components of the baseline assessment and their respective activities are outlined in

tabularform(Table 2-4).

Table2-4Componentsofthebaselineassessment

Components Activities

Asset inventory and priority

setting

• Provide an overall description of existing and proposed

assets

• Describetheconditionofthetargetinfrastructuresystem

Past climate variability and

extremeevents

• Describe the most significant past extreme events

affectingtheassets

• Describetheimpactofthatevent

Climate change threats and

opportunitiesprofile

• Gatherallexistingclimatechangeprojectsrelatingtothe

area

Adaptation audit of past

protectionmeasures

• Describe measures taken by community to learn from

pastextremeevents

• Assess the effectiveness of those past adaptation

measures

Socio-economic status and trends

assessment

• Identifyusersandaffectedcommunities

• Describe the socio-economic trends of relevance to the

asset

Naturalsystemstatusandtrends • Describe the natural systems such as river banks,

catchmentarea

• Describethestatusandtrendsofthesenaturalsystems

Source:DerivedfromADB/ICEM/GON,2014

iii. Impactandvulnerabilityassessment

Themethodconsidersfourimportantfactorsinassessingthevulnerabilityofthetargetsystemand

itscomponentstoclimatechanges:exposure,sensitivity,impactandadaptivecapacity.

Exposureistheextenttowhichasystemisexposedtotheclimatechangethreat.Exposureisbest

assessed by overlapping maps of past extremes such as floods and droughts and of projected

climatechanges in thearea.Exposureto thethreatdependsuponthe locationof thesystemand

threatintensity,frequency,andduration.


18

Sensitivity is the degree to which a system will be affected by, or responsive to, the exposure.

Sensitivityforaninfrastructureassetmaybeinfluencedbythespecificsiting,geotechnicalcharacter

(bankstability,drainage),integrityofdesign,andintegrityofmaterialsandconstruction.

Thepotential impact is aproductofexposureand sensitivity. Thepotential impactof the climate

threatisratedbasedonthesupporttoolsinthevulnerabilityassessmentmatrix.Adaptivecapacity

isunderstood intermsoftheabilitytopreparefora futurethreatandabilitytorecoverfromthe

impact. Once the impact assessment is complete, the adaptive capacity of the community or

organizationtoprepareforandrespondtotheimpactsneedstobeassessed.

Whenthe impactandadaptivecapacityareconsidered,ameasureofrelativevulnerabilitycanbe

defined. An example of parameters and issues considered in the vulnerability assessment is

presentedinFigure 2-1.

Figure2-1Parametersandissuesofvulnerabilityassessment

iv. Adaptationplanning

Adaptation to climate change refers to the actions to be taken during the preparation of the

irrigation project by the authorities concerned. Adaptation also includes taking advantage of

opportunities that may arise due to climate change, as well as responding to negative impacts.

Adaptationplanning involves developing a rangeof adaptationoptions for eachof the significant

impactsofclimatechangeandthendeterminingprioritiesforimplementation.

Adaptationatfarmlevel

Climatechangeadaptationmeasuresatthefarmlevelinvolvecropplanning,includingtheselection

of crop varieties and crop calendars to adoptnew temperatures and rainfall patterns. Theuseof

cropsrequiringlittlewateranddrought-resistantcropswillalsobepreferred.Increasedagricultural


19

diversification, including better integration of trees, crops, fish and livestock will reduce risk and

increase resilience of farming systems (FAO, 2013). In addition, farmers will favormore efficient

technologiestoreduceevaporationlossesfromthefield.

Adaptationatschemelevel

Actions for adapting to climate change in irrigation schemesneed tobe considered in theoverall

contextofirrigationmodernization(FAO,2013),whichwillenhanceefficiency,waterallocationand

reliable delivery of water. Intermediate storage within the irrigation scheme and access to

alternativesourceofwateraresomeoftheoptionstoimprovereliabilityofwaterforadaptationin

irrigationplanning anddesign. A typical example of adaptationplanning is presentedhere (Table

2-5). Asset: Chisapani Naubasta Irrigation System, Banke, Nepal; Command area- 306 ha;

Beneficiaries-575HH;Maincrops-paddy,wheat,potatoes,pulsesandoilseeds;Maincomponents-

intake,maincanalandcommandarea

v. Mainstreamingadaptationplan

The mainstreaming adaptation plan involves the incorporation of prioritized activities into the

systemstartingfromitsstudythroughoperationandmaintenance.

2.7.4 Integrationofclimatechangeintoprojectcycle

Like the EIA, the impacts of climate change also need to be integrated into the project cycle as

illustratedinsection2.2ofthischapter.Inadditiontomainstreamingclimatechangeadaptation,it

isessentialtoplan,designandincorporatemitigationmeasuresfrompotentialadverseimpactsof

climatechangeintotheprojectcycle.


20

Table2-5Exampleofclimatechangeadaptationplanning

Note:VH-veryhigh,H-high,M-medium,L-lowandVL-verylow

ThreatsImpacts Significance Adaptationoptions Priorityadaptation

High and very

highthreats

Impacts for high and very high

threats

Likelihood

(chances

of

the

impactoccurring)

Seriousnessof

theimpact

Significance

of

theimpact

Focus on structural and

bioengineeringoptions

Feasibility

Effectiveness

Priority

Intakestructure

• Increased

riverflows

• Flashfloods

• Further damage to diversion

weir

• Unable to raise water levels to

intake

• Intakeblockswithdebris

• Sedimententersintomaincanal

VH

H

VH

H

H

H

H

M

VH

H

VH

M

• Rebuild diversion weir with

CCconsideration

• Improve river bed

protection downstream of

corewall

• Increasemaintenance

L

M

M

VH

H

H

H

H

H

Maincanal

• Landslides

• Canalssiltup,becomesblocked M M M • Increasecanalclearance

• Protect landslides with

bioengineering

VH

M

M

H

VH

H

Commandarea

• Increased

temperature

• Increased

rainfall

• Storms

• Draughts

• Increase ETo and then water

demand

• More chance of disease to

wintercrops

• Increase rainfall reduces water

demand

• Stormsdamagecrops

• Draughtseffectscropyields

H

M

H

H

M

VL

M

VL

VH

M

L

M

L

VH

M

• Introduce less water

requiringcrops

• Introduce more disease

resistantcrops

• Opportunity for alternative

croppingpatterns

• Chose varieties less

susceptible to storm

damage

M

M

M

M

M

H

M

H

M

H

M

H


21

2.8 Reportpreparation

For theelaborationof the feasibility study,a reportneeds tobeprepared including the following

chaptersandappendices:

Executive Summary: The executive summary consists of highlights of each chapter including

conclusionsandrecommendations.

Salient features:Thesalient featureswillbepresented ina specified tabular formwhichcontains

relevantdataandinformationontheproject.

Introduction:Thischapterconsistsofbackgroundinformationabouttheproject,theobjectivesand

scopeoftheproject,andcontentsofthereport.

Methodology:Thischaptercoverstheapproachandmethodologyofthefeasibilitystudy,analysis,

design,andreportpreparation.

Project area:Details of theproject areahave tobepresented in this chapter,whichmay include

location,topography,climate,waterresources,upstreamanddownstreamwateruse,waterrights,

agricultural situation of the area covering cropping patterns, existing yields of major crops,

extension service, and socio-economic conditions of the area. This chaptermay also contain the

conditionofexistingirrigationfacilities,includingWUAs.

Proposed project: This chapter will focus mainly on the proposed planning of irrigation

infrastructure to enhance agricultural production. The chapter will contain an agricultural

development plan, irrigation system plan, command area, crop-water requirement, proposed

structures and their design, operation andmaintenance, institutional support, environmental and

climate change impacts assessment and implementation arrangement. The assessment of water

availability will be calculated using this manual along with the water balance calculation. The

proposedcroppingpatternwillbebasedonthefarmers’preferencesandthewateravailableatthe

diversionpointduringthecroppingseasons.

The potential yieldwill be based on the potential crop yield adjusted to the local circumstances,

taking into account present crop yields under optimal irrigated conditions and average district

yields.Aproject specific implementation schedulewill have tobepreparedbasedon specific site

conditions.

EconomicAnalysis: For the assessment of the EIRR and B/C ration economic analysis need to be

carriedoutfortheprojectwhichismainlybasedontheagriculturalbenefitstobeaccruedfromthe

projectandthecostofprojectimplementation.Theagriculturalbenefitwillbecalculatedaccording

tothecropbudgetofthe“with”and“without”projectsituations.Thecostoftheprojectisassessed

withthederivationofratesandquantities.

Appendices: TheappendicestotheFeasibilityReportwillinclude:

AppendixA: Maps,theGISmappingofthecanalalignment,commandareaboundariesand

irrigationstatusoverlaidonthedigitaltopographicmaplayers,

AppendixB: Drawings, applicable typical drawings and drawings for headwork and major

canalstructures,

AppendixC: Cropwaterrequirement,thetablesgeneratedbythewaterbalancecalculation

program,

AppendixD: Agriculturedata,tabularresultsoftheFGDs,


22

AppendixE: Cost and quantity estimation, the tables generated by the cost calculation

spreadsheets,

AppendixF: Economicanalysis,

AppendixG: Projectimplementationplan,asummaryoftheproposedinterventionsdetailed

inthereport,

AppendixH: Photographs.

2.9 ProjectSelectionandPriorityRanking

2.9.1 ProjectSelection

The irrigation schemes to be selected for implementation should be technically feasible,

institutionallyacceptableandeconomicallyviable.

Technicalfeasibility:Theschemeshouldbetechnicallyfeasibleinrelationtotheschemestandard

whichmayinclude:

• Adequatewaterisavailableatthesourcetoirrigatetheproposedcommandarea;

• Nowaterrightsproblems;

• Nomajortechnicaldifficultiesmainlyattheintakeandcanalalignment;

• Soilissuitableforirrigatedagriculture;

• Nomajorlandslides,erosionsinthearea

Institutionalacceptability:Theschemeshouldbebasedonthegenuinedemandofthebeneficiary

farmersinrespectto:

• Majorityfarmersarecommittedtheimplementationofthescheme(80%);

• Writtenagreementexistswithfarmers;

• WUAexistorareintheprocessofformation;

• Suitabilitytodiversifyirrigatedagriculture;

• Availabilityofmarketforinputsandproducts

Economicviability:

• Perunitcostoftheschemeshallbewithinthedefinedlimit;

2.9.2 PriorityRanking

The priority ranking of the scheme is carried out based on techno-economic, social and

environmental criteria. The techno-economic criteria includes cost per acre or total cost of the

scheme,wateravailabilityatthesource,sizeofthecommandareaandlengthofthemaincanal.The

social criteria include the number of beneficiary households, the food security situation, and

beneficiaryinterestintheschemedevelopment.Theenvironmentalcriteriaincludeclimateresilient

scheme, focus on crops requiring less water and proximity to national parks. The details of the

criteriaarepresentedintabularform(Table2-6).


23

Table2-6Evaluationcriteriaforschemepriorityranking

S.N Components Weightage Maximumscore

1 Costperacre/totalcost 15% 10

2 Wateravailabilityatsource 10% 10

3 Sizeofcommandarea 15% 10

4 Lengthofmaincanal 10% 10

5 Numberofhouseholds 10% 10

6 Foodsecurity 10% 10

7 Communityinterest/WUA 10% 10

8 Climateresilient 5% 10

9 Focusoncropsrequiringlesswater 5% 10

10 Proximitytonationalpark 10% 10

Total 100%


24

CHAPTER-3

3. HydrologyandAgro-Meteorology

3.1 HydrologyofBhutan

ThehydrologyofBhutancorrelateswiththemonsoonrainfallpatternfromtheBayofBengal,which

has significant variation in time and space.Monsoon rainfall starts in June, is intense in July and

August,andpetersoutinSeptember.November,DecemberandJanuaryarethedrymonths,while

pre-monsoon showers occur in April andMay. Themean annual rainfall ranges from 500mm to

5,000mm.TheheaviestrainfalloccursinborderareaswithIndia,decreasingrapidlynorthward.All

four major rivers, Amochhu, Wangchhu, Punatsangchhu, and Drangmechhu (Manas), flow to

BrahmaputraRiver.RiverflowsarehighfromJunetoSeptemberandstarttorecedefromOctober.

Theflowoftheriversbecomes lowduringthemonthsofDecembertoApril.The longtermmean

annual flowof these rivers is 2,325m3/s,which is equivalent to73,000millionm

3 per year (NEC,

2003).

The Department of Hydro-Met Services (DHMS) under the Ministry of Economic Affairs is

maintaining a network of hydrological and meteorological stations. The hydrological network

comprises 13 principal stations equipped with cableways, gauges, and pressure transducers

connected to data loggers and 19 secondary stations equipped with staff gauges. Similarly, the

meteorologicalnetworkcomprises20ClassAstationswithafullrangeofweatherparametersand

74 Class C stationswith temperature, rainfall and humidity data. Almost all of these hydrological

stations are located on the major rivers and their main tributaries and seldom fulfill the

requirementsofthedesignofirrigationschemes.

3.2 WaterAvailabilityforIrrigation

3.2.1 Concept

AccordingtotheFoodandAgricultureOrganization(FAO),theavailabilityofwaterfor irrigation is

assessed at 75% to 80% reliability of the full supply. The 80% reliability of the full supply is

equivalenttoa1 in5yearreturnperiodevent.Thismeansthat80%ofthetime(4yearsoutof5

years)thereisatleastenoughwateravailabletomeetfullwaterdemandforirrigation,and20%of

thetime(1yearin5years)therewillbeundersupply.Thedesignerhastoestimateriverflowsfora

particularlocationwhereintakeorheadworkcanbeconstructedwhichisexpectedtoexceed80%

of time.Given the large spatial variation in run-off inBhutanand the significantnumberof small

catchments it isunlikely thatonewill findgaugingstationsat thepointof irrigationdiversion.For

gaugedcatchments, theassessmentofwateravailability is carriedoutwith thehelpof frequency

analysis; forungauged catchments, othermethodsneed tobe adapted. InBhutan, suchmethods

have not yet been developed, and this section tries to identify the best ways to estimate the

availablereliableflowforirrigationfromungaugedcatchments.

3.2.2 FrequencyAnalysis

Frequencyanalysisisastatisticalmethodusedtoshowthatfloweventsofacertainmagnitudemay

onaveragebeexpectedonceeveryn year. It is generally carriedout toestimate thedesign flow

fromrecordedflowdatacoveringmorethan10years.Fromtherecordedmonthlyflowvalues,1in

5 or 80% reliability flow can be obtained from the mean and standard deviation values for a

particularmonthusingthefollowingformula:


25

SQQ mean 8418.080

−=

Where, Q80=80%reliablemonthlyflowinm3/s,

Qmean=meanmonthlyflowinm3/s,

S=standarddeviationofthemonthlyseries

Thefactor0.8418isthe1in5reducedvariateofthenormaldistributionassumingthatthemonthly

flowseries followsanormaldistribution.Theworkedoutexampleof frequencyanalysis toassess

80%reliableflowispresentedhereunder(Table3-1).

Table3-180%ReliableFlowofThimchhuatLungtenphug(m3/s)

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1992

5.19

4.22

4.50

4.60

8.02

15.67

47.59

58.92

41.05

17.69

9.84

6.61

1993

5.41

4.47

4.21

6.85

14.26

25.30

37.01

68.67

51.78

23.42

11.53

7.20

1994

5.51

4.74

5.09

6.13

10.82

10.82

30.78

50.41

42.39

14.91

8.50

6.00

1995

4.75

4.12

5.83

7.18

13.17

30.92

66.40

63.14

54.11

29.35

16.93

10.10

1996

7.87

5.85

6.49

9.87

12.35

26.58

61.09

60.11

63.56

39.27

11.74

8.58

1997

6.05

5.62

4.44

5.36

12.55

25.43

57.93

67.15

51.69

21.40

10.90

7.90

1998

5.38

3.75

3.88

6.77

12.78

22.35

74.06

80.83

45.90

28.37

13.02

7.58

1999

5.43

4.06

3.43

4.14

14.70

34.18

61.32

75.84

55.47

29.93

15.22

7.54

2000

5.93

4.54

4.79

10.13

19.49

37.12

63.66

76.50

62.35

18.13

9.50

6.09

2001

6.50

6.17

5.11

5.82

9.86

28.26

36.62

73.05

52.02

32.55

11.81

6.64

2002

4.96

4.23

3.60

5.68

10.28

34.18

76.91

69.95

37.94

19.43

9.92

6.16

2003

4.61

3.98

3.56

6.47

8.74

31.61

62.41

56.52

53.58

24.39

12.69

7.31

2004

5.17

4.44

5.11

5.32

10.49

32.09

60.52

61.88

34.98

27.51

11.12

6.53

2005

4.71

4.06

3.78

4.22

8.34

11.45

45.86

51.72

29.90

27.31

11.99

6.71

2006

4.72

3.81

3.20

4.12

13.13

29.98

52.61

53.22

49.67

23.22

10.66

6.73

2007

5.07

4.44

5.26

8.49

12.13

16.18

48.45

56.70

58.17

23.17

10.76

6.18

2008

4.39

3.37

3.34

4.98

8.46

36.84

57.85

70.33

40.17

21.01

11.14

7.32

Mean

5.39

4.46

4.45

6.24

11.74

26.41

55.36

64.41

48.51

24.77

11.60

7.13


26

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

SD

0.85

0.76

0.95

1.84

2.90

8.46

12.90

9.26

9.56

6.11

2.05

1.04

80%

Reliable

4.68

3.83

3.65

4.69

9.30

19.28

44.50

56.61

40.47

19.62

9.88

6.25

3.2.3 Similarcatchmenttransformationmethod

Whenthediversionsite is locatednear thegaugedsite, themonthly flowsat thediversionsite is

calculatedusinganarearatio.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛=

g

dgdA

AQQ

Where, Qd=monthlydischargeatdiversionsiteinm3/s,

Qg=monthlydischargeatgaugedsiteinm3/s,

Ad=catchmentareaatdiversionsiteinkm2,

Ag=catchmentareaatgaugedsiteinkm2

Workedoutexample(Table 3-2):

Catchmentareaofthestreamatintakesite(Ad)=7.50km2

CatchmentareaofThimpuChhuatLungtenphug(Ag)=663.0km2

Table3-2Workedoutexampleof80%reliableflow

Description Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Q80atThimpu

chhu

4.68 3.83 3.65 4.69 9.30 19.28 44.50 56.61 40.47 19.62 9.88 6.25

Q80atintake

site

0.05 0.04 0.04 0.05 0.11 0.22 0.50 0.64 0.46 0.22 0.11 0.07

3.2.4 EmpiricalMethodsforUngaugedCatchment

A method to assess water availability from ungauged catchments does not exist in Bhutan. The

WaterandEnergyCommissionSecretariatofNepalhasdevelopedamethod(WECSmethod)forthe

assessment of river flows from ungauged catchment areas larger than 100 km2. This method is

basedonaregressionanalysisofpastdata.AlthoughitwasdevelopedforNepal,andconsidersthe

countryasoneunit, itwouldbeuseful forBhutan,whichhassimilar topographyandhydrological

characteristics. For long term average monthly flows, all catchment areas below 5,000 m are

assumedtocontributeflowsequallyperkm2area.Theaveragemonthlyflowscanbecalculatedby

theequation:

Qmean(month)=Cx(AreaofBasin)A1x(Areabelow5000m+1)

A2x(MeanMonsoonprecipitation)

A3

WhereQmean(month)isthemeanflowforaparticularmonthinm3/s,

C,A1,A2andA3arecoefficientsofthedifferentmonths(Table3-3).


27

Table3-3ValuesofcoefficientsforWECSmethod

Month C A1 A2 A3

January 0.01423 0 0.9777 0

February 0.01219 0 0.9766 0

March 0.009988 0 0.9948 0

April 0.007974 0 1.0435 0

May 0.008434 0 1.0898 0

June 0.006943 0.9968 0 0.2610

July 0.02123 0 1.0093 0.2523

August 0.02548 0 0.9963 0.2620

September 0.01677 0 0.9894 0.2878

October 0.009724 0 0.9880 0.2508

November 0.001760 0.9605 0 0.3910

December 0.001485 0.9536 0 0.3607

Note:Theunitsofflowareinm3/sandapowerof0indicatesthattheparticularparameterdoesnot

enterintotheequationforthatmonth.

Example: If the catchment area is 12.54 km2 and monsoon precipitation is 980 mm, the mean

monthlyflowiscalculatedasfollows(Table 3-4).TheequationforthemonthofJulyis

QmeanJuly=0.02123(basinareabelow5,000m+1)1.0093

X(meanmonsoonprecipitation)0.2523

QmeanJuly =0.02123*(12.54+1)^1.0093*980^0.2523=1.76m3/s

Table3-4Exampleofmeanmonthlyflow

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Flow,

m3/s

0.18 0.16 0.13 0.12 0.14 0.52 1.67 2.08 1.60 0.72 0.29 0.20

3.2.5 Climatechangeconsiderationinwateravailability

Climate changehas significant impactsonwateravailability for irrigation,which isoneof thekey

characteristicsofirrigationschemedesign.Assessmentoftheaccuratereliableflowatthediversion

siteisthemajorrequirementofthedesign.Ingeneralpractice,80%reliabilityofavailablewateris

considered adequate for irrigation,whichmeans that in 4 out of 5 years the assessed flow shall

meet the irrigationdemand. In the contextof climate change,9outof10years reliability canbe

consideredforlocalwateravailability.Themethodsdescribedaboveforungaugedcatchmentsare

based on regional experience and do not perfectly represent Bhutanese hydro-meteorological

conditions.Hence, thedesigneralsoneeds toexploreadditionalmethodsandcomparewith local

conditions.


28

3.3 EstimationofDesignFloodDischarge

3.3.1 ConceptofDesignFlood

Thedischargeadoptedforthedesignofahydraulicstructureisreferredasdesignflooddischargeor

design flood. The estimation of design flood is essential for the design of all hydraulic structures

suchasbarrages,weirs, intakes,bridges, causeways, culverts, crossdrainage structures, and river

trainingstructures.Themagnitudeoffloodastructureisrequiredtopasswithoutgettingdamaged

dependsonitsimportance.Thereturnperiod(T)ofthefloodflowistheindicatorwhichdetermines

therelativeimportanceofthevarioushydraulicstructures.

Thereturnperiodisthatperiodofoccurrenceflood(QT)whichwillbeequaledorexceededoncein

T-yearandtherecurrenceintervalistheaveragenumberofyearswithinwhichagivenfloodevent

isequaledorexceeded.Forexample, if the10yearreturnperiodfloodofariver is100m3/s, it is

assumedthat100m3/swillbeequaledorexceededonce in10years. It isexpressedasQ10=100

m3/s. The criteria for the selection of the return period are based on technical and economic

considerationswhicharesummarizedasfollows:

• Importanceofstructuretobeconstructed,

• Effectofovertoppingofthestructure,

• Potentiallossoflifeanddownstreamdamage,and

• Costofthestructure

Referencebooksonhydraulicstructuresandmanualshavesuggestedreturnperiodsaspresented

below(Table3-5).

Table3-5SuggestedReturnPeriod

S.N Typeofstructure ReturnPeriodinyears

1 Irrigationsupplyreliability 5

2 Roadculverts 5-10yearsdependinguponthetypeofcrossings

3 Highwaybridge 10-50yearsdependinguponthetypeofrivers

4 Irrigationintake 10-25years

5 Irrigationweir/barrage 50-100dependinguponitssize

6 Crossdrainagestructures 10-25years

There are various methods of estimating design flood discharge, some of these are illustrated

below.

3.3.2 FloodFrequencyAnalysis

Floodfrequencyanalysisisastatisticalmethodtoshowthatfloodeventsofcertainmagnitudemay

onaveragebeexpectedonceeverynyear. It isgenerallycarriedouttoestimatethedesignflood

from the recorded flow data of more than 10 years. The most commonly used methods for

frequencyanalysisare:

• Normaldistribution,


29

• Lognormaldistribution,and

• Gumbel’sdistribution

i. NormalDistribution: Thegeneralquantileestimationequationisasfollows:

xTavTKxX σ×+=

Where,KTisthefrequencyfactor,obtainedfromstandardtablecorrespondingtoprobabilityof

execeedence,p(=1/T)andCs;

xavisthemeanoftheoriginaltimeseries,

σxisthestandarddeviationoftheoriginaltimeseries;

CsisthecoefficientofSkewnessandinNormaldistributionCsequalstozero

ii. LogNormalDistribution: Thegeneralquantileestimationequationisasfollows:

( )yTavT KYeX σ×+=

Where,KTisthefrequencyfactor,obtainedfromstandardtablecorrespondingtoprobabilityof

execeedence,p(=1/T)andcoefficientofSkewness,Cs

Yavisthemeanofthelog-transformedtimeseries,

σyisthestandarddeviationofthelog-transformedtimeseries,

CsyisthecoefficientofSkewnessandinLogNormaldistributionCsequaltozero

iii. Gumble’s Distribution or Extreme Value Type-I Distribution (EVI): The general quantile

estimationequationisasfollows:

TTYuX α+=

Where, αµ ×−= 5772.0u

μ=xavisthemeanoforiginaltimeseries,

( ) πσα /6×=x

σxisthestandarddeviationoforiginaltimeseries,

YT=(-ln(-ln(1-1/T)))and(1-1/T)istheprobabilityofnonexceedenceand

Tisthereturnperiod

Thedetails of thesemethods are available in theHandbookofAppliedHydrologybyVent Te

Chow,1964.

3.3.3 DesignExampleofFloodFrequencyAnalysis

CalculatethedesignflooddischargeofThimpuRiverfora50yearreturnperiodfromthefollowing

data(Table3-6).

