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Design mannual for small scale irrigation scheme book

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Surendra Maharjan
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Published on Apr 3, 2016

Design manual for small irrigations in Nepal

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 Design mannual for small scale irrigation scheme book

1. This ​Design Manual for Small Scale Irrigation Scheme​ is approved by the Government of Nepal (Ministry Level) on 2071-03-23 Published by:
Ministry of Federal Affairs and Local Development Department of Local Infrastructure Development and Agricultural Roads (DoLIDAR) In
association with: HELVETAS Swiss Intercooperation Nepal Local Infrastructure for Livelihood Improvement (LILI) Technical Team: Mr. Kumar
Thapa, Senior Divisional Engineer/ DoLIDAR Mr. Raju Shrestha, Engineer/ DoLIDAR Mr. Susan Shakya, Technical Coordinator/ LILI-
HELVETAS Swiss Intercooperation Nepal This Publication is available at: Ministry of Federal Affairs and Local Development Department of
Local Infrastructure Development and Agricultural Roads (DoLIDAR) Shree Mahal Pulchowk Lalitpur, Nepal Contact Phone: 977-1-5555001,
5555362, 5543197 Fax: 977-1-5555724 Email: contact@dolidar.gov.np ; dg@dolidar.gov.np
2. TABLE OF CONTENTS 1. Introduction 1 1.1 Concept of Irrigation 1 1.1.1 Surface Irrigation 1 1.1.2 Sub-surface Irrigation 1 1.1.3 Sprinkle
Irrigation 1 1.1.4 Sources of Irrigation 2 1.2 Basic Terms Used In Small Scale Irrigation 2 1.3 Scope of the Manual 3 2. Project Study and Survey
5 2.1 Introduction 5 2.2 Identification Study 5 2.2.1 Introduction 5 2.2.2 Desk study 6 2.2.3 Field visit 6 2.2.4 Analysis and Reporting 6 2.3
Feasibility study 7 2.3.1 Introduction 7 2.3.2 Desk study 7 2.3.3 Field Survey Work 7 2.3.4 Field data analysis 8 2.4 Topographical Survey 9 2.5
Hydrological survey 10 2.6 Socio-economic Survey 10 2.7 Agricultural Survey 10 3. Hydrological Analysis and Water Requirement Assessment
13 3.1 Maximum Design discharge 13 3.2 Flood Frequency Analysis 13 3.3 Regional Analysis 13 3.3.1 Sharma and Adhikari Method 14 3.3.2
Rational Method 14 3.4 Water Availability for Irrigation 14 3.4.1 General 14 3.4.2 Frequency Analysis 15 3.4.3 MIP Method 15
3. 3.5 Discharge Measurement 15 3.5.1 General 15 3.5.2 Float method 16 3.5.3 Bucket and Watch method 17 3.5.4 Current Meter 17 3.6 Water
Requirement Assessment 17 3.6.1 Crop Water Requirements 17 3.6.2 Irrigation Water Duty 18 4. Diversion Headworks and Intakes 19 4.1
Selection of Site for Headwork or Intake 19 4.2 Types of Intake 20 4.3 Design of Simple Side Intake 20 4.4 Bottom Intake 21 4.4.1 General 21
4.5 Diversion Weir 22 4.5.1 General 22 4.5.2 Crest Level of Weir and Under-sluice 22 4.5.3 Length of Weir and Afflux 23 4.5.4 Effect of
Retrogression 23 4.5.5 Hydraulic Jump 23 4.6 Gabion Weir 24 4.6.1 Gabion weir 24 5. Canal Design 27 5.1 Design Concept 27 5.2 Design of
Unlined Canals 27 5.2.1 Design Procedure 27 5.2.2 Side Slopes 28 5.2.3 Permissible Velocity 28 5.2.4 Bed Width to Depth Ratio 29 5.2.5 Canal
Slope 29 5.2.6 Free Board 30 5.2.7 Minimum Radius of Curvature 30 5.2.8 Design Example of Unlined Canal 30 5.3 Lining of Canals 31 5.3.1
Type of Canal Lining 31
4. 5.3.2 Design Concept of Lined Canal 31 5.3.3 Design Procedure of Lined Canal 32 5.3.4 Design Example of Lined Canal 32 5.4 Design of
Covered Canals 33 5.4.1 Design Concept 33 5.4.2 Design of Piped Canal 33 5.4.3 Design Examples of Piped Canal 34 6. Sediment Control
Structures 37 6.1 Gravel Traps 37 6.2 Sand Traps 38 6.2.1 General Configuration 38 6.2.2 Sand Trap Design 39 7. Canal Structures 43 7.1
Head Regulator 43 7.2 Cross Regulator 43 7.3 Proportional Divider 43 7.4 Outlets 44 7.4.1 Design of Pipe Outlet 44 7.5 Drop Structures 44 7.6
Escape Structure 45 7.7 Bridges and Culverts 46 7.8 Retaining Wall 47 7.8.1 General 47 7.8.2 Design Concept of Dry and Gabion Retaining
Wall 47 7.8.3 Design Procedure of Masonry and Concrete Retaining Wall 47 8. Cross Drainage Structures 51 8.1 Type of Cross-Drainage
Structure 51 8.2 Design Considerations 51 8.2.1 Design Discharge 51 8.2.2 Waterway of the Drain 51 8.2.3 Contraction and Expansion of
Waterway 51 8.2.4 Head Loss through Structure 51 8.3 Design Concept of Aqueduct 52 8.4 Design Concept of Superpassage 53 8.5 Design
Concept of Canal Siphon 53
5. 8.6 Design Concept of HDP Pipe Crossing 54 8.7 Design Example of HDP Pipe Crossing 56 9. Shallow Tube Well Irrigation 57 9.1
Groundwater Potential in Nepal 57 9.2 Design concept 58 9.3 Suggested Design Criteria 59 9.4 Design Procedure of STW 59 9.5 STW Design
Example 60 9.5.1 Design Data 60 9.5.2 Assumptions Made 60 9.5.3 STW Planning 60 9.5.4 Tube-well Design 60 9.5.5 Pump and Motor Design
61 9.5.6 Distribution System 61 10. Micro Irrigation 63 10.1 Introduction 63 10.2 Pond Irrigation 63 10.2.1 Design Concept 63 10.2.2 Design
Example of Pond Irrigation 64 10.3 Sprinkle Irrigation 64 10.3.1 Sprinkle System Components 64 10.3.2 Micro Sprinkle in Nepal 65 10.4 Drip
Irrigation 66 10.4.1 Components of Drip System 66 10.4.2 Simple Drip System in Nepal 67 10.5 Small Scale Lift Irrigation 68 10.5.1 General 68
10.5.2 Diesel or petrol engine pumps 69 10.5.3 Hydraulic Ram 70 10.5.4 Treadle Pump 71 11. Agriculture Benefit Assessment 73 11.1
Introduction 73 11.2 Benefit Matrix Approach 73 11.3 Components of Benefit Matrix 73 11.3.1 Command Area 73
6. 11.3.2 Crop Yields 73 11.3.3 Agriculture Inputs 73 11.3.4 Economic Prices 74 11.4 Procedure of Benefit Assessment 74 11.4.1 Total Net
Agricultural Returns "Without Project" 74 11.4.2 Total Net Agricultural Returns "With Project" 74 12. Project Cost Estimate 77 12.1 Introduction
77 12.2 Derivation of Unit Rates 77 12.3 Bill of Quantities and Abstract of Costs 79 13. Economic Analysis 81 13.1 Introduction 81 13.2
Economic Indicators 81 13.3 Basic Assumptions 81 Annex: 1 83 Annex: 2 89 Annex: 3 95 Annex: 4 101 Annex: 5 102 Typical Drawings Sample
103
7. DoLIDAR/MoFALD -Nepal ABBREVIATIONS ADB : Asian Development Bank CA : Catchment Area CCA : Cultural Command Area CWR :
Crop Water Requirement DDC : District Development Committee DHM : Department of Hydrology and Meteorology DOI : Department of
Irrigation DOLIDAR : Department of Local Infrastructure Development and Agricultural Roads DTO : District Technical Office EIRR : Economic
Internal Rate of Return FAO : Food and Agriculture Organization FM : Frequency Modulus FMIS : Farmer Managed Irrigation System GCA :
Gross Command Area GPS : Global Positioning System HDP : High Density Polythene LILI : Local Infrastructure for Livelihood Improvement
MOA : Ministry of Agriculture MOFALD : Ministry of Federal Affairs and Local Development NCWR : Net Crop Water Requirement O&M :
Operation and Maintenance PDSP : Planning and Design Strengthening Project SDC : Swiss Development Cooperation STW : Shallow Tube
Well VDC : Village Development Committee WB : World Bank WECS : Water and Energy Commission Secretariat WUA : Water Users
Association
8. DoLIDAR/MoFALD -Nepal Chapter-1 1. Introduction 1.1 Concept of Irrigation Irrigation is the application of water to the farm land for the
purpose of producing crops whenever and wherever the rainfall is not sufficient to meet the requirements of the crops. Irrigation system can be
classified according to size, source of water, technology, management type etc. In-terms of management there are three broad types of
irrigation systems are practiced in Nepal i.e. government managed, farmer managed, and jointly managed system. However basically almost all
small scale irrigation schemes are under farmer managed system. There are basically three types of water application technologies for irrigation:
· Application of water at the surface of the field-Surface irrigation; · Application of water below the surface of the field-Sub-surface irrigation and ·
Application of water from above the field or in the form of rain- Sprinkle irrigation 1.1.1 Surface Irrigation In this system water is applied at the
surface of the fields. This is the oldest and most commonly practiced system of irrigation. Surface irrigation methods are categorized in the main
groups: · Free flooding irrigation, · Boarder strip flooding irrigation, and · Furrow irrigation In free flooding field plots are divided into small plots
nearly leveled and water is applied at higher level to allow free flooding. Irrigation of paddy fields is the example of free flooding irrigation. In
boarder strip flooding the land is divided into number of strips. The size of strip depends upon the technology used for mechanization. Shorter
and narrower strips are more efficient. Irrigation of wheat fields and mechanized paddy fields are the examples of this irrigation method. In
furrow irrigation furrows are constructed at certain intervals and crops are planted in rows. The length and width of furrow depends upon the
field condition and the technology used. Irrigation of potato and vegetables are the examples of furrow irrigation. 1.1.2 Sub-surface Irrigation In
this type of irrigation water is applied at the roots of the plants. This method is economical in water utilization as evaporation losses are minimal.
