FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCES
ERER DAM & IRRIGATION DEVELOPMENT PROJECT
INTERIM (DESIGN CRITERIA) REPORT
(Final) October, 2008
CONCER 7 ENGINEERING A ND
CONSUL! INGENTERPRISEP.I C (CECE)
ENGINEERS INA IER RESOURCES & AGRICUI TURAL DEVELOPMENT PLA NNERS
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A C R O N Y M S
Annual Average Daily Traffic
BCR
Benefit Cost Ratio
BM
Bench Mark
CCA
Culturable Command Area
CP
Control Point
DCF
Discount Cash Flow
DGPS
Digital Global Positioning System
DS
Design Speed
D/S ERA EIRR ERL GCA IRR MDDL MO FED MWL Nl’V PBM RC
SH1PL U/S UTM WU'DSE
Dam Stream
: Ethiopian Road Authority
: Economic Internal Rate of Return
Full Reservoir Level
Gross Command Area
Internal Rate of Return
Minimum Draw Down Level
Ministry of Finance and Economic Development
Maximum Water Level
: Net Present Value
Protect Bench Mark
Reinforced Concrete
Synergies Hydro India Private Limited
(Jp Stream
: Universal Transverse Marketer
: Water Works Design and Supervision Enterprise2.5.1.    Dam Site Foundation Investigation and Criteria for Design Parameters 34
2.5.2 Construction Material Investigation.................................................................... 34
2.5.3 Major Hydraulic Structures- Foundation Investigation for Design Criteria.... 35
2.6  Road Infrastructure.................................................................................................... 35
2.6.1 Road Traffic Volume and Load........................................................................... 36
2.6.2 Road Network...................................................................................................... 37
2.6.3 Road Standard in the Irrigation Scheme.............................................................. 37
2.6.4 Geometric Design................................................................................................ 40
2.7 Building Infrastructure and Service Centers...........................................................45
2.7.1 Planning Units of Service Center.........................................................................45
2.7.2  Selection of Construction and Finishing Material.............................................. 47
2.8 Irrigation and Drainage............................................................................................... 47
2.8.1 General................................................................................................................. 47
2.8.2 Command Area.................................................................................................... 48
2.8 3 Irrigation Water Requirements...........................................................................  49
2.8        4 Canal System................................................................................................ 56
2 8.5 Irrigation Structures............................................................................................. 64
2.8.6 Drainage System.................................................................................................. S6
2.9   Financial and Economic Analysis.............................................................................92
2.9.1  Estimation of Cost & Benefit Stream................................................................. 92
2.9.2  Methodology....................................................................................................... 92
2.9.3  Crop Budgets...................................................................................................... 97
2.9.4 Financial Viability Criteria.................................................................................. 97
2.9.5  Expected Output................................................................................................. 97
2.9.6  Project Viability Indicators.................................................................................97
2.9.7  Key Evaluation Parameters...............................................................................100
List of Table
Table 2.1 Design Standards VS Road Classification and AADT................................. 39
Table 2.2 Geometric Design Surface Type and Carriageway & Shoulder Width .41 
Table 2.3 Geometric Design Parameters for Design Standard DS6 (Unpaved)............42
Table 2.4 Geometric Design Parameters for Design Standard DS7 (Unpaved)............. 422.5.1.    Dam Site Foundation Investigation and Criteria for Design Parameters....... 34
2.5.2 Construction Material Investigation................................................................. 34
2.5.3 Major Hydraulic Structures- Foundation Investigation for Design Criteria.... 35
2.6 Road Infrastructure.................................................................................................. 35
2.6.1 Road Traffic Volume and Load........................................................................ 36
2.6.2 Road Network................................................................................................... 37
2.6.3 Road Standard in the Irrigation Scheme........................................................... 37
2.6.4 Geometric Design............................................................................................. 40
2.7        Building Infrastructure and Service Centers................................................ 45
2.7.1 Planning Units of Service Center..................................................................... 45
2.7.2  Selection of Construction and Finishing Material........................................... 47
2.8 Irrigation and Drainage............................................................................................47
2.8.1 General..............................................................................................................47
2.8.2 Command Area.................................................................................................48
2.8.3     Irrigation Water Requirements..................................................................... 49
2.8.4  Canal System................................................................................................... 56
2.8.5 Irrigation Structures.......................................................................................... 64
2.8.6  Drainage System.............................................................................................. 86
2.9   Financial and Economic Analysis......................................................................... 92
2.9.1 Estimation of Cost & Benefit Stream.............................................................. 92
2.9.2 Methodology....................................................................................................92
2.9.3  Crop Budgets................................................................................................... 97
2.9.4 Financial Viability Criteria...............................................................................97
2.9.5  Expected Output.............................................................................................. 97
2.9.6  Project Viability Indicators............................................................................. 97
2.9.7  Key Evaluation Parameters........................................................................... 100
List of Table
Table 2.1 Design Standards VS Road Classification and AADT..............................39
Table 2.2 Geometric Design Surface Type and Carriageway & Shoulder Width......41
Table 2.3 Geometric Design Parameters for Design Standard DS6 (Unpavcd)...^v.42 
Table 2.4 Geometric Design Parameters for Design Standard DS7 (Unpaved)......... 42Table 2.5 Geometric Design   parameters for Design Standard DS8 (Unpaved).........43
Table 2.6 Geometric Design   parameters for Design Standard DS9 (Unpaved).........43
Table 2.7 Geometric Design   parameters for Design Standard DS 10 (Unpaved)...... 44
Table 2.8 Functional and Space Requirement of Buildings in the Serive Centers....... 45
Table 2.9 Cropping Pattern for Erer Dam and Irrigation Project...................................50
Table 2.10 Optimal Length of Furrows for Different Kind of Soils Found In the
Command of Erer Project.............................................................................55
Table 2.11 Recommended Slops for Different Types of Soils...................................... 56
Table 2.12 Limiting Radii for Different Canal Capacity............................................... 57
Table 2.13 Bank Side Slopes for Main Canal................................................................ 62
Table 2 14 Recommended Field Drain Dimensions.......................................................86
Table 2.15 Recommended Dimenstions ofTertialry Drains........................................... 90
Table 2.16 RecommeCECE and CES
1. Introduction
1.1 Project Background
The feasibility study of the Erer dam and irrigation project (WEDSE & SHIPL, 2007), shows that the project was envisaged  to  develop  an  area of some 4000  hectares using the construction of an embankment dam of 36.6m high & 1430m crest length, with total capacity of 50.6 million cubic meter.
The project is intended  to  increase agricultural production  per unit area,  improve the living standards of the local poor farmers and bring about better improvements in socio economic conditions & food security.
1.2 Purpose and Scope of the Present Report
The purpose of the present consultancy service is to undertake the detailed design of Erer Irrigation  project based  on  the feasibility  study  conducted  by  WWDSE in  association with SHIPL in 2007. Accordingly, this report presents design criteria for the envisaged detailed  design  of the engineering  works,  and  also  includes planning  criteria on  the financial & economic analysis.
The preparation of the design criteria is very well linked to the actual field situation. At the same time sufficient effort is made to improve the design approach used durnig the feasibility study report.
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2. Design Criteria
2.1 Topographic Surveying
The topographic survey undertaken during feasibility study has many deficiencies with regards to the benchmarks, leveling survey, feature survey and preparation of drawings. During this study additional surveys/resurveys shall be undertaken to rectify deficiencies in  the previous topographic surveys as well as to  fortify  the topographic survey  for undertaking detail designs of the project.
The following surveys shall be undertaken
•    Establishment of Control Points/Benchmarks
•    Topographic mapping survey/ Level densification survey
•    Cross section and longitudinal section survey
2.1.1.  Establishment of Control Points/Benchmarks
Benchmarks are the primary  control points for the development of the topographic survey. The benchmarks refer to both of the horizontal coordinates (local or real world) and vertical coordinates, on the basis of which complete design is dependent Therefore erection of precision benchmarks which have a life period of at least 10 to 15 years is prime requirement for the project.
The benchmarks established during feasibility study have quite a lot of problems in terms of coordinates as well as their erection on the ground The feasibility study benchmarks are not related  to  the GTS benchmarks of the national geodetic grid.  The benchmark levels are quite confusing as their levels written on ground, levels reported are different. Moreover many  benchmarks having  been  documented  with  same name but different coordinate. The coordinates of the benchmarks reported in feasibility are different from actual ground coordinates. Most of benchmarks are either worn out or removed by the villagers as these are just painted on the rocks or erected by 3 to .4 inch thick cement masonry.
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As a network  of new benchmarks needs to  be established,  it should  be initiated  by erection of new master benchmark or project benchmark to which all other benchmarks are related.  The project benchmark  should  be established  on  one of the existing benchmarks of the Feasibility Study, The difference in datum of the topographic survey of the Feasibility  Study  and  the new real world  datum defined  could  be accurately computed and corrected for the levels of different project components.
a)  Benchmark Establishment Specifications
In  the proximity  of the project area a GTS benchmark  is to  be located,  so  that the coordinates and elevation of the same are transferred to the project area by the help of Total Station to make a project benchmark (PBM). This project benchmark will be the master benchmark for the entire project, which is referred to by all the other secondary and temporary benchmarks established in the project area.
The master benchmark or the project benchmark should be situated close to the dam axis and should fall on one of the existing benchmarks established during the feasibility study. Preferably the project benchmark should be erected on one of the four benchmarks close to  dam axis namely  BM-1/BM-2/BM-3/BM-12A.  The overlapping  of old  and  new benchmarks will establish the correction factor so as to be used to correct the levels of the project components.
Incase of non availability of GTS benchmark in proximity of the project area, project benchmark should be established using precision DGPS with UTM coordinate system, Zone 38N
Using the project benchmark many secondary benchmarks are establishes in each of the major project component locations by the help of total station. The project component locations could on the opposite bank (of project benchmark) on the dam axis, main canal off take, off take of secondary canals, aqueduct location close to the bridge on Harar - Jijiga road and few important locations in the command area like territory canal off takes, location of colonics, new roads and command area extremities.
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Few secondary benchmarks are also established in proximity of the Erer River on both the firm banks, starting from the dam axis to the end of the command area.
Approximately 60 secondary benchmarks are to be established in the entire project area. About 10 secondary benchmarks in the dam site area, about 10 benchmarks in the main canal area, about 30 benchmarks in the command area and about 10 benchmarks on the left bank of the error river.
b) Benchmark Dimensions
The specification for dimensions of the benchmark to be erected in the site are to be guided by the principle that benchmarks should exists for sufficiently long periods, that is far much beyond the construction and design period so that any further modification or extension of the project could be undertaken subsequently.
1)  Project Benchmark Specification
The project benchmark (PBM) should be a reinforced concrete pillar constructed with rich concrete reinforced by four bars of at least 10mm diameter on each comer of the pillar The pillar would be 1.5m long of which lm embedded in the ground and 0.5m, projected above the ground The PBM pillar should be 0.5m x 0.5m section. On the top of the pillar a brass or a stainless steel plate is embedded with a triangle and a dot in the center referring the coordinate of the PBM, label as “Project Benchmark (PBM)” and the coordinate values with the benchmark elevation as on top of the pillar are engraved. Also the coordinates and elevation of the PBM pillar shall be engraved on at least two sides of the pillar so that the values are available in case of theft of the brass/steel plate.
2)  Secondary Benchmark Specification
The secondary benchmarks are erected by transferring the coordinates and elevations to the respective benchmarks with the help of total station. The secondary benchmarks are
• also concrete pillars which are lm long, of which about 70 cm is embedded in the ground and 30 cm projected above the ground with a reinforcement bar of 1 cm diameter
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embedded in the center. The pillar should have a section of 30 x 30 cm. The top of pillar shall have a point and  a circle with  the coordinates and  elevation  values of the pillar written  on  the top  and  the name and  number of secondary  benchmark  engraved  on  it. Also the coordinates, elevation and the name of the benchmark should be engraved on at least two sides of the pillar.
2. 1.2 Topographic Mapping Survey
Topographic mapping survey will be carried out for densification of the levels filled in the gaps of the survey undertaken during the feasibility study as well as an item specific survey for the important features of the project. After the survey levels will be plotted on to the drawing and the datum is rectified by the datum correction factor computed from the project benchmark.
a) Level Densification Survey
Level densification survey will be carried out for filling the gaps in the feasibility study level survey. Gaps in the survey points will be identified after plotting on the drawing with measured points at 1: 1000 scale.
The areas are considered as gaps if these fall under following categories -
•    The survey points are sparser than 50m in the plain areas (areas with less than 10 
% slope).
•    The survey points are sparser than 30m in moderate slopes, undulating areas (areas with 10% to 25% slopes).
•    The survey points are sparser than 20m in the steep sloping areas (areas with 25 to 50 % slopes).
•    The survey points are sparser than 10m in the topographic break line areas (areas
of sudden topography changes like transition zones of plain and slope areas).
Level  densification  survey  will  be  carried  out  in  reference  to  the  nearest  secondary benchmark in the gap' area. The gap area is surveyed to the extent the gaps are removed
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according  to  the  gap  criteria.  In  the  course  of  gap  survey  temporary  benchmarks  are erected as per the survey requirement.
All the topographic features falling  in  the project area are to  be recorded  with  their respective names for feature attributes.  The features include villages boundaries with names, ponds, lakes, tanks, Gardens, plantations, Agricultural fields bunds, roads, rivers and their courses, drainages, major land use classes, bridges, culverts, existing canals if any, existing structures if any, electrical poles and lines etc. will be captured. All features shall be captured  in  3d  coordinates i.e.,  every  point of the feature shall have X,Y,Z coordinates.
b) Item Specific Survey
Item specific survey will be carried out for all important components of the project. The major components of the project are dam site inclusive of spillway and outlet, primary and  secondary  canals and  their structures,  infrastructure like colonies and  roads for construction and maintenance of the scheme and locations of investigation and areas of borrows
(1) Dam Site Survey
Dam site survey  shall be started  from the project benchmark.  Project benchmark  and closest secondary benchmark will the two reference points to start the survey. Survey is conducted such that levels are recorded at a close interval of less then 10m apart
Dam site area close interval Survey  is conducted  for a length  of about 2km (250m additional length to the dam crest length on right and left abutments) and a width of 500m on either side of the dam axis so as to cover dam base width and spillway on down stream, and dam base width and impervious clay blanket on the upstream. The total area of survey for the dam site shall be 2km along dam axis and 1km wide across dam axis. Incase of the change in dam and spillway dimensions the area of survey needs to be extended accordingly.
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(2) Canal Strip Survey
A canal strip survey is conducted for a tentative length of 14.4 km for the primary canal and 16.5 km for two secondary canals. The length may vary up to + 10 % of the proposed length.