Table3-6AnnualMaximumFlowofThimpuChhu(m3/s)

S.N Year Maxflow(m3/s) Date

1 1991 143.40 17-Jul

2 1992 82.00 27-Aug

3 1993 106.70 24-Aug

4 1994 62.16 31-Aug


30

S.N Year Maxflow(m3/s) Date

5 1995 93.04 15-Aug

6 1996 83.81 30-Aug

7 1997 105.80 13-Aug

8 1998 129.94 8-Jul

9 1999 118.07 26-Aug

10 2000 111.55 1-Sep

11 2001 137.00 19-Aug

12 2002 132.64 23-Aug

13 2003 104.54 1-Jul

14 2004 100.03 21-Jul

15 2005 78.88 15-Aug

16 2006 84.32 26-Sep

17 2007 155.09 7-Sep

18 2008 86.68 20-Jul

i. Normaldistribution

Meanofthesamplesize(xav)=106.42m3/s

Standarddeviationofsamplesize(Ϭx)=25.44

Reducedvariateisprovidedbasedonreturnperiod.

Returnperiod(years) 2 5 10 20 50 100

Reducedvariate(KT) 0.00 0.84 1.28 1.64 2.05 2.33

xTavTKxX σ×+=

For50yearsreturnperiodflood(Q50)=106.42+2.05*25.44=158.57m

3/s

ii. Gumbledistribution(type-I)

Meanofthesamplesize(xav)=106.42m3/s

Standarddeviationofsamplesize(Ϭx)=25.44

Scalefactor(α)=Ϭx*0.78=25.44*0.78=19.84

Locationfactor(u)=Xav–α*0.5772=106.42–19.84*0.5772=94.97

Reducedvariateisprovidedbasedonreturnperiod.

Returnperiod(years) 2 5 10 20 50 100

Reducedvariate(YT) 0.37 1.50 2.25 2.97 3.90 4.60

TTYuX α+=

For50yearsreturnperiodflood(Q50)=94.97+19.84*3.90=172.36m3/s

Insummarythecalculateddesignfloodsare:

Normaldistribution,Q50=158.57m3/s

Gumbeldistribution,Q50=172.36m3/s


31

3.3.4 RationalMethod

TheRationalMethodisawidelyusedmethodfordesignfloodestimationofsmallcatchments.This

methodconsiderstheentirecatchmentareaasasingleunitassuminguniformrainfalldistribution.

Thismethodissuitableforsmallcatchmentswithanareaoflessthan25km2.Therationalmethod

formulaisasfollows:

AICQ 278.0=

Where,Qisthepeakdischargeinm3/s,

Cistherunoffcoefficient(roughlydefinedasratioofrunofftorainfall),

Iistherainfallintensityinmm/h,

Aisthecatchmentareainkm2

TheRationalformulaadaptsfollowingassumptions:

• Thepredictedpeakdischargehassameprobabilityofoccurrenceasthatofrainfallintensity,

• Therun-offcoefficientisconstantduringrainstorms,and

• Therecessiontimeisequaltothetimeofrise

Themaximumrun-offrateinthecatchmentisreachedwhenallpartsofthewatershedcontribute

totheoutflow.Thishappenswhenthetimeofconcentration(Tc) isreached,whichisregardedas

the time taken for the run-off at the most remote part of the catchment to reach the point of

outflow.Therearemanymethodsavailabletocalculatethetimeofconcentration.Eachmethodhas

its own parameters to consider. Themostwidely usedmethod for the calculation of the time of

concentrationistheRamser-Kirpichmethod,whichisexpressedasfollows:

385.077.00019.0

−××= SLT

c

Where,ListhelengthofthemainstreaminmandSistheweightedslopeofthemainstream.

Rainfall intensity (I) is computed from the rainfall intensity-duration curves of a known location

basedonthetimeofconcentration(Tc)ofthestreaminhours.Theaveragerainfallintensity(I)has

durationequal to thecritical stormduration,normally takenasa timeofconcentration (Tc). The

Feeder RoadManual, Volume I, Hydrology has prepared rainfall intensity curves for 10 selected

stationsofBhutanandthesecurvescanbeusedtodeterminetherainfall intensityofaparticular

areaforthedesignreturnperiod.Fornaturalcatchments,thevaluesofCvaryfrom0.2to0.4.

3.3.5 EmpiricalMethods

Empiricalrelationsarebasedonthestatisticalcorrelationbetweentheobservedpeakflowandthe

catchmentareainagivenregionandarethereforeregionspecific.Themethodusedincentraland

northernpartsofIndiaistheModifiedDickensMethod.

(i) ModifiedDickensMethod

3/2CAQ f =

AAP

PTC

S/)6(100

4)/1185log()6.0log(342.2

+=

+=

Where,

Aisthecatchmentareainkm2,


32

Asisthesnowcoveredareawithinthecatchmentareainkm2,

Tisthereturnperiodinyears

ThemostwidelyusedempiricalmethodsforcalculatingfloodflowinNepalaretheWECSandDHM

Method and the Sharma and AdhikariMethod. The Sharma and AdhikariMethod is the updated

version of theWECS and DHMMethod. Thesemethods are based on regression analysis of the

availabledata.

(ii) WECSandDHMMethod

WaterandEnergyCommissionSecretariat(WECS)ofNepaldevelopedthemethodin1990basedon

theregressionanalysisofavailableDepartmentofHydrologyandMeteorology(DHM)dataofNepal.

Theequationsusedforthecalculationof2-yearand100-yearreturnperiodfloodsare:

( ) 8783.0

300028767.1 AQ =

( ) 7342.0

3000100639.14 AQ =

Where, Q2is2-yearreturnperiodflood,

Q100is100-yearreturnperiodflood,

A3000iscatchmentareabelow3000maltitude,

Basedonthealgebraicevaluationsusedforlognormaldistribution,thefollowingrelationshipcanbe

usedtoestimatefloodsatotherreturnperiods.

)exp(ln 2 lf SQQ σ+=

Where,

326.2/)/ln( 2100 QQl =σ

Sisthestandardnormalvariate(Table3-7).

Table3-7StandardNormalVariate

ReturnPeriod(T)inyears StandardNormalVariate(S)

2 0

5 0.842

10 1.282

20 1.645

50 2.054

100 2.326

Hence, the 50-year return period flood (Q50) is estimated as follows:

( )[ ]326.2/)/ln(054.2lnexp 2100250 QQQQ +=


33

(iii) SharmaandAdhikariMethod

( ) 86.030002

29.2 AQ =

( ) 72.03000100

7.20 AQ =

Where,

Q2is2-yearflood

Q100is100-yearflood

A3000isthecatchmentareabelow3000maltitude

Forestimating the floodsofother returnperiods, the relationships shown in theWECSandDHM

methodcanbeused.Hence,the50-yearreturnperiodflood(Q50)isestimatedasfollows:

( )[ ]326.2/)/ln(054.2lnexp 2100250 QQQQ +=

(iv) SCSMethod

TheU.S.SoilConservationService(SCS)methodisapplicableinsmallcatchmentsandiswidelyused

in irrigationprojects for catchmentsbelow100km2. The runoff fromnatural catchmentsmaybe

estimatedusingthefollowingformula:

( )2

4 a

a

IP

IPQ

+

−=

Where,Qistherunoffinmm,

Pisthedesignrainfallinmm,and

Iaistheinitiallossduetoinfiltration,interceptionandsurfacestorage

Theinitialloss(Ia)isdefinedbyacurvenumber(CN)whichisequalto:

Ia=5.08(1000/CN-10)mm

CN represents the hydrological soil group, the type of land use and the antecedent moisture

condition. A range of CN values appropriate to normal antecedent moisture conditions and

applicable Himalayan conditions is presented in Table 3-8. A high value of CN reflects low initial

lossesandcorrespondinglyhighrunoff,whereasalowvalueindicatesdiminishedrunoff.

Thesurfacerunoffcategoriescanbedescribedas:

Rapid-Earlygrowthstageofcrops

- Poorplantdensity

- Poorgroundcover

- Overgrazedareas

Slow-Latergrowthstageofcrops

- Denseplantintensity


34

- Goodgroundcover

- Lushgrazingareas

Themeanslopemaybecalculatedfrom

100×=L

ESlope

δ

Where, Eδ is the difference in elevation (in m) between the highest point on the watershed

nearesttothestreamheadanditsoutfallpointandListhelengthofthemainstream.Theslope

categoriesaregiveninTable3-8.

Table3-8HydrologicalSoilGroupsandCNValues

TypeofSoil InfiltrationRate Class

DeepSandy High A

ShallowSandyandMediumTextured Moderate B

ShallowwithMedium/HeavyTexture Slow C

ClayandShallow/hardpans VerySlow D

Land use or

Cover

SurfaceRunoff HydrologicalSoilClass

A B C D

Fallow

Row Crops e.g.

Maize

Rapid 77 86 91 94

Moderate 72 81 88 91

Slow 67 78 85 89

BroadcastCrops

e.g.UplandRice

Rapid 65 76 84 88

Slow 63 75 83 87

Pasture or

range

Rapid 68 79 86 89

Moderate 49 69 79 84

Slow 39 61 74 80

Woods Moderate 36 60 73 79

Slow 25 55 70 77

Table3-9SlopeCategories

Category RangeofSlope

Flat 0-5

Moderate 6-10

Steep >10


35

Tochoose thedesign rainfalla rangeofgrowth factorscorresponding tovarious returnperiods is

presentedinTable3-10.

Table3-10Growthfactorsformaximumrainfall

ReturnPeriod(Years) GrowthFactor(G)

5 1.3

10 1.5

25 1.8

50 2.0

100 2.3

150 2.4

200 2.5

Note:G=0.32lnT+0.78

Where, GisthegrowthfactorforGumbeldistribution,

Tisthereturnperiodand

lnisthenaturallogarithm

The growth factors are used to adjust themean annualmaximum24 hour rainfall values. As the

rainfall data represents a single location, theymust be reduced according to the areal reduction

factor, which depends on the size of the catchment area in question. Factors for a range of

catchmentareasaregiveninTable3-11.

Table3-11ArealReductionFactorfor24HoursPointRainfall

CatchmentAreakm2 ArealReductionFactor

10 1.00

50 0.99

100 0.98

200 0.96

500 0.91

1000 0.86

5000 0.76

ThestepstodeterminethefloodflowusingtheSCSmethodissummarizedasfollows:

Step1:Chooseacurvenumber(CN)fromTable3-8,takingintoaccounthydrologicalsoilclass,cover

andrunoffcategory.


36

Step2:Findthecatchmentarea(A)inkm2,mainstreamlength(L)andtheslopeinpercentage(δE/L

x100).

Step3:ChooseaslopecategoryfromTable

Step 4 : Choose a design 24-hour rainfall adjusted according to Table 3-10 to give the required

returnperiodandadjustaccordingtoTable3-11forthecatchmentarea.

Curve numbers (CN) of 60, 70 and 80 have been chosen and for each there are three slope

categories:flat,moderateandsteep.

OnceCNvalueandslopecategoryhavebeenchosen,theremainingtaskistolocatetheintersection

of thecatchmentareaordinatewiththedesignrainfallcurve inorder toreadoff thedesignpeak

flood.

3.3.6 FloodEstimationfromTrashMarks

TheTrashmarkmethodisoneoftheconventionalmethodsofestimatingfloodflowwhichisused

whenotherdataandinformationareunavailable.Thismethodisbasedontheslopeareasurveyof

theparticularriverstretchwherepastfloodlevelscanberecordedwiththehelpofoldinhabitants

and/or local informants. Itmaybepossibletofinddebris indicatingapastfloodlevelortohavea

level on a tree or building pointed out. Trash mark levels are more accurate than remembered

levels. A river cross sectional survey is needed at two or three sites where flood levels can be

obtainedandthehydrauliccalculationiscarriedoutusingManning’sformula:

n

SRAQ

2/13/2××

=

Where,Aistheareaofrivercrosssectionuptotrashlevelinm2,

Risthehydraulicradius=A/P

Sisthewatersurfaceslope

nistheManning’sroughnesscoefficientandisgiveninastandardtable(VenTeChow,1959)

Pisthewettedmeterinm,

Thewatersurfaceslopeisthatobtainedfromthetrashmarksatthreedifferentsites.Thefloodflow

canbeestimatedfortwoorthreesitesandthenaveraged.

3.3.7 AssessmentofFloodFlowConsideringClimateChange

Climatechangemay increase the intensityofextreme rainfall and flow rates. Irrigation structures

aredesignedforacertainlevelofexceedenceofrainfall,flowratesandwaterlevels.Theseallare

basednotonlyontheaccurateassessmentofthedataqualitybutalsoadequateknowledgeofthe

consequencesofanexceedence.Hence,adequatedataandknowledgeisessentialduringthedesign

of hydraulic structures such as intakes and cross drainage works. One suggested approach is to

selectahigherreturnperiodfordesignfloodflowassessment,whichisnotalwayseconomical.In

addition, the computeddesign flooddischarge canbe increasedby adding 10 to 20% in order to

assessdesignfloodwithclimatechangeconsiderations.


37

3.4 WaterRequirementAssessment

3.4.1 CropWaterRequirements

Thecropwaterrequirement isdefinedasthequantityofwaterutilizedbytheplantduring its life

time. It is estimated with the evapotranspiration (ETo) of a reference crop. The steps used to

calculatethecropwaterrequirementareasfollows:

i. Collectclimaticdatafromnearbymeteorologicalstation,

ii. Decidecroppingpattern,

iii. Calculatereferencecropevapotranspiration(ETo)(fromFAOmethodsandtables),

iv. Usecropcoefficients(fromFAO),

v. CalculateevapotranspirationofcropsETcrop(mm/day)

vi. Allowforlandpreparationloss(riceandwheatonly)(Lp),

vii. Allowfordeeppercolation(riceonly)(dp),

viii. Calculatetotalcropwaterrequirements(CWR),

ix. Calculateeffectiverainfall(Pe),

x. Calculatenetcropwaterrequirements(netCWR),

3.4.2 Climaticdata

In order to calculate the reference crop evapotranspiration (ETo), climatic data shall be collected

from thenearest andmost representativemeteorological stations. The required climaticdataare

longtermmonthlyminimumandmaximumairtemperatures,monthlyrainfall,relativehumidityor

vapour pressure, wind speed and sunshine duration. In addition, locational information such as

latitude,longitudeandaltitudearenecessary.

3.4.3 CroppingPattern

Theproposedcroppingpatternfor irrigationschemesshouldbebasedonagro-climaticconditions

andfarmers’preferences.ThepossiblecroppingpatternspracticedinBhutanarepresentedinthe

tablebelow(Table3-12).

Table3-12PossibleCroppingPatternsofBhutan

S.

N

Agro-ecologicalZones Dates

Crops Seedbed Transplant/Plant Harvest

1 Cooltemperate(2600mto3600mamsl)

i Potato March August

ii Maize March October

iii Wheat November June

iv Buckwheat September June

2 Warmtemperate(1800mto2600mamsl)

i Paddy April/May May/June October

Fallow

ii Paddy April/May May/June October

Legumes/Oilseeds December April

iii Paddy April/May May/June October

Wheat Nov/December May


38

S.

N

Agro-ecologicalZones Dates

Crops Seedbed Transplant/Plant Harvest

iv Potato December April

Vegetables April August

V Orchard Roundtheyear

3 DrySub-tropical(1200mto1800mamsl)


Fallow


Wheat Nov/December May


Vegetables/Legumes November February/March

iv Paddy April/May May/June October

Potato November March/April

v Orchard Roundtheyear

4 Humidsub-tropical(600mto1200mamsl)


Fallow


Wheat December April/May


Potato November March

iv Paddy April/May May/June October

Vegetables/Legumes January April

v SpringMaize February June

Paddy May/June June October

vi Orchard Roundtheyear

5 Wetsub-tropical(150mto600mamsl)


Fallow


Paddy January February May/June


Wheat Nov/December April

iv Springmaize February June

Paddy April/May May/June October

v Paddy April/May May/June October

Potato December April

vi Paddy April/May May/June October

Vegetables/Legume November February/March

Source:DerivedfromDOAinformation,2015


39

3.4.4 ReferenceCropEvapotranspiration(ETo)

The Reference Evapotranspiration (ETo) represents the potential evaporation of a well-watered

grasscropknownasthereferencecrop.Dependingupontheavailabilityofclimaticdata,thereare

fourmethods for calculatingEToofwhich themostwidelyusedmethod is thePenman-Monteith

method. The FAO has developed software to calculate ETo, the latest version is Cropwat-8. This

softwareisuser-friendlyandeasilyavailableonFAOwebsites.AworkedoutexampleofCrowat-8is

presentedhere(Table3-13).

Table3-13WorkedoutexampleofCropwat-8

3.4.5 CropCoefficient(Kc)

The crop coefficient (Kc) is the experimentally determined ratio of crop evapotranspiration to

reference crop evapotranspiration (ETc/ETo). The crop coefficient considers the leaf anatomy,

stomata characteristics, aerodynamic properties, and albedo of the crop under the same climatic

conditions.Inaddition,thecropcoefficientfactor(Kc)servesasanaggregationofthephysicaland

physiologicaldifferencesbetweencropsandthereferencedefinition(FAO,1998).Thefactorsthat

determine theKc are crop type, climate, soil evaporation, and crop growth stages. TheKc values

varywithcropgrowingstages,whichare:

• Initialstage,

• Cropdevelopmentstage,

• Mid-seasonstage,and

• Lateseasonstage

Cropcoefficients(Kc)ofvariouscropsproposedinthismanualarederivedfromtheFAOIrrigation

andDrainageManualno56andarepresentedbelow(Table3-14).


40

Table3-14CropCoefficientsofSelectedCrops

Crops Initialstage Cropdevelopment

stage

Mid-season

stage

Lateseasonstage

Maize 0.45 0.80 1.05 0.80

Rice 1.10 1.10 1.05 0.95

Wheat 0.43 1.05 1.15 0.40

Potato 0.42 0.80 1.15 0.77

Mustard 0.45 0.85 1.00 0.72

Legumes 0.40 0.95 1.05 0.95

Vegetable

(winter)

0.28 0.54 0.95 0.89

Vegetable

(summer)

0.34 0.93 1.05 0.91

Orange 0.85 0.85 0.85 0.85

Apple 0.80 1.00 1.10 0.80

Source:DerivedfromFAO,1998

3.4.6 CropEvapotranspiration(ETcrop)

Cropevapotranspirationiscalculatedbymultiplyingthereferencecropevapotranspiration(ETo)by

thecropcoefficients(Kc)oftheparticularcrop.AllofthecalculationsaretabulatedonExcelsheets

on15daysbasisoftheyear.

KcEToETcrop ×=

WhereETcropisthecropevapotranspirationinmm/day,

EToisthereferencecropevapo-transpirationinmm/day,and

Kcisthecropcoefficientsoftheparticularcropforgivenastageofcropdevelopment

3.4.7 LandPreparationRequirement

Sincelandpreparationonpaddyfieldsisdoneduringthedryperiod,somewaterisrequiredforthat

work. For non-paddy crops,much lower landpreparation requirements are assumed, and can be

metbysoilmoisturestorage,except inthecaseofwheat, forwhich60mmisappliedto improve

germination. Landpreparation requirements forpaddyovera15-dayperiodarepresentedbelow

(Table3-15).

Table3-15LandPreparationRequirements

15-dayperiods 1st 2

nd 3

rd 4

th

Pre-monsoonpaddy 75 75 50 50

Followpaddy 55 50 50 -


41

15-dayperiods 1st 2

nd 3

rd 4

th

Monsoonpaddy 55 55 50 50

Wheat 60 - - -

3.4.8 DeepPercolationLosses

Deeppercolationlossesareconsideredonlyforpaddyriceanddependonsoiltypes(Table3-16).In

addition, evaporation losses from the paddy field are also considered based on pan evaporation

dataofeachlocation.

Table3-16EstimatedDeepPercolationLosses(mm/day)

SoilTexture NewlyIrrigated LongtermIrrigated

Sand,loamysand >20 >20

Sandyloam 20 10

Veryfinesandyloam,loamsilty

loam,sandyclayloam

20 5

Siltyclayloam,clayloamsiltyclay,

clay

5 2

3.4.9 TotalCropWaterRequirements(mm/15day)

The Total CropWater Requirement (CWR) is the sum of crop evapotranspiration (ETcrop), water

requirementforlandpreparation(Lp),lossesduetopercolation(Dp)andlosesduetoevaporation

(Ep).

CWR=ETcrop+Lp+Dp+Ep

3.4.10 ReliableRainfallandEffectiveRainfall

Rainfall contributes to a greater extent in satisfying cropwater requirement depending upon the

location. Howmuchwaterwill come from rainfall and howmuchwill be covered by irrigation is

unfortunately difficult to predict as rainfall varies from season to season and year to year. In

addition,notalloftherainthatfallsonthefield isusedbytheplant.Theamountofrainfallused

effectively for plant growth is termed effective rainfall, and is calculated from the 80% reliable

rainfallofthearea.

80% Reliable Rainfall: The amount of rainfall which can be depended on in 1 out of 5 years,

correspondingtothe80%probabilityofexceedence,istermedReliableRainfall.TheFAOsuggests

using80%reliablerainfallforirrigationsystemdesign.Rainfallswith20%,50%and80%probability

ofexceedencearedefinedaswetyear,normalyearanddryyearrainfallrespectively.Tocompute

dryyearrainfall (80%reliable) fromthetimeseriesmeteorologicaldata, the followingprocedures

havebeenadopted:

i. Tabulateyearlyrainfalltotalsofthegivenperiod,

ii. Arrangedataindescendingorderofmagnitude,

iii. Tabulateplottingpositionsaccordingtothefollowing:


42

1

100

+

×=N

mFa

Where,Faistheplottingposition,

mistheranknumber,

Nisnumberofrecords

iv. Plotvaluesinlog-normalscaleandobtainlogarithmicregressionequation(Figure3-1),

v. Calculateyearvaluesof80%probability(P80),

vi. Determinemonthlyvaluesofdryyearaccordingtothefollowingrelationship:

Pav

PPP

dry

iavidry ×=

Where,Pidryisthemonthlyrainfalldryyearformonthi,

Piavistheaveragemonthlyrainfallformonthi,

Pdryistheyearlyrainfallat80%ofprobabilityofexceedence

Pavistheaverageyearlyrainfall,

Figure3-1WorkedoutExampleofReliableRainfall

Thepartoftherainfallwhichiseffectivelyusedbythecropafterrainfalllossesduetosurfacerunoff

anddeeppercolationistermedasEffectiveRainfall(Peff).TheFAOdevelopedsoftware,Cropwat-8,

hasdifferentoptionsforcomputingeffectiverainfall.ThemostwidelyusedmethodistheUSDASoil

ConservationServicemethod.Aworkedoutexampleof80%reliablerainfallandeffectiverainfallof

theParoDCSstationispresentedhere(Table 3-17).

Dry(80%)


43

Table3-17Workedoutexampleofreliableandeffectiverainfall(mm)

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Average

Rainfall

7.4

15.1

20.0

32.1

55.6

83.3

136.9

118.6

89.1

52.2

1.7

2.8

P80

Reliable

5.7

11.6

15.4

24.7

42.8

64.2

105.5

91.4

68.7

40.2

1.3

2.2

Effective

Rainfall 5.6 11.4 15.1 23.7 39.9 57.6 87.7 78.0 61.1 37.7 1.3 2.2

3.4.11 NetCropWaterRequirements(mm/15day)

Thenetcropwaterrequirement(NetCWR)iscalculatedasthedifferencebetweentotalcropwater

requirementsandeffectiverainfall(Pe).

NetCWR=CWR–Pe

Where,NetCWRisthenetcropwaterrequirementinmm/15dayandPeistheeffectiverainfallin

mm.

Hence, the net crop water requirement for paddy crops is crop evapotranspiration plus land

preparation requirements (including evaporation from land preparation) plus deep percolation

minus effective rainfall. The net crop water requirement for dry root crops is the crop

evapotranspiration plus, in the case of wheat, land preparation requirements minus effective

rainfall.

3.4.12 CanalSystemEfficiencies

During the irrigation process, considerable water loss occurs through evaporation, seepage and

deeppercolation.Theamountlostdependsupontheefficiencyofthesystem.Inordertocalculate

the intake water requirements, field irrigation losses, distribution canal losses, and conveyance

canallossesmustbetakenintoaccount.

(i) Fieldefficiency(Fe):Irrigationfieldapplicationefficiencyisexpressedastheratioofwater

storedintherootzonetothewaterapplied.Whenirrigatingdryrootcrops,itisdifficultto

apply thewater evenlyover thewhole field. To allow for this, a field efficiencyof 60% is

assumed.Forrice,astheirrigationisbasedonfloodingfieldefficiencycanbeassumedto

be75%to85%toallowforlimitedrun-offandoverspill.

(ii) Distribution efficiency (De): The distribution efficiency is the efficiency of the water

distributioncanalsandconduitssupplyingwaterfromtheconveyancesystemtoindividual

fields. Irrigationwater is lost along the field channels from the off-take structures due to

seepage, percolation, evapotranspiration, evaporation, and/or leakage. Distribution

efficiencyisconsideredtobe70%to80%dependinguponthetypeofdistributionsystem.

(iii) Conveyanceormaincanalefficiency(Ce):Irrigationwaterisalsolostalongdeliverycanals

suchasthemaincanalandthesecondary/tertiarycanalfromtheintaketotheoff-takesof

distribution system. It is causedby seepage,percolation, evapotranspiration, evaporation,

leakageand/oroverdiversionofirrigationwater.Conveyanceefficiencyisconsideredtobe

70%to80%.


44

Overallirrigationsystemefficiencyrangesfrom29%forsmallscalehillirrigationschemesto54%for

well managedlargeirrigationsystems.