But it has limited application due to technological constraints and economic point of view. This type of irrigation is suitable for water scarce area
and for growing cash crops. Drip or trickle irrigation is being practiced in Nepal for growing vegetable crops in small holder farmers. 1.1.3
Sprinkle Irrigation In this type of irrigation water is applied from above the surface of the field in the form of rain or spray. It is mechanical form of
irrigation and needs pressure flow for spraying water. It is widely used in developed countries for watering of gardens and lawns. In Nepal
sprinkle irrigation has been practicing in tea gardens since long. It is also being used for growing vegetable crops in water scarce area and in
areas of porous soil. Page 1
9. DoLIDAR/MoFALD -NepalPage 2 1.1.4 Sources of Irrigation The main sources of water supply for irrigation are: · Normal flow of rivers, lakes,
spring sources; · Stored flood water in reservoirs or rainfall stored in tanks; and · Wells including tube wells Irrigation water is generally diverted
from source to the field by means of a conveyance system. The design of small scale irrigation covers water acquisition structure, conveyance
system, storage system, and application method. 1.2 Basic Terms Used In Small Scale Irrigation Afflux : the rise of water level on the u/s of the
weir or barrage after its construction, Alignment : center line of the canal to be followed for excavation or profiling, Apron : floor to protect the
surface from erosion, Aqueduct : cross drainage structure made over drainage to pass the canal water, Barrage : structure across the river with
gates to raise the water level for irrigation canals, Bench mark : geodetic reference point taken from national survey of the country, Breast wall :
vertical wall immediately above the face of orifice or gate, Catchment area: area from which rainfall flows into a drainage line, Chute : high
velocity conduit for conveying water, Command area : area which can be irrigated or cultivated, Cultural command area (CCA): total area which
can be cultivated, Gross command area (GCA): total area which can be economically irrigated by irrigation system, Cropping intensity:
percentage of irrigated area cropped within the command area in a year, Cropping pattern: crop planting sequence and crop mix throughout a
year, Cross drainage works: structure necessary to cross the irrigation canal across the natural drainage, Cut-off : wall constructed below the
floor of the structure to reduce percolation, Desilting basin : structure for creating pool of water to settle down suspended sediments, Drop :
structure for dropping the flow of the canal to dissipate its surplus energy, Embankment : earthwork raised above the natural ground to protect
flood water, Free board : margin between a canal bank and full supply level, Guide bank : training bank constructed at the site of bridge or
barrage to guide the river, Head regulator : control structure constructed at the head of the off-take canal, Headwork : combination of structures
constructed at the diversion point of the river, Hydrograph : graph showing the stage, flow of water with respect to time, Intake : structure to
divert water from the source to the canal system, Lining : method of sealing the section of canal to prevent seepage, Main canal : canal that
takes off water from the headwork or from the intake, Marginal bund : embankment constructed along the river at a short distance from the
margin, Off-take : structure to take off the water from parent canal to a relatively small canal, Percolation : flow of water in sub-soil due to the
force of gravity or pressure head, Pier : intermediate support of the bridge or culvert, Pitching : covering of the sloping surface with stone,
concrete blocks, bricks, etc, Regime : state of the river flowing in non-scouring and no-silting condition, Retaining wall : vertical wall to retain the
soil mass behind it, Runoff : part of rainfall that reaches the stream, drain from the catchment,
10. DoLIDAR/MoFALD -Nepal Page 3 Scour : removal of material from the bed of the channel by flowing water, Sediment : non-floating material
transported by water (sand, silt, gravel and clay), Seepage : percolation of water into the soil underneath the structure, Side slope : sloping
distance of a canal section usually measured in a ratio, Silt : water-borne sediment consisting of fine earth, sand or mud, Siphon : closed
inverted conduit for cross-drainage structure, Sluice : conduit for carrying water at high velocity, Spillway : passage for the flow of surplus water
in a weir or conduit, Stilling basin : structure for creating pool of water to disseminate the surplus energy of water, Super-passage : structure
over the canal to bypass the drainage flow, Tail-water : water just downstream of the structure, Uplift : upward pressure of water on the base of
structure, Waterway : distance required for flowing water, e.g. Lacey​s waterway, Weep-holes : horizontal holes on the walls to allow flow of
seepage water behind it, Weir : barrier built across the river to raise the water level upstream, Wing wall : splayed extension of an abutment wall
of the culvert, 1.3 Scope of the Manual Several reference books and manuals are available for planning and design of irrigation schemes. Most
of these references focus on large and medium irrigation schemes. The design of small irrigation schemes is specific with respect to its size and
nature. Based on the experience of planning, designing and implementing small scale irrigation schemes by various organizations, it is high time
to focus on the design of small scale irrigation. In this aspect this manual tries to address the following scope of work: · Review available design
manuals and design guidelines and identify various types of possible components/structures, which are normally required for small irrigation
schemes; · Prepare of stepwise procedures for the design of small irrigation canals and associated structures; · Prepare acceptable standard
design manual of small scale irrigation schemes with necessary drawings.
11. DoLIDAR/MoFALD -Nepal Page 5 Chapter-2 2. Project Study and Survey 2.1 Introduction Study and survey is the basic requirement of any
engineering project. The success and failure of the project implementation depends on the accuracy its study and survey. In irrigation projects
the major studies carried out for project implementation are: · Identification study, · Pre-feasibility study, · Feasibility study, and · Detailed study
For small scale irrigation schemes it may be sufficient to carry out two levels of the study: · Identification study, and · Detail Feasibility study
Survey work is an integral part of the scheme study and is carried out in all stages of the study. The magnitude and accuracy of the survey work
depends on the stage of the study and its accuracy to be needed. The principal types of survey required for small scale irrigation schemes are: ·
Topographical survey, · Engineering survey, · Hydrological survey, · Socio-economic survey, · Agricultural survey This chapter deals with the
requirements of the study and survey for the implementation of small scale irrigation schemes. 2.2 Identification Study 2.2.1 Introduction The
identification of the scheme is the very first step of project implementation process. The district level offices (District Technical Office) should
disseminate the information about the project, its plans, procedures and requirements for implementation of the project through meetings with
VDCs, DDCs, and local FM to the farmers of the district. Prior to start of identification study, the request from the farmers should be reviewed to
establish whether they fulfill the minimum set requirements. The criteria for selection of scheme for identification should be prepared by DTO
based on its principles and objectives. The criteria for the selection of the project for identification may be: · At least two-third of the beneficiary
households should sign the project request form; · There are no potential water right disputes; · The beneficiaries commit to take future O&M
responsibilities; · Sufficient water is available; After the very first initial screening identification study of the project need to be processed.
Engineering and socio-agricultural staff should visit the proposed project site and assess whether the project should be carried on the feasibility
study or rejected. The project identification study is carried out in three steps: · Desk study
12. DoLIDAR/MoFALD -NepalPage 6 · Field visit (Walkthrough survey) · Analysis and reporting 2.2.2 Desk study At this step the project location
should be identified on the topographic maps (1:50,000/1:25,000 scale). The team should collect and review the demand form/previous
reports/study/information related to the project. 2.2.3 Field visit After desk study, message should be sent to the farmers before the field visit
informing them about the tentative date of visit. The tools and equipment to be taken on the field visit may be: · Copy of project request form; ·
Project identification questionnaire; · Topographical maps; · Note books and necessary stationary; · GPS; · Stop watch; · Calculator; · Camera; ·
3 m long measuring tape 2.2.4 Analysis and Reporting Based on the data and information collected during the field visit the team needs to
analysis the project findings and finalize the identification study. The analysis should be based on technical, economical and social aspects of
project implementation. The analysis may cover the following aspects: · Length of main canal; · Size and type of command area (terrace/plain); ·
Water right problems; · Type of soil; · Major technical difficulties (cross drainage/landslide/unstable zones); · Poor farmer​s presence; · Farmer​s
interest Based on the analysis the team has to prepare a report stating the project recommendation for further actions. The recommendation may
be based on: · Genuineness of demands; · Command area; · No technical complication; · Year round irrigation water availability and no water
dispute; · Potential for increased agriculture production; · No environmental adverse effect; · Rehabilitation schemes/ schemes requiring
relatively minor improvements;
13. DoLIDAR/MoFALD -Nepal Page 7 2.3 Feasibility study 2.3.1 Introduction The feasibility study is basis for project implementation and is
carried out by a team of experts having engineering, agriculture, and socio-economic professionals. This study normally forms the basis of
financing by external funding agencies or by the Government. The study assesses the technical feasibility, economical viability and institutional
suitability of the project implementation. The feasibility study is carried out in the following steps: · Desk study · Field survey work · Field data
analysis · Outline design · Cost estimate · Economic analysis · Project reporting and recommendation 2.3.2 Desk study The desk study is a
review of the work carried out at the project identification stage. This study needs to see generally two aspects. · List of outstanding matters to
be studied, and · Any possible scheme alternatives There may be several outstanding issues not touched during previous studies, which should
be addressed during feasibility study. The scheme alternatives need to be reviewed in the feasibility study, which may include the following: For
new schemes: · Alternative water sources from different rivers, pumped supply, ground water, supplementary rivers, etc · Alternative intake site
on one river · Alternative canal alignment For rehabilitation schemes: · Extension of command area · Combining several schemes · Revised
canal alignment · Revised intake site 2.3.3 Field Survey Work The field survey work may differ slightly based on the type of the scheme whether
it is new or rehabilitation. The following are the main activities to be carried out during the field survey work: · Cutting a trace along the canal line
for new scheme · Intake site survey · Discharge measurement · Canal alignment survey · Long section · Work inventory · Major structure site
survey · Command area survey (GPS survey) · Conceptualize the Irrigation System and seek opinion of Farmers
14. DoLIDAR/MoFALD -NepalPage 8 · Agricultural survey · Socio-economic survey · Environmental survey etc 2.3.4 Field data analysis The
collected data and information has to be analyzed, on return to the office, before the final decision can be taken on the project: i. Command area
Estimation of gross command area is often difficult to work out. Following methods can be used to find the command area. · Data obtained from
the farmers · From topographic map · From project identification study · Use of GPS ii. Scheme water balance Before starting the canal design, it
is important to check the water balance to conform whether variable water is sufficient to the proposed command area in all seasons. Following
are the steps to follow: · Determine future cropping pattern- crops, planting, dates and harvesting dates; · Calculate the irrigation water
requirement for each time period(15 days); · Assess the effect on the irrigation water requirement by adjusting the cropping pattern on time
period forward and backward; · Measure the catchment area in the intake sites from the topography maps; · Calculate the 80% reliable flow by
measuring the river discharge at the intake site and the hydrographs of the area; · For each time period, compare the irrigation water
requirement with 80% reliable flow to check the availability of water; · Adjust the cropping pattern, calculate irrigation water requirement for it and
check the availability of water. Repeat this method and determine the optimum command areas; · For each crop, determine the maximum area
that can be irrigated. If the whole area cannot be reliably irrigated command should be reduced. If necessary, alternative cropping pattern to
allow a larger irrigable area for the winter and spring crop should be discussed with district agricultural office. iii. Feasibility design a. System
layout Using topographic maps a plan of the system should be made showing the source, primary canal alignment, major structures and main
part of the command area. A schematic line diagram of system layout need to be prepared; b. Canal design For feasibility level design, typical
canal cross sections can be related to canal discharge and ground conditions. For rehabilitation designs, canal designs, canal design are only
required for reaches where complete remodeling is needed.
15. DoLIDAR/MoFALD -Nepal Page 9 c. Structure design It should be prepared for intake and major structures. For minor structures typical
standard drawing may be used d. Standards for feasibility level design · Head Works: Sketches (Plan, typical section) showing location and
levels and dimensions should be prepared and principal quantities estimated, · Layout: A layout plan of the scheme and command area showing
primary and secondary canal alignment should be prepared, · Canals · Canal routes, capacities , slope, levels and dimensions should be
established, · A longitudinal section of the canal should be prepared, · Typical cross sections should be drawn, · Earthwork quantities should be
worked out, · Structures · Drawings of the typical structures should be prepared, · Each size and type of structure should be assessed, ·
Drawings of major structures showing location, levels and dimension should be prepared, · Principal quantity should be calculated, 2.4
Topographical Survey Topographical survey is the process of determining the position both in plan and elevation of natural features of the area.