Strip  survey  of similar specification  shall be earned  out for both  the primary  and secondary canals. Strip survey shall be carried for a width of 60m i.e., 30m on either side of the center line of the canal alignment. Cross sections are taken every 100m along the canal alignment.
Minimum of seven  levels shall be taken  along  each  cross section  i.e.,  one in  the middle/center line of the strip and three each on either side of the center line (i.e , levels are taken 10m apart in each cross section in the strip) at every 100m along the canal alignments. In the places of undulations about 10 level points across the strip will be captured.
Concrete Pillars, 50 cm long, 20 x 20 cm cross section with a steal pin shall be provided and fixed such that each pillar is 40 cm underground and 10cm projected above ground at intervals of every  2  Km.,  along  the alignment of the canals.  The pillars of similar dimensions are also fixed at the Intersection Points (angle Points) of the canal alignments. These concrete pillars shall be marked with paint and given a Temporary Bench Mark (TBM) number and elevation value. The coordinates and elevations of these TBMs shall be recorded and listed.
(3) Structures
Major structures along the canals shall require special close grid survey. These surveys are undertaken for the following canal structures.
•  At  change  of  5.5  km  on  the  primary  canal  an  ‘Escape  Structure”  is  proposed which provides the provision of escaping the canal water in to the drain during emergencies. This escape structure is located at a point where the primary canal
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crosses main drain. At the drain crossing the canal, 50m x 50m grid survey shall be done with a level spacing of 5m
•      The  primary  canal  crosses  the  Harar  -  Jigiga  road  and  shall  pass  under  the approach  road  to  the  bridge  on  Erer  River.  At  this  crossing  a  siphon  shall  be constructed for the primary canal. At the location of the siphon of the primary canal 100m x 100m grid survey with level spacing of 5m shall be done
•     At the tail of the primary canal i.e., at chainage 14.4km one head regulator each for the two offtaking secondary' canals are proposed At this chainage a 100m x 100m grid survey with level spacing of 5m shall be done.
•     Both the secondary canals pass across the Decho River in the command area. At each of the canal crossing above the Decho River an aqueduct is provided. At the locations of aqueducts at a chainage of 0.91 km on the first Secondary Canal and at 2.45 km chainage of the second Secondary Canal a 100m x 100m grid survey shall be carried out with level spacing of 5m.
(4) Colonies Survey
It is assumed that the colonics for construction and maintenance of the scheme will be located close to the dam site or within the command area. Hence the survey conducted for the dam site area/ command area will serve the requirement for the colony design
(5) Road Survey
Road strip survey will be carried out for any new roads to be constructed in the project area It is presumed  that existing  roads will be strengthened  both  in  the dam site approach area and the command area For the strengthening of the existing road the level densified topographic map will serve the requirement
(6) Survey of Investigation and Borrow Locations
During the level densification survey of the project area all the locations of the geological investigations like drilling and pitting, soil investigation locations and locations of the borrow areas are recorded 
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2.1.3 River Cross Section and Longitudinal Profile Survey
Survey will be conducted for cross sections and longitudinal profile of the Erer River.
a)  Cross Section Survey
Cross section  survey  will be conducted  on  the Erer River to  estimate the carrying capacity of the river in proximity of the command area so as to design flood protection embankments if any required in the proximity of the command area
Cross section  survey  shall be conducted  on  Erer River starting  from the upstream of command  area to  the tail reaches of the command  at every  1km interval.  The cross sections should extend up to 500m on the either side of the Erer river center or up to +5m elevation of the highest flood level marks if available, which ever is less.
Cross sections shall be surveyed by measuring the levels every 10m in the river and 20m in the gentle sloping banks and plain areas
b)  Longitudinal Profile Survey
Longitudinal profile is developed for the Erer River from the Maximum Water level in the reservoir to the tail of the command area by taking levels every 100m.
2.1.4 Drawings and Survey Outputs
All drawings are prepared with 1: 1000 meter scale with contour developed at 0.5 m interval in the dam site and command areas. However to avoid very close contour in steeper areas lm interval contour could be developed The outputs shall be in the form of AutoCAD Drawing  in  UTM coordinates,  Zone 37  N and  Datum WGS-84  i.e.,  the drawing shall be in real world coordinates.
All the topographic features shall be in their respective layers only. The levels shall be in separate layer marked with 3D points and their elevation in the form of text. The text of the levels and the topographic features will be in separate and respective layers. Contours developed shall be in separate layer with elevation values in the each of the contour line, 
i.e., the contours should be 3d lines. 
,
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All lines shall be contiguous with  no  breaks and  no  duplicates.  Polygon  layers should have proper closed polygons with topology duly built. Longitudinal profile of the primary and secondary canals along the alignment will also be developed and supplied in the form of AutoCAD drawing
2.2   Dam Engineering
2.2.1 Selection of Type of Dam
a) General
The selection of the type of dam mostly depends on the following:
1) Topography
Topography dictates the first choice of the type of dam. For example:
■   A narrow U- shaped valley, i.e., narrow stream flowing between high rocky walls, would suggest a concrete overflow dam.
■   A low, rolling plain country, would naturally suggest an earth fill dam with a separate spillway.
■   The availability of a ‘spillway site’ is very important while selecting a particular
kind of a dam
2) Geology and Foundation Condition
The foundation has to carry the weight of the dam. The strength, thickness and inclination of strata; permeability,  fracturing: and  faulting  are all important considerations in selecting the dam type.
The various kinds of foundations generally encountered are discussed below:
■   Solid rock foundations: Almost every kind of dam can be built on such foundations.
■   Gravel foundation: It is suitable for earthen and rock fill dams.
■   Silt and fine sand foundation: They suggest the adoption of earth dams or very low gravity dams.
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■ Clay foundation: They are not fit for concrete gravity dams or for rock-fill dams. They may be accepted for earthen dams, after special treatment
3) Availability of Materials
In  order  to  achieve  efficient  construction  of  the  dam,  the  materials  required  for  its construction must be available locally and at short distances from the construction site.
b) Selection of Erer Dam as an Earth Fill Dam
Since the topography of the dam site is low rolling plain and not narrow U shaped valley, the choice of concrete gravity dam has been ruled out. As described in the feasibility study, the geological profile along the proposed dam axis depicts the three distinct parts of the foundation  viz,  left bank,  river channel and  the right bank.  On  the left bank, weather and fresh bed rock may be available at shallow depth (1- 6 m). On the right bank, the exploration results reveal the presence of sandy clay, gravel layer, and then sand layer up to a depth of 27m. The river channel section is covered by alluvium (30-35m thick) comprising of silty sand mixed with clay.
Presence of overburden to a considerable depth overrules the choice of concrete gravity dam and rock-fill dam. The choice is thus restricted to construct an earth fill dam. Construction material survey, as detailed in the Feasibility Study shows that the material required for a zoned earth fill dam is abundantly available in proximity of the dam site. Based on the above considerations, the earth fill dam is the best solution for Errer dam
2.2.2 Reservoir Operation Levels
The minimum draw down level (MDDL) of the reservoir will be guided by the operating head required to drive the design discharge through the irrigation outlet, which in turn will be set near to  the new zero  elevation  at dam site corresponding  to  reservoir sedimentation at the end of 50 years of reservoir use. After fixing the MDDL, the series of computed  river inflows and  the required  outflows from the reservoir would  be simulated to assess the required live storage and thus to assess the full reservoir level ( FRL). The FRL having being fixed, the flood routing studies would estimate the flood lift
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for passing the design flood over the spillway for the selected crest length of ungated spillway, thus, arriving at the maximum water level (MWL) of the reservoir for passing the design flood.
2.2.3 Basic Design Requirement
The basic principle of design  is to  produce a satisfactory,  functional structure at a minimum total cost. Consideration must be given to maintenance requirements so that savings of construction do not result in excessive maintenance costs. An earth fill dam must be safe and stable during all phases of the construction and the operation of the reservoir To accomplish this, the following criteria must be met.
•     The slopes of the embankment must be stable during construction and under all condition conditions of reservoir operation and earthquake.
•      Seepage  flow  through  the  embankment,  foundation  and  abutments  must  be controlled  to  prevent  excessive  uplift  pressure,  piping,  instability,  sloughing, removal of material by solutioning, or erosion of material into cracks, joints, or cavities. The amount of water lost through seepage must be controlled so that it does not interfere with planned project function
•     The upstream slope must be protected against erosion by wave action, and the crest and downstream slope must be protected against erosion due to wind and rain.
•     Sufficient spillway and outlets capacities should be provided so as to avoid the possibility of overtopping during design flood
•     Sufficient free board must be provided for wind set-up and wave action. Free board must also take into account the settlement of embankment under the effect of dynamic loading
2.2.4     Design Criteria for Various Component of Earth Dam
a) Free Board
Free board will be worked out to arrive at the top of embankment level. Free board will be calculated as per Indian standard. The free board will be calculated both for normal
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W = 3.6 x - 3 (meter)
For Erer Dam, W = 3.6 x 3/- 33.5-3 
W = 11.6-3 = 8.6 m
Based on above, a crest width of 9 m has been selected for Erer dam
d)  Berms
Berms are provided in the upstream and downstream slopes of the dam at every vertical elevation of about 10-15m or so. Berms serve the following purposes:
■   To break the continuity of the slopes, thereby reducing surface erosion in case of downstream slope
■   To provide level surface for construction and maintenance operations.
■   To prevent undermine of the lower edge of rip-rap in case of upstream slope.
The berms will slope towards the inner edge to prevent rainwater from flowing over the outer edge and down the slopes of the dam. A slope of 1 in 50 is recommended for this purpose.
e)   Core
The core provides impervious barrier within the body of the dam Impervious soils are generally suitable for core. However, soils having high compressibility and liquid limit
are not suitable as they are prone to swelling and formation of cracks. The top level of core will be lm above MWL to prevent seepage by capillary siphoning Soils having 
organic content are also not suitable. The minimum top width of the core should be 3 m.
f) Casing
The function of casing is to impart stability and protect the cores. The relatively pervious materials,  which  are not subject to  cracking  on  direct exposure to  atmosphere,  are suitable for casing. The soils normally very suitable for casing are as follows:
•    Well graded gravels, gravel- sand mixture; little or no fines.
•    Well graded sand, gravelly sands; little or no fines.
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However it is the genera! practice to utilize the materials available in their natural state as far as practicable by  proper zoning.  It is to  be ensured  that casing  is laid  on  stripped surface. All loose materials, roots etc. are to be removed before placing casing materials.
2.2.5   Design Criteria for Dam Foundation and Measures for Seepage Control
a)  Dam Foundation Condition
The foundation of the proposed Erer dam will rest on pervious foundation material in the river section and on both the flanks covering a length of about 1000m along dam axis and the remaining portion of 400m length on the left flank and right abutment will rest on patchy  / bouldery  rock  outcrops.  The pervious material is alluvial in  nature, unconsolidated,  loose,  erodible,  non-cohesive,  and  largely  of granular character composed  of gravel,  pebble,  cobble,  sand  with  subordinate silt and  clay.  This type of natural material covers the river section, most of the right flank and 50% of the left flank. It is 30-35m deep along river section and 10-15/20m deep on the flanks.
b)  Measures for Seepage Control
1) General
The various measures commonly adopted for seepage control through the foundation are
•    Open excavated positive cut-off
•    Sheet pile cut-off
•    Slurry trench cut-off
•    Diaphragm walls
•    Grout curtain
•    Upstream impervious blanket
•    Relief wells and filter trenches
•    Combination of more than one method mentioned *’
The selection of seepage control measures for a particular case depends on the nature of foundation strata, the degree of heterogeneity and uncertainty in foundation
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characteristics, the economic value of the water stored, the risk element as influenced by the height of the dam, reservoir volume and potential damage to important properties, lines of communication, and important towns. The choice of the seepage control system would also depend on feasibility of testing the field performance by observations in the initial stages of construction  of operation  and  the scope for taking  corrective action before the serious damage occurs. Many a times, more than one of the above mentioned measures are adopted in a single dam, either by adopting different measures in different reaches or a combination in the same reach.
2) Measures for Seepage Control through Foundation of Erer dam
The type of the method to be adopted will be decided on the basis of detailed foundation investigations and the properties of overburden and rock of the strata at various depths specially its permeability and relative densities for assessing liquefaction potentials.
2.2.6 Design Criteria for Internal Drainage System
Internal Drainage System is designed based on the type of base material, height of water in  the reservoir and  topographical feature of the dam site Internal drainage system comprises of the following:
a)         Inclined Chimney Filter
Incline filter will be provided on downstream face of the impervious core. Apart from collecting seepage emerging out of the core, it prevents the failure of dam by piping in the eventuality of hydraulic fracturing of the core.
b)         Horizontal Filter
It collects seepage from the incline filter and carries it to the toe drain It also collects seepage from the foundation and minimizes possibility of piping along the dam seat.
c)         Filter Criteria
The filter materials used for drainage system shall have the following criteria:
• Filter materials shall be more pervious than base material.
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•    Filter materials shall be of such gradation that particles of base material do not totally migrate through to clog the voids in filter material.
•    Filter material should help in formation of natural graded layers in zone of base
soil adjacent to the filter by readjustment of particles.
The following limits are recommended as per USBR to satisfy the following filter stability criteria and to provide ample increase in permeability between bases and filter (at least 10'2 cm/sec). These criteria are satisfactory for use with filters of both natural sand and gravel or crushed rock.
D  of the filter
15
1.
Djsof the base material Disof the filter
5 to 40 provided that the filter does  not contain more than 5 percent of material finer than 0.074 mm.
2.
--------------------------- = 5 or less. Dgs of the base material
d)Rock Toe
Rock toes are provided mainly to facilitate drainage of the seepage water and to protect the lower part of the downstream slope from tail water erosion. The general practice is to provide up to 20 percent of the hydraulic head or 1 m above maximum tail water level. Rock toe consists of suitable stones with filters.
e)Toe Drain
An open drain paved with stones with suitable filter material along the downstream toe of an earth dam to collect seepage from the horizontal filter will be provided. The minimum depth of the toe drain shall be 0.6 m with gradual increase depending upon the site conditions However the maximum depth of the toe drain shall not exceed 1.2 m for ease of construction and maintenance.
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2.2.7  Design Criteria for Slope Protection
a) Upstream Slope Protection I
Riprap is the protection to the embankment material against erosion due to wave action, velocity of flow, etc. It is of either dumped rock riprap or hand packed rock riprap. It should be hard, dense and durable. Since the rocks are available close to the dam site dumped riprap will be used. USBR recommends a 1.0 m thickness of dumped riprap for major dams. The design criteria recommended by Taylor will be followed. Suitable filter will be provided below the riprap. Riprap shall be provided from 2.0m below the MDDL to the top of the dam.
b) Downstream Slope Protection
The downstream slope needs to be protected against erosion by wind and rain-wash 30 cm thick stone pitching shall be placed on the downstream slope as a protective measure.