3.4.13 IntakeWaterRequirement

Theintakewaterrequirementisthemaximumfieldcropwaterrequirementdividedbythesystem

efficiencies.

TheFieldCropWaterRequirement(l/s/ha)=NetCWR(mm/15day)/(Fe*129.6)

Where, 129.6 is the conversion factor to convert mm/15day into l/s/ha, since 8.64mm/day = 1

l/s/haandFeisfieldefficiency.

IntakeWaterRequirement(IWR)=FieldCWR/(Cse),l/s/ha

Where,Cseisthecanalsystemefficiency,whichmaybetakenas30%to50%.

The detailed worked out example of the crop water requirement is presented hereunder (Table

3-19).

3.4.14 Considerationofclimatechangeincropwaterrequirement

Considerationofthepotential impactsofclimatechangeoncropwaterrequirementssuggestthat

farmoperationsshouldbesuitedtothelocalconditionsinordertoreducetheriskofcropdamage.

Thismayrequireadjustingthetimingofcropplantingandharvestingdates,changingthecropping

intensity, and proposing crop varietieswith lowwater requirements andwhich can tolerate heat

anddrought.Inaddition,irrigationwaterrequirementscouldbeminimizedbyincreasingirrigation

system efficiency and using crop coefficients on the lower side of the given range. Furthermore,

deficitirrigationshallalsobeintroducedincaseofacuteclimatechangeconditions.

3.4.15 IrrigationWaterDuty

The duty of irrigation water is the amount of water required for irrigation of a unit of land for

specificcropataspecifictime.Theirrigationwaterdutydependsupon:

• Typeofsoil,

• Percolationrateofsoil,

• Rainfall,and

• Croppingpractice

In Bhutan, the peakwater requirement occursmostly during the landpreparation period in June

andrangesfrom1.6l/s/hato4.3l/s/ha(Table3-18).

Table3-18Irrigationwaterduty

Soiltype Clay Loam Sandyloam

Percolationrate(mm/day) 0-5 5-10 10-15

Rainfall<2,500mm/year Irrigationduty

(l/s/ha)

2.4 3.3 4.3

Rainfall>2,500mm/year 1.6 1.9 2.2

Source:IrrigationEngineeringManual,1998


45

Table3-19Workedoutexampleofcropwaterrequirement

Project: Irrigation Master Plan Location: Paro DCS Cropping pattern: Rice Potato

Month

Half Month 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2

Cropping Pattern Potato Rice Potato

Rice---Potato 20%

ET0 mm/day 1.46 1.46 1.96 1.96 2.53 2.53 3.02 3.02 3.34 3.34 3.20 3.20 3.14 3.14 3.14 3.14 2.83 2.83 2.54 2.54 1.84 1.84 1.37 1.37

Crop coefficient (Kc) 1.01 1.13 1.13 1.08 0.94 0.77 1.10 1.10 1.10 1.10 1.05 1.05 1.05 0.95 0.95 0.95 0.42 0.55 0.79

ETcrop mm/day 1.47 1.65 2.21 2.12 2.38 1.95 0.00 0.00 0.00 3.67 3.52 3.52 3.45 3.30 3.30 3.30 2.69 2.69 2.41 0.00 0.00 0.77 0.75 1.08

ETcrop (mm/half month) 22.86 25.57 31.01 29.64 36.86 30.20 0.00 0.00 0.00 56.95 52.80 52.80 53.54 51.10 51.10 51.10 40.33 40.33 37.40 0.00 0.00 11.59 11.68 16.78

Land preparation (mm) 55.00 55.00 50.00 50.00

Deep percolation (mm) 75.00 75.00 75.00 75.00 75.00 75.00 75.00 75.00 75.00 75.00

Pan evaporation (mm/day) 5.00 5.00

Pan evaporation (mm/15day) 75.00 75.00

Total crop water requirement 22.86 25.57 31.01 29.64 36.86 30.20 0.00 0.00 0.00 261.95 257.80 177.80 178.54 126.10 126.10 126.10 115.33 115.33 112.40 0.00 0.00 11.59 11.68 16.78

(mm/half month)

80% Reliable Rainfall 2.57 3.32 4.03 5.09 6.49 8.18 10.16 13.41 17.93 22.44 26.97 36.99 52.49 55.10 44.81 37.52 33.22 27.75 21.08 14.10 6.82 2.73 1.83 1.77

Effective Rainfall 0.00 0.00 0.00 0.00 0.00 0.43 1.28 3.05 5.75 8.46 11.19 18.46 30.29 32.08 23.83 18.19 15.16 11.65 7.65 4.24 1.41 0.00 0.00 0.00

Net irrigation water requirement

mm/half month 22.86 25.57 31.01 29.64 36.86 29.77 0.00 0.00 0.00 253.48 246.61 159.34 148.25 94.03 102.28 107.92 100.17 103.68 104.75 0.00 0.00 11.59 11.68 16.78

Overall system efficiency 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30

Total water requirement

(mm/half month) 76.2 85.2 103.4 98.8 122.9 99.2 0.0 0.0 0.0 844.9 822.0 531.1 494.2 313.4 340.9 359.7 333.9 345.6 349.2 0.0 0.0 38.6 38.9 55.9

Water requirement of the

system (m3 per ha) 762 852 1034 988 1229 992 0 0 0 8449 8220 5311 4942 3134 3409 3597 3339 3456 3492 0 0 386 389 559

Water requirement (l/s/ha) 0.59 0.66 0.80 0.76 0.95 0.77 0.00 0.00 0.00 6.52 6.34 4.10 3.81 2.42 2.63 2.78 2.58 2.67 2.69 0.00 0.00 0.30 0.30 0.43

Monthly water requirement

(l/s/ha) 3.12 2.70 2.62 1.35 0.15 0.370.62 0.78 0.86 0.00 3.26 5.22

Jul Aug Sep Oct Nov DecJan Feb Mar Apr May Jun

Total Water Requirement Computation


46

3.5 WaterBalanceAssessment

3.5.1 General

Water balance is the difference of inflows and outflows at the particular point of river diversion.

Inflows are rainfall and runoff while outflows are water demand for irrigation, household water

supply, evaporation and evapotranspiration from vegetation. The water balance is calculated at

eachdiversionpointinordertoascertaintheamountofavailablewaterthatcouldbeusedinthe

proposedirrigationschemewithoutimpactingtheexistingusesofwater.Themaincomponentsof

thewaterbalanceareavailableriverflowattheintakesite,thewaterrequirementforirrigationas

per the proposed cropping pattern and intensity, and downstream mandatory releases for

householdsupplyandenvironmentalflow.

3.5.2 AvailableFlowattheIntakeSite

The available river flow at the intake site is assessed according to themethodology described in

section3.2ofthischapter(Chapter-3).Fromtheavailableflow,the80%reliableflowisassessedfor

diversion to the irrigationschemeunderconsideration.Theavailable flowat the intakesite is the

balanceoftheavailable80%reliableflowminusmandatoryreleasesforupstreamanddownstream

existinguses,includingenvironmentalflow.

3.5.3 ExistingDownstreamRequirements

Existing water uses need to be considered while designing the irrigation scheme. The users and

farmersusing thewater for irrigationand/ordrinkingwater in thevicinityof theproposed intake

siteshouldfirstagreetothedevelopmentoftheproposedirrigationscheme.Thedesigner/planner

needstoassessallofthewithdrawalsofwaterupstreamanddownstreamoftheproposedintake

site.Theexistingwaterwithdrawalscanbeassessedbymeasuringtheflowatitsintakesiteandby

consultingthecommunity.Forthemandatoryenvironmentalflow,atleast10%ofthenon-monsoon

flowshallbereleasedfromtheproposedintakesite.

3.5.4 WaterBalance

Theavailablewaterat thesourcestream/rivershouldmeettherequirementsofallexistingwater

uses, proposed intake water requirements, and the mandatory downstream flow. If the water

balanceispositiveintermsofmeetingtheserequirements,theschemeistechnicallyfeasiblefroma

water availability point of view. If the water balance is negative, it is essential to change the

croppingpatternsand/orreducethecommandareaoftheproposedschemeasitisnotpossibleto

changetheexistingwaterrequirementsorthemandatorydownstreamflow.

3.6 DischargeMeasurement

3.6.1 General

Theamountofwaterpassingatapointonthestreamorchannelduringagiventimeisafunctionof

thevelocityandcross-sectionalareaandisexpressedbytheContinuityequation:Q=AV,whereQis

the stream discharge, A the cross-sectional area, and V the flow velocity. The discharge

measurement concerns themeasurement of the flow velocity and cross sectional area. For small

irrigation schemes, threemethodsareappropriate for thedischargemeasurement: floatmethod,

bucketandwatchmethod,andcurrentmetermethod.


47

3.6.2 Floatmethod

Thefloatmethodmeasuresthesurfacevelocityofflowingwater,andthemeanvelocityisobtained

using a correction factor. The basic idea is to measure the time taken to float an object over a

specifieddistance.

Vsurface=traveldistance/traveltime=L/t

Because surface velocities are typically higher thanmean or average velocities, V mean = k Vsurface

wherekisacoefficientthatgenerallyrangesfrom0.65forroughbedsto0.75forsmoothbeds.The

proceduresfordischargemeasurementareasfollows:

• Step1Chooseasuitablestraightreachwithminimumturbulence(minimumdistanceshouldbe

20m);

• Step2Markthestartandendpointsofthechosenreach;

• Step3Ifpossible,traveltimeshouldexceed20secondsbutbeatleast10seconds;

• Step4Dropyourobjectintothestreamupstreamoftheupstreammarker;

• Step5Startthewatchwhentheobjectcrossestheupstreammarkerandstopthewatchwhen

itcrossesthedownstreammarker;

• Step6Repeatthemeasurementatleast3timesandusetheaverageinfurthercalculations;

• Step7Ifyouonlydothefloatmethod,youneedtomeasurethecross-sectionalareasatthe

startandendpointofyourreach;

• Step8Averagecross-sectionalareas;

• Step9Usingtheaverageareaandcorrectedvelocity,youcannowcomputedischarge

Q=AVmean

Figure3-2Dischargemeasurementwithfloatareamethod

3.6.3 BucketandWatchmethod

The bucket and watch method is a simple volumetric measurement of water at the given time

period.Thismethodisapplicableforspringsourceswithadischargelessthan1lps.Thevolumeof

watercanbemeasuredbydivertingtheentireflowintoameasuringvesselofknowncapacity(for

example,aplasticbucket).Thetimetakentofillthevesselisnoted.Atleastthreetofivereadings

are necessary for themeasurement of the water and average value is computed. The discharge

calculationmaybecarriedoutusingthefollowingformat(Table3-20).


48

Table3-20DischargeMeasurementwithBucket-WatchMethod

Projectname: Location:

Sourcename: Dateofmeasurement:

Description Measurementnumber

A Vesselcapacity(liter) 1 2 3 4 5

B Timetofillthevessel(sec)

C Discharge(l/s)=A/B

D AveragedischargeQ(l/s) Q=(C1+C2+C3+C4+C5)/5

3.6.4 CurrentMeter

A current meter is a device used to measure the velocity of flowing water. It has a propeller

mountedonashaftwhichallowsthedevicetomovefreelyregardlessofthedepthofthewater.

Thespeedofrotationisthefunctionofthevelocityofwater.Themanufacturerofthecurrentmeter

providesthecorrelationbetweenthenumberofrotationsandwatervelocity.Themeasurementof

velocity ismore accurate and reliable and the currentmeter is extensively used in streamswith

uniformcrosssections.Thismethodisusefultomeasurevelocitiesintherangeof0.2m/sto5m/s.

Theproceduretomeasurethevelocityusingacurrentmeterisasfollows:

• Chooseasuitablesiteforthemeasurement;

• Secureameasuringtapeacrossthesection;

• Measurethechainageatbothbanksandcalculatethestreamwidth;

• Dividethestreamwidthintosections;

• Ifthereisstaffgauge,takeagaugereadingatthestartoftheflowmeasurement.Checkatthe

endthatthemeasurementisthesame;

• Measuretheflowinthecenterofeachsection:

o Measurethewaterdepth,

o Ifthedepthislessthanorequalto0.3mmeasurethevelocityat0.6d,

o Ifthedepthofwaterismorethan0.3mmeasurethevelocityat0.2dand0.8d,

• Calculatethevelocityfromthemachinemanufacturerstable;

• Calculatetheflowareaofeachsection;

• Calculatethedischargeineachsectionandsumdischargestogivethetotals,

• ThetotalstreamdischargeQ=a1v1+a2v2+a3v3+……+anvn


49

CHAPTER-4

4. IntakesandHeadworks

4.1 Introduction

Intakestructuresarehydraulicdevicesbuiltattheheadoftheirrigationcanals.Theirrigationcanals

in the context of the small scale irrigation include onlymain canals. The intakes formediumand

largeirrigationschemesaredifferentandbeyondthescopeofthismanual.Themainpurposeofan

intakeistoallowtheabstractionofwaterfromthesourcewithaminimumsedimententry.There

areanumberofdifferenttypesof intakes;theselectionofthemostappropriatetypedependson

thelocation,scaleandfunctionoftheirrigationscheme.Intakescanbefromrivers,reservoirs,and

lakes.

Any hydraulic structure which supplies water to the off-taking canal is called a headwork. A

headworkmaybeastorageheadworkoradiversionheadwork.Astorageheadworkcomprisesthe

constructionofadamontheriver,andstoreswaterwhenthereisexcesssupplyandreleaseswater

according to irrigation demands. A diversion headwork serves to divert the required amount of

water to the canal from the river. This manual covers the design of side intakes and diversion

headworks.

4.2 TypesofIntakes

Typesof intake structures aremainly distinguishedby themethodused todivertwater from the

sourceriver:

• Bankintakes,

• Sideintakeswithcrossweirs,

• Canaldiversionworks,

• Bottomintakes,

• Frontalintakes,

• Submergedintakes,and

• Towerintakes

4.2.1 BankIntake

Bankintakesarelocatedonthebanksoftheriverandtheirfacesarealignedwiththebanks.Bank

intakesareappropriateforirrigationwherewaterfluctuationintheriverislowandasmallportion

oftheriverflowisabstractedtothecanal.The inflowintothe intakestructuredependsuponthe

waterlevelintheriver.Inexistingcommunitymanagedschemes,temporaryarrangementsareused

toraisethewater level.Theintakes,whichareattheriverbedlevel,havecoarsescreens,control

gates and settling chambers to trap coarsematerial. Bank intakes are referred as side intakes or

lateralintakes.

4.2.2 Sideintakewithcrossweir

Sideintakeswithcrossweirareessentialforriversandstreamswhereasubstantialportionofthe

flow is tobediverted.Theweir issituated intheriverwhichdamsupthewater level toensurea

constantminimum depth of water upstream of theweir and allowswater to be diverted to the


50

canalsystem.Basedonthematerialusedfortheirconstruction,weirsaremasonryweirs,concrete

weirs,orwoodenweirs.

4.2.3 Canaldiversionworks

Canal diversion works are weirs or barrages constructed across the rivers to maintain adequate

levelstoabstractwatertowardsthemaincanal.Barragesareconstructedacrossthemainriverat

theheadworkofthenewcanals.Barragesandassociatedheadworksconsistofanundersluicefor

sluicingsedimentsandaheadregulatortocontroltheflowofwatertothemaincanal.

4.2.4 Bottomintake

Bottom intakes are located on the river bed and the water to be diverted is taken through a

collectionchamberbuiltintheriverbottomcoveredwithtrashscreens.Thebarsofthescreenare

laid in the direction of river flow and inclined in a downstreamdirection. The coarse sediment is

keptoutofthecollectionchamberandtransportedfurtherdownstream.Particleswhicharesmaller

thantheopeningsofthescreenbarsareintroducedintothecollectionchambertogetherwiththe

water, and are excluded from the settling basin through a flushing device constructed a short

distancedownstream.Thebottomintakecanbeconstructedatthesamelevelastheriverbedorin

theformofasill.

4.2.5 Frontalintake

Frontal intakes are designed to abstract clear water from mountain rivers. These intakes are

successfully utilized in Turkey. The intake has two layers: the upper layer draws clear water for

irrigationandthelowerlayercontinuouslyflushesthesedimentinfrontoftheintake.

4.2.6 Submergedintake

Asubmergedintakeislocatedatthebottomoftheriverorreservoir.Itcomprisesabellmouthand

bend set in a concrete block with a concrete draw-off pipe. The inlet is protected from the bar

screenandsethighenoughabovethebedtoallowsedimentstopassoneithersideoftheinlet.This

typeofintakeisoftenusedindrainageoutletofsmallreservoir.

4.2.7 Towerintake

Towerintakesarefreestandingstructures,setindeepwaterconnectedbyanaccessbridgetothe

bank.Tower intakesareused inoutletworksof the reservoirswith thearrangementsofgatesor

valves.

Irrespectiveofwhether the intakestructuresareselectedwithorwithoutdamming the river, the

structuresshouldbedesignedsothat:

• Attimesoflowestdischarge,therequiredamountofwatercanalwaysbediverted,

• Allfloodscanbeevacuatedwithoutdamageofthestructures,and

• Theamountofwaterflowingintothecanalislimitedtotheamountofwatertobediverted.

The selection of the type of intakes dependsmainly of the objective of the project, specific site

conditions, and the requirements of the irrigation scheme. The general criteria that affect the

selectionoftheintake/headworkare:


51

• Effectivenessinexcludingsedimententryintothecanal,

• Easeandrelativecostofstructure,

• Effectoffloodsintermsofdamagerisk,

• Effectoffloodsonsedimententry,and

• Flowcontroltothecanalsystem

4.3 SelectionofSiteforIntakesandheadworks

The site for the headwork or intake should be selected to ensure that the required flow can be

diverted to the canal at all stages of the river. The main considerations in the selection of the

headworkorintakesiteareasfollows:

• Channel dimensions- narrow floodwaywith a relatively straight approach reach, enabling the

rivertodeveloplinearflow,

• Thalwegdeflection-themainchannelwithlowflowisreferredasthethalweg.Thestabilityof

thethalwegisofprimeimportanceinselectingsuitableintakesites,

• Outersideofabend-secondarycurrentsdevelopontheoutersideofbendswhichreducethe

entryofbedloadintotheintakeanddeflectthebedcurrentsawayfromtheintake(Figure4-1),

• Confluence of two rivers- the most suitable location for an intake is downstream of the

confluenceoftworivers,

• Boulderdeposits-thelocationofbigboulderdepositisstableinsteeprivers,

• Bankstability-bankswith rockoutcrops,orarmoredboulderbanksprotecting the intake from

erosion,

• Riverslope-intakelocationisstableatpoolreachoftheriverslope,

• Stablegeologicalsection-intakeisstablewheregeologyisstable.

Figure4-1Layoutplansofsideintakes


52

4.4 SideIntake

4.4.1 DesignConcept

A side intake is an opening on the bank of a river which extracts water from the river for the

irrigationcanal.Sideintakesarelessexpensivethanothertypesofintakesandarelesscomplicated.

Side intakesmust be correctly located formaximum safety and effectiveness. Theymust be safe

fromboulder impactandfloodwaterentry.Themostappropriate locationforaside intake is the

outerbendoftheriverornearthenaturalpools(Figure4-2).Sideintakesaresensitivetoriverflood

waterlevels.

Figure4-2IdleLocationsforSideIntakes

4.4.2 DesignProcedure

In itssimplestform,aside intakeconsistsofanungatedorifice inamasonrywall.Thedesignofa

sideintakeinvolvesthesizingofanorificeandthedesignofalinkcanal.Thedesignproceduresare

asfollows:

Step-1 Select a suitable opening size in relation to the required discharge. For rectangular

opening, the size width to depth ratio should be 2:1. The size of the orifice is determined

considering the low flow of the river and needs to be checked to ensure safety during high flow

periods.Theopeningsizemustbemoreorlesssimilartothesizeofthelinkcanalthatconnectsthe

intaketothesettlingbasin.Atrashrack isalsonecessarytopreventdebrisfromentering intothe

canal.

Step-2Computedischargeusingformulae:

Freeflowcase:Freeflowcanoccuronlywhenthedownstreamflowdepthinthecanalisequaltoor

lessthansubsequentdepth,h2(Figure 4-3).

⎟⎠

⎞⎜⎝

⎛−×××= 63.02

12/3

a

hgabCCQ vd

Where,Cdisthedischargecoefficient=0.60,

Cvisthevelocitycoefficient=1,

bisthewidthoftheorifice,

aistheheightoftheorifice,

h1isthedepthofwaterupstreamoftheorifice,


53

Figure4-3Freeflowandsubmersedfloworifice

Freeflow Submersedflow

Subsequentdepth,h2isafunctionofFroudenumber,F,

3

3

gh

VF =

[ ]1812

1 2

3

2 −+= Fh

h

Whenthedownstreamwaterdepth inthecanal isgreaterthansubsequentdepth, theorificewill

becomesubmersedandsubmergedflowequationwillapply.

Submersedflowcase:

( )21

2/32 hhgabCCQ vd −×××=

Step-3Checkthemaximumintakeflowathighfloodlevel.Thelinkcanalandautomaticspillofthe

settlingbasinmustbeadequatetocopewiththeexpectedintakeflowatthehighfloodlevelofthe

river. Iftheseconditionsarenotsatisfied,thefollowingoptionsshouldbeconsideredtore-design

theintake.

Option-1Trydifferentcombinationsoforificesize,linkcanalandautospillingcapacityofthesettling

basin;

Option-2Provideacontrolgateattheorifice;

Option-3Provideadoubleorificeintake

Thebasichydraulicdesignforaside intakewilldependonthesizeofsedimentandhenceonthe

desirablenominaltrapvelocity.Thebasicdesignparametersofthesideintakearepresentedbelow

(Table4-1).

Table4-1Velocityandheadlossacrossgraveltrap

Description CoarseSand Fine/medium

gravel

Medium/coarse

gravel

Velocity(m/s)

Nominal trap

velocity

0.15 0.25 0.50


54

Description CoarseSand Fine/medium

gravel

Medium/coarse

gravel

Sillvelocity 0.15 0.25 0.50

Firstorificevelocity 0.25 0.42 0.83

Outletgatevelocity 0.60 0.60 0.60

HeadLoss(mm)

Atsill 3 8 32

Atfirstorifice 8 22 88

Atoutletgate 46 46 46

Totalheadloss 57 76 166

Taketotalheadloss 60 80 170

Source:PDSPManual,1990

4.4.3 DesignExampleofSideIntake

Designabankintakestructureforacanalwithdesigndischargeof50lps.Assumethatthesediment

tobehandledismainlyfine/mediumgravel.

Designdata:

• Canaldesignwaterlevel 101.00m

• Riverminimumlevel 101.20m

• Rivermaximumlevel 103.00m

i)Calculatenettrapcross-section

FromtheTable4-1,nominaltrapvelocity =0.25m/s

Trapcrosssectionalarea(Ate)=0.05/0.25 =0.20m2

Adopting1mwidetrap,thedepthoftrap=0.20/1.0=0.20m

Afurther0.30mdepthisprovidedtoallowforstorage

TraplengthL=10xAte=10*0.20=2.00m

ii)Designofupstreamorifice

FromTable4-1,forfine/mediumgravel,upstreamorificevelocity=0.42m/s

Orificearea(A) =Q/V=0.050/0.42=0.119m2

Lettheorificebe0.60mwideand0.20mdeep,hencethearea(A)=0.12m2

Usingtheorificeformulathelossofheadattheorificeiscalculated.

hgBDQ Δ= 27.0

( )gDB

Qh

249.022

2

×××=Δ

Where,Q=50l/s,B=0.60m,andD=0.20m

Then,Δh=0.018m,sayΔh=0.02m

iii)Designdownstreamorifice

Letoutletgatevelocitybe0.50m/s

Then,orificearea=0.05/0.50=0.10m2


55

Letorificebe0.5mwideand0.20mdeep,

( )gDB

Qh

249.022

2

×××=Δ

Where,B=0.50mandD=0.20m

Then,Δh=0.0025/0.096138=0.026,sayΔh=0.03m

iv)Designofsill

Fromthetablesillvelocity=0.25m/s,

Areaacrossthesill=0.050/0.25=0.20m2

Ifsillwidth=1.0m,thenthedepthofwateroverthesill=0.20/1.0=0.20m

Headlossacrossthesill=g

KV

2

2

Where,Kiscoefficient=2.5,andVisvelocity=0.25m/s

Then,headloss=0.008m

v)Checkminimumriverlevel

Totalheadloss =0.02+0.03+0.01=0.06m

Minimumriverwaterlevel =101.00+0.06=101.06m

Actualminimumriverwaterlevel=101.20m

Thereforethestructurelevelcorrespondingwaterlevelsareacceptable.

vi)Checkstructureformaximumriverlevels

Assumecanalcantake50%extraflow,thatis75lps

Fora50lpscanalwithz=1:1andn=0.025andbedslope1:500,B=0.30m,D=0.25m

Flowdepthinthecanalfor75lpsis0.32m

Thereforemaximumwaterlevelinthecanal=101.00-0.25+0.32=101.07m

Headlossthroughthedownstreamorifice( )gDB

Qh

249.022

2

×××=Δ =0.0.08m

Headlossthroughtheupstreamorifice( )gDB

Qh

249.022

2

×××=Δ =0.02m

Headlossacrosssill(nearly) =0.02m

Totalheadloss=0.08+0.02+0.02=0.12m

Maximumriverwaterlevel =101.07+0.12=101.19m

Actualmaximumriverwaterlevel=103.00m

Astheheadovertheorificeishigh,acontrolgateonthecanalorificeisnecessarytopreventflood

flowfromenteringthecanal.