The extent of topographical survey depends upon the size, type, and level of the project study and includes: · Benchmark survey, · Traverse
survey, · Alignment survey, and · Longitudinal and cross sectional survey The topographical survey standard for small scale irrigation schemes
may be as follows (Table-2.1). Table 2-1: Topographical Survey Standards S.N Principal use Details 1 Scheme identification · Use existing
maps but supplement with site inspection 2 Feasibility study · Use existing maps, site sketches, · Carry out traverse survey (GPS), · Benchmark
survey (0.50 km interval in the hills and 1.0 km interval in Terai), · Carry out detailed site survey for structures-intake/headwork, major structures
· For command area topographical survey take 100 m interval grid for contour interval of 0.25m in Terai and 0.50 to 1.0 m in the hills · Carry out
canal alignment survey with l-section and x-sections 3 Construction stage · Use design drawings, · Check and agree levels and dimensions, ·
Carry out additional survey for details of the site if required
16. DoLIDAR/MoFALD -NepalPage 10 2.5 Hydrological survey Hydrological survey is carried out to provide data and information for the
assessment of water availability for irrigation, and assessment of flood flows for the intake/headwork and cross drainage works. The data and
information includes are flow measurement, catchment area size, conditions of catchment, river or drainage sections and profile. The general
land use pattern of the catchment area may also be collected during this survey. 2.6 Socio-economic Survey The socio-economic survey is
carried out to determine the social structure of the community and its economic status. The survey includes the collection of quantitative and
qualitative data and information on social structure, socio-cultural institutions, and economic activities of the farmers of the scheme command
area. Some of social and economic indicators of the community are as follows: Social indicators: · Willingness/commitment- verbal request,
formation of committee, submission of request form; · Ethnic composition- homogeneity, diversity; · Education- literacy, school and college,
awareness about irrigated agriculture, prior experience on irrigation; · Rural organization- Parma, Guthi, etc; · Family size- male/female,
economically active members, · Migration- temporary, permanent, foreign/urban areas; Economic indicators: · Land holding size- land less,
marginal land (< 1 ha), land lords(> 5 ha); · Main occupation- agriculture, service, labor, foreign service, business; · Source of income-
agriculture, service, remittance etc; · Expenditure- food, cloth, schooling, festivals, livestock, agriculture LILI/HELVETAS has developed socio-
economic and technical survey formats for the pre-feasibility study and detail study of small irrigation schemes. These formats can be used for
the survey and study of small irrigation schemes and hence annexed for reference (Annex:2-1 and Annex:2-2). 2.7 Agricultural Survey For small
irrigation schemes agricultural survey may include data and information regarding the soil type, land use and agriculture practices of the
command area to be proposed. The soil survey may include the assessment of the type of soil in the command area and its suitability for
irrigated agriculture. The soils may be alluvial, sandy, gravel and boulder mixed. The land use survey may include the general assessment on
land use in the command area which may classify in percent the agricultural land, forest land, grazing land, wetlands, National Parks and
reserve forest area. The agriculture survey includes the collection of data and information on: · Existing cropping pattern, · Existing crop yields, ·
Inputs used and its availability, · Marketing facility and labor situation
17. DoLIDAR/MoFALD -Nepal Page 11 With the completion of detail feasibility study of the scheme studied, the compiled report should prepare
and the report shall comprise the following headings: · Analysis of Irrigation Scheme Area: - Scheme area Background - Location and
Accessibility - Geological and Environmental Aspects - Lay Out of the Scheme · Technical Survey: - Location and condition of Water Source(s)
and Assessment - Description of proposed Structures - Availability of Construction Material and Labors · Socio-economic and Agricultural
Survey: - Social and Economic Situation - Agricultural Situation (existing and proposed command area, cropping pattern and intensity) -
Availability and Linkage to agricultural Service Providers · Detail Cost - Estimate - Approved District Rates and Rates at the Sites - Rate
Analysis - Abstract of Cost and Bill of Quantities - Summary of Labor and Materials · Economic Analysis - Crop Budget without and with
Irrigation Scheme - Benefit/Cost Ratio and Economic Internal Rate of Return (EIRR) · Conclusion and Recommendation: · Annexes: - Details of
compiled data, Designs and Calculations - Working Drawings - Minutes of Community Meetings and Consent Letters of Land Donation -
Photographs of sites/location (Intake, command area, Alignment, community meeting) - Soft Copy of GPS data For the preparation of design
report, refer Guidelines for Detail Survey, Design and Cost Estimate Report of Small-scale Farmer Managed Irrigation System.
18. DoLIDAR/MoFALD -Nepal Page 13 Chapter-3 3. Hydrological Analysis and Water Requirement Assessment 3.1 Maximum Design discharge
The criteria for the selection of maximum design discharge are based on technical and economic considerations. The major criteria for the
selection of design flood are: · Importance of structure to be constructed, · Effect of overtopping of the structure, · Potential loss of life and
downstream damage, and · Cost of the structure Books on Hydraulic Structures and manuals have suggested the return period as below (Table-
3.1). There are various methods of estimating maximum design discharge or design flood. Table 3-1: Suggested Return Period 3.2 Flood
Frequency Analysis The flood frequency analysis is a statistical method to show that flood events of certain magnitude may on average is
expected once every n year. It is generally carried out to estimate the design flood from the recorded flow data of more than 10 years. The most
commonly used methods for frequency analysis are: · Gumbel​s distribution, · Log Pearson ​III distribution, and · Log Normal distribution The
details of these methods are available in the Handbook of Applied Hydrology by Vent Te Chow, 1964. 3.3 Regional Analysis When the recorded
hydrological data of the river is absent or too short a regional analysis is adopted to estimate the flood flow, and low flow of required return
periods. In this method a hydrological homogeneous region is considered from statistical point of view. There are various methods of estimating
flood flow of given return period based on regional analysis. In Nepal following methods are used to estimate the flood flow: · WECS/DHM
(1990) Method- based on regression analysis, · Tahal et al (2002) Method ​ based on Index Flood Method, · Sharma and Adhikari (2004) ​ based
on regression analysis S.N Type of structure Return Period in years 1 Irrigation supply reliability 5 2 Road culverts 5-10 years depending upon
the type of crossings 3 Highway bridge 10-50 years depending upon the type of rivers 4 Irrigation intake 10-25 years 5 Irrigation weir/barrage
50-100 depending upon its size 6 Cross drainage structures 10-25 years
19. DoLIDAR/MoFALD -NepalPage 14 In addition, there are rational method and empirical methods such as Modified Dickens method, Ryve​s
method. The most commonly used methods in Nepal is Sharma and Adhikari Method based on regional analysis and Rational Method as
empirical method. 3.3.1 Sharma and Adhikari Method This method is derived using hydrometric data of DHM up to 1995 and is considered the
updated version of WECS and DHM Method. The formulae are as follows: For estimating the floods of other return periods, the relationships
shown in the WECS and DHM Method can be used. Hence, the 50 year return period flood (Q50) is estimated as follows: 3.3.2 Rational Method
The Rational Method is a widely used method for flood flow estimation of small sized-basins. The rational method formula is as follows: Where,
Q = peak discharge in m3/s C = runoff coefficient (roughly defined as ratio of runoff to rainfall) I = rainfall intensity in mm/h A = catchment area in
km2 For the natural catchments, the values of C vary from 0.2 to 0.4. The average rainfall intensity I has a duration equal to the critical storm
duration, normally taken as a time of concentration. Rainfall intensity-duration-frequency data normally is used to estimate the rainfall intensity
(I). If such data are not available, the value of I is taken to be between 60 to 70 mm per hour for small catchments in Nepal. These values are
comparable to other studies in Nepal and they correspond to about 50 year return period maximum rainfall intensity. 3.4 Water Availability for
Irrigation 3.4.1 General The assessment of water availability for irrigation is carried out on 80% reliability of full supply. This means 80% of the
time there is at least enough water available to meet full demand of irrigation. For gauged river the reliability assessment is carried out by
frequency analysis while for ungauged river regional regression analysis for long term mean flow is adopted in Nepal.
20. DoLIDAR/MoFALD -Nepal Page 15 3.4.2 Frequency Analysis For gauged sites the 80% reliable monthly flows are obtained using the
following formula: The factor 0.8418 is the 1 in 5 reduced variate of the normal distribution assuming that the monthly flow series follows a
normal distribution. In the case of the diversion site being near the gauged site, the monthly flows at the diversion site is calculated using an
area ratio. 3.4.3 MIP Method The Medium Irrigation Project (MIP) Design Manual (1982) has developed regional hydrographs using spot
measurements of the ungauged rivers. This method is widely used in irrigation planning and design in Nepal. The country has been divided into
seven hydrological regions (Figure 3.1); each region has a monthly hydrograph showing mean, 20% reliable and 80% reliable flows. The
ordinates of the regional hydrographs have been non-dimensionalised by dividing each by the April flow. For using the non- dimensional
hydrograph to predict the mean and 80% reliable flows, it is essential to measure the flow at site during dry season (September to May) to
eliminate the influence of storm flow. The detail of MIP method can be found in PDSP Manual M3. 3.5 Discharge Measurement 3.5.1 General
The amount of water passing a point on the stream channel during a given time is a function of velocity and cross-sectional area of the flowing
water and is expressed by Continuity equation: Q=AV, where Q is the stream discharge, A is cross-sectional area, and V is flow velocity.
Discharge measurement follows the continuity equation which is concerns with the measurement of the flow velocity. For small irrigation
schemes, three methods are appropriate for discharge measurement: Float method, Bucket and watch method and Current meter method.
21. DoLIDAR/MoFALD -NepalPage 16 Figure 3-1: Hydrological Regions of Nepal 3.5.2 Float method This method measures surface velocity of
flowing water. Mean velocity is obtained using a correction factor. The basic idea is to measure the time taken to float the object in a specified
distance. Vsurface = travel distance/ travel time = L/t Because surface velocities are typically higher than mean or average velocities, V mean = k
Vsurface where k is a coefficient that generally ranges from 0.8 for rough beds to 0.9 for smooth beds (0.85 is a commonly used value). In
mountain streams with lots of roughness elements, the value may be much lower e.g. 0.67. The procedures for discharge measurement are as
follows: Step 1: Choose a suitable straight reach with minimum turbulence (ideally at least 3 channel widths long); Step 2: Mark the start and
end points of the chosen reach; Step 3: If possible, travel time should exceed 20 seconds but be at least 10 seconds; Step 4: Drop your object
into the stream upstream of the upstream marker; Step 5: Start the watch when the object crosses the upstream marker and stop the watch when
it crosses the downstream marker; Step 6: Repeat the measurement at least 3 times and use the average in further calculations; Step 7: If you
only do the float method you need to measure the cross-sectional areas at the start and end point of your reach. Step 8: Average cross-sectional
areas; Step 9: Using the average area and corrected velocity, you can now compute discharge, Key 2. Hills to north of Mahabarats, rivers rising
north of shiwaliks, inner Terai. 1. Mountain catchments. 7. Rivers draining from Churia range to the Terai. 3. Pokhara, Nuwakot, Kathmandu,
Sun Kosi tributaries. 4. Lower Tamur valley 5. Rivers draining Mahabarats. 6. Kankai Mai basin.