2.2.8     Slope Stability Analysis for Earth Dam
The earth dam shall be safe and stable during all phases of construction and operation of the reservoir. The analysis shall be done for the most critical combination of external forces, which are likely to occur in practice. As per IS. 7894 the stability of upstream and downstream slopes should be checked for the following conditions which are found to be critical for stability of an earth dam.
•    Construction condition with or without partial pool (for u/s and d/s slopes)
•    Sudden drawdown (for u/s slope)
•    Steady seepage (for d/s slope)
•    Earthquake condition (for u/s and d/s slopes)
As per the Indian standard, minimum factor of safety desired under various conditions are given below:
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SI No.
Conditions
Critical Slope
Minimum    Factor of Safety
1
Construction  condition  with  or without partial pool
U/S and D/S
1.0
2
Sudden   Draw   down   with   tail water at maximum
U/S
1.3
3
Steady   seepage   with   reservoir full
D/S
1.5
4
Steady  seepage  with  earthquake forces
U/S and D/S
1.0
Material properties to  be used  in  stability  analysis shall be based  on  the laboratory analysis of soil samples collected  from the selected  borrow areas.  Effective shear parameters shall be used in the analysis The method of analysis of slope stability will be circular arc method  In  this method  of analysis the surface of rupture is assumed  as cylindrical or in the cross section by an arc or a circle. This method, also known as Swedish or Slip Circle method, is generally adopted for analyzing of earth dam
The analysis will be carried  out with  the help  of software ‘SLIDE’ developed  by Rocscience Inc, Canada or software developed by core group of engineers from the field of Water Resources Engineering in India or any other acceptable software
2.2.9 Instrumentation
Instrumentation is very important aspect so as to monitor the post construction functions of dam. Instruments to be used should be simple and easy to install. Malfunctioning of
any instrument at any stage may lead to wrong interpretation of behavioral aspect of the dam. A few instruments for measurement of movement and pore pressure in the body and foundation of the dam are recommended as below:
•       Instruments   to   measure   movements:   instruments   will   be   installed   for measurement of vertical and lateral movement of dam embankment and also for surface movement.
•     Piezometers will be used for measurement of pore pressure. Pore water pressure indicates whether the various zones in earth dam arc functioning properly or not
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and  also  indicate  the  effectiveness  of  inclined  and  horizontal  filters.  A  few piezometers are, therefore, will be installed at critical points of cross section.
• In light of the location and the size of the dam it is worth to have an instrument to measure  strong  motion  record.  The  record  of  strong  motion  will  facilitate  the establishment of network of data centers for long term dam safety evaluation.
2.2.10  Diversion Flood Arrangement
River  diversion  arrangement  during  construction  period  will  be  provided  considering design flood of 100 years return period as per IS 14815:2000.
2.3  Dam’s Appurtenance structures (hydraulics aspect)
2.3.1.  General
Overall the hydraulic aspects of the design  will deal principally  with  application  of universally  accepted  methodologies for determination  of the configuration  of the dam appurtenance structures for best economy  and  functional efficiency.  The required efficiency is to be attained by provision of:-
•    adequate discharge capacity
•      adequate   overflow   capacity   to   limit   infringement   on   freeboard   during emergency
•    appropriate hydraulic setting to permit excess energy dissipation
•    proper transitions to minimize hydraulic head losses
2.3.2  Geometry and sizing
a) Spillway
The selected structure is an un-gated spillway located on the left abutment of Erer earth dam where the elevation of the natural ground is around 1380 masl. It consists of an approach channel, control structure, discharge channel (inclined chute), a stilling basin as an  energy  dissipator,  and  an  exit channel,  as its components.  Generally  the spillway design must fulfill the following criteria:-
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•     It must have adequate discharge capacity in order not to create possibility of the dam being overtopped by flood waters. In addition to providing ample spillway capacity, sufficient freeboard must also be provided above Normal Waler Level which is set at spillway crest level
•    It must be hydraulically and structurally safe
•    The material of the spillway must be erosion resistant
•      The  spillway  discharge  should  not  be  allowed  to  erode  or  undermine  the downstream toe of the dam
•    It should be provided with some device for dissipation of excess energy
•     The  spillway  discharge  should  not  exceed  the  safe  discharge  capacity  of  the downstream river channel to avoid over bank flooding
Besides, the following specific criteria will be adopted in defining the shape and size of the selected spillway type:-
1. Crest Length and Height
Results of the flood routing study determine the design head above the spillway crest required  for the estimated  spillway  discharge based  on  crest length/outflow,  crest length/surcharge relationships and  inflow hydrograph.  Half Probable Maximum Flood routed through the reservoir is adopted as spillway design flood discharge. The outflow discharges for various lengths of spillway  and  surcharge head  would  determine Maximum Water Level in  the reservoir.  The spillway  discharge equation  is used  to determine the surcharge height,
Q= CLH3/2
L= gross crest width of spillway
L = effective width obtained by considering abutment contraction effects
c
Q= outflow flood for selected crest width obtained from flood routing study
C= coefficient of discharge
For un-gated spillway there is no need for provision of piers. Thus access to the spillway crest for pedestrians and vehicles alike is from the right bank of Erer River along the dam crest. Hence in the absence of piers, only abutment contraction effects are considered.
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2.  Crest Profile
A vertical upstream faced straight un-gated overflow weir with ogee crested profile is selected as a control section of the spillway. The upstream and downstream crest profile coordinates (See page 365, Design of Small Dams, United States Bureau of Reclamation, Third Edition, 1987) will be defined by the following cquations:-
For downstream crest co-ordinates
in which K and n arc constants whose values depend on the upstream inclination and on the velocity of approach Ho is the total design hydraulic head i.e. (surcharge head [ho] + approach velocity head [h ]).
a
For upstream crest co-ordinates
y=
Q.724(x-0 270H )
ll   +Q 126
Wd)
^ _ .4315(/Z J
0
J75 (x +0.270/7J “
The curve extends (x = -0.270 H<j) upstream and (y = -0.126 H<j) downstream from the crest point.
Alternatively the upstream and downstream crest profile coordinates could be defined using  Waterways Experimental Station  (WES) standard  curves as presented  by  the
USBR. 
3.  Discharge Channel
\
The most significant design criteria for the discharge channel are:-
•    The structure should be stable under high and low flow conditions
•    The configuration of the structure should relate to the water surface profile
•     Water should be contained with in the structure proper to effect control of flow and also prevent structure undermining
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Discharge channel dimensions are primarily governed by hydraulic requirements, but the selection  of the profile,  cross sectional shape,  etc is influenced  by  the geological and topographical characteristics of the site.  Velocities on  the discharge channel will be limited to 3O.5m/s. If the limiting velocities are exceeded a combination of measures will be employed  until acceptable hydraulic characteristics are maintained  to  prevent chute floor cavitation That can be realized by making changes to the chute floor slope and in extreme cases by provision of aeration devices.
Water surface profile upstream and downstream of the overflow weir will be determined. The flow profile on the spillway surface determines the height of side walls required to retain the flow in the discharge channel. Super critical flow condition shall be maintained for the entire length of the rectangular spillway chute section downstream of the straight ogee crested free overflow weir.
Freeboard should be provided to the chute side walls for the half-Probable Maximum Flood  (PMF) which  is adopted  as design  discharge for the spillway  with  due consideration to the effect of air entrainment which would call for additional allowance in the height of sidewalls. The adequacy of the freeboard provided for the discharge channel shall be verified using the following empirical expression-
Freeboard = 0.61 + 0.04v*d,/3
Where  v  and  d  are  mean  velocity  and  depth  of  water  in  the  chute  reach  under consideration in m/s and m respectively
Irrigation Outlet
Arrangement of the different components of an  irrigation  outlet would  vary  with  the layout selected. However, the Erer dam irrigation outlet an outlet consists of an intake device, a conveyance structure, a control and a terminal device.
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• • *
The irrigation outlet will be placed on the right abutment of the dam, at chainage 1266 along dam axis, so that it can be more conveniently connected directly to the Primary canal that feeds the gravity irrigation canal network which is also located entirely on the right bank of the Erer River. In addition to releasing the impounded water as and when needed for irrigation and maintenance of the environment, the irrigation outlet work shall be used to:-
•    deplete the reservoir i.e. pass a part of the design flood to the downstream, as a supplement to the spillway in case of emergency
•    empty the reservoir up to its sill level to permit inspection, to make needed repairs or to maintain the upstream face of the dam or the spillway, and
•    control rise of the reservoir during initial filling
The following hydraulic criteria will be applied in the design of various components of the Erer dam irrigation outlet and while establishing its location in plan and elevation:-
•     Irrigation  outlet  should  be  so  located  in  order  to  minimize  the  length  of  the conveyance structure. The intake and outlet works need to be located as close as possible  to  the  upstream and  downstream extremities  of the Erer dam profile, leaving bare minimum space to accommodate the intake and outlet works and required working space between terminal structure and dam toe.
•      The  irrigation  outlet  works  should  be  placed  on  firm  foundation  so  that serviceability is not affected by settlement. The final alignment of the irrigation outlet  will  be  decided  based  on  topographic  and  geological  considerations. Besides, structural joints should be inserted every 12 m in order to allow possible differential settlement, without leakages.
•     The irrigation outlet should prevent piping along the contact between the outside surface of the conduit and the embankment. Projecting collars will be provided on the conduit to reduce the seepage along the outside of the conduit. The common rule is that the collars should increase the seepage length by at least 25 percent. Thus 2Nx > 0.25L where N is the number of collars, x is the projection of the collar measured from the outside surface of the conduit and L is the length of the conduit.
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•     Number of bends on the irrigation outlet should be kept as minimum as possible under the given topography. However where bends are to be provided smooth transitions should be provided to minimize head losses.
•     The outlet conduit will be sized for a discharge of 3.05m /s which just covers the quantity earmarked for irrigation and maintenance of the environment only. Thus the irrigation outlet size should be sufficient such that the peak water demand of
3.05 m3/s can be withdrawn with the reservoir at its Minimum Draw Down Level (MDDL). In other words the outlet discharge capacity should guarantee release of adequate water to meet requirements downstream under varying upstream head and even if the reservoir is at MDDL. There is neither a separate water supply outlet nor provision for the release of riparian flows.
•     It  should  be  possible  to  regulate  the  irrigation  outlet  discharge  according  to varying  downstream  requirements.  Regulation  is  to  be  effected  by  means  of control gates capable of operating at part gate openings.
•     The  irrigation  outlet  should  be  positioned  at  an  elevation  that  would  allow unimpeded flow of water by gravity to the Primary Canal, equally the longitudinal profile of the Primary Canal should also be suited to receive water by gravity from the irrigation outlet.
•    The inlet to the irrigation outlet should be placed adequately high for it ought to function free from interference of sediment deposits and at the same time its inlet should be kept sufficiently below the minimum draw down level of the reservoir to prevent vortex formation when the water is drawn with reservoir at MDDL.
•    The irrigation outlet should release water at a safe velocity in the primary canal so as to protect it from erosion In other words appropriate hydraulic setting that permits dissipation of excess energy should be provided.
•      Outlet  conduit  limiting  velocity  on  its  upper  side  is  limited  by  the  head corresponding to the difference in elevation between the reservoir Normal WaterCECE and CES
ability to guarantee flushing of the conduit. Thus the lower and upper limits range from a minimum of 2m/s to about 17m/s respectively.
• Use of the outlet work for diversion during construction would not be ruled out supplementary to other means.
2.3.3 Flow Analysis
Since  flow  conditions  and  their  analysis  are  different for  a spillway  and  an  irrigation outlet each structure is treated separately and presented below.
a) Spillway
The approach  channel shall be hydraulically  dimensioned  so  that sub-critical flow condition exists along its length for a range of discharge values. Critical flow condition shall be maintained at the ogee crested control section. After passing over the control structure, the water enters the discharge channel (chute) as supercritical flow The slope of the chute shall be kept just sufficient to meet the flow requirement of supercritical flow for as long a distance from the crest as possible, without getting its bed into filling, given that its bed  shall any  way  be kept in  cutting  through  out its length.  Computation  to determine the water surface profile in the discharge channel shall be performed using the specific energy  equation.  Determination  of velocity  in  the discharge channel involves calculation  of the water surface profile down  the chute by  Step Method starting with known velocity and depth at the start of the chute The initial depth may be calculated from the known head, discharge, and weir height.
b) Irrigation outlet
The overall size of the irrigation  outlet is determined  by  its hydraulic head  and  the required  discharge capacity.  For a given  discharge,  if head  is increased,  the cross sectional area of the conduit can be reduced. In other words, the conduit size can be reduced if the minimum draw down level, at which the outlet is located, is kept low.
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For a given head and discharge, the conduit size can also be reduced by diminishing the various losses and hence the composite coefficient of all losses. Hence, the total head losses encountered in the outlet works should be calculated for the detail design. These losses include those caused  by  trash  racks,  conduit entrance,  conduit friction,  gates, transitions and  bends.  The net effective head  responsible for flow shall then  be determined.  Moreover the outlet works system will be checked to determine reservoir evacuation requirements.
If the outlet is submerged at the exit, the head is equal to the difference between the upstream and downstream water levels. When the outlet discharges free, the head is equal to the difference between the upstream water level and the centre of the outlet exit. 
Flow through the irrigation outlet will be controlled by hydraulically hoisted control and emergency gates. As proposed by the feasibility design, the in take shall be low level but will have dry shaft that will house the gates.
The access bridge shall be constructed out of reinforced concrete pier and beams while the deck shall be wooden.
The location of the intake structure will remain at the site proposed by the feasibility study
The stilling basin should be designed for a discharge of 3.05m /s when the water level in the reservoir stands at Normal Water Level At this level the gates would have to be operated so that flow upstream of the control gates would be under pressure and that on the downstream of the gates that are located at some intermediate point along the conduit would be open channel flow.
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2.3.4 Energy dissipation
The Irrigation  outlet will end  in  well designed  energy  dissipating  structure before the water is conveyed to the primary canal. Similarly the spillway flood discharge coming down the chute will end up in a well designed energy dissipating structure before the water reaches the Humee stream and ultimately the Erer river course. In both cases the criteria for the design of the respective energy dissipators is
•    Essentially of reducing the high velocity flow to a velocity low enough to prevent erosion of downstream channel.
•    The need to come up with the most efficient and at the same time, the cheapest
type of structure
In  this  respect  the  stilling  basin  is  selected  as  being  the  preferred  energy  dissipating device at the end of the spillway chute and irrigation outlet conduit respectively.