56

Figure4-4Sideintakedesign

4.5 DoubleOrificeIntake

4.5.1 DesignConcept

Adoubleorifice intakehastwoorifices,oneat theriver intakeproperandasecondorificeat the

endofthelinkcanalorsettlingbasinwithasidespillway.Thefirstorificeisthefirsttocontrolthe

flow.Duringhighstageoftheriverthefirstorificedeliversexcessflowtothelinkcanal.Thisexcess

flowwouldbespilledfromaside-spillwaytowardsthedownstreamendofasectionofthelinkcanal

andsluicessedimentfromtimetotime.Atlowriverlevels,therewillbearelativelysmallsediment

supply; the link canal, if properly sized,will act as a sediment trapwhichmay be cleared out by

removing the stop-logs from the sediment ejection channel. At higher stages, the continuous

operation of the side spillway will lead to a significant proportion of the bed material being


57

transportedthroughthelinkcanal.AtypicalsketchofadoubleorificeintakeispresentedinFigure

4-5.

Figure4-5Typicalsketchofadoubleorificeintake


58

4.5.2 Designprocedure

(i) AssessthedesignwithmaximumallowableflowsQ1andQ2toenterthecanaldownstream

ofthesecondorifice,withcorrespondingwaterlevelsatthecanalentrance,WL1andWL2.

Lettheenergyheads,h1andh2,bethesameasthesewaterlevels,

(ii) Chooseasizeforthesecondorifice,A2,togiveamaximumvelocity,say2m/s i.e.forthe

maximumallowableflowtoenterthecanal,Q2,

(iii) UsingA2, calculateorificehead loss fromgraphatdesigncanal flow,Q1;add toh1 togive

energyhead,h3inthestructure,

(iv) Set the excess flow spillway crest level allowing 0.05 m as free board during normal

conditions,

(v) Asthere isnofriction loss inthechannel,thefirstorificedownstreamenergy levelcanbe

takenash3.Calculatetheheadlossatthefirstorifice(itissuggestedthatbothorificesare

madethesamesize),andcalculatetheenergyhead,h4,upstreamofthefirstorifice(Figure

4-6),

(vi) Compare this energy head loss, h4, with the minimum river level during the irrigation

season. If there is excess available head, the system can delivermore than the required

flow,

(vii) Now considermaximum canal flow,Q2, with second orifice downstream energy level, h2,

assumingthelinkcanalhasscouredout,

(viii) Calculatetheheadlossatthesecondorificeandaddtoh2togiveh3,theenergyheadinthe

structure,

(ix) Calculatetheheadoverthecrestofexcessspillwayusingbroadcrestweirformula,

Thedownstreamenergylevelatthefirstorificecanbetakenash3,

(x) CalculatetheorificeheadlosswithflowrateQmax,addtoh3togiveh4andcomparewith

maximumriverlevel.Iftheriverlevelishigher,therewillbeagreaterinflowandtheside-

weiristooshort.Ifthemaximumriverlevelislower,thecrestlengthoftheweircalculated

aboveismorethanadequate.

(xi) Amend the assumed value of spill water above accordingly and repeat until the head

upstream of the first orifice agrees with maximum water level, or it is obvious that the

design needs to be reconsidered, i.e. by changing one or both orifice sizes, or the ratio

Q2/Q1,ortherelativecanalandriverlevels,etc.

Figure4-6Doubleorificeintake

Fordesignflow Formaximumflow


59

4.5.3 DesignExampleofDoubleOrificeIntake

Designflowincanal(Q1) =250l/s

Maximumallowableflowincanal(Q2) =350l/s

Canaldesignwaterlevel(h1) =502.00m

Canalmaximumallowablewaterlevel(h2) =502.11m(seebelow)

Structurelength =30.00m

Minimumriverlevel =502.50m

Maximumriverlevel =504.00m

Maximumflowenteringstructure =700l/s

Calculations

Foracanalwithadesignflowof250l/s,waterslopeof1:500,n=0.025,

Z=1:1,b=0.45andd=0.45.

Formaximumallowableflowincanalof350l/susingformula:

b

a

b

a

d

d

Q

Q=⎥

⎦

⎤⎢⎣

⎡60.0

60.0

⎥⎦

⎤⎢⎣

⎡×=

b

a

baQ

Qdd

=0.45*(0.35/0.25)^0.60=0.55m

Maximumcanalwaterlevel=502.00+(0.55–0.45)=502.10m

AreaofflowatdesignflowA1=b*d1+m*d1^2=0.45*0.45+1*0.45^2=0.41m2

VelocityatdesignflowVdes=Qd/A1=0.25/0.41=0.62m/s

AreaofflowatmaximumflowA2=b*d2+m*d2^2=0.55*0.45+1*0.55^2=0.55m2

VelocityatmaximumflowVmax=Qmax/A2=0.35/0.55=0.64m/s

Fordesigncanalflows

h1=WL1=502.00m

Formaximumallowablecanalflows

H2=WL2=502.10m

Assumemaximumvelocityof2m/sthroughorifice(i.e.formaximumallowableflowincanal).

Formaximumallowableflow,Q2=0.35m3/s

A2=0.35/2.0=0.175m2

Letorificedimension: Widthbe0.45mandthedepthbe0.40m

Theorificewidthissameasthatofcanalbedwidthandthedepthallowsadequatesubmergenceof

theorificeatallflows.

Fordesigncanalflows

ForQ1=250l/s,B=0.45m,D=0.40m

Headloss,

( ) gDB

Qh

270.02

2

×××=Δ

=0.20m

Waterleveld/soffirstorifice,h3=h1+Δh=502.00+0.20=502.20m

Toallowafreeboardof0.05m,makecrestlevelofexcessflowspillway502.25m

AssumeinletorificeissamesizeasoutletorificeB=0.45mandD=0.40m

Headloss


60

( ) gDB

Qh

270.02

2

×××=Δ

=0.20m

Waterlevelu/soffirstorifice,h4=h3=∆h=502.20+0.20=502.40m

Checkforminimumriverlevel,h4islessthanminriverlevelof502.50m(h4<NWL)

Riverlevelisadequate

Formaximumallowablecanalflow,Q2of350l/s

H2=502.10(seestep(i)above)

Formaximumcanalflowsheadloss

( ) gDB

Qh

270.02

2

×××=Δ

=0.39m

Waterleveld/soforificeatmaximumflow

h3=h2+Δh=502.10+0.39=502.49m

Headacrossexcessflowspillway=waterd/soffirstorifice–spillwaycrestlevel

hw=502.49–502.25=0.24m

Assumeflowenteringfromtheriverinflood,

Qf=2xQ2=2*350=700l/s

AmountofflowtobespilledQspill=700-350=350l/s

Lengthofspillwaycrest,LW=Qspill/(1.4*h3/2)=0.35/1.4x(0.24)1.5

=2.08msay2.20m

Formaximumflowenteringfromriver

ForA=700l/s,B=0.45m,D=0.40mheadloss,∆h

( ) gDB

Qh

270.02

2

×××=Δ

=1.57m

Waterlevelu/soffirstorificeatmaximumflow=h3+∆h=502.49+1.57=504.07m

Maximumriverlevel=504.00mwhichislessthan504.07m

Thereforestructuredesignisacceptable.

4.6 BottomIntakeorTrenchIntake

4.6.1 DesignConcept

Abottomintakeortrenchintakeislocatedintheriverbedthatdrawsoffwaterthroughracksinto

thetrenchwhichconveystheflowtothemaincanal.Themaincharacteristicofthebottomintakeis

that it hasminimum impacts on river levels. This typeof intake ismostly suitable in hilly regions

whererivershavelowsedimentconcentration.Themaincomponentsofabottomintakeare:

• Weirtocontrolminimumriverflowsandtomaintaintheflowtowardstherack,

• Steelracktoextractwaterfromtheriver,

• Bottom trench immediately below the rack to lead the water extracted by the rack into the

canal,

• Floodcontrolgateimmediatelydownstreamofthetrench,

• Feedercanaltocarryallsedimententeringtheintaketothesettlingbasin,

• Settlingbasinwithasidespillandsedimentflushingarrangement,


61


DataRequired

• Maximumdiversiondischargerequired(Qc),

• Designflooddischarge(Qd),

• Existingwidthoftheriver(P),

• Bardiameter(a),

• Barspacing(b)(asperthedesirablesedimentsizerequiredtobeeliminated),

• Approachflowdepth(ho),

The design procedure of the bottom intake is related to the design of the size of the trench and

collectionchamberincludingsedimentflushingarrangements(Figure 4-7).

i. Selecttheheightoftheweir(P)abovetheriverbed

ii. Fixthelengthoftheweirbasedontheexistingwidthoftheriver,

QdP 75.4=

WherePistheLacey’swaterwayinmand

Qdisthedesignflooddischargeinm3/s,

iii. Calculatetheheadovertherackcrestusingtheformula,

okhh3

2=

Wherehoistheapproachflowdepthinm,andkisconstantdependingupontheinclinationof

thetrashrack(β),

β 0 2 4 6 8 10 12 14 16 18 20 22 24 26

k 1.00 0.98 0.96 0.94 0.93 0.91 0.89 0.88 0.87 0.85 0.84 0.83 0.81 0.80

iv. Calculatecoefficientofdischargethroughtrashrackusingtheformula,

Whereaisthewidthofthebarsection,

bisthecentertocenterdistancebetweenbars

βistheinclinationofthetrashrack

v. Calculatewidthandlengthofthetrenchopeningortrashrackusingtheformula,

WhereCisthedischargecoefficient,

µisthecontractioncoefficientofthetrashrackanddependsuponthetypeofbars,

bisthewidthofthetrashrack

Listhelengthofthetrashrackand

histheheadovertherackcrest

β2/3cos60.0 ××=b

aC

ghLbCQA 23

2×××××= µ


62

Thecalculationof thewidthand lengthof the trash rackshouldbebasedona trialanderror

method and the final dimensions should be determined by whether the structure is easy to

operateandtherequiredflowisensured.Forpracticalpurposes,thelengthofthetrashrackis

increasedby20%.

vi. Calculatethesizeofthecollectioncanal-alongthetrashrackwidth(weirwidth)theamountof

waterforirrigationfallingthroughtherackopeningincreaseslinearlyandreachesitsmaximum

valueattheendofthecrosssection.Forreasonsofsimplicity,thisendcross-sectionisusedfor

thedimensioningofthecollectioncanal.

The water depth in the collection canal is calculated using a continuity equation along with

Manning’sformula,

2/1

3/2

2

1S

dB

dB

ndBAVQ ×⎟

⎠

⎞⎜⎝

⎛

+

×××==

WhereBisthewidthofthecanalandisthesameasthelengthofthetrashrack,

disthewaterdepthinthecollectioncanaltobeassessed,

nistheroughnesscoefficient,and

Sistheslopeofcollectioncanal

Waterdepthcanbecalculatedusingtrialanderror.

vii. Calculatethefreeboardofthecanalfrom

dFb

25.0= wheredisthedaterdepthinm,

viii. Fixthetotaldepthofcollectioncanal

Figure4-7Bottomintakedesign

Layoutofweir Flowthroughracks


63

4.6.3 DesignExampleofBottomIntake

Data

Canaldischarge(Q)=400l/s

Designflooddischarge(Qd)=12m3/s

Averagewaterdepthatlowflow(ho)=0.30m

Existingwidthoftheriver(W)=15m

Calculations

CalculatethelengthofweiracrosstheriverfromP=4.75sqrt(Qd)

P=4.75*sqrt(12)=16.45madoptweirlengthastheexistingwidthofriveras15m

Adoptheightofweiras0.30mabovetheriverbedlevel,

Adoptslopeoftrashrackas8o(β)

Adoptflowforflushing(Qf)=1.5*Q=1.5*400=600l/s

Designflowconsideringcloggingof40%(QA)=1.4*Qf=1.4*600=840l/s

Calculateheadovertheu/sedgeofthetrashrackfrom

h=2/3*k*howherekcontractioncoefficientofrack;for8ok=0.927(fromabovetable)

hoistheaveragedepthofwateratlowflow(approachflowdepth)=0.30m

h=2/3*0.927*0.30=0.19m

Calculatecoefficientofdischarge,

Whereaisthewidthofbarsection=0.02m(adopt)

bisthecentertocenterdistancebetweenbars=0.04m(adopt)

βistheinclinationoftrashrack=8o

C=0.60*0.02/0.04*cost^3/28=0.30

Calculatesizeoftrashrackfromformula,

WhereCisthedischargecoefficient=0.30fromcalculation,

µ is thecontractioncoefficientoftrashrackanddependsupontypeofbars, forordinaryrackµ=

0.85

bisthewidthofthetrashrack

Listhelengthofthetrashrackand

histheheadovertherackcrest=0.19mfromcalculation

Substitutingvaluesweget0.84=2/3*0.3*0.85*b*L*√(2*9.81*0.19)

0.84=0.32b*L

Solvingthisequationweget

b(m) 2 4 6 8 10

L(m) 1.30 0.65 0.43 0.32 0.26

Selectwidthoftrashrack,b=4m,andlengthoftrashrackis0.65m

Fromoperationalpointofviewthelengthoftrashrackistaken(Lt)=1.2L

Lt=1.2*0.65=0.78mandhenceadoptas0.80m

β2/3cos60.0 ××=b

aC

ghLbCQA 23

2×××××= µ


64

Calculatethedimensionsofcollectioncanalfrom,

2/1

3/2

2

1S

dB

dB

ndBAVQ ×⎟

⎠

⎞⎜⎝

⎛

+

×××==

WhereBisthewidthofthecanalandissameasthelengthofthetrashrack=0.80m,

disthewaterdepthinthecollectioncanaltobeassessed,

nistheroughnesscoefficient=0.02forconcreteand

Sistheslopeofcollectioncanal=0.03(Sistakenas30%)

ForQ=0.84m3/sweget, 0.84=0.80*d*1/0.02*(0.03)^1/2*(0.80*d/(0.80+2*d)^2/3

Fromtrialanderror,d=0.355m

Calculatefreeboardas0.25*d=0.25*0.355=0.09m

Totalheightofcanalatcollectionchamber=0.355+0.09=0.44m

AdoptD=0.45m

Hence,designisacceptable.

4.7 DiversionHeadwork

4.7.1 Introduction

Adiversionheadworkmaybe aweir or a barragedependingupon theobjective of the irrigation

scheme.Weirsaresolidwallsacrosstherivertoraisethewater levelandtodivertwater intothe

irrigationcanal.Weirsmaybeprovidedwithsmallshuttersonthetop.Duringfloods,astheentire

dischargeoftheriverhastopassoverthecrest,thereisconsiderableaffluxontheupstreamdueto

thesolidwallacrosstheriver.

Abarragehasasimilarstructureasaweirbutthegatealoneaffectstheheadingofthewateronthe

upstream.Thebarragecrestlevelisontheaverageriverbedlevelandhasminimumafflux.Froman

operationalpointofview,abarrage isbetterthanaweir;however, thecostofabarrage ismuch

higherthanthatofaweir.

4.7.2 Idealsitefordiversionheadwork

Anidealsitefordiversionheadworksshouldhavethefollowingcharacteristics:

• Narrowandwelldefinedriversection,

• River should have high and non-submergible banks in order to minimize the cost of river

training,

• Maximumcanalcommandwithmoderateearthworks,

• Suitabilityofriverdiversionduringconstruction,

• Thesiteshouldbesuchthattheweirorbarragecanbealignedrightangledtotheriverflow,

• Suitablelocationforallcomponentsoftheheadworks,

• Diversionheadworksshouldnotsubmergevaluablepropertyupstream,

• Goodfoundationshouldbeavailableatthesite,

• Thesiteshouldbeaccessibleandconstructionmaterialavailable,

4.7.3 ComponentsofaHeadwork

The diversionweir is a hydraulic structure across the river whichmaintains the pond level at its

upstreamsothatwatercanbedivertedtotheoff-takingcanal.Generally,adiversionheadwork is

constructed in perennial rivers which have adequate flow throughout the year. A diversion

headworkmayeitherbeaweirorabarragedependinguponthetypeandobjectivesofthescheme.


65

Forsmallscaleirrigationschemes,weirsaresuitable.Accordingtotheconstructionmaterial,weirs

maybe classifiedasmasonryweir, concreteweir andgabionweir.According to the shapeof the

crest,weirs are classified as vertical dropweir,Ogeeweir, broad crestedweir, and sharp crested

weir.Thelayoutofthediversionweiranditscomponentsarepresentedbelow(Figure4-8).

1. Weir 2.Dividewall 3.Fishladderforlargeweir 4.Undersluice 5. Head regulator

6.Abutments 7.Guidebund 8.Canal

Figure4-8Definitionsketchofweiranditscomponents

4.7.4 Typesofweirs

Basedontheflowcharacteristics,weirsaregrouped intotwotypes:sharpcrestedweirandbroad

crested weir. Sharp crested weirs are used as flow measuring devices in irrigation while broad

crestedweirs are used for diversionworks. There are three categories ofweirs, distinguished by

howtheyareconstructedandthematerialsused:(i)Verticaldropweirs,(ii)Rockfillweirs,and(iii)

Concreteweirswithslopingglacis.

(i) Verticaldropweirsarewalltypestructuresonahorizontalconcretefloor.Shuttersmayalsobe

providedonthecrestswhicharedroppedduringtheflood.Waterispondeduptothetopofthe

shuttersduringnon-floodperiods.Verticaldropweirsaresuitableforsmallirrigationdiversions,

atypicalsketchofoneispresentedhere(Figure4-9).

Figure4-9Verticaldropweir

1

2

3

4 56

7

8


66

(ii) Rockfillweirs consist of a number of corewalls across the river,with the space between

thesewallsfilledinbyrocks.Theseweirsareeconomicalinareaswhererocksareavailable.

A typical sketch of a rockfill weir is presented here (Figure 4-10). For small irrigation

schemes,gabionweirswithcorewallsarealsousedwhichareeconomicalbutsusceptibleto

flooddamage.

Figure4-10Sketchofrockfillweir

(iii) Concreteweirswithslopingglacisarecommonforbothsmallandlargeirrigationdiversions.

Thecresthasslopingglacisbothontheupstreamanddownstreamsides.Energydissipation

isachievedwithgreatdealatslopingglacisandthusconcreteweirsareextensivelyusedin

irrigationdiversionsandinlargedrops.Atypicalsketchofaconcreteweirispresentedhere

(Figure4-11).

Figure4-11Sketchofconcreteweir

4.7.5 Designconceptofweir

The design of a weir consists mainly of two parts: hydraulic design and structural design. The

hydraulicdesignshalldecidethefollowingparameters:


67

• Lengthofwaterway,dischargeintensityandafflux;

• Safeexitgradient;

• Lacey’ssiltfactor, Dmf 76.1= where,Dm-averagesizeofsedimentparticleinmm;

• Depthsofcutoffsinrelationtobothscourdepthandexitgradient;

• Levelandlengthofdownstreamhorizontalfloor;

• Thicknessofdownstreamhorizontalfloorwithreferencetoupliftpressureandhydraulicjump;

• Lengthandthicknessofprotectionworks

Inadditiontotheaboveparameters,thehydraulicdesignofweirinvolvesthefollowingconcepts:

Crestlevelofweirandundersluice

Fixationofthecrestleveloftheweirisgovernedbytwofactors:therequiredcommandlevelofthe

canal and available low flowof the river during thedry season. If theminimum flowof the river

exceeds thedesigndischargeof the canal, the crest level of theweir is set lower than thewater

leveloftheriver.Thecrestleveloftheundersluiceisfixedatthelowestbedleveloftheriverinthe

deepestchanneland isgenerallykept1mbelowthecrest levelof theweir in smallandmedium

rivers(Figure 4-12).

Figure4-12Typicalweirs

Concreteweir Gabionweir

Lengthofweirandafflux

The lengthof aweir isdirectly correlatedwith theaffluxanddischarge intensityof theweir. The

longer the lengththesmaller is thedischarge intensity.The lengthoftheweiralsodependsupon

the topography of the weir site, river characteristics, and provision of energy dissipation at

downstreamend.Laceyhasprovidedformulaforwaterway:

QP 83.4= Where,Qisthehighflooddischargeinm3/s

For boulder stage rivers, Lacey’s formula may not work properly and the designer has to judge

consideringthefollowingmainpoints:

• Averagewettedwidthoftheriverduringflood;

• Formationofshoalsupstreamoftheweir;

• Energydissipationdevicesdownstreamoftheweir;


68

The discharging capacity of the undersluice is determined with the consideration of 20% of the

designfloodortwicethecanaldischargeandcalculatedas:

( ) 2/31.0705.1 HHnLQ −=

Where,Listotalclearwaterwayinm,

nisnumberofendcontraction,

Hisheadoverthecrestinm

Effectofretrogression

Retrogression is the lowering of the river bed downstream of the weir after its construction. In

designing the weir, the effect of retrogression needs to be considered. For small rivers the

retrogressionistakentobe0.30mto0.50m.

Designofhydraulicjumpbasin

Hydraulicjumpoccurswhenasupercriticalflowofwaterchangesintoasub-criticalflow.Thisisthe

mostefficientmethodofdissipatingthekineticenergyofflowingwater.Hydraulicjumpisstablein

slopingglacisandhence it isessential toprovide slopingglacisat theweir surfacewith the slope

rangingfrom1:3to1:5.Withtheanalysisofhydraulicjump,thefollowingparametersaredesigned:

• Downstreamfloorlevel,

• Lengthofhorizontalfloor,

• Downstreamprotectionworks,and

• Upliftpressure

4.7.6 DesignProcedureofWeir

Thefollowingdataareessentialforthedesignofdiversionweirs:

• Maximumflooddischarge(Qd)-basedondesignreturnperiodflood,

• Stagedischargecurveoftheriver,

• Minimumwaterleveloftheriver,

• Cross-sectionoftheriveratweirsite

• Averagesedimentsize,andsiltfactor

Step1 Fixationofcrestlevelsofweirandundersluice;

Weircrestlevel=minimumwaterleveloftheriver

Undersluicecrestlevel=deepestriverbedlevel

Step2 Calculaterequiredwaterwaysofweirandundersluice,

Laceywaterway, QPw 83.4=

Dividetotalwaterwaysintoweirandundersluicewithconsiderationofdividewalland

piers,

Step3 Calculatedischargeintensityofweir,scourdepthandvelocityheadtherein,

q=Q/L,3

2

35.1 ⎟⎟⎠

⎞⎜⎜⎝

⎛=

f

qR andapproachvelocity,V=q/R

Step4 Determinetheheadoverthecrestofweirandundersluice=u/sTEL–crestlevel

Step5 Calculate the discharges passing through the undersluice (Q1) and weir (Q2) for both

floodflowandpondlevelflowandcomparewithdesignflooddischarge,

( ) 2/3

111110.0705.1 HnHLQ −=


69

( ) 2/3

222210.084.1 HnHLQ −=

WhereL1isthelengthofundersluice,

L2isthelengthofweir,

H1istheheadoverthecrestofundersluice,

H2istheheadoverthecrestofweir

Step6 Determine head loss for different flow conditions (high flood flow and pond flow);

HL=u/sTEL–d/sTEL

Step7 Determineparametersofhydraulic jumpofundersluice for floodflowandpond level

conditionsconsideringwithandwithoutconcentrationandretrogression

HydraulicjumpcalculationsarecarriedouteitherusingBlenchcurvesorspecificenergy

curvesorbytrialanderrormethod.

Step8 In trialanderrorprovide trial valueofpost jumpdepthandcalculatepre jumpdepth

fromformula:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

×+−=

2

2

2

2

21

4

1/2

2

1D

D

qgDD

Step9 Calculatespecificenergyofu/sandd/sofjumpfromformula:

( )222

222/ DgqDEf ×+=

( )122

112/ DgqDEf ×+=

Step10 Calculateheadloss,Hl=Ef1-Ef2andequalwithcalculatedvaluesofheadlossinstep6,

Step11 Calculatelevelofjumpformation,lengthofconcretefloorandFroudenumberforeach

flowcondition,

Levelofjump=d/sTEL-Ef2

Lengthofconcretefloor,L=5(D2-D1)

Froudenumber,

1gD

qF =

Step12 Adoptd/sfloorlevelasminimumofjumplevelsandadoptd/sfloorlengthasmaximum

ofconcretefloorlengthsfordifferentflowconditions,

Step13 Determine the u/s and d/s scour depths from the formula of scour consideration,2/1

2

35.1 ⎟⎟⎠

⎞⎜⎜⎝

⎛=

f

qR ;determinedepthofu/ssheetpileorcutoffas1to1.25Randd/ssheet

pileorcutoffas1.25to1.5R;

Step14 Workout total floor lengthandexit gradient (GE),determinevalueof

λπ

1 from the

equationH

dGE

=λπ

1 for a given value of GE and known values of d and H. From

correspondingvalueof

λπ

1 readthevalueofαfromKhosla’sexitgradientcurve.

Step15 Calculatethetotallengthoffloor(b)=αxd;dispositionoftotalfloorlengthmaybeas

follows:

Cisternlength=5to6(D2-D1)


70

Glacislength=3to5times(crestlevel–cisternlevel)for1:3to1:5slopeofglacis;

Balanceforu/sfloorlength,

Step16 Determineupliftpressureactingonthefloor;%pressuresatu/sandd/ssheetpilelines

areworkedoutas:

%pressureatupstreamsheetpileline

For known value of b and d1,b

d1

1=

α, having knownα read out values ofΦD andΦE

fromKhosla’spressurecurve.