22. DoLIDAR/MoFALD -Nepal Page 17 Q = AV Figure 3-2: Discharge Measurement with Float Method 3.5.3 Bucket and Watch method Bucket
and watch method is simple volumetric measurement of water at the given time period. This method is applicable for spring source having
discharge less than 1 lps. A plastic bucket or any vessel can be used to measure the volume of water by diverting the entire flow into measuring
vessel of known capacity. The time taken to fill up the vessel is noted. At least three to five readings are necessary for the measurement of the
water and average value is computed. The discharge calculation may be carried out using following format (Table-3.3). Table 3-2: Discharge
Measurement with Bucket-Watch Method 3.5.4 Current Meter Current meter is a device used to measure the velocity of flowing water. It has a
propeller mounted on a shaft which allows moving freely to the propeller at any depth of water. The speed of rotation is the function of the
velocity of water. The manufacturer of current meter provides correlation between number of rotations and velocity of water. The measurement
of velocity is more accurate and reliable and the current meter is extensively used in streams having uniform cross sections. 3.6 Water
Requirement Assessment 3.6.1 Crop Water Requirements The crop water requirement is defined as the quantity of water utilized by the plant
during its lifetime. It is estimate with evapo-transpiration (ETo) of reference crop. The steps used to calculate the crop water requirements are as
follows:
23. DoLIDAR/MoFALD -NepalPage 18 I. Decide cropping pattern, II. Calculate reference crop evapo-transpiration (ETo) (from FAO methods
and tables), III. Use crop coefficients (from FAO), IV. Calculate evapotranspiration of crops ETcrop (mm/day) V. Allow for land preparation (rice
and wheat only) loss (Lp), VI. Allow for deep percolation (rice only)(dp), VII. Calculate evaporation from land preparation (rice only)(Eo), VIII.
Calculate total crop water requirements (CWR), IX. Calculate effective rainfall (Pe), X. Calculate net crop water requirements (net CWR), The
detail of crop water requirement can be referred on PDSP Manual, M3. 3.6.2 Irrigation Water Duty The duty of irrigation water is the amount of
water required for irrigation of unit land area for specific crop. The duty of irrigation water depends upon: · Type of crop grown; · Climate season;
· Type of soil, and · Efficiency of cultivation methods In Nepal water requirement practices vary significantly. In the hills and mountains FMIS
water requirement is as high as 5.0 lps/ha, while it is up to 4.0 lps/ha in Terai. For the design of small irrigation schemes the fallowing water
requirement values are suggested (Table-3.7). Table 3-3: Irrigation Water Duty Source: Pokhrel, 1998 Water requirement at Head works
(diversion point) = Water requirement at field + Conveyance loss
24. DoLIDAR/MoFALD -Nepal Page 19 Chapter-4 4. Diversion Headworks and Intakes 4.1 Selection of Site for Headwork or Intake The
selection of site for the headwork or intake relates to ensure the required flow to be diverted to the canal at all stages of the river. The main
considerations in the selection of the headwork or intake site are as follows: · Channel dimensions- narrow floodway with a relatively straight
approach reach, enabling the river to develop linear flow, · Thalweg deflection- the main channel with low flow is referred as thalweg. The
stability of thalweg is the prime importance in selecting the suitable intake sites, · Outside of a bend- at the outside of bend secondary currents
develop which reduce the entry of bed load into the intake and deflect the bed currents away from the intake (Figure 4-1), · Confluence of two
rivers- at the downstream of the confluence of two rivers the intake location is mostly suitable, · Boulder deposits- in steep rivers boulder
deposits occur when river emerges from the gorge and at its downstream river bed is stable due to reduced tractive force, · Bank stability-banks
with rock outcrops, or armored boulder banks protect the intake from erosion, · River slope-intake location is stable at pool reach of the river
slope, · Stable geological section- intake is stable where geology is stable. Figure 4-1: Layout of side intake on hills and Mountain Rivers
25. DoLIDAR/MoFALD -NepalPage 20 4.2 Types of Intake The main types of intakes are: · Side intake with no river controls; · Side intake with
simple stones or brushwood weir; · Side intake with some form of river controls; · Parallel side intake with spur wall; · Intake from behind the
boulder; and · Bottom intake 4.3 Design of Simple Side Intake The design of simple side intake for small scale irrigation systems in the hills
consists of: · Design of feeder canal, and · Sizing of an orifice to pass the desired discharge The feeder canal shall have capacity of surplus
discharge for flushing the sediments from sand trap or de-silting basin. The orifice with or without gate is provided on the headwall having
sufficient height to protect the canal from flood entry. The sizing of orifice opening will consider the required discharge to pass and to control the
entry of flood flow (Figure 4-2). The basic hydraulic design for a simple intake will depend on the size of sediment and hence on the desirable
nominal trap velocity. The basic design parameters of the side intake are presented hereunder (Table-4.1). Table 4-1: Velocity and Head Losses
across Gravel Traps
26. DoLIDAR/MoFALD -Nepal Page 21 B = orifice width, m D = orifice height, m h = orifice head loss,m g = acceleration due to gravity, (m/s2 )
C = discharge coefficient = 0.7 Figure 4-2: Side intake with two orifices 4.4 Bottom Intake 4.4.1 General A bottom intake captures the water from
the bed of the river. It is a run-of- the river diversion structure which diverts stream flow by allowing water to drop into the trench constructed
transverse to the stream flow. It allows the diverted water to flow through an orifice to the main canal. This type of intake is mostly suitable in hilly
region and also termed as Bottom Rack Intake. The main components of a bottom intake are: · Weir to control minimum river flows and to
maintain the flow towards the rack, · Steel rack to extract water from the river, · Bottom trench immediately below the rack to lead the water
extracted by the rack into the canal, · Flood control gate immediately downstream of the trench, · Feeder canal to carry all sediment entering the
intake to the settling basin, · Settling basin with a side spill and sediment flushing arrangement,
27. DoLIDAR/MoFALD -NepalPage 22 4.5 Diversion Weir 4.5.1 General The diversion weir is simple hydraulic structure across the river to
maintain the pond level at its upstream which needs careful design with respect to hydrologic, hydraulic and structural analysis. According to the
material of construction weirs may be grouped as masonry weir, concrete weir and gabion weir. According to the shape of crest the weirs are
classified as vertical drop weir, Ogee weir, broad crested weir, and sharp crested weir. The layout of the diversion weir and its components are
presented hereunder (Figure-4.2). Figure 4-3: Layout of Weir and its Components 1. Weir 2. Divide wall 3. Fish ladder for large weir 4. Under
sluice 5. Head regulator 6. Abutments 7. Guide bund 8. Canal The design of weir consists mainly two parts: hydraulic design and structural
design. The design of weir shall decide the following parameters: 4.5.2 Crest Level of Weir and Under-sluice Fixation of crest level of the weir is
governed by two factors: the required command level of the canal and to divert the low flow of the river during dry season. If minimum flow of the
river exceeds the design discharge of the canal the crest level of weir is set at lower than the water level of the river. The crest level of the under
sluice is fixed at the lowest bed level of the river in the deepest channel. The crest level of the under sluice is generally kept 1 m below the crest
of the weir in small and medium rivers.
28. DoLIDAR/MoFALD -Nepal Page 23 4.5.3 Length of Weir and Afflux Length of weir has direct correlation with the afflux and discharge
intensity of the weir. Longer the length smaller be the discharge intensity. The length of weir also depends upon the topography of the weir site,
state of river, and provision of energy dissipation at downstream end. Lacey has provided formula for waterway as wetted perimeter: For boulder
stage rivers this Lacey​s formula may not work properly and designer has to judge considering the following main points: · Average wetted width
of the river during flood; · Formation of shoals upstream of the weir; · Energy dissipation devices downstream of the weir; Figure 4-4: Concrete
weir and undersluice The discharging capacity of the broad crested weir is calculated as: Where, L is total clear waterway in m, n is number of
end contraction, H is head over the crest in m 4.5.4 Effect of Retrogression Retrogression is the lowering of the river bed downstream of the weir
in early stages after construction. In designing the weir the effect of retrogression needs to be considered. For small rivers the retrogression is
taken to be 0.30m to 0.50m. 4.5.5 Hydraulic Jump Hydraulic jump occurs when super critical flow of water changes into the sub-critical flow. This
is the most efficient method of dissipating kinetic energy of super critical flow of water. Hydraulic jump is stable in sloping glacis and hence it is
essential to provide sloping glacis at the weir surface with slope ranging from 1:3 to 1:5. With the analysis of hydraulic jump following
parameters are designed: · Downstream floor level; · Length of horizontal floor; · Downstream protection works; and · Uplift pressure
29. DoLIDAR/MoFALD -NepalPage 24 4.6 Gabion Weir 4.6.1 Gabion weir Gabions weir have been used extensively in the past, for irrigation
headworks, but the result has not been encouraging. The gabion weirs are vulnerable to damage by boulders moving during floods, and after a
few wires are broken the entire gabion structures may collapse. Most of the gabions weirs constructed in hill irrigation systems in Nepal stand for
only one or two years and the performance is very poor. Instead of providing the facility to abstract the water, it creates problems of outflanking
and degrading the river bed when it is washed away by the floods. Gabion weirs are suitable in streams with gentle slope and stable bed
conditions usually sandy and muddy. It is nearly impossible to find such kind of situation in the hilly rivers of Nepal. Hence, gabion weirs
basically are semi permanent structures and their use could be limited in the following three locations at head works: · For river bed control; · To
make an orifice controlling the amount of water diverted into the intake; and · As protection for the inlet structure from erosion by flow over the
spillway and from the gate. Compared with masonry, gabions have the advantage of a degree of flexibility, but as described above, they must be
considered as semi permanent structures. They cannot be designed to withstand such high discharge intensities as permanent structures, nor
can they have life of more than 10 years or so years in gravel bed rivers bearing in mind the gabion weirs' vulnerability to breaking and
corrosion. The economy of gabions is in the use of local stones found on the stream itself. The saving in capital cost is, however, offset by
additional maintenance costs and the inevitable rebuilding. If properly constructed, gabions could be suitable for use in reaches of rivers, which
are sometimes unstable, with flood flows causing movement of the riverbed. Gabion weirs may be constructed as simple gravity weirs with
alternative downstream arrangements: · With a downstream sill or counter-weir · With both a stilling basin floor and downstream sill · With
depressed stilling basin floor and flexible apron Gabions are highly permeable, and when used for hydraulic structures, require particular
attention to ensure the foundation material is not washed out a common cause of failure of gabion structures is due to the filter layers being
inadequate. A filter of impermeable membrane must be constructed underneath the gabions. The preferred form of filter is a synthetic fabric ( a
type of geotextile). Alternatively a sand and gravel filter may be used. The sand layer should be at least 150 mm thick. Failure of gabions may
also occur by the wire cage through the impact of stones and boulders carried over the weir. Stepped weirs are not suitable for small channels
with low flows and light bed load. The weir crest may need protection with a concrete coping. A gravel and rock fill against the upstream face of
the weir is recommended. Gabions must not simply be laid on existing bed of banks surfaces; they require proper foundations as would be
required for masonry weirs.