The criteria for choice of the energy dissipation type will be governed by the tail water depth and the characteristics of the hydraulic jump itself while its design will be affected by  factors like the nature of foundation,  magnitude of the flow and  its recurrence, velocity of flow, orientation of flow, and depth-discharge relationship of the watercourse at the site of the structure.
Consideration  will be given  to  ensure that established  hydraulic relationship  in  a rectangular stilling  basin  between  the initial depth  (pre- jump  depth) and  the sequent depth (post-jump depth) required for the formation of a hydraulic jump are satisfied
The initial Froude number Iq will be maintained to be greater than unity to effect the formation of a hydraulic jump. Formation of a hydraulic jump just below the crest in the discharge channel will be avoided.  This will be effected  throughout the length  of the channel by keeping the channel flow at supercritical state (i.c. depth < critical depth).
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The tail water depth at the site will be analyzed on a case by case basis in order to achieve the formation of a perfect jump. Other types of energy dissipating devices will be considered only if the tail water depth could not approximately equal to that required for a perfect jump formation.
Undermining of the structure and lowering of the riverbed should be prevented Suction pressure on  the floor occurs due to  the hydraulic jump  formed  for the dissipation of energy in a stilling basin provided at the end of the spillway chute. For the calculation of the suction  pressure,  the location  of hydraulic jump  and  the water surface profile upstream and downstream of the point at which the hydraulic jump is formed will be determined.  The total length  of impervious floor should  be designed  to  satisfy  the requirements of exit gradient, scours and economy.
2.3.5 Spillway detailing
The details regarding sub surface drainage, uplift, anchorage, joint arrangements are to be carried out using practically tested details.
In cases of strong rock formation, drainage and associated uplift may not be a problem as the up lift pressure possibility is low and counter acting anchorage could be made easily. However, incase of soil formation under the chute slab, there would be an excessive uplift that has to be taken care of by providing perforated drain pipes or naturally graded filter below the chute at specified spacing. It might also be required to increase the minimum thickness of 25 cm in order to counter balance excessive up lift pressure.This will be compared with the option of providing anchorage blocks at specified spacing 
For reasons of avoiding cracks of concrete walls and chute floors joints shall be provided at design spacing usually not exceeding 12m.
2.4 Danis Appurtenance Structures (Structural Aspects)
2.4.1 Properties of Material
This section describes briefly the properties of materials and design stresses that’would be used in the structural design of all hydraulic structures.
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AJI canal & drainage structure would be designed, wherever possible in masonry. There is  an  abundant  supply  of  rock  in  both  irrigation  project  areas  from  which  stone  and aggregate can be quarried .Structures can be constructed of stone masonry using a 1:4 cement/sand mortar. Where these is a severe hydraulic requirement such as the face of a weir-type structure with risk of damage from high flow velocities, coarse sediment and gravel, the surface layer should be constructed with dressed stone in tight fitting square blocks (30 cm square). In canal drop structure, quarried stone with the split face exposed in a random but tight-fitting pattern, may be used. The exposed face of standard masonry in hydraulic structures may have joints pointed.
The density of masonry acting as a stabilizing force is taken as For masonry acting as a load on a structural element
the density is taken as
Maximum permissible compressive stress Maximum permissible shear stress
2.4.2 Design Stress
a)Reinforced Concrete
20km/m3
21km/m3 (6.9N/mm2)
2
(0.69N/mm )
Reinforced Concrete (RC) would be used for those structures and elements of structures that are required  to  resist bending  moments,  e.g.  bridge decks,  operating,  aqueducts, breast walls, high retaining walls etc. The design of reinforced sections will be by the “Elastic” or “Modular Ratio” method. As a result, the characteristic strength for RC for the irrigation and drainage structures will be taken as C2O, C25, C30. The designs will be carried out according to the requirements of EBC S-2, Ethiopian Building code standard for structure use of concrete. In the case of head works structures, the concrete grade would  be higher (C20-C30) whereby  elements may  be designed  using  the ultimate strength method if this leads to more economical design.
For the design of tile reinforced concrete structures the following stresses will be adopted 
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3) Reinforcement for crack control
In principal structures such as the spillway; where it is desirable to provide an additional measure of crack  control over and  above that normally  provided  by  code minimum reinforcement requirements, the quantity of crack control reinforcement may be obtained from the formula:
A, = Kc Kfrf.ef Act/oin accordance with EBCS-2
d) Soils
Detailed knowledge of all the soils that would be encountered under foundations backfill material behind  retaining  walls and  for small structures,  may  not be possible due to resource constraints.  Soil property  data would  be available from the geotechnical investigation for the major works and head works. For preparing standard designs for the small structures the following soil parameters are assumed:
1) Backfill material (behind retaining walls)
Density of saturated material
Density of moist material
Angle of internal friction 0 (clayey material) Angle of internal friction, 0 (granular material)
Internal cohesion (clay)
Taken as zero in granular material
-         2.15 t/m3
-         1.75 t/m3
-         15-30°
-         27-35’
- 100-500 kg/m2
Active  pressures  would  be  calculated  with  the  Rankine  formula.  This  formula  for  a cohesion less soil is:
Pa = Wh
Where:
w is the unit weight of the material K2 = (I-Sin 0)/(l + Sin 0)
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2) Foundation Pressures
The  allowable  bearing  pressures  will  be  estimated  from  geotechnical  analyses. Approximate allowable bearing pressures for firm clay and stiff clay arc 75-150 and
150-300 KN/m2  respectively. The allowable pressure for hard sound rock can be as high as 10000 Kn/m2
2.5  Geotechnical Investigation
The  geotechnical  criteria  and  investigations  thereof  arise  from  the  basic  engineering 
requirements as outlined below for the proposed 36.6 m high earth dam:
The dam should be safe and stable against:
•     Internal erosion, expected excess loss of water and high exit gradient leading to piping and sloughing. The toe of the dam is expected to remain dry as far as possible or with allowable clean water flow
•      All   conditions   of   loading   including   saturated   and   differential   settlement conditions due to variation of material in various sections of dam.
•    Erosion by wave action at the upstream slope of the earth dam
•    Overtopping by inflow design flood and wave action; i.e. adequate free board.
•     Seepage through dam embankment, foundation material and dam abutments be controlled against internal erosion
•      Slopes  of  the  zoned  sections  of  the  dam  embankment  to  be  stable  during construction and under all conditions of reservoir operations
•    Safe design of dam including service factor of the site conditions.
The spillway should be stable against:
•    All foundation conditions; rock or weak founding material, with or without consolidation grouting to sustain loading.
•    Erosion of foundation material and training walls during spill discharge.
•    Uplift of the chute, if any to decide the nature of tying with rock. 
The canal structure should be stable against
•    Settlement and erosion because of foundation conditions; weak rock or soil
•    Seepage characteristics and uplift pressure.
•    Slope stability of embankments
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The design criteria, as outlined above, are only guidelines and require to be validated by the design engineer at the time of actual designing of the civil works. Further, the design criteria require also  to  be revalidated  after the availability  of geotechnical data being proposed in the phase I report.
2.5.1. Dam Site Foundation Investigation and Criteria for Design Parameters
In light of prevailing thick previous alluvial sediments in the river section and both the flanks besides patchy  rock  outcrops near both  abutments,  an  exploration  pattern  is proposed to decipher the nature and thickness of the alluvial sediments at depth
The thick alluvial sediments and the bedrock are proposed to be explored by sufficient number of bore holes going to depth that will reach bed rock.
Water percolation tests are proposed to be carried out by:
a) Falling Head or Rising Head method for alluvial sediments/overburden sections.
2.5.2  Construction Material Investigation
For assessing suitability of various types of soil, the engineering requirements for earth dam design are as provided below:
Non-dispersive,  non-swelling,  low permeability,  and  low plastic clays,  silty  clays and clayey  silt for the impervious core section  of the dam Lab  test will decide these parameters. Shallow depth trial pits and auger holes to depths of 3 - 5 m are adequate for field permeability and penetration test
For shell zone,  filter material,  riprap  and  rock  toe: adequate quantity  of material is available and their suitability has already been assessed in the feasibility report. Selection of impervious core material is crucial on this project.
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2.5.3  Major Hydraulic Structures- Foundation Investigation for Design Criteria
To Design suitable foundation of canal structure on soil, study of the bed and bank of the canal against erosion  and  water losses are essential.  The engineering  requirements for such needs arc:
a)      Strength test - by shear test and penetration tests
b)
Density of foundation material
c)
Sieve analysis
d)
Specific gravity
e)
Atterberg limit
f)
Proctor test
g)
Triaxial shear test on Unconsolidated undrained soil samples or direct
shear test
h)      Field permeability tests
The tests maybe carried out in consultation with Irrigation Engineer.
2.6  Road Infrastructure
The Ethiopian  Roads Authority’s Standard  Specification  2000  will be adopted  for the design, specifications, and pay items of different functional roads serving the Erer Dam and Irrigation scheme. Introduction on the approach of the design was illustrated in the inception report. Here detailed approaches and parameters would be discussed.
The road  standard  design  criteria of the project would  adhere to  the ERA standard specification The standard considers pavement structure, drainage, road feature and other ancillary items of each road type. Design of the different road services, specification of works, workmanship, material and equipment would be part of the bidding documents for the road construction works.
Considering the provision of access at all levels, it would be very important to review and meet the requirements of road  design  criteria under the Ethiopian  Roads Authority (ERA). The Standard and Specification 2000 of ERA will be the base for the road design, and specification of the construction of all components of the road infrastructure.
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However, for lower design cases, where the beneficiary themselves may undertake the construction and maintenance works of such structures the standard may not be followed.
2.6.1 Road Traffic Volume and Load
Additional factor which  will influence the road  design  standards,  design  speed  in particular, is the volume and composition of traffic on the road. The design of a road should  be based  in  part on  actual traffic volumes.  Traffic indicates the need  for improvement and directly affects the features of design such as widths, alignments, and gradients of the road. Traffic data for a road or section of road, including traffic trends, is generally available in terms of annual average daily traffic (AADT).
A design class, or standard, is selected Functional hierarchy is also exists in such away that traffic aggregates moves from feeder to main collector and a collector intern to link trunk roads. However, the actual flows will vary from site to site and it is important to make sure that the designation of a road by functional type should not give rise to over- design of the road then the traffic levels actually encountered.
For Erer irrigation  project,  the roads are expired  to  be low volume roads.  Thus importance of geometric studies is less. In such circumstances, it is appropriate to adopt inexpensive standards to  development of a system of such  a feeder roads system at minimums cost.
When the irrigation scheme commence operational it will create additional or generated traffic. For such type of traffic, the farm out puts and inputs would be calculated based on the expected land development. Other social and economical factors that could also affect the flow of traffic in the road network would be considered. Other type of traffic on the road is the diverted ones which might use the road due to the improvement of the routes from other alternative roads
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2.6.2 Road Network
Generally,  road  infrastructure network  would  follow the irrigation  canal network  In addition, road access for efficient operation and maintenance of the dam and irrigation system will be provided Additionally, social and economical centers in the project area will be considered and the requirement of the beneficiaries must be met. The resettled farmers, which are dis-placed from the reservoir area and other falloffs will have access to the road network.
The functional roads serving  different purposes in  the project area will have its own standard, which will consider the volume of traffic expected during the operation of the farm. The network of the road in the port area will be connected to either major road or near by town for further connectivity with other outlet The naming of each road segment will be adopted  during  the preparation  of design,  bills of quantities,  construction  and maintenance phases.
Hierarchy of the road network could be categorized according to their major functions. All roads would be provided with adequate side drains to protect from runoff water and it will be disposed at the nearest natural channel.
The capacity of the drain will be detected later taking in to account water discharge that will occur during high storm.
2.6.3 Road Standard in the Irrigation Scheme
As mentioned  above,  the Ethiopian  Road  Standard  and  Specification  2000  will be adopted for different classes of road to be provided in the project area. The entire road under consideration will have a specific geometric design to fulfill the major function it serves. Both the physical characteristics and turning capabilities of vehicles are controls in geometric design. Vehicle characteristics and dimensions affecting design of the road include power to weight ratio, minimum turning radius and travel path during a turn, and vehicle height and width. The road elements, which will be affected include: the selection of maximum gradient, lane width, horizontal curve widening, and junction design of the
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road. At present, vehicle fleet in Ethiopia includes a high number of four-wheel drive utility vehicles and overloaded trucks However, for this particular project farm tractors and trailer are conserved to be common in the road..
The road design in Ethiopia, as per the ERA standard first identifies the function of the road  it serves.  The functional classification  of the road  in  the country  includes five classes. The most appropriate classes of road, in the farm and access road of the Erer project could be categorized as Feeder Road, where the AADT in most cases is less than 100  vehicles.  Additional factor influencing  the development of road  design  standards, 
particular the design speed, is the volume and composition of traffic on the road. The design of a road should have to be based in part on actual factual traffic volumes. Traffic indicates the need for improvement and directly affects features of design such as widths, alignments, and gradients.
Using the road functional classification selection and the design, traffic flow, the feeder road standards would be appropriate for Erer irrigation project at the initial phase Table
2-1 below shows the feeder road with the design standards and expected AADT.
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Table 2-1 Design Standards vs. Road Classification and AADT
Road
Functional
Classification
Design
Standard
No.
Design
Traffic
Flow
(AADT)
Design Speed (km/hr)
Flat
Rolling
Mountainous
Escarpment
DS6
50-100
60
50
40
30
F
c.
E
D
E
R
DS7
30-75
60
50
40
30
DS8
25-50
60
50
40
30
DS9
0-25
60
40
30
20
DS10
0-15
60
40
30
20
The Design Speed is used as an index to link road function, traffic flow and terrain to the design parameters of sight distance and curvature This ensures that a driver is presented with  a reasonably  consistent speed  environment.  In  practice,  most roads will be constrained to minimum parameter values over short sections or on specific geometric elements.
The DS-9 and DS -10 standard which are the lowest standard, will be applied to road section of the tertiary and quaternary canal roads respectively.
The  singled  lane  width  of  this  road  would  be  3.3  meters,  where  in  most cases  farm tractors and carts may use the road section frequently. In case of two directional flows where one traffic face against the other, a passing lane would be provided within a sight distance or say at every 400 to 500 meters alternatively. The width of the lane could be widened at village section, where there would be a need for parking lanes as well as pedestrian walk way. The surface could also be paved, if there is necessity, with natural
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gravel to be used effective during the wet seasons. Otherwise if there is no need of using the road during the rainy season, the surface may be keeps natural soil surface.