%pressureatthebottomofsheetpile=100-ΦD

%pressureatthebottomoffloor=100-ΦD

%pressureatdownstreamsheetpileline

Forknownvalueofbandd2,b

d2

1=

α,readvaluesofΦDandΦEcorrespondingto

α

1

fromKhosla’spressurecurvewhichwouldbe%pressureatthebottomofthefloorand

sheetpilerespectively,

Step17 Correctionduetofloorthickness

Thethicknessoftheflooratthelocationofthesheetpilesaretentativelyassumedfor

correctingthevaluesofΦCintheu/sandΦEinthed/s.Ift1isthefloorthicknessatu/s

sheet pile of depth d1, correction due to floor thickness = ( )CD

d

tφφ −

1

1 which is

positive.Ift2isthefloorthicknessatd/ssheetpileofdepthd2,correctionduetofloor

thickness= ( )DE

d

tφφ −

2

2 whichisnegative,

Step18 Correctionduetomutualinterferenceofsheetpilesisworkedoutbytheformula:

b

Dd

b

DC

+×='

19

Step19 Afterknowingthecorrected%pressuresatkeypoints,depthofsheetpilesorcutoffs,

thicknessofflooriscalculatedatdifferentpointsfromthed/sendofthecutoffwall.

4.7.7 DesignExampleofConcreteWeir

Designdata:

Highflooddischarge,Q 35m3/s

Averagebedlevelofriver 303.8 m

HFLbeforeconstruction 305.75 m

PermissibleAfflux 0.5 m

PondLevel 305.25 m

Siltfactor 1

Safeexitgradient 0.20(1/5)

Concentration 20%

Retrogression 0.5 m


71

Fixationofcrestlevelandwaterway

Crestlevelofundersluiceistakenat 303.80 m

Crestlevelofweiristakenat 305.25 m

Lacy'swaterwayPw=4.83x√Q=4.83x√35 = 28.63m

Waterwaysforundersluiceandweir

Undersluiceportionbothside

1bayof1meachL1= 1m

Overallwaterway 1m

Weirportion

1bayof17meachL2= 17m

Overallwaterway 17m

Assume2dividewalls0.5mthickness= 1m

Overallwaterway,L=2x1+17+1= 20m

U/SHFL=d/sHFL+Afflux =305.75+0.50= 306.25 m

Dischargeintensity,q=Q/L=35/20= 1.75 m3/s

Scourdepth,R=1.35x(q2/f)1/3=1.35x(1.75^2/1)^1/3= 1.96 m

Approachvelocity,V=q/R =1.75/1.96= 0.89 m/s

Velocityhead,hv=V2/2g= 0.89^2/2x9.81= 0.04 m

u.s.TEL=H.F.L.+velocityhead =306.25+0.04= 306.29 m

Headovertheundersluicecrest,H1=u/sTEL–crestlevel=306.29–303.80=2.49m

Headovertheweircrest,H2 =u/sTEL–crestlevel=306.29–305.25=1.04m

DischargepassingthroughundersluiceandweirareQ1andQ2respectively,

Q1=1.705(L1-0.1nH1)H13/2 =1.705*(1-0.1*0*2.49)*2.49^3/2=6.70m3/s

Q2=1.84(L2-0.1nH2)H23/2=1.84*(17–0.1*0*1.04)1.04^3/2=33.17m3/s

TotaldischargepassingthroughheadworkQd=2xQ1+Q2=2*6.70+33.17=46.57m3/s

SinceQd>QthecrestlevelandwaterwayareOK

Loosenessfactor=L/Pw =20/28.63= 0.70

Flowatpondlevel

Headovertheundersluicecrest,H1=Pondlevel–crestlevel=305.25–303.80=1.45m

Headovertheweircrest,H2 =Pondlevel–crestlevel=305.25-305.25=0.00m

Dischargepassingthroughundersluice,Q1

Q1=1.705(L1-0.1nH1)H13/2 =1.705*(1-0.1*0*1.45)*1.45^3/2=2.98m3/s

Q2=1.84(L2-0.1nH2)H23/2 =1.84*(17–0.1*0*0)0=0.00m3/s

TotaldischargepassingthroughheadworkQd=Q1+Q2 =2.98m3/s

Dischargeintensity,q=Q1/L1=2.98/1=2.98m3/s/m

Normalscourdepth,R=1.35x(q2/f)1/3=1.35*(2.98^2/1)^1/3=2.80m

Velocityapproach,V=q/R =2.98/2.80=1.07m

Velocityhead,v=V2/2g =1.07^2/2*9.81=0.06m

u/sTEL=pondlevel+velocityhead=305.25+0.06=305.31m


72

d/swaterlevelforQ= 2.98m3/s= 304.75 mfromstagedischargecurve

d/sTEL=d/swaterlevel+velocityhead=304.75+0.06=304.81 m

Headloss=u/sTEL–d/sTEL=305.31–304.81=0.50m

Furtherdesigniscarriedoutintabularform(Table4-2andTable 4-3).

Table4-2Designofundersluice

Descriptions Formaximumflood Forpondlevel

Without

concentration

and

retrogression

With20%

concentration

and0.50m

retrogression

Without

concentration

and

retrogression

With20%

concentration

and0.50m

retrogression

Dischargeintensity,q=1.7xH3/2

6.68 8.02 3.15 3.78

Headrequiredforq 2.49 2.81 1.45 1.70

d/sHFL 305.75 305.25 304.75 304.25

d/sTEL(d/sHFL+velocityhead) 305.79 305.29 304.81 304.31

u/sHFL(d/sHFL+afflux) 306.25 306.25 305.25 305.25

u/sTEL(u/sHFL+velocityhead) 306.29 306.61 305.31 305.50

Headloss 0.50 1.32 0.50 1.19

Postjumpdepth,D2-trialvalue 2.66 3.42 1.74 2.21

Pre-jumpdepth,

⎟⎟⎠

⎞⎜⎜⎝

⎛+

×+−= 2

2

2

2

21

4

1/2

2

1D

D

qgDD

0.95 0.89 0.52 0.49

d/sspecificenergy,

( )222

222/ DgqDEf ×+=

2.98 3.70 1.90 2.36

u/sspecificenergy,

( )122

112/ DgqDEf ×+=

3.48 5.02 2.41 3.55

Ef1-Ef2=Headloss(shouldbe

equaltoabovevalue)

0.50 1.32 0.50 1.19

Levelofjump=d/sTEL-Ef2 302.81 301.59 302.91 301.95

Lengthofconcretefloor

L=5(D2-D1)

8.56 12.62 6.09 8.62

FroudenoF=q/(g*D31)0.5

2.31 3.04 2.70 3.55

d/sfloorlevelisprovidedat(minimumofjumplevel) 301.59m

d/sfloorlength(maximumoflengthofconcretefloor) 12.62mroundupto13m

DepthofCut-offfromscourconsideration

Dischargepassingthroughundersluice 6.68m3/s


73

Overallwaterwayofundersluice 1.00m

Dischargeintensity 6.68m3/s/m

Scourdepth,R=1.35x(q2/f)1/3=1.35*(6.68^2/1)^1/3= 4.80m

u/scut-off

Onu/ssideallowcut-offdepthas1.1R =1.1*4.80=5.28m

RLofbottomofscourhole=u/sHFL–cut-offdepth=306.25–5.28=300.97mSay 300.90 m

d/scut-off

Ond/ssideallowcut-offdepthas1.25R=1.25*4.80=6.00m

RLofbottomofscourhole=d/sHFL–cut-offdepth=305.25–6.00=299.25m

Say299.00 m

Totalfloorlengthandexitgradient

Theexitgradientshouldbecheckedfortheconditionthatthecanaliscompletelyclosedwhenhigh

floodispassingintheriver;thisprovidesworststaticcondition.

Maximumstatichead=Pondlevel–d/sfloorlevel=305.25-301.59= 3.65m

Depthofd/scutoff=d/sfloorlevel–RLofbottomofscourhole=301.60-99.00=2.60m

Exitgradient, H

dGE

=λπ

1

Hence, λπ

1

=1/5*2.60/3.65= 0.142

FromKhosla'sexitgradientcurve,α(alfa)=8.9

Requiredtotalfloorlength,b=αxd=8.9*2.6=23.14m

Adopttotalfloorlength=24.00 m

Thefloorlengthshallbeprovidedasperfollowingproportion:

d/shorizontalfloorlength,=13.00m

d/sglacislengthwith3:1slope,3*u/sfloor–d/sfloor=3(303.80-301.60)=6.60m

Balanceshouldbeprovidedasupstreamfloor=4.40m

Totalfloorlength=13.00+6.60+4.40=24.00m


74

Figure4-13Designsketchofundersluice

Pressurecalculations

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

Determinetheupliftpressureactingonthefloorand%pressuresatu/sandd/ssheetpiles:

%pressureatu/ssheetpileline

Upstreamcutoff,d1=303.80-300.90= 2.90 m

Lengthoffloor,b=24.00mandu/scutoffdepth,d1=2.90m

Then,1/α=d1/b=2.90/24.00=0.121

FromKhosla’scurves,φD=88.5 %

φC= 83.4%

φD-φC=88.50–83.40=5.10%

Applycorrectiontotheabovevalues,

Thicknessofu/sfloor,t1=0.50m

φCcorrectionfordepth=t1/d1(φD–φC)=0.50/2.90*5.10=0.88 (+ve)

φCcorrectionforinterferenceofd/scutoff

Khosla’sformula,⎟⎠

⎞⎜⎝

⎛ +×⎟⎟

⎠

⎞⎜⎜⎝

⎛=

b

dD

b

DC

1

50.0

1

19

d1=303.80-0.50-300.90=2.40m

D=303.80-0.5-299.00=4.30m

b1= 23.50 m

b= 24.00 m

C=19*(4.30/23.50)^0.5*(2.40+4.30)/24.00=2.25%(+ve)

φCcorrected =83.40+0.88+2.25= 86.52 %say 86.50 %


75

Downstreamcutoff,d2=301.60-299.00=2.60m

Lengthoffloor,b=24.00mandd/scutoffdepth,d1=2.60m

Then,1/α=d2/b=2.60/24.00=0.108

FromKhosla’scurves,φE1=15.8%

φD1= 11.0%

φE1-φD1=15.80–11.00=4.8%

φE1correctionfordepth=0.60*4.80/2.60= 1.11(-ve)

φE1correctionforinterferenceduetou/scutoff

d=301.60-0.60-299.00=2.00m

D=301.60-0.60-300.90=0.10m

⎟⎠

⎞⎜⎝

⎛ +×⎟⎟

⎠

⎞⎜⎜⎝

⎛=

b

dD

b

DC

1

50.0

1

19

=19(0.10/24.00)^0.5*(2.00+0.10/24.00)=0.11 %(-ve)

φEcorrected =15.80-1.11-0.11=14.58%say14.60%

φC=86.50% andφE=14.60%

Floorthickness

Themaximumstaticheadwilloccuronthefloorwhenthereisnoflowinthecanalbutmaximum

floodispassingdownriver.Inthiscasethemaximumstatichead=3.65m

Downstreamfloor

At1mfromd/send

Unbalancedhead=(86.50-14.60)/100*3.65*1.00/24.00+14.60*3.65/100=0.64m

Floorthickness=0.64 /(2.24-1)=0.52m

Providefloorthickness=0.6 mat1mfromd/sendofcut-off

Upliftpressure=1000*0.64=642.25kg/m2

Slabweight=2500*0.70=1750.00 kg/m2

Netupliftpressure=642.25–1750.00=-1107.75 kg/m2

CalculatemomentMr=w*l2/12=-194087396Nmm

d=(Mr/Rb*b)0.5=546mm,say600mm

Provide50mmcoverbothside,sothattotalthicknessofslabis=700mm

Floorthicknessprovided=700mm

At13.00mfromd/send


Floorthickness,t=1.95/(2.24-1)=1.58m

Providefloorthicknessas1.60mat13mfromd/sendofcut-off

Upliftpressure=1000*1.95 =1954.42kg/m2

Slabweight,= 2500*0.30=750.00 kg/m2

Netupliftpressure,=1954.42–750=1204.42kg/m2

Mr=w*l2/12=25092144Nmm

d=(Mr/Rb*b)0.5=196mmsay200mm


76

Provide50mmcoverbothside,sototalthicknessofslabis=300 mm

Floorthicknessrequired=300mmatendoffloor

Protectionworksbeyondimperviousfloor

Upstreamprotection

Blockprotection

Scourdepth,R=1.35(q2/f)1/3=1.35(4.80^2/1)^1/3=4.80m

Anticipatedscour=1.50*R=1.5*4.80=7.20m

Upstreamscourlevel=u/sHFL-anticipatedscourlevel=306.25–7.20=299.05m

Scourdepth,Dbelowu/sfloor=crestlevelofundersluice–scourlevel

D=303.80-299.05=4.75m

Volumeofblockprotectiontobegiven(D)=4.75m3/m

Provide1.5mx1.5mx1.0mconcreteblockover0.50mhickgravel

Thelengthrequired=4.75/(1.0+0.50)=3.16m

Providethreerowsoftheaboveblockinalengthof3.00m

Launchingapron

Quantityoflaunchingapronshouldbe2.25Dm3/m

Thicknessoflaunchingapron=1.50m

Thelengthrequired=2.25*4.75/1.50=7.12m

Providelaunchingapronas7.00mlength

Downstreamprotection

Scourdepth,R=4.80m

Anticipatedscour=2*R=9.60m

Downstreamscourlevel=HFL–retrogression–anticipatedscour

=305.75–0.50-9.60=295.65m

Scourdepth,Dbelowd/sfloor=d/sfloorlevel–scourlevel=301.60-295.65=5.95m,Say6.00m

Invertedfilter

Thelengthofinvertedfilter=Dmandthicknessequaltothed/slaunchingapron

Provide rowof 1.50mx1.50mx1mconcreteblockwith10cmgapfilledwithpeagravelover

0.5mthickgradedfilter

Thelengthrequired=5.95/(1.0+0.50)=3.96m=4m

Launchingapron




Providelaunchingapronas9.00mlength


77

Table4-3Designexampleofweir

Descriptions Formaximumflood

Without

concentrationand

retrogression

With20%concentration

and0.5mretrogression

Dischargeintensity,q=1.84xH3/2

1.95 2.34

Headrequiredforq 1.04 1.17

d/sHFL 305.75 305.25

d/sTEL(d/sHFL+velocityhead) 305.79 305.29

u/sHFL(d/sHFL+afflux) 306.25 306.25

u/sTEL(u/sHFL+velocityhead) 306.29 306.42

Headloss 0.50 1.13

Postjumpdepth,D2-trialvalue 1.325 1.687

Pre-jumpdepth,

⎟⎟⎠

⎞⎜⎜⎝

⎛+

×+−= 2

2

2

2

21

4

1/2

2

1D

D

qgDD

0.35 0.33

d/sspecificenergy,

( )222

222/ DgqDEf ×+=

1.44 1.79

u/sspecificenergy,

( )122

112/ DgqDEf ×+=

1.94 2.92

Ef1-Ef2=Headloss(shouldbeequal

toabovevalue)

0.50 1.13

Levelofjump=d/sTEL-Ef2 304.36 303.51

Lengthofconcretefloor

L=5(D2-D1)

4.88 6.79

FroudenoF=q/(g*D31)0.5

3.01 3.97

Downstream floor level isprovidedat theminimum levelof jump formationoraveragebed level

minusafflux=303.30m

d/sfloorlengthisprovidedasmaximumoflengthoffloor= 6.79mroundupto7.00m

DepthofCut-offfromscourconsideration

Dischargepassingthroughtheweir(Qd)=33.17m3/s

Totalwaterwayofweir=17.00m

Dischargeintensity=1.95m3/s/m


78

Scourdepth,R=1.35x(q2/f)

1/3=1.35*(1.95^2/1)^1/3=2.14m

u/scut-off

Onu/ssideallowanticipatedscouras1.1R=1.1*2.14=2.36m

RLofbottomofscourhole=u/sHFL–anticipatedscourdepth=306.25–2.36=303.89m

AdoptRLofscourhole=302.00m

d/scut-off

Ond/ssideallowanticipatedscouras1.25R=1.25*2.14=2.68m

RLofbottomofscourhole=d/sHFL-anticipatedscourdepth=305.25–2.68=302.57m

AdoptRLofd/sscourhole=301.50m

Totalfloorlengthandexitgradient

Theexitgradientshouldbecheckedfortheconditionthatthecanaliscompletelyclosedwhenhigh

floodispassingintheriver;thisprovidesworststaticcondition.

Maximumstatichead=pondlevel–d/sfloorlevel=305.25-303.30=1.95m

Depthofd/scutoff=303.30-301.50=1.80m

Exitgradient,H

dGE

=λπ

1

Hence,

λπ

1 =1/5*1.80/1.95=0.185

FromKhosla'sexitgradientcurve,α(alfa)=10.38

Requiredtotalfloorlength,b=αxd=10.38*1.80=18.68m

Adopttotalfloorlength=18.00 m

Thefloorlengthshallbeprovidedasperfollowingproportion:

d/shorizontalfloorlength,=7.00m

u/sglacislengthwith1:1slope=1*(crestlevel-u/sfloor)=305.25–303.80=1.45m

Crestwidth=1.50m

d/sglacislengthwith3:1slope,3*u/screstlevel–d/sfloor=3*(305.25-303.30)=5.85m

Balanceshouldbeprovidedasupstreamfloor=2.20m

Totalfloorlength=7.00+1.45+1.50+5.85+2.20=18.00m


79

Figure4-14Designsketchofweir

Pressurecalculations

Letthefloorthicknessintheupstreambe0.4mandnearthed/scut-offbe0.40m

Upstreamcutoff,d1=303.80-302.00=1.80m

Lengthoffloor,b=18.00mandu/scutoffdepth,d1=1.80m

Then,1/α=d1/b=1.80/18.00=0.10

FromKhosla’scurve,φD=88.8%

φC= 84.0%

φD-φC=88.80–84.0=4.80%

Applycorrectiontotheabovevalues,

Thicknessofu/sfloor,t1=0.40m

φCcorrectionfordepth=t1/d1(φD–φC)=0.40/1.80*4.80=1.07(+ve)

φCcorrectionforinterferenceofd/scutoff

Khosla’sformula, ⎟⎠

⎞⎜⎝

⎛ +×⎟⎟

⎠

⎞⎜⎜⎝

⎛=

b

dD

b

DC

1

50.0

1

19

d1=303.80-0.40-302.00=1.40m

D=303.80-0.40-301.50=1.90m

b1= 17.50 m

b= 18.00 m

C=19*(1.90/17.50)^0.5*(1.40+1.90)/18.00=1.13%(+ve)

φCcorrected =84.0+1.07+1.13=86.20%

Downstreamcutoff,d2=303.30–301.50=1.80m

Lengthoffloor,b=18.00mandd/scutoffdepth,d1=1.80m


80

Then,1/α=d2/b=1.8/18.00=0.100

FromKhosla’scurves,φE1=14.70%

φD1= 10.20%

φE1-φD1=14.70–10.20=4.50%

φE1correctionfordepth=0.40*4.50/1.80=1.00(-ve)

φE1correctionforinterferenceduetou/scutoff

d=303.30-0.40-301.50=1.40m

D=303.30-0.40-302.00=0.90m

⎟⎠

⎞⎜⎝

⎛ +×⎟⎟

⎠

⎞⎜⎜⎝

⎛=

b

dD

b

DC

1

50.0

1

19=19(0.90/17.50)^0.5*(1.40+0.90/18.00)=0.54%(-ve)

φEcorrected =14.70-1.00-0.54=13.16%say13.20%

φC=86.20% andφE=13.20%

Floorthickness

Themaximumstaticheadwilloccuronthefloorwhenthereisnoflowinthecanalbutmaximum

floodispassingdownriver.Inthiscasethemaximumstatichead=305.25–303.30=1.95m

Downstreamfloor

At1mfromd/send


Floorthickness=0.34/(2.24-1)=0.27m

Providefloorthickness=0.40mat1mfromd/sendofcut-off

Upliftpressure=1000*0.34=335.70kg/m2

Slabweight=2500*0.70=1750.00 kg/m2

Netupliftpressure=335.70–1750.00=-1414.30 kg/m2

CalculatemomentMr=w*l2/12=-247798530Nmm

d=(Mr/Rb*b)^0.5=617mm,say600mm

Provide50mmcoverbothside,sothattotalthicknessofslabis=700mm

Floorthicknessprovided=700mm

At7.00mfromd/send


Floorthickness,t=0.81/(2.24-1)=0.65m

Providefloorthicknessas0.80mat7.00mfromd/sendofcut-off

Upliftpressure=1000*0.81 =810.46kg/m2

Slabweight,=2500*0.30=750.00 kg/m2

Netupliftpressure,=810.46–750.00=60.46kg/m2

Mr=w*l2/12=1259587Nmm

d=(Mr/Rb*b)^0.5=44mmsay200mm

Provide50mmcoverbothside,sototalthicknessofslabis=300mm

Floorthicknessrequired=300mmatendoffloor


81

Protectionworksbeyondimperviousfloor

i. Upstreamblockprotection

Scourdepth,R=1.35(q2/f)

1/3=1.35(2.14^2/1)^1/3=2.14m

Anticipatedscour=1.5*R=1.50*2.14=3.21m

u/sscourlevel=u/sHFL–anticipatedscourdepth=306.25–3.21=303.04m

Scourdepth,Dbelowu/sfloor=u/sfloorlevel-scourlevel=303.80–303.04=0.76m

Volumeofblockprotectiontobegiven=Dm3/m=0.76m

3/m

Provide1.0mx1.0mx0.30mblockover0.20mthickgravel

Thelengthrequired= D/(blockthickness+gravelthickness)=0.76/0.50=1.53m

Providetworowsoftheconcreteblockinalengthof2.00m

ii. U/slaunchingapron

Quantityoflaunchingapronfor2:1slopebe2.25Dm3/m

Thicknessoflaunchingapron=blockthickness+gravelthickness=0.50m


Providelaunchingapronof3.00mlength

iii. D/sblockprotection

Scourdepth,R=2.14m

Anticipatedscourdepth=2*R=4.29m

Downstreamscourlevel=HFL-retrogression–anticipatedscourdepth

=305.75-0.50-4.29=300.96m

Scourdepth,Dbelowd/sfloor=d/sfloorlevel–scourlevel=303.30-300.96=2.34m

iv. D/sInvertedfilter

Thelengthofinvertedfilter=Dmeterandthicknessequaltothed/slaunchingapron

Provideinrowof1.00mx1.00mx0.60mconcreteblockwith10cmgapfilledwithpeagravelover

0.40mthickgradedfilterinalengthof2.00m

v. D/slaunchingapron



Thelengthrequired=2.25*D/1.00=2.25*2.34=5.26m

Providelaunchingapron6.00minlength

4.8 EnergyDissipationinHydraulicStructure

4.8.1 Energydissipation

The energy ofwater flowing overweirs/spillways needs to be absorbed or dissipated before it is

returnedtotheriverorcanalwhereunprotectedorsoftfoundationsexist.Theobjectiveofenergy

dissipation is thus to destroy the energy of a mass of water so that it does not create serious


82

erosion.Thedevicesusedfordissipatingtheenergyofflowingwaterarecalledenergydissipaters.

The dissipation of energy is performed by transforming super-critical flow into sub-critical flow

through hydraulic jumps, turbulence, and impacts. The most widely used energy dissipaters in

irrigationworksare(i)USBRbasins(type-Itotype-IV),(ii)Vlugterbasin,(iii)SAFBasin,(iv)Impact

block(straightdrop)basin,(v)Slottedgratingdissipater,(vi)Impacttypestillingbasin,(vii)Plunge

pool, (viii) Stilling well, (ix) Baffled spillway, and ( x) Deflector bucket. Among these energy

dissipaters(i)to(v)and(x)aremostlyusedinspillways,whereastypes(vi)to(ix)areusedinoutlets.

Thedeflectorbucketsaresuitable foruse inheadworkstructures,butnotgenerallyused incanal

works. The detailed sketches of these energy dissipaters are presented in Figure 4-15 . In this

section,onlythehydraulicjumpbasinsarediscussedfurther.

4.8.2 Hydraulicjumpbasins

Hydraulic jumpbasinsarethemosteffectivedevices todissipatetheenergyof flowingwaterand

are widely used in all types of hydraulic structures. The form and flow characteristics of the

hydraulicjumpwhichoccursinastillingbasincanberelatedtotheFroudenumber(F=V/(gd)0.5)of

the flow entering the basin. The selection of particular energy dissipater depends on the Froude

number,typeofstructureandeconomy,whichisillustratedintabularform(Table 4-4).

Table4-4Selectionofenergydissipaters

Type Consideration/Applicationlimits

USBRTypeI WiderangeofFroudenumberandvelocity(V<15m/s)

Longbasinandsimpleconstruction,speciallysuitableforFr<2.5

USBRTypeII ForV>15m/s

Longbasinswithchuteblocks

USBRTypeIII ForFr>4.5,V<15m/s

Shortbasin,butcomplicatedbyfloorandchuteblocks

USBRTypeIV ForFrbetween2.5-4.5,V<15m/s


Vlugter Forcanalfallswheredrop<4.0m

Simpletodesignandconstruction,suitableformasonrystructure

SAF Forsmalllowheadstructures


Impactblock Fordrop<2.0m

Generallyeconomic,suitableforcanalfalls

Slottedgrating ForFrbetween2.5-4.5

Smallstructure,butcomplicatedbygrating

Baffledspillway Notailwaterrequirement

Complicatedbyblocks,economic

Bucket Needsmoretailwaterthanjumptypebasins

Goodfoundationandprotectionrequiredinviewofscourholeformation


83

Figure4-15Sketchesofdifferenttypesofenergydissipaters


84

4.9 PumpedIrrigationSystem

4.9.1 Introduction

Smallscalepumpedirrigationsystemsaremadeupofthefollowingcomponents:

• Watersource,

• Pumpandpowerunit,

• Distributionsystemand

• Methodofirrigation

Thewatersource,distributionsystemand irrigationmethoddeterminetheenergydemand,while

thepumpandpowerunitsupplythepower.Thewatersourcemaybeariver,lakeorgroundwater.