30. DoLIDAR/MoFALD -Nepal Page 25 Comments: The best results for this type of construction are obtained by exercising the utmost care in
the filling of the gabions and in arranging their layout in courses in such a way that the distribution of the mesh in the structure is balanced with
respect to its characteristic functions. The deformation caused by shear can be effectively reduced by increasing the number of panels aligned
perpendicular to the face of the wall and therefore parallel to the soil thrust and water thrust. Every effort shall be made to keep voids and bulges
in the gabions to a minimum in order to ensure proper alignment and neat square appearance. Well constructed gabions with proper panels
inside are monolithic and continuous. Compression and shear tests conducted by MACCAFERRI have shown that gabions could withstand a
compression load of 300 to 400 tons per square meter. The deformation of gabions is mainly due to shear stress developed in the boxes. The
values of shear modulus of elasticity were found varying from 25 to 40 ton per square meter. (MACCAFERRI, Nepal Pvt. Ltd). Figure 4-5: Plan
and Section of a Gabion Weir
31. DoLIDAR/MoFALD -Nepal Page 27 Chapter-5 5 Canal Design 5.1 Design Concept The design of irrigation canal involves: · Determination of
canal capacity; · Determination of cross sectional parameters; · Determination of bed level and slope of alignment; For large irrigation system
canal design of capacity starts from the field level to main canal and up to intake. For small scale irrigation the design of canal capacity may be
carried out by: Q = NCWR x CCA/8.64 Where, Q ​ design capacity of canal at the intake in lps; NCWR ​ net crop water requirement in mm/day
CCA ​ cultural command area in ha; If crop water requirement is not assessed, the capacity of canal can also be determined by the duty of
irrigation water. Q = Duty x CCA Where, duty or water requirement in lps/ha; Example: Duty = 3 l/s/ha; CCA = 20 ha; The capacity of canal (Qd)
= 3 x 20 = 60 l/s Canal cross sectional parameters are bed width, water depth, side slope, roughness coefficient, and free board. The
longitudinal slope of the canal is determined based on the size of the canal and topography of the alignment. 5.2 Design of Unlined Canals 5.2.1
Design Procedure The design of unlined canals is carried out by various methods. For small irrigation schemes it may be appropriate to use
Manning​s formula. The design procedure of the canal follows a series of steps:
32. DoLIDAR/MoFALD -NepalPage 28 The design of various sections of the canal may be carried out in excel sheets. Various parameters of
The design of various sections of the canal may be carried out in excel sheets. Various parameters of canal design are briefed in the
forthcoming headings. 5.2.2 Side Slopes The side slopes for the trapezoidal canal section (Figure 5-1) is determined on the basis of slope
stability of soil. The side slope should minimum as far as possible. If the canal in fill the side slope should be slightly flatter than in cut sections.
Following are recommended side slopes of the canal. Soil type Side slope (m) (vertical: horizontal) Hard rock 1:0.25 Soft rock 1: 0.50 Heavy clay
1:0.50 to 1:1 Loam 1:1 to 1: 1.50 Sandy loam 1:1.5 to 1:2 Figure -: Typical Canal Section 5.2.3 Permissible Velocity The optimum design
velocity is chosen from the experience and knowledge of the soil conditions. The velocity should also be checked using the Tractive Force
formula. The lower velocity allows to grow the weeds in the canal section than consequently decreases the canal capacity while higher velocity
tends to erode the bed of the unlined canal. Hence, the velocity should be in permissible limits based on the type of soil, and capacity of canal.
For unlined canals the velocity should be in between 0.45 m/s to 0.75 m/s. The unit Tractive Force (T) is given by the equation: T = CWRS
Where, C is the coefficient taken as 1 for bed and 0.76 for sides, W is the specific weight of water, 1000 kg/m3, R is the hydraulic radius of the
canal section, m S is the water surface slope The permissible Tractive forces of various soil conditions are presented hereunder (Table 5.1):
33. DoLIDAR/MoFALD -Nepal Page 29 Table 5-1: Permissible Tractive Force 5.2.4 Bed Width to Depth Ratio The bed width to depth ratio
depends upon the capacity of the canal. For small irrigation canals having the discharge less than 0.50 m3 /s the ratio is recommended to be
unity. For discharges greater than 0.50 m3 /s, wider canals could be provided if necessary. 5.2.5 Canal Slope The canal slope is designed for
non-silting and non-scouring velocity in canal and is calculated using Manning​s Formula: Where, R is hydraulic radius =A/P, S is canal slope and
n is roughness coefficient, For small irrigation schemes the slope of the canal may limit up to 1:1500 in Terai and 1:500 in the hills. However, for
existing FMIS the slopes may be steeper ranging from 1:100 to 1:500 depending upon the canal size, soil type, and topography. The value of
Manning​s roughness coefficient depends upon the type of bed materials, condition of the canal and its geometry. The recommended values of n
are presented hereunder (Table 5.2). Table 5-2: Values of Roughness Coefficient
34. DoLIDAR/MoFALD -NepalPage 30 5.2.6 Free Board Free board is difference of bank top level and full supply level of the canal. It is
necessary to accommodate surplus discharge of canal coming from upstream reach. Free board depends upon the capacity of canal. The
recommended values of free board are given below. For Q less than 0.10 m3/s Fb=0.20 m; For Q between 0.10 to 0.50 m3/s Fb=0.30 m 5.2.7
Minimum Radius of Curvature The irrigation canal should be straight as far as possible from hydraulic point of view. However, in practice it is not
possible to align straight canal and needs to bend the canal providing appropriate curvature. The minimum radius of curvature depends upon the
canal capacity and topography. For small irrigation schemes the acceptable radius for unlined canal is taken from three to seven times of the
water surface width. In rocky section the radius may be three times while it is larger in general types of soil. 5.2.8 Design Example of Unlined
Canal Data: Command area = 20 ha, Water requirement in hills = 5lps/ha Soil type = ordinary soil
35. DoLIDAR/MoFALD -Nepal Page 31 Hence, design is acceptable and canal parameters are: Bed width = 0.30 m, Water depth = 0.30 m, Free
board = 0.15 m, Side slope (V:H) = 1:1 Longitudinal slope = 1/500 The design of canal section is usually carried out in excel sheet with trial and
error method. 5.3 Lining of Canals 5.3.1 Type of Canal Lining Canals are lined to prevent seepage and leakage. There are various types of lining
the choice of which is based on the purpose, nature of the scheme and availability of the material. The main features of the lined canal are
presented in tabular form (Table 5-3). Table -: Main features of lined canal 5.3.2 Design Concept of Lined Canal Design of lined canal is carried
out using continuity equation and Manning​s formula considering as rigid boundary canal section. The longitudinal slope for small canals depends
upon the topography and existing condition of the canal. For rectangular section of canal the bed width to water depth ratio is taken as unity for
design purpose. The side slope of the lined canal is zero in most of the cases. However, the lined canal may be of trapezoidal section with side
slope ranging from 1:0.50 to 1:1.
36. DoLIDAR/MoFALD -NepalPage 32 5.3.3 Design Procedure of Lined Canal The design procedure for the lined canal is almost same as that of
unlined canal for small irrigation schemes. Bed width = 0.40 m, Water depth = 0.40 m, Free board = 0.20 m, Longitudinal slope = 1/400 The
design of canal section is usually carried out in excel sheet with trial and error method. LILI has prepared two sheets for canal design: one for
earthen canal and another for lined canal design (Annex: 5-1).
37. DoLIDAR/MoFALD -Nepal Page 33 5.4 Design of Covered Canals 5.4.1 Design Concept Covered canals are generally designed to be free
flowing open channels like lined canal section. For the hydraulic computation, Manning​s formula is used which is described in previous
headings. Head losses in the transition between the open canal section and the covered canal section need to be considered during the design.
The energy losses can be computed as: Where, Hi, Ho are the head losses at the inlet and outlet respectively in m, Ci, Co are loss coefficients
for the inlet and outlet; Vp is the velocity in the covered section in m/s, V is the velocity in the open canal in m/s, g is the acceleration due to
gravity = 9.81 m/s2 The value of Ci and Co is different for different types of transitions. Values for some of the transitions are as follows (Table
5.4). The most practical values of Ci and Co are 0.50 and 1.00 respectively. Table 5-4: Ci and Co values for different transitions 5.4.2 Design of
Piped Canal Piped section may flow full or part full. For piped canals flowing full, the capacity is determined by the: · Pipe diameter (internal); ·
Pipe length; · Difference in water level between upstream and downstream (i.e. the head loss). Pipes are designed for a velocity in the range of
1.0 to 3.0 m/s. Pipe design charts are used to calculate the head loss due to friction within the pipe. Entry, exit and bend losses are added. Total
head loss (HL) is given by: HL = Hi + Hfr + Hb + Ho Where, Hi, Ho are entrance and exit head losses (m) Hfr is the friction head loss over the
length of the pipe (m) Hb is the head loss in bends (m)
38. DoLIDAR/MoFALD -NepalPage 34 The entrance and exit losses are calculated using formula: Most pipe entrances are abrupt and values of
Ci = 0.5 and Co = 1.0 are generally used. Friction losses may be calculated using the charts given in Figures 10.12 to 10.15 of Chapter 10.3,
Rigid Boundary Canals from D2 Field Design Manual Vol I. Losses in bends, Hb, are given by:
39. DoLIDAR/MoFALD -Nepal Page 35 Table 5-5: Hydraulic Properties of Pipes Flowing Part Full A check needs to be made on the hydraulics
of the pipe to see whether the pipe will submerge or not. The Froude number in pipes flowing part full (and covered canals) should be kept below
0.55 to avoid standing waves. The Froude number is calculated from formula
40. DoLIDAR/MoFALD -NepalPage 36 If pipe invert is set at canal bed level, then water depth above invert = 0.5 m. This is about 1.4 x
diameters, i.e. the pipe may submerge (greater than 1.2 times pipe diameter). To avoid submergence, the pipe invert would have to be set above
canal bed level which is not satisfactory for sediment transport. Therefore, increase pipe diameter to 0.6 m (to match the canal depth) and set
invert of pipe at bed level. Set outlet to ensure that pipe is not submerged. Allow 0.2 x diameter freeboard i.e., set pipe invert at (downstream
water level ​ 0.8 x diameter).
41. DoLIDAR/MoFALD -Nepal Page 37 Chapter-6 6. Sediment Control Structures 6.1 Gravel Traps In small irrigation schemes in the hills a
gravel trap behind the intake is provided to exclude the gravel from entering into the canal. The design of gravel traps differs from that of settling
basins because the former handle coarse material which enters near the bed, rather than suspended material which has to be settled through
the depth. The main design principle is that the water velocity through the basin should be less than that which is competent to move the
smallest size of gravel. Table 6.1 gives nominal and design velocities for various particle sizes. Table 6-1: Flow velocity through Gravel Trap
Gravel traps may be emptied by hand or by flushing. For small schemes, hydraulic flushing will not usually be appropriate; manual clearing is
more likely to allow an economic design with more assured operation (Figure-6.1).