The next hierarchy in the road network would be the road provided for the primary and secondary canals and other social and economical centers under the scheme. All roads of the DS-10 will be connected to these roads and more intensive canal maintenance are expected most of the operation of the irrigation system will be performed at these levels, are farm in put and out puts would be transported through these routes prior to and after every  farming  and  harvesting  periods respectively.  Thus a need  for higher and  more durable roads standard is required for this section.
Rationally  the selection  of these roads may  fall under the DS-7  and/or DS-8  design standards,  with  passing  bays provided  within  sight distance or at every  400  meters interval alternatively.  These roads would  be provided  with  good  gravel material of minimum 20 cm thick and compacted to a certain level of degree which would be defined during the detail design of the road infrastructure.
The higher level of the road network under the Erer irrigation project is the connection of the above roads to the higher standard of roads or to the nearest town where the Ethiopian road  network  existed  This level of the road  network  could  be design  to  the DS-6 standard This standard connects all the lower roads of the scheme to the nerst man road or world town
2.6.4 Geometric Design
A cross-section  of a road  will normally  consist of the following  section^ carriageway, shoulders or curbs,  drainage features,  and  earthwork  profiles.  The appropriate class of roads as defined on the above heading will have the following carriageway and shoulder width at rural and urban sections. See Table 2-21.
Design  elements  such  as  lane  and  shoulder  widths,  horizontal  radius,  super  elevation, sight distance and gradient are directly related to, and vary, with design speed Thus all of
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the geometric design parameters of a road are directly related to the selected design speed.
The carriageway, shoulders in rural and urban sections, parking lane and foot way widths are defined on the table 2-2 below for all selected design standards.
Table 2-2 Geometric Design Surface Type and Carriageway & Shoulder Width
Design
Standard
Surface
Type
Width
(m)
Rural
shoulder
width (m)
Urban shoulder width (m)
Shoulder
Parking 
Lane***
Foot
way
00
n/a
3.5~*
2.5
DS6**
Unpavcd
6.0
1.0 (earth)
n/a
n/a +
n/a +
DS7
Unpavcd
4.0
0.0
n/a
n/a +
n/a +
DS8**
Unpavcd
4.0
0.0
n/a
n/a +
n/a +
DS9**
Unpaved
4.0
0.0
n/a
n/a +
n/a +
DS10**
Unpaved
3.3
*♦ Shoulders included in the carriageway width given in Table 2-1
To be provided where urbanization requires this facility
+ Where these classes of roads pass through urban areas, the road shall be designed to Standard DS6 
++ The actual shoulder width provided sliall be determined from an assessment of the total traffic flow and level of non-motorized traffic for each road section
+++ Depending on the development of the town & Includes a shoulder
For each design standard, parameters shown in tables 2.3 to 2.7 shall be applied
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Table 2.3 Geometric Design Parameters for Design Standard DS6 (Unpaved)
Design Element
Unit
Flat
Rolling
Mountainous
Escarpment
Urban/Peri-
Urban
Design Speed
km/h
60
50
40
30
50
Min. Stopping Sight
Distance
m
85
55
45
30
55
Min. Passing Sight
Distance
m
225
175
125
75
175
% Passing Opportunity
%
20
20
15
0
20
Min. Horizontal Curve
Radius
m
125
85 ”
50
30
85
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
6
7
10
10
7
Max. Gradient (absolute)
%
8
9
12
12
9
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Super elevation
%
8
8
8
8
4
Crest Vertical Curve
k
18
10
5
3
10
Sag Vertical Curve
k
18
12
8
4
12
Normal and Shoulder
Crossfall (Unpaved)
%
4
4
4
4
4
Right of Way
m
30
30
30
30
40
Table 2.4 Geometric Design Parameters for Design Standard DS7 (Unpaved)
Design Element
Unit
Flat
Rolling
Mountainous
Escarpment
Urban/Peri-
Urban
Design Speed
km/h
60
50
40
30
50
Min.       Stopping       Sight Distance
m
85
55
45
30
55
Min         Passing         Sight Distance
m
225
175
125
75
175
% Passing Opportunity
%
20
20
15
0
20
Min. Horizontal Curve
Radius
m
125
85
50
30
85
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
6
7
10
10
7
Max. Gradient (absolute)
%
8
9
12
12
9
Minimum Gradient
°/o
0.5
0.5
0.5
0.5
0.5
Maximum Superelevation
%
8
8
8
8
4
Crest Vertical Curve
k
18
10
5
3
10
Sag Vertical Curve
k
18
12
8
4
12
Normal and Shoulder
Crossfall (Unpaved)
%
4
4
4
4
4
Right of Way
m
30
30
30
30
30
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Table 2.5 Geometric Design Parameters for Design Standard DS8 (Unpaved)
Design Element
Unit
Flat
Rolling
Mountainous
Escarpment
Urban/Peri-
Urban
Design Speed
km/h
60
50
40
30
50
Min.       Stopping       Sight Distance
m
85
55
45
30
55
Min.        Passing        Sight Distance
m
225
175
125
75
175
Min. Horizontal Curve
Radius
m
125
85
50
30
85
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
6
7
10
10
7
Max. Gradient (absolute)
%
8
9
12
12
9
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Superelevation
%
8
8
8
8
4
Crest Vertical Curve
k
18
10
5
3
10
Sag Vertical Curve
k
18
12
8
4
12
Normal and Shoulder
Crossfall (Unpaved)
%
4
4
4
4
4
Right of Way
m
20
20
20
20
20
Max. Spacing of Passing
Bays
m
500
500
500
500
500
Design Vehicle
DV 2/3
Tabic 2.6 Geometric Design Parameters for Design Standard DS9 (Unpaved)
Design Element
Unit
Flat
Rolling
Mounta
inous
Escarpment
Urban/Pcri
- Urban
Design Speed
km/h
60
40
30
20
40
Min. Stopping Sight
Distance
m
85
45
30
20
45
Min Passing Sight
Distance
m
225
125
75
50
125
Min Horizontal Curve
Radius
m
125
50
30
15
50
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
6
7
13
13
7
Max. Gradient (absolute)
%
8
9
15
15
9
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Design Element
Unit
Flat
Rolling
Mounts
inous
Escarpment
Urban/Peri
- Urban
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Superelevation
%
8
8
8
8
8
Crest Vertical Curve
k
18
5
3
2
5
Sag Vertical Curve
k-
18
8
4
2
8
Normal and Shoulder
Crossfall (Unpaved)
%
4
4
4
4
4
Right of Way
m
20
20
20
20
20
Max. Spacing of Passing
Bays
m
500
500
500
500
500
Design Vehicle
DV 2/3
Table 2.7 Geometric Design Parameters for Design Standard DS10 (Unpaved)
Design Element
Unit
Flat
Rolling
Mounta inous
Escarp ment
Urban
Design Speed
km/h
60
40
30
20
40
Min Stopping Sight
Distance
m
85
45
30
20
45
Min. Passing Sight
Distance
m
225
125
75
50
125
Min Horizontal Curve
Radius
in
125
50
30
15
50
Transition Curves Required
No
No
No
No
No
Max. Gradient (desirable)
%
6
7
14
14
7
Max. Gradient (absolute)
%
8
9
16
16
9
Minimum Gradient
%
0.5
0.5
0.5
0.5
0.5
Maximum Superelevation
%
8
8
8
8
8
Crest Vertical Curve
k
18
5
3
2
5
Sag Vertical Curve
k
18
8
4
2
8
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Design Element
Unit
Flat
Rolling
Mounta inous
Escarp ment
Urban
Normal and Shoulder
Crossfall (Unpaved)
%
4
4
4
4
4
Right of Way
m
20
20
20
20
20
Max. Spacing of Passing
Bays
m
500
500
500
500
500
Design Vehicle
DV 1
2.7 Building Infrastructure and Service Centers
2.7.1  Planning Units of Service Center
The service center shall consist of the following functions;
•    Administration
•    Residences
•    School and
•    Other ancillary functions
Table  2.7  below  shows  functional  and  space  requirements  of  buildings  in  the  service centers.
Table 2.8Functional and Space Requirement of Buildings in the Service Centers
Function
Tentative
area
Remark
I
Residences-
Three bed rooms
Two bed room
One bed room
Number to be decided
106 m2
85 m2 
40m2
2
Guest     houses     with     common kitchen and dining
Each 25m2
Five guest houses
3
Toilet and changing room blocks
Number    as    needed    as many
4
Administration block*
250m2
5
Multi purpose hall
300m2
For training and meetings
6
Laboratory and office for botany
40 m2
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Function
Tentative
area
Remark
7
Sheds for nursery
25 m2
No
8
Cafeteria with kitchen and store
100-150m2
For -100 peor
9
Stores
50-100m2
Grain store
Fertilizer etc
Chemical
10
Generator room
2 5 m2
11
Transformer room
25m2
12
Pump house
6 m2
13
tube well house
15 m2
14
Crop protection
Chemical store
foreman labors
20m2
15
maintenance shop
15m2m
16
agricultural         machinery shed and store
100m2
Shed for tractors if any.
17
water reservoir and supply
—
18
Guard houses
Each 6m2
19
Inspections blocks
6m2
To be places random on the project site
20
school
3 block
each 165 
2 
m
2
★The administration block comprises of the following functions with their tentative areas. Project manager (25m )
Secretary (12)
Audit (20)
Administration (30)
Finance- accounts, cost and budget follow up Personnel (12)
Marketing services Purchase
Sales
General store
Crop produce Spare parts
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Gravity system
Water management
Agricultural department (30)
Agronomy
Secretary
Foreman labors
Infrastructure services (30)
Toilets
Janitors
2.7.2   Selection of Construction and Finishing Material
The availability of some materials in the vicinity would help an easier construction of the service centers, the idea of easier maintenance of the structures by the local residents and farmers also should be mandatory in selecting the construction material
2.8 Irrigation and Drainage
2.8.1 General
The Erer Irrigation System is most likely to satisfy all the requirements of the modern irrigation system mentioned as below:
CECEandCES
r
Chemicals
Office supplies equipment
Fuel and lubricants
Research and capacity building service Research
Capacity building
Planning and programming service (20) Irrigation development dep  t (30)
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•    The irrigation system is technically feasible, economically viable, socially acceptable and environmentally sustainable
•    It is capable of supplying adequate water to the planned crops at every stage
of their growth and development.
•     It ensures equitable distribution of water in the entire command.
•    The system is capable of possessing enough flexibility for meeting urgent
needs of present and future.
2.8.2 Command Area
a)  Gross Command Area
The area which  can  be economically  irrigated  from a scheme without considering  the limitation  of quantity  of water is known  as Gross Commanded  Area (GCA) In  Erer irrigation  project the gross commanded  area is located  on  both  banks of the river. However on account of limitations imposed by the availability of water only about 4500 ha area is chosen as gross commanded area. Out of this about 3700 will be culturable commanded  area These figures will be refined  on  the basis of detailed  topographical surveys which are being taken up for detailed design of the project..
b)  Irrigation Intensity
The percentage of CCA proposed to be annually irrigated is called Irrigation Intensity. Extensive irrigation is preferred to intensive irrigation on a smaller area on account of two reasons. Firstly quantity of water may be limited and secondly over irrigation entails harmful effects like water logging,  soil salinisation  etc.  Normally  annual intensity  of irrigation may be limited to about 80% of CCA . However in areas of low rainfall it may be raised to 125% to accommodate double or triple cropping. A higher intensity may be allowed if an effective drainage system is provided in the irrigated area. In Erer irrigation project irrigation  intensity  of 170  % is being  assumed  with  the provision  of effective surface drainage system.
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c)  Irrigation Area
Irrigation area is the product of CCA and Irrigation Intensity that is to be worked out separately for each crop- season.
2.8.3    Irrigation Water Requirements
a)  Crop Waler Requirement
Crop water requirement is computed on the basis of well known water balance equation as given  below ETo  is worked  out by  using  universally  accepted  Penman-Monthieth equation. CROWAT 4.3 software which is developed most recently is used to work out crop water requirement
WR = (ETo* Kc ( C,T,F )—ER (R,ET,S ) + Ti ) / E
Where:
WR     - Gross water requirement
ETo    - Reference crop evapotranspiration (computed by Penman- Mothieth equation )
Kc       - Crop coefficient- a function of C,T,F
C         - Crop type
T         - Stage of crop development
F         - Frequency of soil wetting in the first crop development stage only ER      - Effective rainfall (which has been assumed as 80 %)
R        - Rainfall
ET
- Actual evapotranspiration
S
- Soil moisture storage factor
Ti
- Technical irrigation( Application Loss )
E
- Irrigation efficiency
The nearest climatological stations close to Erer irrigation project area arc Harar, Dire
Dawa,  Bisidimo  and  Jijiga.  The climatic parameters viz.  mean  monthly  rainfall, 
temperature,  sunshine hours,  wind  velocity  and  relative humidity  as recorded  at these
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stations  are  utilized  in  working  out  project  climatological  parameters  after  making suitable adjustments for difference in altitude etc
b)  Cropping Pattern
The nature, scope and extent of crops to be grown in a particular area is determined by climate, soils, availability of water and requirement of food, fiber, financial resource etc. Basically people settled in a given area decide the type of crops required to be grown for their own consumption.
The main  crops to  be grown  in  the command  area should  be determined  in  due consultation with the Agriculture Department and the agriculturists, looking to the crops being grown presently and cropping pattern to be introduced for irrigated fanning. Crop Calendar,  crop  water requirement and  cropping  intensities should  be finalized  in consideration  with  socio-economic factors of the area and  also  on  the basis of the experience gained in irrigation projects implemented in the country. Based on discussion with farmers of project area, the preliminary surveys and review of previous studies, the cropping  pattern  proposed  by  the agronomist for the project is as shown  in  table 2.9 below.
Table 2.9 Cropping Pattern for Ercr Dam and Irrigation Project
Season 1 Meher Season
Crop type
Planting date
Area%
Maize
July
30
Sorghum
July
5
Ground nut
July
20
Rice
July
10
Haricot Bean
July
5
Vegetables
July
10
Alfalfa
July
5
Sugarcane
July
10
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Season 1 Meher Season
Orchard
July
5
Total
100 %
Season 2 Belg Season
Maize
Feb
30
Tomato
Feb
5
Rice
Feb
12
Potato
Feb
5
Okra
Feb
5
Sweet potato
Feb
|5~
Vegetables
Feb
5
Haricot beans
April
3
Sugarcane
July
10
Alfalfa
July
5
Orchard
July
5
Total
90
Total
Cropping intensity
For Season-1 and Season-2 170% ( 20 % crops are perennial crops)
c) Irrigation Scheduling
Irrigation schedule is designed to start irrigation from the first planting date of the crops when all of the readily available soil moisture has been used. The irrigation amount will be equal to  soil moisture deficit.  The soil moisture deficit will return  to  zero  after the irrigation.