Theamountofwaterabstractedandtheheighttowhichitmustbeliftedfromtheriverorborehole

addtotheenergydemand.PumpsaremostlydrivenbyelectricmotorsinBhutan.

The distribution system conveyswater from the pump to the farmer’s field and consists of open

channels or pipes. Some systems are a combination of open channel and pipes. The choice of

distributionsystemhasasignificanteffectonenergydemand.Theirrigationmethodmaybesurface

irrigation, sprinkler or drip irrigation. Surface irrigationmay be supplied either by a pipe or open

channel.Sprinklerordripirrigationsystemswouldnormallyusepipeddistributionsystems.

Themostcommoncombinationsofthecomponentsforanirrigationsystemare:

• Pump openchannel surfaceirrigation

• Pump pipesupply surfaceirrigation

• Pump pipesupply sprinklerordripirrigation

4.9.2 Typeofpumps

Apumpisadevicetoliftwatertoahigherlevelortotransportitfromonepointtoanother.Pumps

requiresomepowertoperformtheseworkswhich issuppliedbyadrivingdevice (electricmotor,

internalcombustionengine)throughamediumofatransmission.Hence,apumpingsetconsistsof

apowersupplier,atransmissionandapump.

Amongvarioustypesofpumpsto liftwater,Turbotypepumpscomprisingcentrifugal,mixedflow

andaxial flowpumpsaremostfrequentlyused inpumpingworksforwatersupply, irrigation,and

drainageservices.Thesepumpsallfunctionontheprinciplethattheenergyrepresentedbyheadis

imparted to the water pumped through the rotational motion of an impeller. The principle of a

pump’sfunctioningisrepresentedbyEuler’sequation.

Figure4-16SketchesofPumps

Centrifugal Mixedflow Axialflow

Inaradialflow(centrifugal)pump,thewaterleavestheimpellerinplanesperpendiculartotheshaft

axisdue to centrifugal force. Thewater then is guided througha volute casing into thedischarge

nozzle,whileapartofthevelocityheadisconvertedintothestatichead.Insomecasesdiffusersare

arrangedaroundtheimpeller.


85

In amixed flow pump, thewater leaves the impeller in a direction diagonal to the shaft axis on

conicalsurfaces.Thewaterleavingtheimpelleriseithercollectedbyavolutecasingorreceivedbya

diffusercasing.

Inanaxialflowpump,thewaterflowsthroughapropellertypeimpelleroncylindricalsurfacesand

adiffusercasingisexclusivelyusedforrestoringpressure.

Mostofthepumpsarecoupledwithelectricmotorsorenginesbeingdrivenfromthepumpshaft

end. Submersible motor pumps are driven by motors integrated into the pump casing and

immersed in the suction sump or deep wells. These pumps are exclusively vertical shaft types

featuringeaseinhandlingwhenusedforportableapplications.

4.9.3 TotalDynamicHead

A total dynamic head or total pumping head is the head that the pump is required to impart to

waterinordertotheheadrequirementoftheparticularsystem.Thetotalheadismadeupofstatic

suctionhead,staticdischargehead,requiredpressurehead,andfrictionhead(Figure 4-17).

Figure4-17Sketchoftotaldynamichead

4.9.4 PumpCharacteristicCurves

Mostmanufacturersprovidefourdifferentcharacteristiccurvesforeverypump:thetotaldynamic

headversusdischarge(TDH-Q)curve,theefficiencyversusdischarge(Eff-Q)curve,thebreakpower

versus discharge (BP-Q) curve, and net positive suction head required versus discharge (NPDH-Q)

curve.Allfourcurvesaredischargerelated(Figure 4-18).


86

Figure4-18Sketchesofpumpcharacteristiccurves

Total dynamic head versus discharge curve relates the head to the discharge of the pump. This

curve shows that thepumpcanprovidedifferent combinationsofdischargeandhead.When the

headincreasesthedischargedecreasesorviceversa.

Efficiency versus discharge curve relates the pump efficiency to the discharge. The efficiency is

definedasoutputworkoverinputwork.

BPC

TDHQ

BP

WP

workInput

workOutputE pump

×

×===

WhereEpumpisthepumpefficiency,

BPisthebreakpowerinkworHP(1kW=1.36HP)theenergyimpartedbytheprimemovertothe

pump,

WPisthewaterpowerinkw,theenergyimpartedbythepumptothewater,

Qisthedischargeinm3/shourorl/s,

TDHisthetotaldynamicheadinm,and


87

Cisthecoefficienttoconvertworktoenergyunitandequalsto102ifQismeasuredinl/sand367if

Qismeasuredinm3/hour.

Break power versus discharge curve relates the input power required to drive the pump to the

discharge.

Thenetsuctionheadrequiredversusdischarge:Someoftheatmosphericpressureatwaterlevelis

lost in theverticaldistance to theeyeof the impeller, some to frictional losses, and some to the

velocityhead.ThetotalenergythatisleftattheeyeoftheimpelleriscalledtheNetPositiveSuction

Head(NPSH)andtherequiredenergytomovewater intotheeyeoftheimpeller iscalledtheNet

Positive Suction Head Required (NPSHR). It is the function of the pump speed, the shape of the

impeller and the discharge. In order to avoid cavitation of the pump, available NPSH should be

greaterthanNPSHR.

Manufacturers also provide the combined characteristic curves for total head, shaft power and

efficiency plotted on the ordinates against capacity on the abscissa. The forms of characteristic

curvesarecloselyrelatedtothevaluesofthespecificspeedinwhichrespectivecharacteristicsare

giveninpercentageofthevalueatthebestefficiencypoint.

4.9.5 PowerandEnergyforPumping

In general, the power for the pump set is electricity obtained either from the central power grid

systemorgeneratedfromitsownsource.Nominaloperatingconditionofthepowersupplyis400

Volt,3PhaseA.C.with50Hzfrequency.Theamountofenergyrequiredtoliftthewaterdependson

thevolumetobeliftedandtheheadrequired:

367

HVE

w×

=

WhereEistheenergyinkilowatthour(kWh),Vwisthevolumeofwaterinm3,andHistheheadin

m,

Poweristherateofusingenergyandiscommonlymeasuredinkilowattorhorsepower(1kW=1.36

HP).Poweriscalculatedas:

t

EP =

WherePisthepowerinwatt,Eistheenergyinwatthourandtisthetimeinhour.Fromtheenergy

andpowerequationsdischargerelationisobtainedas:

QHP 81.9=

WhereQisthedischargeinliterpersec,andHisthetotalheadinm

4.9.6 Pumpsinseries

Connecting pumps in series applies to the caseswhere the same discharge is required butmore

headisneededthanthatonepumpcanproduce.Fortwopumpsoperatinginseries,thecombined

headequalsthesumoftheindividualheadsatacertaindischarge.

4.9.7 Pumpsinparallel

Pumpsareoperatedinparallelwhenforthesamehead,variationindischargeisrequired.Atypical

examplewouldbeasmallholderpressurizedirrigationsystemwithmanyusers.

4.9.8 Selectionofpumps

Awiderangeofpumpsareavailableonthemarket.Theselectionoftherightpumpfortherightuse

depends on a series of inter-related factors. Centrifugal pumps have the largest application in

irrigationcomparedtoothertypeofpumps.Themostcommonlyusedcentrifugaltypesofpumps


88

are horizontal end suction, vertical submersible and vertical turbine pumps. Mixed or axial flow

impellersaresometimesusedinlowlift,highcapacityoperationslikeliftingwaterfromacanal.In

general practice, the selection of the pumps requires the use of manufacturers’ characteristics

curves.

Priortoselectingthepumpingsystem,itisessentialtocalculatetotalheadoveranumberofstages

and finalize the complete layout of the system. All components of the system including valves,

bends, manifolds, reducers, etc. may be incorporated to evaluate total head for each stage of

pumping,summingupstaticheadandfrictionhead.Amarketsurveyforvarioustypesofpumpsand

theiravailabilitywillhelp to identify thecost-effectivepump foragiven site.The selectionof the

prime mover governs the selection of a pump, whether it is mechanically driven or electrically

driven. In our case, we use electrically driven pumps. In the case of electrically driven pumps,

directly coupled ACmotors are generally available in themarket.While selecting a pump, try to

ensurethatthepowerisadequatefortherequireddischargeandhead.

Centrifugalpumpsare themost commonlyusedengine-poweredpumps for small-scale irrigation.

Theyaremainlydesignedtoproviderelativelyhighheadandlowdischargeforagivenpower.Some

sort of compromisehas to be acceptedbetween requireddischarge, total head and geographical

location.

4.9.9 Sitingofpumpingstation

The pumping station is a building which provides a foundation for the pumping set and its

accessories andprotects them fromadverseweather conditions. It also houses the electrical and

other auxiliary equipment and supports suction and discharge piping. The selection of a suitable

locationforapumpingstation isvery important in irrigationdevelopment.Several factorshaveto

beconsideredwhileselectingthepumpingsite.

• Reliabilityofthewatertomatchwiththeproposedcroppingpatternofthecommandarea,

• Outsidethefloodlevelincaseofriverwaterabstraction,

• Adequatewaterdepthforsuctionpipe

4.9.10 SelectionofPipesandFittings

Pipesareanintegratedpartofapumpingsystem.Inanyliftscheme,theselectionofproperpipesis

asimportantasthatofthepump.Notonlydopipesrepresentamajorpartoftheinitialinvestment,

theirimproperselectionwilladverselyaffectoperationandmaintenancecosts.Thecompletefailure

ofaschemecanresultfromimproperpipeselection.

Inselectingpipes,considerationmustbegiventotheirdiameter,materialandcost,keepinginview

theworking condition and availability of such pipes on themarket. The basic considerations that

finallyleadtotheselectionofpipesarediscussedbelow:

• Workingpressure,

• Waterhammer-introducebreakpressuretankorsurgetank,

• Internalpressure-adequatepipethickness,

• Externalpressure-considertrafficload,earthpressure,

• Frictioninpipes-smallfrictionfactor,

• Qualityofwater-siltfreewater,

• Costofpipe-lowcost,

• Availability


89

4.9.11 Maincivilworksforpumpedirrigation

Intake

Intake is necessary to guide thewater towards the jackwell or sumpwell. It is alsonecessary to

maintainthewaterlevelfluctuationsintheriver.

Jackwell/Sumpwell

Thewaterdivertedfromtheriveriscollectedinajackwellorsumpwellfromwhereitispumpedto

the irrigationcommandarea.The term jackwell isusedwhen thepumphouse is locateddirectly

over the well. In such cases, vertical turbine pumps are mostly used. When the pump house is

locatedon the ground close to thewell the term sumpwell is used. Forpermanent lift irrigation

schemes,properlydesignedpumpingwellsareprovidedforinstallingthepumpingsets.

Sumpwellsare constructedon thebanksof the river toprotectpumpingunits from floodwater.

Theyareusuallyusedtopumpwaterusingcentrifugalpumpsfromriversandstreamsinwhichthe

flood levels are not high and the water level in the river is within the suction lift of centrifugal

pumps.

Pumphouse

Thepermanentpumphousecanbeprovidedifthewaterlevelintheriverismoreorlessconstant

throughouttheyear.Generallythepumphouseisrectangularinshape.Thepumps,motors,panel

box for electrical components, starters and other accessories are located in the pump house

separately. The dimensions of the pump house are based on the specific requirements of the

installation.Thismayvaryfromsitetositedependinguponthestreamhydrologyalso.

DeliveryChambers/dischargetank

The delivery chamber or discharge tank is necessary at the head of the distribution system.

Generallyitcanbedesignedforoneortwominuteretentioncapacityoftheincomingflow.

4.9.12 Designprocedureofpumpedirrigation

i. Assessthedesigndischargeofthesystembasedonpeakwaterrequirementsandthecommand

area,

ii. Ascertain thewater availability at the source,which should have adequate flow tomeet the

cropwaterrequirements,

iii. Draw the proposed pumped irrigation layout mentioning intake, sump well, pump house,

pipelinealignment,dischargetank,etc,

iv. Drawthelongitudinalprofileofthesystembasedonthelayoutplanmentioningbedlevelofthe

source,high flood level,pumphouse level, criticalpointsalongpipealignment,anddischarge

tank,

v. Calculate the lengthof thepumpingmainlineand leveldifferencebetween thewater levelat

sourceandthedischargetank,

vi. Determinethenumberofpumpingunits,operatinghoursperdayanddesigndischargeofeach

pump,

vii. Determinethestagesof thepumpingsystembasedontheavailableheadandcapacityof the

pump,

viii. CalculateeconomicsizeofpumpingmainfromLea’sformula,

QD ×= 22.1

WhereQisthedesigndischargeinm3/s,


90

ix. Adaptnearestpipediameteravailableinthemarket,

x. Calculatevelocityofflowinthepipe,2

4

D

QV

π=

WhereQisthedesigndischargeandDisthepipediameter,

xi. Adopt economic diameter of pipe considering permissible velocity for the particular type of

pipe,

xii. Calculatetotalheadlossinpipeflow,

Totalheadloss=frictionheadloss+bendloss+entranceloss+exitloss

FrictionheadlossfromDarcy-Weisbachformula,gD

VLfh f

2

2×

=

Wherehfisthefictionheadlossinm,

fisthefrictioncoefficientandcanbecalculatedusingvariousformulas,

Listhepipelengthinm,

Disthepipeinsidediameterinm,

Visthevelocityofwaterinm/s,and

gistheaccelerationduetogravity=9.81m/s2

ColebrookWhiteequationforfrictionfactor,⎟⎟

⎠

⎞

⎜⎜

⎝

⎛+−=

fN

De

f R

51.2

70.3

/log

1

Swamee-Jainequationforfrictionfactor,

2

90.0

74.5

70.3

/log

25.0

⎟⎟⎠

⎞⎜⎜⎝

⎛+

=

RN

Def

Whereeistheroughnessheightofpipematerial,

Disthediameterofthepipe,and

NRistheReynoldsnumber,λ

DVNR= ,whereλistheviscosityofthewater

xiii. Calculatetotaldynamicheadforpumping,

Totaldynamichead=Leveldifferencebetweensourceanddischargetank+Headloss+Suction

head,

xiv. Selectthetypeofpumptobeinstalledinthesystem(centrifugal,submersible),

xv. Selectthepumpcapacitybasedonthemanufacturer’scharacteristiccurves,

xvi. Basedonthepumpmodelanditspowercalculatetheefficiencyofthepump,

BP

WPE pump =

WhereWPisthetheoreticalpower,

BPisthebreakhorsepowerbasedonthepumpmodel,

xvii. Calculatethewaterhammerpressureatthedeliverypipe,

WhereVisthevelocityofflowinpipeinm/s,

gistheaccelerationduetogravityinm/s2,

⎥⎦

⎤⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−+

=

mEt

d

Kw

g

VPh

2

11

.

1


91

wisthespecificweightofthewaterinN/m3=9810N/m

3,

KistheeBulkmodulusofwaterinN/m2=2060N/m

2,

disthediameterofpipeinm,

tisthethicknessofthepipeinm,

EistheYoung’smodulusofelasticityofpipeinkN/m2,

1/misthePoissonratio,

xviii. Comparethewaterhammerpressurewiththeallowablepressureofpipematerial,

4.9.13 DesignexampleofPumpedirrigationsystem

Designdata:

Commandarea 10ha

Sourceofwater PerennialRiver

Minimumwaterlevelatthesource 505.00m

Maximumwaterlevelatthecommandarea 640.00m

Lengthofmainlineprofile 320m

Calculations

Thepeakwaterrequirementbe1.25l/sperha

Thetotalwaterrequirementbe10*1.25=12.50l/s

Assumethepumpshallrunfor12hours,thenpeakdischarge=12.50*2=25l/s

Letusprovidethreepumps inparallel;twoforoperationandoneforreserve,thendischargeofa

singlepumpunitbe,q=12.50l/s

Calculate economic size of pumpingmain from Lea’s formula, QD 20.1= = 1.2*√0.0125 = 136.4

mm

AvailablesizeofGIpipe=125mm

Calculateheadlossforpipediameter125mmandlengthof320m

Velocityinpipe,2

4

D

Q

A

QV

π== =4*1000/(3.14*125^2)=1.020m/s

UsingDarcy-Weisbachequationheadloss,g

V

D

Lfhf

2

2

×=

WherefiscoefficientoffrictionwhichiscalculatedfromMoody’sdiagram

ForGIpipeabsoluteroughness,e=0.15mm

Listhelengthofpipe,L=320m

Disthediameterofpipe,D=125mm

Visthevelocityinpipe,V=1.02m/s

FromMoody’sdiagramf=0.036

Then,hf=0.036*320/0.0125*1.02^2/2*9.81=4.88m

Takebendandjoinlossesas10%ofthefrictionalloss,thetotalloss,hf=4.88*1.1=5.36m

Totalheadforpumping=leveldifference+headloss+suctionhead

Letthedepthofsumpwellbeat2.0mandminimumwaterlevelbelowthesumpwellbe1.20m

Thentotalhead=(640.00-505.00)+5.36+2.00+1.20=143.56m

Based on the discharge to be pumped and dynamic head, select pump from manufacturer’s

characteristiccurve,

SelectKSBorequivalentpumpwithmodelno:UPHA293/8+HBC

Powerofpump=30kW=40HP,


92

Discharge45m3/hourfortotalheadof144m,

Totallengthofpump=2.36m,

Calculatetheefficiencyofthepump,P

gHQρη = =0.0125*9.81*143.56/30=58.60%

Calculatethewaterhammerpressureatthedeliverypipe,

WhereVisthevelocityofflowinpipe=1.02m/s,

gistheaccelerationduetogravity=9.81m/s2,

wisthespecificweightofthewater=9810N/m3,

KistheeBulkmodulusofwaterinN/m2=2060N/m

2,

disthediameterofpipe=125mm,

tisthethicknessofthepipe=5.4mm,

EistheYoung’smodulusofelasticityofpipe=210kN/m2,

1/misthePoissonratio=0.27

Ph=1.02/sqrt(9.81/9810(1/(2060*1000000)+0.0125/0.0054*(210*1000000*1000)(1-

0.27*0.50))/1000*1000=1.34N/mm2,=13.62kg/cm

2

Totalpressure=staticpressure+waterhammerpressure,

Staticpressure=(headdifference+depthofsumpwell+waterlevelbelowsumpwell)/acceleration

duetogravity=(135+2.0+1.2)/9.81=138.20/9.81=14.08kg/cm2

Totalpressure=14.08+13.62=27.70kg/cm2

GIpipewithheavydutycanwithstandthispressure,

Hence,thedesignisacceptable.

⎥⎦

⎤⎢⎣

⎡⎟⎠

⎞⎜⎝

⎛−+

=

mEt

d

Kw

g

VPh

2

11

.

1


93

CHAPTER-5

5. CanalSystemDesign

5.1 Introduction

Before the start of the survey and design, the canal system has to be planned. Canal system

planningincludesdecidingwhattypesofcanalsanddrainsarerequiredandwhattypeofsystemto

use.Italsocomprisesdecidinghowthesystemwillbeoperatedandwhatcontrolstructureswillbe

requiredtodistributewateruptothefarmer’sfield.Inaddition,systemplanninginvolveslayingout

thesystemofcanalsanddrainsonsuitablebasemaps.Hence,an irrigationsystemcomprises the

canalstobringwatertothefieldsandthedrainstotakeawaytheexcessirrigationorrainfallwater.

5.2 TypeofCanals

Basedon their hierarchy, canals are named themain canal, branch canal, tertiary canal and field

canals.Themaincanaltakesoffwaterfromtheheadworkorintakewhileabranchcanaltakesoff

waterfromthemaincanal.Similarly,atertiarycanaltakeswaterfromthebranchcanalandfeeds

fieldcanals.

Basedonthealignment,thecanalsarecategorizedasacontourcanalandaridgecanal.Acontour

canal isalignedalongthecontoursandprovides irrigationonthevalleysideonly.A ridgecanal is

aligned along the ridge and provides irrigation on both sides. Generally,main canals are contour

canalsandbranchcanalsareridgecanals.

Based on the shape of the cross sectional area, the canals are named as trapezoidal canal,

rectangular canal, triangular canal, and circular canal. The shape of the canal depends upon the

capacity,nature,topography,andpurposeofthecanal.

Basedonthematerials inthesectionboundary,thecanalsarelinedcanalandunlinedcanals.The

choiceof linedorunlinedcanaldependsuponthetype,nature,purposeanddevelopmentcostof

thescheme.Generallylinedcanalsaredesignedtoreducetheseepagefromthecanalsectionalong

itsconveyance.Thetypesoflinedcanalsandtheirrelevancearepresentedinseparatesections.

5.3 Layoutplanning

The layout plan is the arrangement of canals, drains, and structures required to achieve water

distribution and remove excess water from irrigation fields. The layout planning establishes the

optimum arrangements, positions, levels of canals and structures. It also considers physical,

technical, and social constraints of the specific site. Generally two types of canal layouts exist

(Figure5-1):

Bifurcatinglayout:Inbifurcatingsystemwaterisdividedamongtwoorthreelargegroupofcanals,

subdivided into twoor threesmallgroupofcanalsandsoon.This typeof layoutsystemexists in

communitymanagedsystemsinvalleysandplains.

Hierarchical layout: In hierarchical system water is divided into large blocks and then after sub-

dividedintosmallerunits.Generally,thistypeoflayoutisexistedinmodernirrigationsystems.The

layoutofcanalsanddrainsfallowridgesandvalleysrespectively.


94

Figure5-1CanalLayouts

Bifurcatinglayout Hierarchicallayout

InsmallhillirrigationschemeslikeinBhutan,thefollowingtwotypesoflayoutsystemsaresuitable

forplanningpurpose(Figure5-2):

a. Thefirstsystemhasaprimaryormaincanalwithaseriesofdirectoff-takesalongthemain

canal.Thistypeofcanallayoutexistedinthecommunitymanagedirrigationsystems;

b. The second type of system has small and medium sized off takes along the main and

secondarycanals.Thenumberofsecondarycanalsdependsuponthesizeofthe irrigation

scheme.

Figure5-2SmallScaleCanalLayouts

Figure5-2showsthatinthetype-Alayout,alloftheofftakesarelocatedalongthemaincanalwhile

in the type-B layout, secondary canal or branch canals also exist. The layout planning of a canal

systeminvolvesthreekeyparameters:

• Linkageofcommandareaandintake,

• Alignmentoftheprimarycanal,and

• Typeofcanallayout

Thelocationofthe intakeisthekeydeterminantofthecommandareaandinmanycaseswillset

thelimitoftheareatobeirrigated.Establishingtherelationshipsbetweentheintake/headworksite

andthecommandareaintermsoflevelsandwaterquantitiesinvolvesatrialanderrorapproach.


95

Routeselectionofacanaldependsontopography,geologicalconditionsalongthealignment,canal

type, type and size of irrigation project. Primary canals in the hills follow the contours and pass

through ridges, valleys, deep gullies, natural drains, land slide zones and vertical to moderately

sloped terrain. In selecting the alignment of the primary canal, it is essential to establish the

geological features along the alignment, and define the stable and unstable portions. The route

shouldbemodifiedifunstableanddifficultfeaturesareobserved.Ifthealignmenthastopasssuch

features,appropriatesolutionsneedtobeadopted.

Typeofcanal layoutalsodecidesthesystemplanning.Themajor layoutsystemsbeingadoptedin

hilly environment are discussed above. However, there may be an alternative layout system

applicableinhillyterrainlikeBhutan(Figure5-3).Thistypeof layoutfeedstheisolatedpocketsof

thecommandareaalongtheprimarycanal.

Figure5-3AlternativeLayoutforSmallScaleCanals

5.4 DesignConceptofIrrigationCanals

Ingeneralprinciple,irrigationcanalsliealongridgesanddrainslieinvalleys.Thedesignofirrigation

canalsinvolvesthefollowingsteps:

• Drawthecanallayout,

• Measureschemecommandarea,

• Determinecanalcapacityordesignflow,

• Prepareschematicdiagramofcanalsystem,

• Designcanalparametersforeachreachofthecanalsystem,

• Workoutcanalwaterlevelsatcriticallocations,

• Drawthepreliminarycanallongprofile,

• Fromstructuresdesignfinalizecanalwaterlevels,

• Drawupfinalcanallongprofile,

• Drawupcanalcrosssections

Forlargeirrigationsystems,canalcapacitydesignstartsfromthefieldleveltomaincanalandupto

the intake.Thisdesignapproach involvescalculationsofwaterrequirementsat the field leveland

capacitiesoftheirfeedercanalssumminguptothemaincanal.Forsmallscaleirrigation,thedesign

ofcanalcapacityiscarriedoutby:

Q=NCWRxCCA/8.64

Where,Q–designcapacityofcanalattheintakeinlps,

NCWR–netcropwaterrequirementinmm/day,

CCA–culturalcommandareainha,


96

If cropwater requirement is not assessed, the capacityof a canal can alsobedeterminedby the

dutyofirrigationwaterordepthofirrigationwaterforthemaincropstobeplanted.

Q=DutyxCCA

Where,dutyorwaterrequirementinlps/ha,

Example:

Duty=3l/s/haandcommandareais20ha,

Thecapacityofcanal(Qd)=3x20=60l/s

Priortostartingthecanalsystemdesignalongwiththestructuredesign,aschematicdiagramofthe

canalschemeisdrawnindicatingcanalsegmentsandlengths,majorstructures,andthecommand

area of the main branch canals and direct off-takes. Based on the schematic diagram of the

proposedcanalsystem(Figure 5-4)canalparametersaredesignedforeachreach.Thecanalscross

sectionalparametersarebedwidth,waterdepth,sideslope,roughnesscoefficient,andfreeboard.

Thelongitudinalslopeofthecanalisdeterminedbasedonthesizeofthecanalandtopographyof

thealignment.