42. DoLIDAR/MoFALD -NepalPage 38 Figure 6-1: Typical Gravel Trap for Small Schemes 6.2 Sand Traps 6.2.1 General Configuration Sand
traps operate as settling basins and have three parts: inlet zone, settling zone and outlet zone (Figure-6.2). The design configuration of inlet
zone affects the performance of sand traps. To achieve optimum hydraulic efficiency and effective use of the settling zone, the inlet needs to
distribute inflow and suspended sediment uniformly over the cross-sectional area of the settling zone. The size of settling zone depends upon
the capacity of canal and its location. The minimum length to width ratio (L/W) is taken as 2 to 3.
43. DoLIDAR/MoFALD -Nepal Page 39 Figure 6-2: Typical Configuration of Sand Trap In irrigation systems, it is generally feasible to achieve a
better L/W ratio, say 8 to 10. Basin shape can be improved by subdivision with longitudinal divide walls, which may also be desirable for
operational reasons. 6.2.2 Sand Trap Design For small irrigation schemes where there is no data available for the design of sand trap a
simplified method is adopted here. i. Assess the suspended solids concentration and their mean size From general knowledge and site
inspection assess the maximum suspended solids concentration and the probable D20 and D50 sizes. If flood flows can be totally excluded by
closing the intake then provide the criterion for gate closure. Otherwise, a peak concentration at least as high as the maximum reached annually,
should be assumed. The design of a settling basin is very sensitive to the particle size of the sand to be trapped. Samples of the sediment
entering an existing canal, or in the river at the intake site, should be collected and analyzed for particle size distribution. For design of the
settling basin, the D20 particle size should be used (D20 size being that which 20% of the sample by weight is smaller than). It is not considered
practical to provide a settling basin for D20 particle sizes less than 0.15 mm. For design of the scouring velocity in the settling basin, the D50
size should be used. If there is no available information, the following values can be used for design (Table-6.2). i. Assess transporting capacity
of the canal Consider the design of the canal in terms of its transporting capacity. For example, in a small existing canal assume a maximum
capacity of 100 mg/l of 0.3 mm diameter sand during the month with the highest sediment load.
44. DoLIDAR/MoFALD -NepalPage 40 Table 6-2: Recommended values of sediment particles ii. Calculate the required trapping efficiency The
difference between (i) and (ii) gives the trapping efficiency required. iii. Calculate the required surface area of the settling basin The design
curves (PDSP Manual D2) for D20 particle size can be used to assess the settling basin area. These curves have been simplified to be
appropriate for small schemes. Choose the basin length and width to give a length/width ratio of 6 to 10 (Figure 6.3). iv. Flow depth Choose a
flow depth sufficient to ensure no re-scour of deposits when there is full storage v. Storage requirements Consider storage requirements in
terms of frequency of clearing out, and allow this volume below the nominal basin depth obtained in (v) above. The storage volume, Vs
45. DoLIDAR/MoFALD -Nepal Page 41 iv. Finalize the size, geometry and location, and check that it remains consistent with the assumption
made in (iv) Figure 6-3: Typical Layout of Sediment Trap
46. DoLIDAR/MoFALD -Nepal Page 43 Chapter-7 7. Canal Structures 7.1 Head Regulator Head regulator controls the amount of water flowing
into the off-taking canal. Head regulator is structured at the head of each canal taking water from the main canal. Head regulator is equipped with
vertical sliding gates, Road Bridge, and flow measuring devise depending upon the size of the canal, and type of the scheme. In small scale
irrigation scheme head regulator may not have road bridge and flow measuring device. 7.2 Cross Regulator Cross regulator heads up water
level in the parent canal and allows diverting part of this water to the off-taking canals. Cross regulator is structured at the downstream of off-
taking canal and is equipped with vertical sliding gates and road bridge for vehicular transport. In small scale irrigation schemes cross regulators
are not necessary to construct. 7.3 Proportional Divider Flow divider divides the incoming flow of the canal two into branching canals. Flow
divider may be proportional divider or division box. Flow divider may be equipped with gates or has no gates. Most of FMIS in Nepal have
proportional dividers with no gates. These proportional dividers divide water according to set norms and practices of the community. This
practice leads to the rigid system of water distribution and farmers have no opportunities to take extra water. The system runs automatically with
no need to open or close or adjust the flows. Figure 7-1: Definition Sketch and Photograph of Proportional Divider The width of the structure is
proportional to the designed discharge of each off-taking canal which is visible clearly by the farmers. The downstream condition of the
proportional divider should be of free flow condition in all off taking canals. In other words the downstream water level of the canal should not
influence the flowing pattern of the proportional weir. The design of proportional divider is concerned with the fixing of the width of the weir and
provision of energy dissipating arrangement at the downstream of the off-taking canals. The width of the weir is calculated considering the broad
crested weir: Where, B is the width of the weir, and H is the height of the crest; the width of the weir is proportional to the discharge in free flow
condition. The downstream energy dissipation is calculated as per the hydraulic jump calculations. For small scale structures the length of the
basin is 5 to 6 times the height of the drop (difference in water levels of upstream and downstream canals).
47. DoLIDAR/MoFALD -NepalPage 44 7.4 Outlets Outlets are the small structure used to supply the water from canals to the field. Depending
upon the type, size and importance of the scheme outlets are of various types. The main design objective of these outlets is to supply irrigation
water equitably up to the tail end of the schemes. In large irrigation systems outlets are both gated and un-gated. The choice of the gate
depends upon the necessity of the operational flexibility. For small scale irrigation outlets are simple pipe outlets fixed at the head of the field
channel. The control of water could be managed by manual inspection or letting to flow continuously. In hill schemes pipe outlets provide
sufficient drop to the irrigated area which is considerably lower than the parent canal. 7.4.1 Design of Pipe Outlet The design formula for a
submerged orifice is: Where, Q is outlet discharge in lps, A is the area of the pipe in m2 H is the head difference between parent canal water
level and water level of outlet canal C is coefficient and equals to 3,300 for pipe shorter than 6 m and 2,800 for pipe longer than 6 m (for
submerged flow) & 2760 (for free flow) Thus for a 100 mm diameter HDP pipe with a 0.25 m head difference, the discharge is: Q = 3300 x 0.102
x 0.250.5 = 13 l/s (for submerged flow) Q = 2760 x 0.102 x 0.250.5 = 11 l/s (for free flow) 7.5 Drop Structures In general drop structures are
required in a canal when the gradient is steeper then that allowed by the tractive force. If drops are not provided, erosion will occur in the canal.
In the hills, if the canal is in rock, quite steeply graded canals can be made with no need for bed protection, but some form of stilling basin should
be cut in the rock at the end of the chute. Figure -: Definition Sketch of Chute Drop
48. DoLIDAR/MoFALD -Nepal Page 45 For the small drops (up to 2.0 m), the straight drop fall is the simplest and most appropriate drop fall
structure, for larger drops, various other options are possible: - Chute - Cascade - Pipe drop Design recommendation The common hydraulic
principle in all types of drop structures is the dissipation of Excess energy as the water drops from a high level to a low level. All of the water's
excess energy must be dissipate inside the drop structure before water is allowed back into the downstream canal. Small scale irrigation system
in remote hills, construction of large drop structure is difficult and undesirable. Below 2 meters vertical or chute drops and below 4 meters
cascade or pipe drops are more appropriate. - In case of straight drop, drop should be preferably made in multiples of 0.5 m i.e. 0.50 m, 1.00 m,
1.50 m or 2.00 m. - Except the pipe drops, the crest width/flume width/cascade width should be made approximately equal to the upstream canal
bed - Generally rectangular chute should be used for slopes of up to 1:2 - A cascade for slopes from 1:1 to 1:2, or a pipe drop for slopes steeper
than 1:1 or for very long drops may be more appropriate - In case of cascade drop, the drops should not exceed 1 m each and steps walls
should be at least 0.50 m high - In case of pipe drops, an inlet box with upstream overflow weir for safe disposal of excess flows and outlet
stilling box are required. The maximum pipe velocity for HDPE pipes should be 2 m/sec - The structures should be made out of 1:3 cement
plastered masonry - Pitching should be provided downstream of the structure 7.6 Escape Structure Escape structures are the safety structures
for the release of excess water of the canal. In addition escape structures are provided for emergency release of canal water. Excess water can
enter into the canal both in the dry and rainy seasons. In small irrigation schemes overflow escapes are suitable in the hills while in the Terai
gated escape structure may also be suitable depending upon topography and size of the canal. For remote canal alignment overflow type
escapes are suitable and are located just upstream of the natural drainage channel. Gated escape structures need human intervention to adjust
and control flow hence it is not recommended in the remote locations. Generally the interval of escape structure is based on the size of the canal
and topography. In the hills it is recommended to provide escapes at an interval of 500 m whenever natural drainage channels are available.
Design Concept of overflow escape structure: i. Overflow escapes are designed as side channel weir, Q = 1.7 W h 3/2 Where, Q is the flow
across the crest (m3/s), W is the crest width (m), and h is the water depth across the crest (m) ii. The crest level is set 0.05 m to 0.10 m above
design water level of the canal,
49. DoLIDAR/MoFALD -NepalPage 46 iii. The head over the crest should not exceed 50% of the freeboard, iv. The length of the crest is
calculated for 50% of the canal discharge or estimated excess discharge, For small irrigation scheme escapes are overtopping sections of the
canal at the location where escaped water can channelize to the natural drainage channel. Over topping sections are provided at the upstream
of the Superpassages or at the center of the aqueducts. 7.7 Bridges and Culverts Bridges and culverts are required to carry foot, vehicular and
animal traffic across the canals. If crossings are not provided, the canal bank will be rapidly eroded, causing local instability to the canal.
Crossing should be provided where all the major paths, roads and tracks cross the canal alignment. The culverts may be concrete pipe culvert
and box culvert. The bridges may be foot bridges, large bridges , and walkways. The choice of pipe culvert or box culvert depends upon the size
of the canal and its importance for vehicular movement. The pipe culverts are designed to flow full and have the fall of 0.05 m in normal
condition. The design head loss through the culvert for preliminary assessment is taken between 0.05 m to 0.10 m. For the detail assessment of
the head loss is carried out with the consideration of friction loss in the pipe, and inlet and outlet loss at upstream and downstream of the pipe.
The flow velocity is recommended to limit within 1 m/s in order to minimize the downstream scour and protection works therein. In Nepal two
categories of concrete pipes of standard sizes are available for the purpose pipe culverts: NP2 and NP3. Generally, NP2 pipes are used for road
works while NP3 pipes are used for irrigation works. The detailed characteristics of standard concrete pipes is found in IS Code 458:2003.