The water application time is planned when all (100%) of the readily available moisture has been used up In this situation the crop will not become moisture stressed. The water application  depth  is calculated  to  refill the soil moisture storage so  that the soil may return to its field capacity. (100% of the readily available moisture is replenished).
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Monthly ETo and monthly rain fall are distributed using polynomial curve fitting. To generate rainfall events, each 5 days of distributed rainfall is accumulated as one storm. 
For total and readily available soil moisture the calculation procedure is indicated below TAM = Total Available Moisture = (FC% - WP %)* Root Depth [mm]
RAM = Readily Available Moisture = TAM * P [mm]
Where:
FC = Field Capacity
WP = Wilting Point
P = Depletion Percentage
rf) Irrigation Efficiency
As per International Irrigation and Drainage Committee the efficiency in use of water can be separated in to three components.
1)  Storage Efficiency
Storage efficiency  is the ratio  of water diverted  for irrigation  or for ay  purpose and volume of water entering  the reservoir.  Losses in  reservoir are caused  by  seepage, spillage from spillway  and  evaporation.  On  the basis of experience of reservoirs which are operational storage efficiency  of 85  to  90  % is achievable.  The same has been considered in reservoir operation.
2)  Conveyance Efficiency
Conveyance Efficiency  is the ratio  of water reaching  irrigation  field  and  water diverted from the supply  source Conveyance losses comprise of seepage losses,  evaporation losses (total called  Absorption  losses) and  leakage loss from irrigation  system.  This is comprised  of two  efficiencies i.e conveyance efficiency  of canals and  conveyance efficiency of field channels. For design of canal system of Erer project each is assumed to 90 % giving total conveyance efficiency as 81 %
As per practice the conveyance losses of canals are worked  on  the basis of wetted perimeter of canal and  absorption  loss per million  sq.m,  of wetted  area.  The absorption 
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loss  from  field  channel  is  taken  as  some  percentage  of  water  delivered  to  the  field channel. Therefore both are considered separately for Erer irrigation system.
3)  Field Application Efficiency
Field  Application  Efficiency  is the ratio  of the volume of water used  by  plant by evapotranspiration  and  volume of water reaching  the field.  Water used  in  transpiration includes the effective rainfall also. This efficiency is assumed as 70 % for Erer project considering 30% loss.
Total Efficiency of Water Use for Irrigation is multiplication of above three efficiencies. The overall efficiency of Erer irrigation system is assumed as 55 % (0 9*0.9*0.7=56.7 say 57 %).
ej Irrigation Methods
The methods of application of water to the land can be broadly classified into surface, subsurface, sprinkler and drip or trickle irrigation methods. The choice of the method of application of water in a particular case will depend upon a number of factors. In general, the available water supply, the type of soil, the topography of the land, and the type of crop to be irrigated influence the choice of the irrigation method.
Sprinkler, trickle or drip irrigation methods are highly expensive to install. Therefore, these are neither practiced in the project area nor recommended for adoption. Hence the choice is narrowed down to choosing one out of two surface irrigation methods i.e. basin irrigation and furrow irrigation.
1) Basin Irrigation Method
Basin irrigation is adopted in following situations.
•    For gentle and uniform land slope
•    For close growing crop
•    On heavy soils where water is absorbed very slowly.
•    When leaching is required to flush salts from soil profile
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Basin irrigation is not to be adopted in the following situations:
•    Where precise land grading and shaping is required
•    Where crops are sensitive to wet soils around their stems
In  view of restrictions imposed  on  account of above situations basin  irrigation  is not generally recommended. However in some pockets where situations are favorable basin irrigation can be tried on limited scale.
1) Furrow Irrigation Method
This method is widely adopted in Ethiopia. The irrigation water is applied at a specific rate of flow in to shallow, evenly spaced channels called “furrows” created between rows of the plants. The water reaches the roots of plants by percolation. In this method the water is not applied to the entire surface area of land, only to % the surface is wetted and hence evaporation losses are low.
This method  is successfully  used  in  irrigation  of sugar cane,  cotton,  tobacco,  potato, maize,  sorghum,  vegetable etc.  The system needs low capital investment and  has low evaporation rates. Also cultivation is easier in heavy soils and there no wastage of land as in basin irrigation
The irrigation supplies in furrow are cut when the water reaches to about 2/3 length of furrow. The filled water in each furrow irrigates the remaining 1/3 length of furrow This procedure economizes the use of irrigation  water and  precludes the chances of over irrigation at the tail ends of furrows The irrigators learn this practice in a short time. When  furrow irrigation  is practiced  salt concentration  accumulates on  the ridges and leaching is not possible. Land leveling in most cases is required
Kinds of Furrow Irrigation
There are three kinds of furrow systems o Level furrows
o Graded straight furrows o Graded contour furrows
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The adaptability of furrow irrigation to a specific site depends on climate, soils, topology, crops to be grown and water supply. Level furrows are used in fine textural soils having very low infiltration rates. The water is pounded in furrow till it is absorbed by the soil. Graded straight furrows are laid along the prevailing land slope where major gradient is less then 2%. In contour furrows the water is carried across a sloping field rather than down  the slope if major gradient exceeds 2%.  In  areas where surface drainage is necessary, the furrows can be used to dispose off runoff from rainfall. When rainwater in conserved furrows act as effective means to catch and preserve the water from rainfall.
Furrow Spacing
Furrow spacing depends on the type of crop grown and machine used for planting and cultivation.
o For potato, maize, cotton furrows spacing are varying from 60-90cm. 
o For vegetables (carrots, onions, lettuce) 30-40cm.
o In sandy soils the furrow spacing should not be more than 50-60 cm
Furrow Length
The optimum length  of furrow is usually  the longest furrow,  which  can  safely  and efficiently  irrigate.  Proper furrow length  depends largely  on  hydraulic conductivity  of soils and land slope Optimal lengths may be chosen from the following table:
Tabic 2.10 Optimal Length of Furrows for Different Kind of Soils Found In the
Command of Erer Project
Soil Type
Furrow Slope
%
Clays
Loams
Sands
d* in cm
d in cm
d in cm
7.5
15
5
10
5
10
0.05
300
400
120
270
60
150
0.10
350
440
ISO
330
90
190
0.20
370
470
220
370
120
250
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0.30
390
500
280
400
150
280
0.50
380
500
280
370
120
250
1.0
270
400
250
300
90
220
d* = depth of irrigation application
Furrow Slope
A  minimum  furrow  grade  of  0.05%  is  needed  to  ensure  surface  drainage.  Following slopes may be adopted in different types of soils.
Table 2.11 Recommended Slops for Different Types of Soils
Types of Soils
Recommended Slops (%)
Clay
0.05 -00 20
Loam
0.20-0.40
Sand
0.25-0.60
Furrow Stream
This can be adjusted after the furrow irrigation system is installed The size of furrow stream varies from 0.5 to 2.5 liters per second. Portable rubber siphons with a size of 30 to  50mm diameter and  150mm head  are employed  to  ensure proper stream size The Maximum non - erosive flow rate in furrow is estimate by following empirical formula:
Qm = 0.6 / S
Where Q   = Maximum non-erosive stream in l/sec
m
S = furrow slope in %
2.8.4  Canal  System
a) Canal Alignment
In relatively plain area attempts are made to align canals along ridgeline of two adjacent catchments Branch canals and distributaries may also be aligned along the ridgelines of subsidiary catchments of main stream. This configuration will preclude the chances of
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providing  costly  cross-drainage  works  besides  facilitating  the  provision  of  drainage channels along the lowest lines.
In  areas  where  ridgeline  takes  a  sharp  loop  align  the  canal  in  straight  line  and  make special arrangement to irrigate the area bounded by ridgeline and canal
Canals should be as straight as possible because provision of sharp curves leads to scour on the outside and siltation on the inner side of canal section in the curved length of canal. In hilly areas attempt to align the canal along the suitable contour line except for necessary longitudinal slope that has to be given to it.
If  it  is  not  possible  to  provide  straight  alignment  for  topographical  reasons,  simple circular curves may be provided with following limiting radii
Table 2.12 Limiting Radii for Different Canal Capacity
3
Capacity' of Canal (m /sec)
Limiting Radius (m)
Less than 0.3
100
0.3-3
150
3-15
300
The canal should be kept in balanced earthwork as far as possible for economy. Super elevation of the water surface at bends be calculated by following formula:
h = Bv2/ gR
Where:
h = height of super elevation in m.
B = Bed width of canal in m.
v - velocity of flow in m/sec
g = acceleration due to gravity = 9.81 m/sec2
R - Radius of curvature at centerline of canal
Secondary and tertiary canals should take oft from main canal from or near the points where the canal crosses the watershed Thus secondary canals shall be taken along the secondary watershed and tertiary canal shall be taken along the tertiary watershed. The
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secondary canals shall be so aligned that the maximum area is served by the least length of channel.
The alignment of contour canals especially in upper reaches shall be decided after careful consideration  of economy.  Alternative alignments,  their benefits and  costs shall be considered, while designing.
Deep  cuttings  or  high  embankments  shall  be  avoided  by  suitable  detouring.  Careful judgment shall be exercised in fixing the points of drainage crossing.
The main canals and secondary canals are feeder canals which feed supplies to tertiary canals.  Hence  attempt  may  be  made  for  not  providing  direct  irrigation  from  them otherwise regulation of irrigation supplies in tertiary canals will become a problem. Irrigation  outlets  are  provided  preferably  on  tertiary  canals  In  some  special  cases, however, some outlets may be provided on main and secondary canals if strictly required by  topographical  consideration.  But  this  may  be  done  in  rare  cases  Height  of  banks should be made 10 % greater in order to allow for settlements Such allowance is not to be included in framing earthwork estimates. Attempt shall be made to utilize excavated material from cutting reaches in to filling reaches nearby rather than transporting it from other places. This will affect economy in construction The excavated material may also be used in road construction.
b) Canal Design
The canal and related structure should perform their functions efficiently and competently with minimum maintenance, ease of operation, and minimum water loss. Canal design shall fulfill the following requirements:
•    Conveyance of water in required amount from head to tail in every field.
•    Economical section ensuring non-silting and non -scouring velocities.
•    Minimum seepage losses; lining to be adopted only when cost effective.
•    Minimum maintenance and operation costs.
•    The embankments should be structurally stable.
•    Minimum density of canal network.
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•    Available water is utilized to ensure maximum irrigation efficiency and benefits.
•    Drainage system capable of removing excess rain and irrigation water efficiently without affecting crop yields.
Hydraulic Section
Design Criteria: Kennedy’s Theory, Lacy’s Theory and Manning’s Formula are used in design of earthen canals. For canals of small discharging capacity such as in Erer Project not much change is going to happen in channel dimensions if worked out by either of the formula. Therefore on account of simplicity Manning’s Formula is proposed to be used in designs of Erer Project. As per Manning’s Formula
V= l/n*R2/3S1/2
Where:
V = Velocity in m/sec
R = Wetted Mean Radius = Cross section Area/wetted Perimeter
S = Water Slope
n = Manning Roughness Coefficient. (0.025 for main, branch and tertiary canals and 0.030 for field channels).
The  bed  width  and  depth  ratio  of  the  canals  are  normally  based  on  Lacy’s  Regime Theory. However from practical considerations following criteria be adopted
Q < 0.5n?/sec
B = D (in clay)
B = 2D (in sand)
3
Q = 0.5 - 1.0m /sec
B = 2D (in clay)
B = 3D (in sand)
3
Q = 1.0 - 5.0m /sec
B = 3D (in clay)
B > 3D (in sand)
Where.
B = bed width in m
D = depth of water in m
Q = discharge in m3/sec
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Limiting  Velocities: The higher the velocity, the smaller will be the section and thus affect the economy. But to avoid siltation and scouring of earthen canals the following velocities may be adopted.
Maximum velocity     =          0.75m/sec in all canals
Minimum velocity      =          0.5m/sec in all canals
Working Head: For preparing command statement and deciding the full supply levels in the different component of the canal system the following minimum working heads are required:
o Head over field 
=  15 cm
o Head for field channel at its off take from quaternary = 15 cm
o      Head for quaternary in tertiary canal
= 15 cm
o      Head for tertiary canal in the secondary canal                     = 15 cm
o      Head for secondary canal in the primary canal                    = 30 cm
o      Head for primary canal at main outlet (dam site)         = 60 cm
Longitudinal  Slope:  Lacy’s  following  regime  equation  for  given  discharge  and  silt factor provides guidance in choosing slope for unlined earthen channel.
S = 0.00031 (f3/2Q1/6)
Where
S = longitudinal slope
3
Q = discharge in m /s
F = silt factor =1.76 mrl/2 for silts,
very fine to fine, it varies from 0.4 to 1.0
and for fine sands from 1.25 to 1.50
where.
~ average particle size of the boundary material
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In actual practice slightly steeper slope is provided than the one worked out by above equation.
Critical Velocity Ratio (CVR): According to Kennedy for non silting and non scouring channels in steady regime there is only one velocity called ‘critical velocity denoted by Vo, which is function of depth of water in channel and obtained by following equation:
Vo =0.546 D06
Where:
Vo = critical velocity in m/s
D = depth of water in meter
This equation was applicable to the grades of silt existing in the observation channel (in UBDC canal in Pakistan) To take in to account of varying silt grade another factor called ‘critical velocity ratio’ was introduced which is denoted by ‘m’ and the above equation was amended as below:
V = 0.546m D0 64
Where:
m = V / Vo = CVR
The value of m depends on type of silt. For channels carrying appreciable suspended loads, the value of m should be taken 1.10 at head and 0.85 towards tail end. In case of Erer Project, water is released from the reservoir and expected to be silt free and therefore m may be taken to lie between 1.0 to 0.85.
Free Board
The following minimum free board can be used for different discharges indicated below:
J
Discharge (m /sec)
Minimum Free Board
<0.4
0.2
0.4-1.2
0.3
1.2-5.0
0.4
Bank Width: From stability consideration following may be adopted:
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Discharge (n?/sec)
Top width of the Bank (m)
<0.3
1.00
0.30-0.75
1.25
0.75-3.00
1.50
3.00-6.00
1.75
Saturation Gradient
Types of Soils
Saturation Gradient
Clayey Loam
1:4
Sandy Loam
1:5
Maximum of the two be adopted
Ensure a minimum cover of 0.5m on saturation line
Roads on Banks: Roads are provided on the left bank of canal to facilitate inspection and maintenance
Table 2.13 Bank Side Slopes for Main Canal
Type of Soil
Side Slopes
Cutting
Filling
Loamy Soils
1 : 1
1.5 : 1
Sandy Soils
1.5 : 1
2: 1
For tertiary canals and field channels cutting and filling slopes be kept 1.5:1.
c)  Canal Losses
(i)  Evaporation Losses
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e
Evaporation  losses will depend  on  water surface area,  relative humidity,  and  wind velocity,  temperature and  sunshine hours.  In  hot and  dry  months these losses will be maximum, but seldom exceed 10% of total losses.