5.5 DesignofUnlinedCanals


Thedesignofunlinedcanalsiscarriedoutusingvariousmethods.Forsmallirrigationschemesinthe

hills,itisappropriatetouseManning’sformula.Thedesignprocedureofthecanalfollowsaseries

ofsteps:

i. Determinethedesigndischarge,Q,

ii. Determinethedesignsideslope,m(1vertical:mhorizontal),

iii. Determinecanallongitudinalslope,S,

iv. Determinebedwidth todepth ratio, r (r=B/D;BbeingbedwidthandDbeingwater

depth),

v. Thecalculationshallbecarriedoutbytrialanderrormethod,

vi. AssumebedwidthofthecanalB,

vii. Calculatedepthofwaterdepth, BrD ×=

viii. Calculatecrosssectionalareaofflow, 2mDBDA +=

ix. Calculatewettedperimeterofflow, ( )212 mDBP ++=

x. Calculatehydraulicradius,P

AR =

xi. CalculatevelocityofflowusingManning’sformula, 2/13/21SR

nV ×=

xii. Calculatedischargeofcanalusingcontinuityequation, VAQ ×=

xiii. Checkthecalculateddischargeandcomparewithdesignedone,Qc>Qd

xiv. Comparethecalculatedvelocitywithlimitingvelocity,Vc<Vper

xv. Calculatetractiveforce, SRWCT ×××=


97

Where,Cisthecoefficientequalto1forbedand0.76forsides,Wisthespecificweight

ofwaterinN/m3,Risthehydraulicradiusofthecanal,andSistheslopeofthecanal,

xvi. Comparecalculatedtractiveforcewithpermissiblevaluesaccordingtotheirmaterial,

Excelsheetsarerequiredtodesignvarioussectionsofthecanal.Thedesignparametersofacanal

arepresentedinthesectionsbelow.

Figure5-4SchematicDiagramofCanalSystem


98

5.5.2 SideSlopes

Thesideslopesforthetrapezoidalcanalsectionisdeterminedonthebasisoftheslopestabilityof

soil.Thesideslopeshouldbeas lowaspossible.Thesideslopesshouldbeslightlyflatter infilling

sectionsthanincutsections.Therecommendedsideslopesofthecanalarepresentedbelow(Table

5-1).

Table5-1Recommendedsideslopesofcanal

Soiltype Hardrock Softrock Heavyclay Loam Sandyloam

Sideslope

(Verticalto

horizontal)

1:0.25 1:0.50 1:0.50to1:1 1:1to1:1.50 1:1.5to1:2

5.5.3 PermissibleVelocity

Themaximum flow velocity, above which scouring will occur in the canal, is termed permissible

velocity. This permissible design velocity is chosen from experience and knowledge of the soil

conditions.Alowervelocityallowsweedstogrowinthecanalsection,consequentlydecreasingthe

canal capacity, while a higher velocity tends to erode the bed of an unlined canal. Hence, the

velocityshouldbewithinthepermissiblelimitsbasedonthetypeofsoilandcanalcapacity(Table

5-2). The velocity shall be checkedwith permissible tractive force of the canalmaterial, which is

givenbytheequation:

T=CWRS

Where,Cisthecoefficienttakenas1forbedand0.76forsides,

Wisthespecificweightofwater,1000kg/m3,

Risthehydraulicradiusofthecanalsection,m

Sisthewatersurfaceslopeofthecanal,

Table5-2Maximumpermissiblevelocitiesandtractiveforces

S.N Material Maximumpermissible

velocity(m/s)

TractiveForce(kg/m2)

Clearwater Siltywater

1 Finesand 0.45–0.75 0.13 0.37

2 Siltloam 0.60–0.75 0.23 0.54

3 Alluvialsilt 0.60–0.75 0.23 0.73

4 Ordinaryloam 0.75–1.05 0.37 0.73

5 Finegravel 0.75–1.05 0.37 1.56

6 Coarsegravel 1.00-1.25 1.47 3.28

7 Cobblesandshingles 1.25-1.50 4.45 5.39


99

5.5.4 BedWidthtoDepthRatio

Thebedwidthtodepthratiodependsuponthecapacityofthecanalanddeterminesthehydraulic

efficiencyofthesection.Forsmall irrigationcanals,whenthedischargeis lessthan0.50m3/s,the

ratioofbedwidthtodepthrangesfrom1to2.Fordischargeshigherthan0.50m3/s,widercanals

shouldbeprovidedifnecessary.Inahillyterrain,awidercanalsectionrequiresmorehillcutting.In

addition,inrockyterrain,narrowsectionsarepreferable.Hence,bedwidthtodepthratioisfixed

basedon thescheme’s siteconditions. A typical canal sectionofa trapezoidal canal ispresented

below(Figure5-5).

Figure5-5Typicalsectionoftrapezoidalcanal

5.5.5 CanalSlope

The canal slope is designed for non-silting and non-scouring velocity and is calculated using

Manning’sFormula:

n

SRV

2/13/2

=

Where,Rishydraulicradius=A/P,Siscanalslopeandnisroughnesscoefficient,

For small irrigation schemes, the slopeof thecanalmaybeup to1:500 in thehills and1:1000 in

plainareas.However,forexistingcommunitycanals,theslopesmaybesteeperrangingfrom1:50to

1:500dependinguponthecanalsize,soiltype,andtopography.Atypicallongitudinalprofileofthe

canalispresentedbelow(Figure5-6).

Bedwidth

1:m(sideslope)Waterdepth

Freeboard

Bankwidth


100

Figure5-6Typicallongitudinalprofileofcanal


101

5.5.6 Manning’sRoughnessCoefficient

ThevalueofManning’sroughnesscoefficient(n)dependsuponthetypeofbedmaterials,condition

ofthecanalanditsgeometry.Therecommendedvaluesof‘n’arepresentedbelow(Table5-3).

Table5-3Valuesofroughnesscoefficient

S.N Description Roughnesscoefficient,n

1 Earthencanalrecentlyexcavated 0.016to0.022

2 Gravelbedcanal 0.022to0.030

3 Canalwithshortgrass 0.022to0.033

4 Canalwithdenseweeds 0.030to0.040

5 Canalswithearthbedandrubblesides 0.028to0.035

6 Canalwithstonybottomandweedybanks 0.025to0.040

Source:DerivedfromVenTeChow,1959

5.5.7 FreeBoard

Freeboardisthedifferencebetweenbanktoplevelandfullsupplylevelofthecanal.Itisnecessary

toaccommodatesurplusdischargeofacanalcomingfromuphillreachesorfromotheroperations.

The free board depends upon the capacity of the canal and its recommended values are given

below.

ForQlessthan0.10m3/sFb=0.20m,

ForQbetween0.10to0.50m3/sFb=0.30m,

ForQlargerthan0.50m3/sFb=0.50m

5.5.8 MinimumRadiusofCurvature

The irrigation canal should be as straight as possible froma hydraulic point of view.However, in

practiceitisnotpossibletoalignstraightcanalandcanalsneedtobendtoprovideanappropriate

radiusof curvature,whichdependsupon thecanal capacityand topography. . For small irrigation

schemes, theacceptable radius foranunlinedcanal is taken tobe from three to seven times the

water surfacewidth. In rockycanal sections, the radiusof curvatureshallbeonly three times the

width,whileforgeneralsoil,itisuptoseventimesthewidth.

5.5.9 DesignExampleofUnlinedCanal

Data:

Commandarea=20ha,

Waterrequirementinhills=3pls/ha

Soiltype=Ordinarysoil

Manning’sroughnesscoefficient=0.025


102

Calculation:

Determinedesigndischarge(Q)=commandareaxwaterrequirement=20*3=60lps=0.06m3/s

Forordinarysoiltakesideslope(m)=1:1

Determinebedwidthtodepthratio(r)=1

Assumebedwidth,B=0.20m

Adoptslopeofcanalas1:100asperexistingconditions,

Calculatedepthofflow,D=r*B=0.20m

Calculatecrosssectionalarea, 2mDBDA += =0.20*0.20+1*0.20^2=0.08m

2,

Calculatewettedperimeterofflow, ( )212 mDBP ++= =0.20+2*0.20*sqrt(1+1^2)=0.77m

Calculatehydraulicradius,R=A/P=0.08/0.77=0.10m

Calculatevelocityofflow, 2/13/21SR

nV ×= =1/0.025*0.10^2/3*(1/100)^1/2=0.89m/s

Calculatedischarge,Q=AxV=0.08*0.89=0.071m3/swhichisgreaterthandesigneddischargeof

0.060m3/s,

The velocity of flow is slightly higher than the permissible for earthen canal, however itmay be

suitableforexistingsiteconditionhavinghardclayorrockybed,

Calculatetractiveforce,T=CWRS=1*1000*0.10*1/100=1.04kg/m2whichisinpermissiblelimit,

Hence,designofunlinedcanalisacceptableandparametersare:

Bedwidth=0.20m,

Waterdepth=0.20m,

Freeboard=0.10m,

Canaldepth=0.30m

Sideslope(V:H)=1:1

Longitudinalslope=1/100

ThedesignofcanalsectionisusuallycarriedoutinExcelsheetwithtrialanderrormethod.

5.6 LiningofCanals

5.6.1 PurposeofLining

Canal lining is themethodofsealingthecanalsurfacetopreventseepageand leakagefrom

the section. In addition, canal lining supports unstablebanks andprevents erosion in steep

sections of the canal. Lining is often a major demand of farmers in the rehabilitation of

existing schemesor in the constructionofnew schemes.Theneed forand the typeof canal

lining should be carefully considered during the planning and design stages and wherever

practicallowercostalternativesshouldbeseriouslyconsidered.

5.6.2 TypeofCanalLining

i. DryStoneLining

Drystoneliningiscommoninhillyareaswherestonesareeasilyavailableandwaterisabundantat

thesource.This liningismainlytoprotectagainsterosionandisnoteffectiveforseepagecontrol.


103

Thethicknessoftheliningdependsuponthesizeoftheavailablestonesandrangesfrom20cmto

50cm.Atthedownhillsideofthecanal,drystoneretainingwallsareusedaspartofcanalliningto

stabilizethebankandprotectsoilerosion.

ii. StoneMasonryLining

Stonemasonry lining is themost commonly used lining in the hills and foothill areas. Due to its

effectiveness inpreventingseepageanderosion, it iswidelyusedwherestonesareavailable.The

recommendedthicknessoftheliningrangesfrom30cmto50cmin1:4cementsandmortar.For

smallschemes,liningcanoftenbemadewithdressedstone,butselectedundressedriverstoneswill

be cheaper inmost cases. The inner sides of themasonry liningmaybeplasteredwith sand and

cementmortar,butthis isnotrecommendedforsmallschemes. Insomecases,theinnerbankof

themasonryliningisonlymadewithdrystonesasthisallowsdrainageintothecanalfromuphill.A

stonemasonryliningwiththeconcretebedisthemostcommonformofcanallining.However,due

toqualityofworksinthehills,thesustainabilityofstonemasonryliningisinquestion.

Figure5-7Typicalstonemasonryandconcretelining

Stonemasonrylining Concretelining

iii. ConcreteLining

Cementconcreteliningisthemosteffectivemeansofcanalliningasitpreventscanalseepageand

leakage. Depending upon the workmanship, the roughness coefficient of a concrete lining is

significantly low incomparison tostonemasonry lining,whichallowshigherdischarge inasimilar

section.Concrete liningmaybeplaincementconcrete liningor reinforcedcementconcrete lining

dependingupon the typeof canal tobe linedand thenatureof theproject. The thicknessof the

concrete lining varies from 7.50 cm to 15 cm depending upon the capacity of the canal and the

objectiveofthelining.Forlargecanals,trapezoidalsectionissuitableforconcreteliningwhilefor

small canals, rectangular sections are popularly used in the hills. In addition, concrete liningwith

nominalreinforcementorwithGIwiresisrecommendedaslowcostcanalliningsolutionsforsmall

irrigationschemes.

iv. SoilCementLining

Soilcementliningisalowcostcanal liningtechnologyandisrarelyusedinirrigationcanals.It isa

mixtureofredsoilandcementwith7partssoilandonepartcement.Themortarofthesoilcement

isappliedtothesurfaceofthecanalascementplasterintwotothreelayers.Thethicknessofone


104

layerofplastermaybe2to3cm.Afterplastering,curingisalsoimportantandtheliningshouldbe

coveredandkeptmoistforafewdays.Thistypeofliningiseffectiveinseepagecontrolbutcanbe

damagedbycattlemovementsandothererosion.Soilcementliningisthemosteconomicalliningin

anirrigationproject.

Figure5-8Ferrocementandsoil-cementlinings

Ferrocementlining Soilcementlining

v. FerroCementLining

Ferro cement is a thinwall reinforced concretemade up ofwiremesh, sand, and cementwhich

possesses strength and serviceability. The mixture of cement with pie gravel is placed over the

thoroughly compacted surface of canal laid with plastic sheet and chicken mesh wire. Nominal

reinforcementisalsoprovidedifnecessary.Thistypeofliningiscostlierthanmasonrylining,butis

durableandeffectivetocontrolseepageandleakage.

vi. PlasticLining

Plastic lining is usedwhere the alignment of the canal falls in unstable areas like those prone to

landslides.Amembraneconsistingofasufficientlythickplasticsheetislaidontheproperlyleveled

surfaceofthecanalandacoverofdrystoneorsoil isprovidedtoprotect it fromminorpuncture

and tear. Plastic lining isquiteeffective for seepage control, is economical andeasy to construct.

Thethicknessoftheplasticsheetmaybe200to300micronsorequivalent(0.25mmto0.50mm).

Plasticliningispronetodamageandtemporaryinnature.

vii. SlateLining

Slate lining is a special type of canal lining usedwhere slates are available. This type of lining is

suitableinmountainswheresoilsareerosiveincanalbed.Thisliningiseffectiveforseepagewhen

usedwith cementmortar pointing and cheaper than other types of linings. The thickness of the

liningmaybe10cmto50cmdependinguponthethicknessofslates.

Inshort,cementconcrete lining is themostsustainable typeof liningcomparedtoother typesof

lining,andhenceitisrecommendedtouseeveninsmallcanals.


105

5.6.3 DesignProcedureofLinedCanal

DesignoflinedcanaliscarriedoutusingcontinuityequationandManning’sformulaconsideringas

rigidboundarycanalsection.Thedesignprocedureofthecanalfollowsaseriesofsteps:

i. Determinethedesigndischarge,Q,

ii. Determinethesideslope,m(1vertical:mhorizontal),

iii. Determinecanallongitudinalslope,S,

iv. Determinebedwidthtodepthratio,r(r=B/D;BbeingbedwidthandDbeingwaterdepth),

v. Determinetheroughnesscoefficientbasedontheliningmaterial(Table5-4),

Thedesigncalculationshallbecarriedoutbytrialanderrormethod.

vi. AssumebedwidthofthecanalB,

vii. Calculatewaterdepth, BrD ×=

viii. Calculatecrosssectionalareaofflow, 2mDBDA +=

ix. Calculatewettedperimeterofflow, ( )212 mDBP ++=

x. Calculatehydraulicradius,P

AR =

xi. CalculatevelocityofflowusingManning’sformula, 2/13/21SR

nV ×=

xii. Calculatedischargeofcanalusingcontinuityequation, VAQ ×=

xiii. Checkthecalculateddischargeandcomparewithdesignedone,Qc>Qd

xiv. Comparethecalculatedvelocitywithlimitingvelocity,Vc<Vper

Table5-4Recommendedroughnesscoefficientsforlinedcanal

S.N LiningType Maximum Velocity

(m/s)

Roughness Coefficient

‘n’

1

2

3

4

5

6

Drystone

DryBrick(withoutmortar)

Dressedstonemasonry

Brickmasonry

Randomrubblemasonry(stone)

PlaincementconcreteorRCC

1.0

1.0

2.0

1.5

1.5

2.5

0.025–0.028

0.020–0.025

0.018–0.020

0.017–0.020

0.020–0.015

0.015–0.017

Source:PDSPManualVolumeM8,1990

Thelongitudinalslopeforsmallcanalsdependsuponthetopographyandexistingconditionofthe

canal.Forrectangularsectionofcanalthebedwidthtowaterdepthratiorangesfrom0.5to2.The

side slope of the lined canal is zero in case of rectangular sectionwhile it is 1:0.50 to 1:1.50 for

trapezoidalcanal.


106

5.6.4 DesignExampleofLinedCanal

Data:

Commandarea=20ha,

Waterrequirementinhills=3lps/ha

Typeoflining=stonemasonry

Sectiontype=rectangularsection

Manning’sroughnesscoefficient=0.02

Calculations:

Determinedesigndischarge(Q)=20*3=60lps=0.06m3/s

Forrectangularsectionsideslope(m)=0

Determinebedwidthtodepthratio(r)=1

Assumebedwidth,B=0.25m

Adoptslopeofcanalas1:100asperexistingconditions,

Calculatedepthofflow,D=r*B=0.25m

Calculatecrosssectionalarea,2

mDBDA += =0.25*0.25+0*0.25^2=0.0625m2,

Calculatewettedperimeterofflow, ( )212 mDBP ++= =0.20+2*0.20=0.75m

Calculatehydraulicradius,R=A/P=0.04/0.6=0.083m

Calculatevelocityofflow,2/13/21

SRn

V ×= =1/0.02*0.083^(2/3)*(1/100)^1/2=0.95m/s

Calculatedischarge,Q=AxV=0.0625*0.95=0.60m3/swhichisequaltothedesigneddischargeof

0.060m3/s,

Thevelocityofflowis lessthanthepermissiblevelocityforstonemasonrycanal.Hence,designof

linedcanalisacceptableandparametersare:

Bedwidth=0.25m,

Waterdepth=0.25m,

Freeboard=0.15m,

Canaldepth=0.40m

Sideslope(V:H)=0

Longitudinalslope=1:100

The design of canal section is usually carried out using excel sheets following a trial and error

method.

5.7 CoveredCanals

5.7.1 Introduction

Coveredcanalsareusedwherelocalslipsoccurorarelikelytooccur,whichcouldblockthecanals.

Sometimestheyarealsonecessaryindeepcutsectionsandintheheadreachofacanaltoprotect

thecanalfromfloods.Thecrosssectionalsizeshouldbemadelargeenoughfordesiltingpurposes

and access should be provided on long lengths. If rock falls are likely, the structure should be


107

covered with earth to dampen the fall. Covered canals are generally not suitable for deep slide

areas.Therearevarioustypesofcoveredcanals(Figure5-9),someofthesearepresentedbelow.

i. ConcreteorStoneSlabCover

Inrockyareaswherecanalsectionisperfectlycutintothesizeandconcreteorstoneslabscan

be laid directly over the canal section. The slabs should be of manageable size so that, if

necessary,theycanberemovedduringcanalmaintenance.

ii. Slabsoverstonemasonrylining

Forlengthswheresmallslipsarelikelytooccur,slabsarelaidonmasonrywalls.Generallycover

slabs canbemadeof reinforced concrete. If a slip occurs on anuncovered sectionof a lined

canal, repairs using slabs would be the most economical solution. On long reaches, for

maintenanceaccess,oneormoreslabscanbetemporarilyremoved.

iii. Pipes

HDPpipescanbeusedasacheapformofthecoveredcanalforsmallschemes,butcaremust

be taken to ensure that the pipes are laid on an adequate slope to prevent siltation and

blockage.Inaddition,adequatepipejointsmustbeprovidedtopreventseepage,whichcould

leadto furtherslips.For largercanals, reinforcedconcretepipesofadequatediametercanbe

used for covered canals. Accessmanholes should also beprovided at 20 to 30m intervals of

coveredcanal.

Figure5-9Typicalcoveredcanals


108

5.7.2 DesignConcept

Coveredcanalsaregenerallydesignedtobefreeflowingopenchannelslikelinedcanalsections.For

thehydrauliccomputation,Manning’sformula,whichwasalreadydescribedinprevioussections,is

used.Thevelocityofwatershouldbekeptapproximately1m/sandfreeboardshouldbesufficient

(atleast0.20m).Thewaterdepthinthepipesshouldbekeptas2/3ofthediameterofthepipe.For

smallscaleschemes,HDPpipesmayrunfull.Headlossesinthetransitionbetweentheopencanal

andthecoveredcanalneedtobeconsidered,whicharecalculatedas:

( )g

VVCiHi

p

2

22

−=

( )g

VVCoHo

p

2

22

−=

Where,Hi,Hoaretheheadlossesattheinletandoutletrespectivelyinm,

Ci,Coarelosscoefficientsfortheinletandoutlet;

Vpisthevelocityinthecoveredsectioninm/s,

Visthevelocityintheopencanalinm/s,

gistheaccelerationduetogravity=9.81m/s2

The value of Ci and Co is different for different types of transitions. The values for some of the

transitionsareaspresentedhere (Table5-5).ThemostpracticalvaluesofCiandCoare0.50and

1.00respectively.

Table5-5Coefficientsforinletandoutletofdifferenttransitions

S.N Typeoftransition Ci Co

1 Culvertpipeterminatesinheadwallacrosschannel 0.50 1.00

2 Headwall with rounded transition of which the radius

exceeds0.1timesthedepthofwater

0.25 0.50

3 Brokenbacktransitionwithflareangleofabout1:5 0.20 0.40

Largediameterpipesaregenerallyflowpartfullincoveredcanals.Thefollowingtablepresentsthe

hydraulicpropertiesofthepipesflowingpartfull(Table5-6).

Table5-6Hydraulicpropertiesofpartfullflowingpipe

Waterdepth

frominvert

Wettedarea(A) Wetted

perimeter(P)

Hydraulic

radius(R)

Watersurface

width(Ws)

0.10d 0.04d2 0.64d 0.063d 0.6000d

0.20d 0.11d2 0.93d 0.118d 0.8000d

0.30d 0.20d2 1.16d 0.172d 0.9165d

0.40d 0.29d2 1.37d 0.212d 0.9798d

0.50d 0.39d2 1.57d 0.250d 1.0000d

0.60d 0.49d2 1.77d 0.277d 0.9798d

0.70d 0.58d2 1.98d 0.293d 0.9165d


109

Waterdepth

frominvert

Wettedarea(A) Wetted

perimeter(P)

Hydraulic

radius(R)

Watersurface

width(Ws)

0.80d 0.67d2 2.21d 0.303d 0.8000d

5.7.3 DesignExamplesofCoveredCanal(PipedCanal)

i. Case-IPipeflowingfull

Data:

Discharge,Q=100l/s

Velocityincanal=0.50m/sec.

Pipetype=HDP

Length=70m

2bendsat45oradius=2*diameter

Availableheaddifference=2.0m

Calculations:

Basedonthedesigndischargeandpipetypeselectsizeofthepipe,

FromHDPpipefrictionchart(Figure5-11)select250mmdiameterpipe,

CalculatethecrosssectionareaofthepipeA=πD2/4=3.14*0.25^2/4=0.049m

2

Calculatethevelocityofflow,V=Q/A=0.1/0.049=2.05m/s,acceptableforHDPpipe

Determinefrictionlossfromgraph(Figure5-11),frictionlossis1.10mfor100mlength,

Calculatefrictionlossfor70mlengthofpipe,frictionheadlosshf=70x1.1/100=0.77m

Headlossatentryandexit,( )

g

VVhe

2

5.12

12

22−

= =1.5[2.052–0.5

2]/19.62=0.32m

Headlossatpipebends,g

Vfh bb2

2

= =0.09x2.052/19.62=0.02m

Fortwobendsheadloss,hb=0.02x2=0.04m

Therefore,totalheadloss=0.77+0.32+0.04=1.13m<2m

Thetotalheadlossislessthanavailableleveldifferenceandhencedesignisacceptable.

ii. Case-IIPipeflowingpartfull

Data:

Discharge =200l/s

Depth =0.5m

Pipetype =concrete

Length =150m

Velocityincanal =0.5m/s

Manning’sn =0.015

Simpleheadwallforentranceandexitofthepipe


110

Calculations:

Velocityistobekeptbelow3.0m/s

Thepipewillflowwithminimumof0.20diameterfreeboard.

FromTable5-6areaforwaterdepthof0.8diameter=0.67d2

Calculateminimumflowareinthepipewiththevelocitylimit,A=Q/Vmax

Q=200l/s=0.20m3/sandVmax=3m/s;thenA=0.2/3.0=0.067m

2

And,hence,0.67d2=0.067

Thediameterofpipe,d=0.315m

Requireddiameter=0.315m

Use350mmdiameterpipeasnextavailablesize,

Calculatetheactualvelocityofflowinpipe,V=Q/A=0.2/(0.67Xd2)=0.2/(0.67X0.35

2)

V=2.44m/s

Beforeproceedingwiththecalculationoflosses,checkthepipeinletsettinginrelationtothecanal

depth,

Canaldepth=0.5m

Pipediameter=0.35m

Ifpipeinvertissetatcanalbedlevel,thenwaterdepthaboveinvert=0.5m.Thisisabout1.4times

thepipediameter. Thepipe shall submergewhenwaterdepth is greater than1.2 times thepipe

diameter.Toavoidsubmergence,thepipeinvertwouldhavetobesetabovecanalbedlevelwhich

is not satisfactory for sediment transport. Therefore, increasepipediameter to 0.6m inorder to

matchthecanaldepthandsetinvertofpipeatbedlevel.