50. DoLIDAR/MoFALD -Nepal Page 47 7.8 Retaining Wall 7.8.1 General Retaining walls are the structures necessary along the canal alignment
to protect the canal from slipping or sliding. In general retaining walls are necessary in hill irrigation schemes where canal alignment is not stable
enough. Based on the materials used retaining walls are categorized as follows: · Masonry retaining wall · Masonry with dry stone panels · Dry
stone walls · Concrete retaining walls · Gabion walls 7.8.2 Design Concept of Dry and Gabion Retaining Wall The design of retaining wall
involves the fixation of bottom width, top width, height and material to be used. For dry stone and gabion retaining walls the design criteria is as
follows: i. For shallow depth slide a. Dry stone wall with vertical face Max depth of wall (H) = 4 m, Thickness of wall (W) > 0.2 H, Face angle 4:1
b. Gabion wall with stepping face Max depth of wall (H) = 6 m, Thickness of wall (W) > 0.2 H, Face angle 4:1 ii. For deep depth slide a. Dry
stone wall with sloping face Max depth of wall (H) = 4 m, Top thickness of wall (W) > 0.25 H, Base thickness of wall (B)> 0.70 H b. Gabion wall
with stepping face Max depth of wall (H) = 6 m, Top thickness of wall (W) > 2 m, Base thickness of wall (B)> 0.70 H 7.8.3 Design Procedure of
Masonry and Concrete Retaining Wall The size of the retaining wall for masonry, concrete and RCC is fixed on the basis of stability calculation
which is checked against overturning and sliding. The design procedure involves the following steps: i. Assess the factor of safety · Factor of
safety against overturning 2.0 for normal loading case, 1.50 for extreme loading case · Factor of safety against sliding 1.5 for normal loading
case, 1.2 for extreme loading case,
51. DoLIDAR/MoFALD -NepalPage 48 Figure 7-3: Definition Sketch of different type Retaining Walls ii. Calculate the loads acting on the wall ·
active soil pressure due to horizontal ground, · surcharge load of the soil behind the wall, · hydrostatic pressure behind the wall, · self weight of
the wall, iii. Assess the allowable stress (compressive and tensile) of the wall based on the material of the wall. iv. Assess the filter materials to
be used behind the wall section v. Draw the load diagram vi. Calculate the forces and moments on the toe of the wall vii. Check the factor of
safety viii. Calculate the bearing pressure and check against allowable pressure ix. Check the stress at the junction of the wall
52. DoLIDAR/MoFALD -Nepal Page 49 Figure 7-6: Possible Retaining Examples in Canals
53. DoLIDAR/MoFALD -Nepal Page 51 Chapter-8 8. Cross Drainage Structures 8.1 Type of Cross-Drainage Structure There are five types of
cross drainage structures in irrigation canal alignments: · Aqueduct- when canal passes over the drain, · Superpassage- when canal passes
under the drain, · Siphon- when canal passes under the drain, · Drain culvert- when drain passes under the canal, and · Level crossing-when
canal crosses the drain at the same level 8.2 Design Considerations 8.2.1 Design Discharge The design flood discharge is assessed by
hydrological analysis as described in Chapter-3. In small cross drainage strictures the flood return period may be considered as 5 to 10 years.
For aqueduct 10 years return period may be essential while for Superpassage, siphon, and culvert 5 years return period may be adequate. 8.2.2
Waterway of the Drain The waterway of the drainage is necessary to fix for the structure in order to avoid constriction of the flood flow. In the
hills and mountain areas the waterway is provided within the existing defined banks of the river or drain. In Terai area Lacey​s waterway may be
appropriate; where, Q is the design flood discharge of the drain. 8.2.3 Contraction and Expansion of Waterway The contraction and expansion of
the waterway of the drainage channel should be smooth in order to minimize loss of lead therein. The contraction should not exceed 20% for
drains. The transitions of the aqueduct, siphon, and culvert barrel should not be steeper 2:1 in contraction and 3:1 in expansion. The transitions
are designed on the basis of Mitra​s hyperbolic transition equation: Where, Bx is the bed width at any distance x from the flumed section, Bn is
the bed width of normal canal section, Bf is the bed width of flumed canal section, Lf is the length of transition, 8.2.4 Head Loss through
Structure The loss of head across the structure also governs the choice of structure. Adequate hydraulic head drives the canal flow across the
drainage structure. Aqueducts and Superpassage have less head loss while siphon requires higher head difference between its inlet and outlet.
The velocity through the barrel of the siphon or in the flume section of the aqueduct is limited to 2 to 3 m/s.
54. DoLIDAR/MoFALD -NepalPage 52 8.3 Design Concept of Aqueduct Aqueducts are used where canal crosses over deeply incised streams
or drains. The basic considerations in the design of aqueduct are: · HFL of drain or stream should be sufficiently below the bed level of the canal;
· Banks of the drain should be stable; · For small irrigation schemes the span of aqueduct should limit to 10 m; · Water velocities in the flume
should be from 1.0 to 1.5 m/s; Figure 8-1: Aqueduct in Hills
55. DoLIDAR/MoFALD -Nepal Page 53 8.4 Design Concept of Superpassage There are many types of Superpassage based on their materials
of construction, size of the drainage channel, and need of protection works. The design of each type of Superpassage depends solely on the
topographical conditions and nature of drainage channel. According to the materials of construction Superpassage may be: · Stone masonry; ·
Concrete pipe; and · Reinforced concrete In most small irrigation schemes Superpassages are like a covered canal with guide walls to confine
the drainage flow. However, downstream protection works are essential in several instances to dissipate the excess energy of the drainage
channel during the flood. A typical sketch of the Superpassage is presented hereunder (Figue-8.3). Figure 8-2: Typical Superpassage with u/s
guide walls and d/s protections. 8.5 Design Concept of Canal Siphon Inverted siphons are used to cross the drainage channel by a canal. There
are two types of siphons: · Shallow siphon to cross shallow and wide river with little pressure head, and · Deep siphon to cross deep river
Shallow siphon: Shallow siphons are designed so that the river or drain is kept on the same gradient and the canal flows in a conduit under the
river bed. Small siphons are often designed using concrete pipes in a masonry or concrete surround. Adequate velocity should be provided to
prevent siltation at low flows. For small irrigation canals drop type inlet can also be provided. To prevent choking from floating debris trash rack
is provided at the upstream of inverted siphon. Deep Siphon: Deep siphons are used in the hill schemes to cross the river in deep valleys. The
siphon is taken down the valley side, anchored by blocks, and then crosses the river on the short span bridge. Due to high pressures the siphons
are usually made of steel pipe (Figure 8.4). In some cases bridge is in suspension with pipe held in wire slings below the cable.
56. DoLIDAR/MoFALD -NepalPage 54 8.6 Design Concept of HDP Pipe Crossing The design of HDP pipe consists of three components: ·
Hydraulic design of HDP pipe; and · Structural design of cable suspension Hydraulic Design of HDP Pipe: Pipe crossing section may flow full or
part full. Considering the full flow of the section the following parameters need to be decided first: · Diameter of pipe (internal) based on design
discharge of the canal system; · Length of pipe based on the topography of the drainage section; · The difference in water level between
upstream and downstream or inlet and outlet. The limiting velocity of the HDP pipe is considered less than 3 m/s. Based on this limiting velocity,
and design discharge, the diameter of the pipe and required loss of head therein is assessed from the pipe flow characteristics available in
standard table (standard provided by the manufacturing company). In addition to the loss of head in pipe friction other losses such as trash loss,
entry and exit loss and bend loss need also be assessed by using standard formula and practices. Entry and exit loss is assessed as, He = 1.5*
(V2 pipe-V2 canal)/2g Where, VPipe is velocity of water in pipe in m/s VCanal is velocity of canal at the inlet in m/s Joint loss is assessed as
25% of friction loss for each joint. The bend loss is calculated as Hb = Cb*Vpipe 2 /2g, Where, Cb is bend coefficient, VPipe is velocity of water
in pipe in m/s The trash rack loss is taken as 0.10 for small irrigation schemes. Structural Design of Cable: The design of suspension cable
consists of the calculation of saddle difference, cable inclination, tension in cable, and loads due to cable, pipe, wind load and other loads. ·
Maximum permissible height difference between two saddles (h) = Eh-El, where Eh and El are the elevation of higher and lower saddle
(crossing). H is considered as S/20 to S/25 where S is clear span of the saddle or crossing; · Cable inclinations at higher saddle (ß1)= tan-1 (4b
+ h)/l (degree) and cable inclinations at lower saddle (ß2)= tan-1 (4b - h)/ l (degree), where b is the sag measured at the midpoint of the cable
(m); and maximum cable inclination at saddle of higher elevation is given by, ßmax = tan-1 (4 l /22 + l /25)/ l = tan-1 (0.1818 l + 0.04 l)/ l = tan-1
0.2218 =12.51o , say 13o · Horizontal distance from the saddle of higher elevation to the lowest point of cable is given by ; e = l/2(1+h/4b) ·
Maxm sag at the lowest of cable from the saddle at higher elevation is given by fmax = b +h/2 + h2 /16b · Total horizontal tension (all cable) (HT)
= G* l 2 /8b, where G is load in KN/m · Total Maxm tension (all cable) at the higher saddle is given by, Tmax = HT/cosß1 =HT*(1+tan2 ß1)1/2 ·
Max tension in main cable (T(M) max)= Tmax* Am/(Am+Ah) and Max tension in hand rail cable (T(H)max) = Tmax* Ah/(Am+Ah), Where Am and
Ah are cross sectional areas of main and handrail cables respectively, · To a close approximation when two saddles are at different elevations
the arc length of the loaded cable across the span is given by,
57. DoLIDAR/MoFALD -Nepal Page 55 L = l [1+2/3*(fmin/ l) 2*(1+K)/K + 2/3*{( fmin+h)/ l }2* (1+K)], where L is the total arc length i.e. cable
length across the span to a very close approximation, K = { l /( l +h)}1/2 , l = span of crossing, and fmin is the lowest point of the cable from the
elevation of saddle on the lower side under dead load case. Trial and error procedure is adopted to solve these cable equations. The level span
equations come close to give solutions for the inclined span when fmin is replaced by (fmin+h/2). Since the equations for the level span are
simpler than for inclined span, they can be used frequently for making the first guess for values to use in calculations with the inclined span
equations. For level span, Eh-El = h = 0, C =1 and fmin= Bd = dead load sag, then the equation reduces to L = l [1 +8/3*( Bd/l) 2 ] · For span up
to 80 m, Sag to Span ratio in dead load case = 20 · For span over 80 m, Sag to Span ratio in dead load case = 22 · Free board = Fb= El - HF1-
fmin, where El = Saddle elevation of the walkway cable on the lower side, HF1 = highest flood level, however, Fb = 5m · Factor of safety for the
cable = Tbreak/Tmax = 3 · Where Tbreak=Minimum breaking load for cables and Tmax= Total cable tension at higher foundation level (all
cables) KN · Live load = 400 kg/m2 (4 KN/m2 ) for span = 50m · Wind load = 100 to 150 kg/m2 · Dead load = Weight of all accessories · Other
load are effect due to temperature, snow load etc included in live load · Cutting length of cable = 1.1*Span + back stay length, where back stay
length is the cable length between saddle center and center of dead man or drum(anchorage block) as per foundation consideration. The cross
sectional area of the cable is the actual area of the wire without counting the void space between wires. The value for the area can be estimated
from the information on the cable weight and specific gravity (7.843 t/m3 ) for steel in the wire. The designer should obtain values of E (modulus
of elasticity) and cross-sectional area from manufacturer or designer​s best judgment has to be used. Figure -4: HDP Pipe Crossing
58. DoLIDAR/MoFALD -Nepal 8.7 Design Example of HDP Pipe Crossing Design data: Discharge to be passed through the pipe =0.05 m3 /s
Span of crossing=20.0 m Level difference between the pipe inlet and outlet =2.0 m Total length of HDP pipe required from inlet point to out let
point=50.0 m Velocity of flow in open channel near the inlet point of HDP pipe=0.75 m/s Maxm allowable velocity in pipe < 3 m/s Design steps:
We know, Q=V.A, A=Q/V, From standard Table for Q=0.05 m3 /s, head loss = HL (m/100m) = 0.88/100=0.0088 m, and V= 1.59m/s Flow area of
pipe, A = Q/V = 0.05/1.59 = 0.03145m2 Internal diameter of pipe, Ø= (4*A/3.14)1/2 = (4*0.03145/3.14) 1/2 =0.200m Chose 225 mm outer
diameter HDP pipe from series III (data from HDP pipe factory) Available internal diameter of pipe= 225-2*12.2 =200.6mm, which is close to
chosen Ø=200mm Hence, OK Calculation of losses Headloss due to friction in 50.0m long pipe = 50*HL =50*0.0088 = 0.44m Entry and exit loss
=1.5*(V2 pipe-V2 canal)/2g = 1.5* (1.592 -0.752 )/2*9.81 = 0.15m Trash rack loss = 0.10m Joint loss is estimated as the friction loss in pipe
length equivalent to 0.25 m per joint = 0.25*(50m/5-1) =3.125m (considering length of one pipe to be 5m) Hence joint loss = 0.0088*3.125
=0.0275m Bend loss = Cb* V2 pipe/2g, where (Cb = 0.10) =0.10*1.592 /2*9.81 = 0.0123m per bend Assuming max 4 bends in this case we get,
bend loss = 0.0123*4 = 0.052m Total loss = 0.44 + 0.15 + 0.10 + 0.0275+ 0.052 =0.77 m < 2 m (level difference between inlet and outlet),
Hence, OK Page 56
59. DoLIDAR/MoFALD -Nepal Page 57 Chapter-9 9. Shallow Tube Well Irrigation 9.1 Groundwater Potential in Nepal Groundwater is a part of
hydrologic cycle and considered as reservoir of fresh water. Use of underground water for human needs is being practiced since time
immemorial. There are abundant groundwater resources in Nepal Terai both in the form of shallow and deep groundwater. Shallow groundwater
is extracted from the unconfined aquifer where as deep groundwater is extracted from the confined aquifer of the ground. initiated in Nepal from
1959 under Groundwater Resources of Kathmandu Valley Investigation by Geological Survey of India. Later in 1969 a comprehensive
investigation was started under USAID assistance mainly focusing Terai districts: Nawalparasi, Rupandehi, Kapilvastu, Banke, Bardiya, Kailali
and Kanchanpur. In 1982 D.Duba had evaluated the groundwater potential of Nepal Terai as 11.60 Billion Cubic Meters (BCM). In 1987
Groundwater Development Consultant (GDC) had estimated the total potential of 10.75 BCM. In 1993 JICA had also carried out groundwater
study in Nepal Terai mainly focusing on Jhapa, Siraha and Banke districts. Water Resources Strategy 2002 has prepared hydro- geological
mapping (Figure-9.3) of the country and has assessed the rechargeable groundwater potential between 5.80BCM to 11.50 BCM.