(ii) Seepage Losses
Seepage Losses are influenced  mainly  by  nature and  porosity  of soil,  depth  of soil, turbidity and temperature of irrigation water, age and shape of canal section and position of ground water table.
(m) Conveyance Losses
For  design  purposes  evaporation  and  seepage  losses  are  combined  and  are  called conveyance losses expressed in cumecs (m3/sec ) per million sq. m. of wetted perimeter. For detailed design purposes of Erer project the conveyance loss has been assumed as 2.5 cumecs per million sq m of wetted area. The wetted perimeter of main canal in Erer project will be of the order of 6m. Thus the conveyance losses will be of the order of 15 lit/sec/km For secondary and tertiary canals where wetted perimeter will be of the order of 3 m. this loss will be in the neighborhood of 7.5 lit/sec/km The same are proposed for adoption in preparing capacity statements of canals in Erer project.
d)Canal Embankment Material
For canal embankment materials following criteria are used:
o Liquid limit > 25%
o Plasticity index > 10%
o Gypsum <2% (to avoid solution holes)
o Sulphate < 0 2% (to avoid sulphate attack on ordinary Portland Cement)
e)Berm
Berm  is  the  horizontal  space  left  at  ground  level  between  the  toe  of  the  bank  and excavation.
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o If depth of cutting is less than 5M provide berm width equal to half of the depth of cutting
o If depth of cutting is more than 5M provide additional berm of 0.5 to 10m width just little above the full supply level besides the one at ground level The width of berm at ground level may, however, is such cases be reduced from that required 
9
from the above criterion.
J) Catch Water Drain
For a canal excavated  on  the slopping  ground,  a catch  water drain,  having  designed carrying capacity to carry expected discharge of flood from its catchment and leading to natural stream shall be provided
2.8.5 Irrigation Structures
The canal and  related  structures should  perform their functions efficiently  and competently with minimum maintenance, ease of operation, and minimum water losses. Many types of canal structures are required in an irrigation system to effectively and efficiently convey, regulate, and measure the canal discharge and also protect the canal from storm runoff damage.
a)  Canal Head Works
o Regulates the supplies in the canal
o Controls the entry of sediment in canal
Design Criteria
o Alignment: Align regulator between 90°-120°in respect of axis of the weir.
o Waterway:
•    Keep waterway equal to width of canal
•    In case when waterway works out to more than the width of canal, adjust crest level in such a way that the water way becomes equal to the width of canal
•     In exceptional cases when waterway works out more than the width of canal flared out walls are to the provided in downstream of regulator to join the canal width.
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Waterway of canal head regulator may be worked out with following formula in drowned condition
Q = Cl 1 V2g [(// + haj12 - Aa3/2]+C ld ^2g (H + ha)
2
Where:        Ci and C2 are numerical coefficients having values 0 577
and 0.80 respectively
H = difference of u/s and d/s water level
ha = hade due to velocity of approach
1 = clear length of waterway
d = depth of downstream water level above the crest
o Crest Level: Keep crest level of head regulator at least 0.50m above the crest level
of under sluice.
o Roadway: Provide 3.65m clear width of road between 0 23m wide kerbs to be provided on both side of the road.
•    It shall be R.C.C slab directly resting over piers and abutments continuously.
•    Provide 20cm thick slab which will be adequate to carry single lane traffic expected
•    Steel handrails shall be provided on both the sides of roadway.
•    A minimum of 7 5cm wearing coat shall be provided with 20cm thick slab of
the road.
o Piers: shall be constructed monolithic with the raft foundation.
(i)    Width: The width of pier shall be determined by consideration of following forces:
•    Weight of pier
•    Weight of roadway with live load
•    Horizontal force transmitted from gates impounding water
The worst condition  will be when  there is no  live load  on  the roadway  and regulatory gates are completely closed with water level up to full supply level with  no  water on  d/s side.  No  tension  is allowed  in  any  section  of pier.  A minimum of 1.0 m wide cone, pier or 1 50 m wide stone masonry pier with 30 cm
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C-25 cement concrete cap may be adopted Piers shall have at least one groove for stop-logs besides suitable groove for gates.
(ii)      Length: A minimum length of pier shall be equal to the width of roadway with kerbs plus width of gate operating platform e.g. 3.65+2x0.23+1.5=5.61° where 1.5m is the width of gate operating platform. The top of pier under road slab shall be lower and under gate operating platform shall normally be higher which depends on the height of gates.
(iii)     Gates:
•    The gate shall rest in 7 5cm deep gate groove in the sill
•     The top of gate shall be at least 30cm above the full supply level on the up streamside.
•    Gates shall be fabricated from mild steel and shall be lifted manually.
•     Where  gates  are  less  than  2.0m  wide  a  single  spindle  will  be  provided  to connect to gear unit.
•      Where  gates  are  wider  than  2.0m,  2  lifting  spindles  will  be  provided connecting to a centrally mounted level gear unit with hand wheel.
(iv)     Abutments:
•     Keep top width of abutment not less than 1.0m out of which 0.50m will be bearing for the R C. slab The bottom width shall be kept not less than 2.0m.
•     The  abutment  shall  have  its  front  face  vertical  to  facilitate  the  working  of gates and rear face sloping
•    The base of abutment shall be at least 1.0m below ground level.
•    The section of abutment shall be checked for stability considering its self load, live  load  and  dead  load  transmitted  by  road  slab,  side  thrust  due  to  earth retained and surcharge due to live loads etc.
(v) Length and thickness of Floor and Protection Works
•      Principles  of  design  of  length,  thickness  and  protection  works  of  canal regulator arc the same as that of weir or barrage.
•     D/S floor thickness should be worked out from uplift pressure considerations when high flood is passing down the weir and there is nor flow in canal
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•    Exit gradient at the end of d/s apron will have to be kept as low as possible by providing d/s cut off of mass concrete or sheet piles.
•    To reduce uplift on the floor, an upstream cut off is to be provided.
•    Uplift pressures may be evaluated by Khosla’s method and thickness of d/s floor may be worked out accordingly.
•    Weep holes are required to be provided in the wing walls and abutments
above canal full supply level with suitable graded inverted filters.
b) Regulating Structures
As their name indicates these structures are used to regulate the flow in one or more canals Regulation of flow is important as it could result in canal damage or unfair water distribution.
o A head regulator is provided at the head of channel, which controls the supplies entering in to channel.
o A cross regulator on the other hand is located at the down stream side of an off taking  point  on  main  channel  to  head  up  water  level  to  enable  the  off-taking channel to draw the required supplies.
o Cross regulators are used for diverting supplies so that repairs/ construction may be carried out in the parent channel.
o Cross regulators are also used to ensure safety of lining in the reaches where sub 
soil water levels are high. The cross regulators are also combined with fall, 
o Cross regulators facilitate the road communication since road is normally taken 
over them at little extra cost.
o Head regulators also control the entry of silt in to distributaries besides serving as meter for measuring discharges
When the available working head in off taking canal is more than half of the full supply depth in parent channel, cross regulator may not be provided in conjunction with head regulator.
1) Head Regulator
o The effective waterway of canal head regulator should not have less than 60%. of the bed width of off taking channel
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o Flow velocity should not ordinarily exceed 2.5m/sec through canal head regulator, 
o For cross regulators, the waterway is fixed with the consideration of max 
allowable afflux, which is normally taken about 150mm.
(i) Secondary Canal Head Regulator
There are two secondary canals off-taking from Primary Canal
o Head regulators shall be provided on secondary canals at all points of off taking of tertiary canals.
o These regulators will be regulated by single undershot gate operating under free flow or submerged conditions depending on site conditions.
o These will cross the canal embankments and therefore would have regular road bridge to provide passage to the access road.
o The flow under the gate shall be measured by using upstream and downstream water  depths  and  applying  appropriate  hydraulic  formula  under  submerged conditions.
(ii) Tertiary Canal Head Regulator
o Tertiary canal head regulator structures shall comprise of a single gated intake made of concrete followed by a pipe conduit under the canal embankment opening in to a measuring weir outlet structure.
o As long as the discharge in the secondary canal is constant the gate opening of tertiary' canal head regulator will remain constant, which can be checked from the discharge measuring weir outlet structure.
o The gates on the head regulator will be simple straight lift gates that can be operated by an individual man.
o Preferably gates shall be locked, to safeguard against unauthorized operation/tampering.
The intake and outlet structures shall be kept as simple as possible.
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2) Cross Regulator
Cross regulators are structures located on the parent channel, usually combined with road bridges. They help in absorbing fluctuations in various sections of the canal system and in preventing the possibility of breaching in tail reaches.
o  To  avoid  undesirable  contractions  and  concentrations  of  flow  the  overall  linear waterway  in  unlined  canal  may  be  kept  equal  to  the  bed  width  of  canal  for headless   cross   regulators(i.e.   When   there   is   no   fall   between   upstream   and downstream full supply levels).
o  For  head  less  cross  regulator  on  lined  canals  the  clear  linear  waterway  may  be kept equal to the average width of canal and overall linear water way equal to width of canal at full supply level with marginal adjustments in both.
o Where the regulator is combined with fall the clear water way would depend on following two conditions
i.  For submerged falls, the drowning ratio (i.e. ratio of tail water over crest to head water over the crest) should be greater than 0.8.
ii.   For free fall the discharge per unit length over the crest should be equal to greater than that required for available loss of head and required value of full supply depth down stream ( generally above bed level or above down stream cistern in some cases )
The values of fluming ratio (ratio of clear waterway and designed bed width downstream) should not be less than 0.5
(i) Secondary Canal Cross Regulator
o The secondary canal cross regulators shall be the same gated structures as that of  head  regulator  of  secondary  canal  if  the  discharge  of  secondary  canal  is more than O.5m(i) * 3/sec.
o For smaller discharges weir structures (Duck Bill Weir) are considered adequate.
o In special situation where secondary canal divides in to two, gated head regulators shall be provided
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o Duck Bill Weir is preferred over the straight weir because the level control in the parent channel is affected by minimum afflux. A Duck Bill Weir may or may not be followed by a fall structure depending upon the site conditions
Tertiary Canal Cross Regulator
o  The  cross  regulators  shall  be  Duck  Bill  Weirs  controlling  the  water  levels upstream of head regulators.
o In case of small discharges a simple broad crested weir having the same width as that of parent channel followed by a fall will be adequate
No. and Width of Bays
o For aesthetic reasons the no. of bays be kept as odd. It helps to remove pier at centre which avoids pier in the middle where concentration of flow and scour will occur. In special cases the no of bays may be even.
o If regulation is done by wooden planks the width of bay may be limited to 2.5 m
o If the depth of canal is more than 2.0 m, gate regulation is recommended. The width of bays is kept in accordance with the availability of standard sizes of gates when facilities for manufacture are not available.
Crest Level
o The crest level of off taking channel head is generally kept 0.30m higher than the crest level of cross regulator.
o In small channels the section is not flumed and the sill level is kept at the bed of channel.  It  gives  significant  amount  of  afflux  which  may  be  worked  out  with drowned weir formula.
o In channels whenever significant afflux is required to feed off- taking channel, the parent channel is flumed at the site of regulator from sides The sill is also raised above the bed, reducing the height of gate. The discharge formula applicable in this case will be:
Q = C L H3/2 Where
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<
H = upstream water depth above sill level plus approach velocity head in (m)
L = Length of Sill (m)
3
Q = Discharge (m /sec)
C = 1.71 if jump is formed
C < 1.71 if jump is drowned
•    In lined canals setting of crest above bed (upstream or downstream) should not be less than 15 cm nor higher than 40 % of the normal depth of canal.
•    The coefficient of discharge for broad crested weir (where width of crest is more than 2.5 times the head over crest) is 1.705 (rounded off to 1 71 ). In case of submerged falls the value is required to be reduced on the basis of submergence ratio. Following table is referred
Submergence ratio (%)
0.1
0.2
0.3
0.4
0.5
Ratio of Modified coefficient to C (%)
0.66
0.86
0.93
0.97
0.98
These are only approximate values which can be safely used in design of small structures (discharge < 15 cumecs ). However for important structures the value of coefficient of discharge should be got determined by model tests because this depends on many factors including head over the crest, shape and width of crest, its height over upstream floor, roughness of the surface, position of downstream apron and tail water level. This has been incorporated in the text of design criteria.
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22
• For discharge less than 10 cumec the slope of down stream glacis shall be kept 2.5:1. The upstream should entirely be of a circular curve without any straight portion. The radius of a circular curve is worked out by following formula:
Ru = (3H -X )/2x
Where Ru is the slope of upstream glacis in cm
X is head over crest in m
X is height of crest above upstream bed in m
The curve joining the crest with down stream glacis shall have a radius of 60 cm
Piers
Piers shall be constructed monolithic with .the raft foundation
o  Width  of  Piers:  The  width  of  pier  shall  be  determined  by  consideration  of following forces:
(i)      Weight of pier
(ii)      Weight of roadway with live load
(iii)      Horizontal force transmitted from gates impounding water
The worst condition will be when there is no live load on the roadway and regulatory gates are completely closed with water level up to full supply level with no water on d/s side No tension is allowed in any section of pier A minimum of 1.0 m wide cone, pier or 1.50 m wide stone masonry pier with 30 cm C-25 cement concrete cap may be adopted. Piers shall have at least one groove for stop-logs besides suitable groove for gates, 
o Length of Pier
A minimum length of pier shall be equal to the width of roadway with kerbs plus width of gate operating platform e.g.
3.65+2x0.23+1.5=5.61" where 1.5m is the width of gate operating platform.
The top of pier under road slab shall be lower and under gate operating platform shall normally be higher which depends on the height of gates.
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o Gates
o Abutments
(i)      The gate shall rest in 7.5cm deep gate groove in the sill
(ii)      The top of gate shall be at least 30cm above the full supply level on the up streamside.
(iii)      Gates shall be fabricated from mild steel and shall be lifted 
manually.
(iv)     Where gates are less than 2.0m wide a single spindle will be
provided to connect to gear unit
(v)       Where  gates  are  wider  than  2.0m,  2  lifting  spindles  will  be provided  connecting  to  a  centrally  mounted  level  gear  unit  with hand wheel.
•     Keep top width of abutment not less than 1.0m, out of which 0.50m will be bearing for the R.C slab. The bottom width shall be kept not less than 2.0m.
•     The abutment shall have its front face vertical to facilitate the working of gates and rear face sloping.
•    The base of abutment shall be at least 1 0m below ground level.