Calculatetheslopeofthepipefromformula:

2/13/21

SRn

V = or3/4

22

R

VnS =

Fordepthofflow=0.8xdiameter,hydraulicdepthR=0.303d

Areaofflow,A=0.67d2 =0.67x0.60^2=0.24m

2

Hydraulicradius,R=0.303d=0.303x0.60=0.182m

Velocityofflow,V=Q/A=0.20/0.24=0.83m/s

Slopeofthepipe,S=(0.0152x0.832)/(0.1824/3)=0.0015

Headloss=Sxlength=0.0015X150=0.23m

Forentryandexitlosses,Ci=0.50andCo=1.0

Losses=[1.5(Vp2–V2)]/2g=[1.5(0.832–0.5

2)]/19.62=0.03m

Totalheadloss=0.23+0.03=0.26m

Setoutlettoensurethatthepipeisnotsubmerged.Allow0.2xdiametersasfreeboardi.e.,setpipe

invertat(downstreamwaterlevel–0.8xdiameters).

TheFroudenumberinpipeflowingpartfull(andcoveredcanals)shouldbekeptbelow0.55toavoid

standingwaves.TheFroudenumberiscalculatedfromformula:

F=[αV2/(g.dm)]0.5

Where,dm=flowareainsqm/watersurfacewidthinm,


111

HereV=0.83m/s

dm=0.67d2/0.8d=0.536xd=0.32m

Andthen,F=0.83^2/(9.81x0.32)^1/2=0.39<0.55

Hence,designisacceptable.


112

Figure5-10Designchartforconcretepipes


113

Figure5-11DesignchartforHDPEpipes


114

5.8 DesignofPipelines

5.8.1 Basicconceptonhydraulicdesign

Energy is necessary for themovementofwater fromoneplace to another. A gravity-flowwater

system is powered by gravitational energywhich is determined by the relative elevations of the

system.Aswaterflowsthroughpipes,someenergyislostforever,dissipatedbyfriction.Duetothe

changingtopographicprofileofthesystem,somepointshaveminimalenergy(lowpressure)while

somepointshaveexcessiveenergy(highpressure).Thepurposeofpipelinedesign,therefore,isto

manage properly frictional energy losses in order to deliver the desired amount of flow to the

desired locations. This is accomplished by the careful selection of pipe sizes, strategic location of

controlpoints,breakpressuretanks,reservoirsandoutlets.

i. Relationbetweenheadandwaterpressure

Headisthemeasureofenergyandrepresentstheamountofgravitationalenergycontainedinthe

water. Inmetric system of units, head is alwaysmeasured inmeters. Thewater pressure at any

pointisdeterminedbythedepthofwateratthatpoint(Table5-7).

Table5-7Waterpressureaccordingtothehead

Head(m) 10 20 30 50 60 100

Waterpressure(kg/cm2) 1 2 3 5 6 10

Inapipelinewherewaterisnotmoving,thelevelofwatersurfaceistermedasstaticlevelandthe

pressuresarereportedasstaticheads.Whenwater ismoving inthepipelinewithaconstantflow

thesystemisindynamicequilibrium.

ii. Hydraulicgradeline(HGL)

Hydraulic grade line represents the energy level at each point along the pipeline. The vertical

distancefromthepipelinetotheHGListhemeasureofpressureheadandthedifferencebetween

HGLand static level is theamountofhead lostby the frictionof the flow.Thewaterpressureat

atmosphere is zero and HGL always come to zero wherever comes into contact with the

atmosphere. Since frictional losses are never recovered, the HGL always slopes down along the

directionoftheflow.

iii. Controlvalves

Anexcessive amountof energy can cause thepipe to burst.Onemethod to control an excessive

amountoftheenergyistoinstallcontrolvalvesatstrategicpointsthroughoutthepipelinesystem.

Avalveisadevicewhichcanbeadjustedtocreategreaterfrictionallossesaswaterflowsthroughit.

Therearetwotypesofcontrolvalves:gatevalvesandglobevalves(Figure5-12)usedinpipeline.

A gate valve serves as anon/off control valve for thepurposeof completely cuttingoff the flow.

Generally, it is located at the outlets of the intake, reservoir, break pressure tank, and atmajor

branchpoints.Globevalvesaredesignedtoregulateflowthroughthesystemandarebestlocated

nearthedischargepoints.


115

Figure5-12Gateandglobevalves

GateValve GlobeValve

iv. Frictionalheadloss

Any obstruction to the flow, partial or otherwise, causes frictional losses of the energy. The

magnitude of the energy lost due to the friction against some obstacle is determined by several

factors.Themajorfactorswouldbetheroughnessoftheobstacleandthevelocityoftheflowwhile

minorfactorswouldincludewatertemperature,suspendedparticlesetc.Thediameterofthepipe

andtheamountofflowthroughitdeterminesthevelocityofflow.Thegreatertheflow,thefasteris

thevelocity,andthegreaterthefrictionallosses.Likewise,therougherthesurfaceoftheobstacle,

the greater is the frictional losses. Frictional head losses are provided by pipemanufacturers for

bothHDPandGIpipes.

Frictional head losses of flow through fittings such as elbows, reducers, tees, valves etc are

calculated using various formulas and coefficients. In small scale pipeline design, these frictional

headlossesareavailableasequivalentpipelengths.Theamountofheadlossinthefittingsdepends

upon the shape of the fitting and the flow through it. Head losses are computed with the

determinationoftheequivalent lengthofpipenecessarytocreatethesameamountofheadloss.

Thelengthstodiameter(L/D)ratioforvariousfittingsaregivenhere(Table5-8).

Table5-8Lengthtodiameterratioofdifferentfittings

S.N Fittings L/Dratio

1 Tee(run-side) 68

2 Tee(run-run) 27

3 Elbow(90o) 33

4 Union 7

5 Gatevalve 7

6 Freeentrance 29

7 Screenedentrance 150


116

v. Maximumpressurelimits

Maximumpressure limitsareconsideredwhenselectingpipematerialsandtypetowithstandthe

highestpossiblepressurethanoccursinthepipeline.Foragravitysystem,theworstscenarioisfor

pressureduringshut-offconditions.Class IIIHDPpipeshavemaximumpressure limitsof6kg/cm2

whichmean60metersofhead.ClassIVHDPpipeshavemaximumpressurelimitsof10kg/cm2for

theuse in100metersofhead.Class IVpipeshavehigherwall thicknesswhichwithstandsgreater

pressures.GalvanizedIron(GI)pipescanwithstand25kg/cm2ofpressure,meaning250metersof

head. To limit themaximumpressure, breakpressure tanks couldbe constructed along themain

pipeline.

vi. Minimumpressurelimits

The pressure in the pipe along the section where HGL is underground is a negative pressure.

Negative pressure in the pipe needs to be avoided for the proper design of the system. It is

recommendedtoalignHGL10metersabovethegroundleveltohavetheminimumpressurelimit.

Inanycase,HGLisnotallowedtogoundergroundatall.

vii. Velocitylimits

The velocity of flow through the pipeline is also an important consideration in the design of a

pipeline.Ifthevelocityistoohigh,theflowcancauseexcessiveerosionofthepipe,whileifitistoo

low,itcancreatecloggingofthepipeline.Therecommendedvelocitylimitsare3.0m/s(maximum)

to0.70m/s(minimum).

viii. Air-blocksandwashouts

Anair-block isabubbleofairtrappedinthepipelinewhosesize issuchthat it interfereswiththe

flow ofwater through the section.Whenwater is allowed to fill or refill the pipeline, air cannot

escape from certain section and is trapped. The proper design canminimize the trapped air and

potentialair-blocksby:

• Arrangingpipe sizes tominimize frictionalhead lossbetween the sourceand the first air-

block,

• Usinglargersizepipeatthetopandsmallersizeatthebottomofthecriticalsections,

• Eliminatingorminimizinghigherair-blockswhichareclosertothestaticlevel,

• Providingairvalves,

Atypicallayoutofpipelineispresentedforreference(Figure5-13)

ix. Airvalves

Airvalvesareautomaticairreleasedevices,whichreleasesaccumulatedairpressureofthepipeline.

Inmains,airvalvesaremostlyplacedatcriticalplaces.Astructureconstructedtolocatethevalveis

referred as valve chamber. The typical examples of air valves are presented in Figure 5-14 and

typicalplanofthevalvechamberarepresentedin Figure5-16.


117

Figure5-13Typicallayoutofpipeline

x. Washouts

WashoutsarelocatedatthebottompointsofthemajorU-profiles,especiallythoseupstreamfrom

thereservoirtank.ThewashoutsarethegatevalvemountedontheTeebranchtoamainline.The

number of washouts in the system depends upon the type of source, whether, or not there is

sedimentation tank or reservoir, and the velocity of flow through the pipe line. The size of the

washoutpipes shouldbesameas thatof thepipelineat thatpoint.Thesewashoutsare regularly

openedtocleanthepipe.

Figure5-14Typicalairreleasevalves


118

xi. Breakpressuretank

A break pressure tank (BPT) is a structure to allow the flow to discharge into the atmosphere,

therebyreducingthehydrostaticpressuretozeroandestablishinganewstaticlevel.ABPTisbuilt

alongthemainpipelineatstrategic locations.Theflowingwater isdischarged inthetankandthe

energyoftheflowingwaterisdissipatedwithappropriatedevicesandwaterisbacktoatmospheric

pressure.TheenergydissipationiscarriedoutwithbafflewallsconstructedspeciallyintheBPT.

Breakpressuretanksareconstructedwithcementconcreteorstonemasonrywithorwithoutfloat

valves.Anoverflowsystemwith thecapacity to remove themaximumquantityofwaterwhich is

likelytooverflowshouldbeincluded(Figure5-15).Theoverflowwatershouldbedisposedinsucha

waythatitwillnotcausesoilerosion.

Figure5-15Typicalplanofbreakpressuretank

5.8.2 DesignConceptofMainPipeline

The basic concept of themain pipeline is to calculate the head losses in different reaches of the

pipeline.Foreachreach,itisnecessarytodeterminethedesiredamountofheadtobeburnedoff

andthelengthofthepipeline,calculatethedesiredfrictionalheadlossfactor,andselectthepipe

sizefromthetable.Whendesigningthepipe,thedesignercanbeginfromthesourceorfromthe

end of the pipeline. It is also necessary to assess the requirement of the reservoir based on the

balancestudyofsupplyanddemand.


119

Figure5-16Typicalvalvechamber

5.8.3 Headlossesinpipesandfittings

Thecalculationofheadlossesinthepipesandtheirfittingsisthemostimportantparameterofpipe

line design. Head losses in the pipe line occur due to friction, bends, and sudden expansion and

contractionofpipes.Thecommonlyusedformulasforthecomputationofheadlossduetofriction

are:

• Darcy-Weisbachformula,

• HazenWilliamsformula,

• Manning’sformula,and

• Colebrook-Whiteequation

i. Darcy-Weisbachformula

Darcy-Weisbachisthemostwidelyusedformulainpipelinedesignforshortlengthpipes.

gD

VLh f

2

42

×= λ

Wherehfisthefictionheadlossinm,

λisthefrictioncoefficient,


Disthepipeinsidediameterinm,

Visthevelocityofwaterinm/s,and

Gistheaccelerationduetogravity=9.81m/s2


120

Forcastironpipesλ=[0.02+1/(2,000D)]x1.5

Forsteelpipesλ=[0.0144+9.5/(1000√V)]x1.5

Forexactcalculationofλ intermsofReynoldsnumberandrelativeroughnessMoody’sdiagramis

used(Figure5-17).FortheuseofMoody’sdiagramitisnecessarytocalculatetheReynoldsnumber

fromformula:

γ

DVRe=

WhereVistheaveragevelocityofflowinm/s,

Diameterofpipeinm,and

γisthekinematicviscosityofwaterinm2/s

ii. Hazen-William’sFormula

Hazen-William’sformulaiswidelyusedforlongdistancepipelines.

87.485.1

85.167.10

DC

Q

L

h f

×

×=

Wherehfisthefrictionheadlossinm,


Qistheflowrateinm3/s,

CistheflowcoefficientbasedontypeofpipesandisprovidedinTable5-9,

Dispipeinsidediameterinm

Table5-9Hazen-William'sflowcoefficients

Typeofpipe Flow

coefficient(C)

Typeofpipe Flow

coefficient(C)

Newweldedsteelpipe 140 Cementlinepipe 140

Steelpipeafter20yearsuse 100 Asbestoscementpipe 130

Newcastironpipe 130 Pre-stressedconcretepipe 130

Castironpipeafter20yearuse 100 Plasticpipe 150

iii. Manning’sformula

Manning’sformulaisgenerallyusedinopenchannelflowandinculvertsandisgivenby:

3/4

22

R

Vn

L

h f ×=

Wherehfisthefrictionheadlossinm,


nistheroughnesscoefficientandisgiveninTable 5-10,


121

Visthevelocityofflowinm/s,and

Risthehydraulicradiusofthepipeandisequaltocrosssectionalareadividedbywettedperimeter

ofthepipeinm,

Table5-10Valuesofpiperoughness

Pipematerial Roughnessk,(mm)

Glass,drawncopper 0.003

Steel 0.05

Asphaltedcastiron 0.12

Galvanizedcastiron 0.15

Castiron 0.25

Concrete 0.3to3.0

Plastic 0.03

iv. ColebrookWhiteEquation

The Colebrook-White equation provides a mathematical method for calculation of the friction

factor.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛+−=

fRD

eLog

f e

35.9214.1

1

10

Wherefisthefrictionfactor,

eistheinternalroughnessofthepipe,

Distheinnerdiameterofpipeand

ReistheReynoldsnumber

v. Headlossinpipefittings

Pipelinefittingsandvalvesproduceheadlossesdependingontheirconfigurationswhichstatethat

valueofheadlossisproportionaltothesquareofflowvelocity.

g

Vfh f2

2

=

Wherehfistheheadlossinm,

fistheheadlosscoefficient,

Visthevelocityofflowinm/s,and

gistheaccelerationduetogravityinm/s2

Forsmallscalepipelinedesign,theheadlossinpipefittingsincludingotherlossesenrouteistakenas

thepercentoffrictionheadlosswhichrangesbetween10to20percent.Inthatcase,otherlossesdo

notneedtobecalculatedseparately.


122

Figure5-17Moody'sdiagramforfrictionloss


123

vi. Headlossatpipeinlet

Thelossofheadoccursattheinletofthepipelinewhichisbasedontheshapeandconfigurationof

theinletstructureandiscalculatedasfollows:

g

Vfh ii2

2

=

Wherehiistheheadlossatinletinm,

fi is the inlet losscoefficientwhichdependsupontheshapeofthe inletstructureandprovided in

Table5-11,

Visthevelocityofflowafterinletinm/s,and


Table5-11Headlosscoefficientsatpipeinlet

Inletshape Straight Chambered Rounded Bellmouth

Coefficient

(fi)

0.50 0.25 Circular=0.10

Rectangular=0.20

0.06

vii. Headlossatpipeoutlet

The loss of head also occurs at the outlet of the pipelinewhich is based on the velocity of flow

beforetheoutletandiscalculatedasfollows:

g

Vfho o2

2

=

Wherehoistheheadlossatoutletinm,

foistheoutletlosscoefficientandisequalto1,

Visthevelocityofflowbeforeoutletinm/s,and


viii. Headlossatsuddenexpansion

Thelossofheadalsooccurswiththesuddenexpansionofthepipeline(Figure5-18)whichisbased

onthesizesofthepipesandtheircorrespondingvelocitiesandiscalculatedasfollows:

( )g

Vf

g

VVh sese

22

1

22

21 =−

=

Wherefseisthelosscoefficientforsuddenexpansion(Table5-12),

V1isthevelocitybeforeexpansioninm/s,

V2isthevelocityafterexpansioninm/s


124

Table5-12Headlosscoefficientsforsuddenpipeexpansion

D1/D2 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

fse 0.98 0.92 0.82 0.70 0.56 0.41 0.26 0.13 0.04

Figure5-18Suddenexpansionandcontractionofpipes

ix. Headlossatsuddencontraction

Thelossofheadalsooccursatthesuddencontractionofthepipelinewhichisbasedonthesizesof

thepipesandtheircorrespondingvelocitiesandiscalculatedasfollows:

g

Vfh scsc2

2

2

=

Wherefscistheheadlosscoefficientforsuddencontraction(Table5-13),

V2isthevelocityaftercontractioninm/s,

Table5-13Headlosscoefficientforsuddencontractionofpipe

D2/D1 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

fsc 0.50 0.45 0.43 0.40 0.36 0.31 0.24 0.17 0.09

x. Headlossatgradualexpansion

Thelossofheadalsooccurswiththegradualexpansionofthepipeline(Figure5-19),whichisbased

ontheangleoftheexpansionandiscalculatedasfollows:

( )g

VVfh gege

2

2

21−

=

Wherefgeistheheadlosscoefficientforgradualexpansionandisequalto0.011θ1.22

,

V1 is the velocity before expansion inm/s, V2 is the velocity after expansion inm/s, and θ is the

expansionangleindegrees(Table5-14).

Table5-14Headlosscoefficientforgradualexpansion

θ 7.50 10.00 12.50 15.00 17.50 20.00 22.50

Fge 0.126 0.183 0.240 0.299 0.361 0.425 0.491


125

Figure5-19Typicalgradualexpansionofpipeline

xi. Headlossatelbow

Head loss occurs at bends of the pipelinewhich is based on the turning angle, ratio of radius of

curvaturetothediameterofpipeandiscalculatedasfollows:

g

Vfh bb2

2

=

Wherefbisthelosscoefficientforelbow,

Visthevelocityofflowinm/s,

Thevaluesoffbaregivenbythefollowingexperimentalequationforsmoothsteelpipe,whichare

functionofratioofradiusofcurvature(R)tothediameter(D)andturningangle(θ)intherangeof

0.50to2.5(Table5-15).

Table5-15Headlosscoefficientsforelbowsofpipeline

R/D\θ 22.50o 30

o 45

o 60

o 90

o

0.50 0.989 1.420 1.399 1.615 1.978

0.75 0.289 0.334 0.409 0.472 0.578

1.00 0.147 0.17o 0.208 0.240 0.294

1.50 0.085 0.121 0.121 0.139 0.170

2.00 0.073 0.084 0.103 0.119 0.145

2.50 0.069 0.079 0.097 0.112 0.138

xii. HeadlossatTee

A loss of head also occurs at the Tee of the pipeline (Figure 5-20) which is based on the flow

directionandvelocityinthepipe,andiscalculatedasfollows:

g

Vfh tt2

2

=

WhereftistheheadlosscoefficientforTee,



126

Thevaluesofftdependupontheflowratiosofrespectiveflowdirectionsandcornerchamfers.For

flow from straight pipe towards tee ft is 0.97 and from tee to straight pipe ft is 1.5. The loss

coefficientsforbranchesarealsosimilartothatoftee.

Figure5-20Sketchofflowfromstraightpipetoteeandteetostraightpipe

xiii. HeadlossatOrifice

Headlossoccursattheorificeofthepipelinewhichisbasedonthethrottlingratioandvelocityin

pipeandiscalculatedasfollows:

g

Vfh oror2

2

=

Whereforistheheadlosscoefficientfororifice,


TheheadlossatanorificedependsonthethrottlingratioasgiveninTable 5-16.

Table5-16Headlosscoefficientsfororifice

(D2/D1)2 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

orf 226 47.80 17.50 7.80 3.75 1.80 0.80 0.29 0.06

xiv. HeadlossatValves

At valves connected with pumps and installed on the pipeline, the head losses are produced

accordingtothevalvetypesusedeventhoughtheyarefullyopened.Theheadlossisexpressedas:

g

Vfh vv2

2

=

Where fv is the loss coefficient for valves and depend upon the type of valves used. These

coefficientsaregiveninTable5-17.



127

Table5-17Headlosscoefficientsforvalves

Nominal

size

(mm)

100 150 200 250 300 350 400 500 600

Gate

valve

0.16 0.15 0.11 0.05 Negligible

Butterfly

valve

- - 2.40 1.80 1.40 1.09 0.86 0.60 0.45

Check

valve

1.32 1.27 1.21 1.16 1.11 1.05 1.00 0.98 0.96

xv. Headlossesatscreens

Incaseofinletswithtrashscreens(Figure5-21),theheadlossacrossthescreenshallbecalculated

bythefollowingformula:

g

V

b

shts

2sin3

23/4

×⎟⎠

⎞⎜⎝

⎛×= δ

Wherehtsistheheadlossacrossthescreeninm,

δistheinclinationofthescreen,

sisthethicknessofbarinmm,

bistheopeningbetweenbarsinmm,and

Visthevelocityupstreamofthescreen

Figure5-21Typicalsketchoftrashscreenbars

5.8.4 DesignProcedureofPipedCanal

i. Calculate the design flow of the pipeline as 25% greater than required flow considering

partialblocking,

QrQd ×= 25.1


128

WhereQdisthedesignedflowandQristherequiredflow,

ii. Determinetheelevationdifferenceofthepipelinebetweenstartandendpoints,

iii. Dividethepipelinelengthintostrategicsegments,thefirstsegmentwillbefromsourceto

the first break pressure tank, second segment from first break pressure tank to second

break pressure tank, and the third segment will be from second break pressure tank to

dischargetank,

iv. If thedemandofwater isgreater than theavailable flowat thesourcea reservoirwillbe

necessarytoprovidetobalancetheflow.Ifthedemandofwaterislessthantheflowatthe

sourcestorage reservoirwillnotbenecessary.Thedesignofmainpipelinewillbecarried

outbasedonthemaximumwaterdemandforirrigation,

v. Calculatethediameterofthepipeconsideringthelimitingvelocityandflowinthepipefor

eachsegment,

vi. Preparelinediagramofthepipelinesystemwithdetailsoflevelsandpipediameters,

vii. Theexampleofpipelineworks:

a. Intake,

b. Supplymainpipeline,

i. Intaketofirstbreakpressuretank,

ii. FirstBPTtosecondBPT,and

iii. SecondBPTtodischargetank

c. Distributionsystem

viii. AsampleexcelsheetofpipelinedesignispresentedinTable5-18.

5.9 Canalsalongunstableareas

Byvirtueoftheuniquetopography,fragilegeologyandmonsoonhydrology,Bhutan’shillslopesare

oftenunstabledue todebris flow, landslides and riverbankerosion. The irrigation canalspassing

along the slopes of the hills and mountains in many cases are susceptible to instability of the

alignment.Therearethreetypesofinstabilitythatcanaffectthecanal,whicharealsoexpressedin

Figure5-22.

5.9.1 Deepslideacrossthecanalalignment

Landslidesandoverburdencreepaffectthewholehillsideforlargeareasaboveandbelowthecanal,

andmore or less independently of the influence of canal construction. In such areas, trees have

oftenslipped,andtheupperpartofthetreetrunkgrowsverticallywhilethelowerpartissloping.

Sometimesthegroundwillbefairlybrokenwithlittledensevegetation.

5.9.2 Shallowslideabovethecanal

Ashallowslideabovethecanal isduetotheeffectofthecanalcuttingcausingitsowninstability.

Forexistingcanals,thegroundwillhaveoftenslumpedintothecanalandtherewillsometimesbe

groundwaterseepage.


129

5.9.3 Shallowslidebelowcanal

Thepossibilityofinstabilitybelowthecanalisduetosteepnessandparticularlyexcessporewater

fromthecanalitself.Inexistingcanals,suchareasareusuallyfairlyobvious;thecanalsizeisusually

restricted and has been temporarily stabilized by bamboo or boulders, while continuing seepage

throughoroverthecanalbanksusuallyonlyworsenthesituation.

Figure5-22Threetypesofcanalinstability

Deepslideacrossthecanal Shallowslideabovethe

canal

Shallowslidebelowthecanal

Thegeneralsolutionsforunstableareasofthecanalalignmentaregroupedintofivecategories:

• Liningofthecanal,

• Coveredcanal,

• Retainingwallsonuphillsideaswellasdownhillside,

• Temporarymeasures,and

• Reforestationandgrassing

Thesesolutionmeasuresareapplicabletothespecificsiteconditionsoftheabovementionedcanal

instabilities.ThetypicalinstabilityproblemsandtheirsolutionsaregiveninFigure5-23.


130

Figure5-23Canalinstabilityproblemsandtheirsolutions


131

Table5-18Sampledesignsheetofpipelineworks

Project:

Schemename: Sourcename: Safeyield:

From To Actual Design From To OD ID Class From To

(m) (m) (lps) (m) (m) (m) (m) (m) (m) (mm) (mm) (kg/cm2) % (m) (m) (m/s) (m) (m)

(0) (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (17) (18) (19) (20) (21) (22)

1 CC P1 250 275 4 1000 993 7 1000 7.00 7 90 76.3 6 1.15 3.16 3.838 0.88 1000 996.8375

2 P1 P2 250 275 4 993 947 46 1000 49.84 53 90 76.3 6 1.15 3.16 46.675 0.88 996.8375 993.675

3 P2 P3 220 242 4 947 955 -8 1000 38.68 45 90 76.3 6 1.15 2.78 35.892 0.88 993.675 990.892

4 P3 P4 100 110 4 955 956 -1 1000 34.89 44 90 76.3 6 1.15 1.27 33.627 0.88 990.892 989.627

5 P3 P4 200 220 4 956 947 9 1000 42.63 53 90 76.3 6 1.15 2.53 40.097 0.88 989.627 987.097

6 P4 P5 200 220 4 947 943 4 1000 44.10 57 90 76.3 6 1.15 2.53 41.567 0.88 987.097 984.567

7 P5 FRC 200 220 4 943 942 1 1000 42.57 58 90 76.3 6 1.15 2.53 40.037 0.88 984.567 982.037

Pipeline LengthDesign

dischargeS.N

HydraulicgradelineReducedlevel Level

Diff

Newstatic

level Totalavail

head

Maxstatic

pressure

Pipeused

HDPE

Friction

factor

Head

loss

Residual

head

Flow

velocity

Adapting to Climate Change through IWRM - adb.org · Project title Adapting to climate change...

Documents

Transcript of Adapting to Climate Change through IWRM - adb.org · Project title Adapting to climate change...