60. DoLIDAR/MoFALD -NepalPage 58 Based on the potential groundwater the irrigable area for shallow tube well in Terai comes to about 1.53
million ha. APP had targeted the installation of 176,000 numbers of STW in Terai each irrigating 2.50 ha of land. The present use of
groundwater for irrigation is less 20 percent of its minimum potential. 9.2 Design concept The design of shallow tube well involves the siting of
the well, design of well, selection and design of distribution system, selection of appropriate pumps, and cost estimate of the shallow tube well
construction. The site for tube well construction depends upon the groundwater potential, land suitability for irrigation, and type of well drilling
technology. The design of tube well relates with the design of screen length, casing and gravel pack arrangements during drilling. The selection
of water distribution system depends whether the piped system or channel system would be suitable for the given condition. In the design of tube
well two assumptions need to be made: · Well yield, and · Crop water requirement Highly Productive Aquifer Moderately Productive Auifer No
Groundwater Potential
61. DoLIDAR/MoFALD -Nepal Page 59 Based on the previous experience of similar wells in the locality, yield of the well needs to be assessed.
The yield of the well is simply the expected discharge of the proposed well. It depends upon the hydro- geological strata of the well location,
draw down of water table, length of screen to be provided and size of well boring. The discharge formula of the shallow tube is expressed as:
Where, Q- discharge in m3 /s, K-coefficient of permeability in m/s, H- initial head existing before pump in m, h-operative length of strainer in m,
R- radius of influence in m, and r-radius of well in m In practice the yield of the well is assessed to be 5 to 15 lps depending upon the
hydrogeology of the area. 9.3 Suggested Design Criteria Based on hydro-geological evidence and existing practice of shallow tube well
construction GDC has suggested following design criteria for effective design and implementation. · Minimum screen length of 10 m, · Internal
diameter of well 4 inch or 100 mm for 15 lps target yield or less, · Minimum 12 m blank pipe from ground level to the top of the screen, · Well
development until water is clear, 9.4 Design Procedure of STW Data required: · Topographical maps, · Cadastral maps, · Soil infiltration test
results, · Drilling results (log sheets), · Type of distribution system and · Power source for pump Design Steps: · Assessment of hydro-geology of
the area and suitability of STW development including quantity and quality of water; · Assessment of well yield; · Determination of well
parameters such as screen type, screen length, and length of casing; · Assessment of land suitability for STW irrigation; · Design of cropping
patterns with and without project conditions; · Calculation of crop water requirement based on proposed cropping pattern, · Preparation of
distribution layout based on topography. · Siting of well and access to the field; · Selection of power supply options or availability of electric
supply; · Selection of type and technology of distribution system such as lined canal versus unlined canal, open channel versus piped channel; ·
Design of channel size or pipe size;
62. DoLIDAR/MoFALD -NepalPage 60 · Selection of standard control, distribution, and outlet structures; · Decide STW operating rules; · Cost
 estimation; · Cost benefit analysis; 9.5 STW Design Example 9.5.1 Design Data Proposed irrigated area: 180 ha Location: Mainapokhar, Bardiya
Hydro-geology: As shown in sketch (Figure-9.5) 9.5.2 Assumptions Made Based on the hydrogeology of the area and past experience assume
the discharging capacity of the STW is 5 lps, which will be sufficient to irrigate about 3 ha of land. The water requirement hence is assumed as
1.66 lps/ha. 9.5.3 STW Planning The entire study area consists of 180 ha of land suitable for STW irrigation development. It is proposed to have
60 units of STWs each irrigating 3 ha of land with discharging capacity of about 5 lit/sec. The whole command area is developed in 2 clusters
having 30 units of STW in each. The irrigated area of each cluster would be then 90 ha. 9.5.4 Tube-well Design A typical shallow tube-well
consists of a length of the well string continuous through aquifer. The productive aquifers are screened so that water can enter the well and
other strata are sealed with blank casing. A gravel pack around the well string is necessary to prevent fine sand from the aquifer entering the well
through the screen. Diameter of pipe: 100 mm Diameter of casing: 100 mm Maximum possible depth of tube well: 25 m from ground level The
length of screen is calculated using formula: Where, L ​ length of screen in m, A - area of screen required in m2 A = discharge/entry velocity
Discharge is 5 lps and entry velocity is taken as 3 cm per sec A = 5 * 0.001 /3 * 0.01 = 0.166 m2 Opening percent is assumed to be 11 percent
d- screen diameter in m = 10 cm= 0.10 m Thus, the length of screen is 4.83 m and takes as 5.0 m
63. DoLIDAR/MoFALD -Nepal Page 61 The transmissivity of groundwater aquifer is assumed to be 1,100 m2 per day for more than 60-m
thickness aquifers, which gives the hydraulic conductivity of 18.33 meter per day. The design discharge of STW for irrigation of 3 ha of land is
assumed to be 5 lps (432 m3 per day). Drawdown is estimated from Logan​s equation. Where, S - drawn down in m Q ​ design discharge in m3
/day = 5 lps = 432 m3 per day K ​ hydraulic conductivity in m per day = 18.33 m/day L ​ length of screen in m = 5 m Thus, drawdown S is equal to
6.22 m 9.5.5 Pump and Motor Design The pump and motor design is carried out considering the critical condition of pumping the water from
STW. When maximum suction head approaches to the permissible value the situation is critical. This maximum suction head includes drawn
down in water level including losses in the non-return valve, bends and the pump mechanism. Based on the head to be required the capacity of
pump is calculated. P = (0.746*Q*H)/ (270*E) Where, P = the required capacity of the pump in kW Q = required design discharge of the STW in
m3/hr = 18 m3 /hr (5 lps) H = total dynamic head in meters = head loss in delivery + draw down + depth to water table + other losses = 2.5 m +
6.22 m + 4.5 m + 1 m = 14.22 m, say 15 m. E = pump efficiency usually taken for overall efficiency as 70 percent P = (0.746*18*15)/(270*0.70)
Thus P = 1.06 kW Considering the factor of safety as 25 percent the pump motor capacity comes to: Capacity of the motor = 1.06*1.25 = 1.33
kW or approximately 2 HP Indian and Chinese pumps having 2 HP capacities are available in the market. In addition, there are submersible
pumps for 4-inch diameter STW having 3-inch outlet delivery pipe. The discharge of this type of pump ranges up to 10 lps and 15 lps. Such new
technology pumps are also available in the market. 9.5.6 Distribution System The distribution system proposed is in line with the practice
followed by the farmers. It is considered that a temporary thatch house will be made as a pump house and the distribution system will be of
earthen channel. The length of such distribution earthen channel will be based on the specific field conditions depending on topography and its
relief. In general the length of the distribution system will not exceed 240 m and its configuration will depend upon the actual site condition. The
bed width of the canal will be about 20 cm with depth not more than 20 cm in trapezoidal shape.
64. DoLIDAR/MoFALD -Nepal Page 63 Chapter-10 10. Micro Irrigation 10.1 Introduction Micro Irrigation, in the context of Nepal, is related to the
alternate methods of both irrigation application and water acquisition. This chapter deals with the concept and design of various forms of micro
irrigation feasible for Nepal. 10.2 Pond Irrigation 10.2.1 Design Concept The design of pond irrigation involves the design and assessment of the
following components: · Water availability assessment for the proposed site of pond; · Irrigation water requirement assessment-based on
cropping pattern, irrigation methods; · Design of intake and conveyance canal or piped system-collection chamber, washout chamber, flow
regulating chamber; · Design of pond capacity or water storage reservoir; and · Water application methods- drip irrigation, sprinkle irrigation and
or free flooding method; Figure 110-1: Definition Sketch of pond Pond irrigation scheme shall be proposed in the water scarce area. In other
words it shall be proposed where the source of water for conventional irrigation may not be sufficient. In such schemes the source of water may
be quite small and need to be tapped carefully. The design of water intake shall involve the design of spring intake structure as adopted in
drinking water supply schemes: · Measurement of discharge with bucket and watch method, · Design of overflow weir, · Design of inlet chamber,
and · Sizing of conveyance pipe The capacity of pond shall be fixed on the basis of the following: · Available water at the source, · Water
requirement per day for the proposed crops, and · Agreed pond operation rules with the farmers

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