•     The section of abutment shall be checked for stability considering its self load, live load and dead load transmitted by road slab, side thrust due to earth retained and surcharge due to live loads etc
•     Width of pier and section of abutment shall be worked out with the same considerations as mentioned in section dealing with canal head regulator.
•     Upstream face of the crest should be given a better of 1 in 1. The downstream sloping glacis should not be steeper than 2:1. Comers of the crest shall be rounded up.
•    The crest width shall be kept equal to 2/3 of the He, where He is the head of water over crest plus approach velocity head
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73• For sharp crested weir (where width is less than 2/3 He) coefficient of discharge  in  metric  unit  shall  be  1  84  and  for  broad  crested  weir (where width of crest in more than 2.5 He) it will be 1 705
Exit Gradient and Uplift Pressure
o The structure shall be checked for exit gradient and adequate length of floor and downstream cut off wall should be provided for safe value of exit gradient. An exit gradient of 0.2 to 0.3, depending on type of soil and importance of structure shall be provided for ordinary conditions.
o If overall length of impervious floor is inadequate the downstream cutoff wall has to be deepened to the required extent.
o  The  thickness  of floor  provided  shall be  sufficient to  resist uplift pressure.  The uplift pressure is to be calculated in following two condition:
•      When   upstream   water   level   is   headed   up   to   full   supply   and downstream cistern is dry.
•    When the upstream water level is headed up to full supply and varying discharge  pass  down  stream.  The  maximum  uplift  pressure  would occur at a point where trough of standing wave is formed.
o Pressure relief arrangements shall be provided in case of important structures subjected to high up lift pressure.
Cistern Dimensions
o The length of down stream cistem should be such as to absorb the turbulent flow downstream of hydraulic jump.
o The elevation of cistern floor with respect to crest level shall be determined on the basis of hydraulic calculation
Cutoff Depth
o The upstream cut off shall be provided up to Lacey’s scour depth below the upstream bed level or ground level, which ever is low.
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3
Channel Capacity (m /sec) Min Depth of Cut off (m)
Up to 3.0 
3.1 to 30 
10
12°
o The downstream exit gradient shall be provide from safe exit gradient considerations
Channel Capacity (m /sec) Min Depth of Cut ofT (m)
3
Up to 3.0 3.1 to 30.0 
10
120
c)  Conveyance Structures
In addition to the canal network, it is usually necessary to use structures to convey water along the canal route. Some of this structure specific to the project site are:
1) Aqueduct of the Secondary Canal
Two secondary canals shall be crossing Decho River to extend irrigation on other side of Decho  River.  Aqueduct structures are proposed.  Following  are the design  criteria proposed for design of aqueduct structure.
o The choice of type of aqueduct depends up on consideration of economy which in turn will depend on size of drain to be crossed and size of canal. It will also depend on foundation strata at crossing site, dewatering requirements, envisaged head loss and topography of the site.
o The design discharge for calculating the water way of drain at structure site shall be higher of two i.e. 1.50 year flood and maximum observed discharge at the
structure site. 
1
o In plain the channels arc in alluvium and water way provided is 60 to 80 % of water way calculated by Lacy’s formula as below:
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p w = c Q 1/2
Where:
P  = Wetted perimeter in m
w
C = Coefficient whose value is normally 4 8
Q = Design discharge in m3/sec
This is the distance between two abutments
o  In  the  present  option  the  earth  banks  are  discontinued  over  the  drain  and  canal water is carried in a rectangular RCC trough. The sides of canal are connected in either side of work to the earthen banks preferably by means of transition wall.
Canal is generally flumed to effect economy.
o  The  layout  of  the  aqueduct  is  so  fixed  that  is  preferably  in  straight  reach  of drainage channel. The canal should cross it preferably at right angle.
o Wing walls of drain should have splay of 2:1 and 3:1 in upstream side and downstream side respectively. This spay should not be flatter than 3:1 and 4:1 respectively Drainage wing walls may be suitably connected to the high ground, 
o The transition in canal should be provided with 2:1 and 3:1 splay in upstream and down stream sides but not flatter than 3:1 and 5:1 respectively. However, it
should be ensured that flow follows transition boundaries.
o  The  drainage  channel  should  be  guided  towards  structure  by  suitable  training works like training walls, guide banks, spurs etc. The canal bank adjacent to the cross  drainage  work  should  be  protected  by  suitable  protective  measures  like turfing, pitching and launching apron where ever necessary
o In case of canal trough supported on piers, expansion joint of sufficient width may be provided at the centre of pier.
Maximum uplift pressure and exit gradient are caused by seepage flow from canal when the canal is running full and drainage channel is dry. The structure should be safe against uplift pressure and exit gradient. To safe guard against uplift pressure and exit gradient concrete or brick floor may be provided in canal for sufficient distance on upstream and downstream side of the structure with cut off' walls at ends.
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1
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2) Cross-Drainage Structures
o  In  every  system,  canals  cross  some  natural drains  along  its alignment Provisions are  required  to  be  made  to  cross  the  drain  across  the  canal  safely  without damaging the canal structure and interfering with its functions
o The cross drainage works may be three types depending upon the level at which a drain crosses the canal. If the level of canal bed is higher than the full supply level of drain the drain is designed to cross through an aqueduct or siphon aqueduct or culvert  If  the  situation  is  other  way  round,  the  drain  will  cross  through  super passage or siphon In a special situation when water level in drain and canal is approximately the same the structure in this category will be level crossing.
o Several drains descend from upland areas and cross into the proposed command areas situated on right side of river course. These drains have well defined courses and   cross   Primary   Canal   and   Secondary   canals   at   the   levels   which   are considerably  below  the  bed  levels  of  canals.  Therefore  simple  Pipe  Culverts  or Box Culverts of adequate size shall be provided at drainage crossing points.
o The cross drainage culverts shall have sufficient size to pass 25-year return period flood
o The catchments area of these drains shall be demarcated on topographical sheets in T50,000 scale. US SCS method shall be used for determining 25-year return period flood discharges.
o  The  IDF-  curves  which  have  been  developed  from  data  of  self  recording  rain gauges situated in the region and surrounding area shall be used for finding out the peak discharges.
o The natural drains which cross the canals shall have adequate cross section to pass flood discharge of 10-year return period. If the existing section is smaller than the one  required  for  passing  10-year  return  period  flood  the  drain  section  shall  be resection to the desired size.
d) Protective Structures
Protection must be made in an open irrigation system to protect the canal on the uphill side from damage by storm runoff. Additionally the canal also must be protected from excess flows caused by storm waters entering the canal.
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1) Canal Escapes
These are the structures meant for escaping surplus or excess water for the purpose of safety  of canal and  its structures and  depicting  the canals for repair and  maintenance purposes. There are two types of escapes, Weir Escapes and Sluice Escapes
Choice of Types of Escape
Weir Escapes are constructed  in  masonry  or cement concrete with  or without crest shutters capable of escaping surplus water from canal. Sluice Escapes however comprise of a head regulator of sluice escape, cross regulator on canal located just downstream of location of escape (if entire discharge of the canal is required to be escaped) and escape channel. Sluice escape besides removing surplus water act as scouring sluice and helps to remove silt from the canal Sluice escapes are therefore preferred  over weir escapes generally unless site conditions limit the use of weir escapes.
Surface escapes may be used in case of an escape opposite an inlet when an inlet does not bring considerable amount of silt. Surface escapes also become useful at the tail end of canal where there are fluctuations in withdrawals from canal and excess quantity of water can be suitably disposed off. Sluice escapes are necessary where the canal is required to be emptied  quickly.  Sluice escapes become essential when  the inlet can  bring considerable amount of silt.
Criteria for Location
Following criteria are adopted:
o 1 he location of spillway is decided by availability of suitable drain, depression or river with bed level at or below canal bed level for disposing off surplus water through escape directly or through escape channel.
o Provision of escapes at every 15 to 20 km is desirable on main canals and 10 to 15 km on secondary canals.
o  Escapes  may  be  provided  up  stream  of  major  structures  such  as  aqueducts, railway crossings, major diversion structures etc.
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78o  Escapes  may  be  provided in combination with aqueducts or siphons for affecting the economy.
o  Escapes  are  necessary  at  important  points  where  branches  take  off  from  main canal or several distributaries take off from branch canal In case of lift channels escapes are essential up stream of pumping station.
o  When  canal  is  very  close  to  the  edge  of  river  bank,  its  bed  level near the  head works being considerably lower than the flood plain of the river there is a risk of flood flow entering in to canal by way of breaches. If there is no escape provided the canal system may be severely damaged by excessive flow. The escape may be located at a point downstream of the reach where canal bank is vulnerable to the flood damage to restrict the damage to the reach upstream only.
o When the canal runs along the steep side of the hill or along a steep bank of a comparatively soft material, an escape may be located at the upstream end of the canal  section  where  it  first  approaches  a  steep  bank  In  case  of  a  land  slide occurring in the hill slope the entire canal may be blocked resulting in abrupt rise of  water  level  in  the  upstream  for  a  considerable  distance  This  intern  ca  cause extensive damage to canal section. An escape provided at a suitable place on the upstream of such reach would act as a safety plug to arrest the rise of water level and avoid consequential effects.
o When canal is confined by bank on one side and the unbanked side allows surface inflow,  escapes  need  to  be  provided  at  appropriate  location  in  the  vicinity  to dispose of the water so received.
o Escapes opposite the inlets or at a nearest suitable location may be needed whose drainage water is let in to the canal without reserve capacity to receive such water over and above the authorized full supply discharge of the canal.
o  Certain  quantity  of  heavy  bed  silt  may  find  its  way  through  the  regulator  in  to head reach of canal and thereby reduce the water way. In such cases sluice escape within 5 km of canal head reach may be provided.
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Criteria for Escapes Capacity
The flow requirement to be diverted through escape may vary from small quantities to total discharge of canal No general rules are laid for deciding the discharge capacity of escape.  The criteria are location  and  requirement specific Following  guidelines are adopted:
o If the canal passes near important town or important installations where breach of canal can affect costly properties or human lives the capacity of escape should be equal  to  maximum  flow  which  can  pass  in  the  canal.  In  other  cases  it  should seldom  be  greater  than  the  half  of  discharge  of  canal  or  not  less  than  the difference between maximum discharge of canal at the proposed escape site and maximum flow at next escape.
o When the escape is used mainly to empty the canal for maintenance, the capacity should be fixed taking in to account the number of days in which the canal is to be emptied.
Design Considerations
The structural as well as hydraulic design shall be same as that of other regulators and weirs.
o The safety of escape structure shall be checked for exit gradient. The maximum exit gradient shall be worked out on the basis of difference in maximum water level in canal and minimum water level in escape channel.
o The escape structure shall be safe against uplift pressure and sliding
o The water way required shall be computed using appropriate discharge coefficient considering the conditions of flow
o In case of surface escape the sill is provided at full supply level. The waterway shall be fixed taking in to consideration the depth of flow not exceeding VS of the free board provided in canal.
o In case of sluice escapes it is desirable to keep sill as low as possible depending upon   the   permissible   bed   level   of   escape   channel   This   will   enable   quick emptying of canal and silt removal besides providing economical waterway
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o The energy dissipation arrangements shall be made adequately to cater for all
flow conditions and operation.
o Adequate protection works shall be provided on down stream side of structure
same as in case of other regulatory structures
(ii)  Tail Escapes
At the end of every canal, whether secondary or tertiary, a tail escape is provided. This is very simple structure made in masonry or concrete as broad crested weir followed by a masonry cistern whose crest is kept at the full supply level (FSL) of the last reach of the channel. The length of broad crested weir is proposed to be about 2.0 times the width of the end channel at FSL so that effluxes caused by weir are minimized. Since no gated cross regulators are proposed to be provided on the secondary and tertiary canals, their cross sections will remain uniform from head to tail. The tail escape channel, which will ultimately join the existing drain falling in its alignment, will have the same section at that of its parent channel.
e) Outlets (Field Channel Off-take)
Out let must fulfill following requirements
o It shall be structurally strong and shall have no moving parts so that
■   It does not require periodic attention
■   It is not tampered by farmers or unauthorized persons
o It shall draw its fair share of silt carried out by parent channel
o It shall work efficiently with small working head.
o It shall be economical.
Various types of canal out lets have been evolved from time to time to obtain suitable performance. No one has come out to be suitable universally. In fact it is very difficult to  achieve good  design  with  respect to  flexibility  and  sensitivity  because of various indeterminate conditions both  in  distribution  channels and  the water courses,  namely, discharge levels,  silt charge,  capacity  factor,  rotation  of channels,  regime condition  of distributing channels etc affect their functioning. Even a particular outlet considered
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suitable  upstream  of  control  structure  in  a  channel  may  not  remain  suitable  for  a considerable distance on the same channel
Generally pipe outlets both in both semi modular or non modular type are in practice. Besides open flume outlets or crump’s adjustable proportional module of semi modular type are also being used at places There are three types of outlets practiced in the world. These are Modular Outlets, Non-modular Outlets and Semi-modular Outlets. In Modular Outlets the discharge remains constant and does not depend on the water head in the canal and  water course This type of outlets are either with  moving  parts or without moving parts The latter type is called Rigid Module. The Modular Outlets with moving parts are very complicated and difficult to design These are liable to be tampered by farmers or disturbed by floating materials.
Pipe outlets of circular or rectangular cross section fall in Non-modular Outlet category’ if the discharging  end  is submerged.  If the discharging  ends discharges freely  in  the watercourse it falls in  Semi Modular category’ Open  Flume Outlets or Adjustable Proportionate Module (APM) outlets are other types of semi-modular outlets.  In  the present case Pipe Outlets are proposed  to  be provided  for supplying  water to  field channels. The discharge formula for non-modular pipe outlet is as under
q = CAy[2gh
Where:
g = acceleration due to gravity in cm/sec2
A = Cross-sectional areas of pipe in cm2
C — Coefficient of discharge which depends on length, size and Roughness of pipe
h= working head of outlet in cm
The available sizes of pipes shall be used which are near to the size as worked out as above. It is common practice to place the pipe outlet at the bed of the distributing canal so that the outlet draws its share of silt Pipe outlets are adopted in the initial stage of irrigation in any area. Therefore the pipe outlets are proposed in this project for adoption
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In  rainy  season  when  additional  supplies  are  available  pipe  outlets  are  also  used  to distribute those supplies.
Although the theoretical discharge to each field is fixed, there will be variations in the flow from time to time. To ascertain as to whether the irrigation supplies in a particular canal system are efficient the flow measurements at last off take points are of greatest importance
J9 Drop Structures
Canals are constructed with permissible bed slopes so that the velocity of water in the canal will neither produce scouring nor silting of the canal It is not always possible to carry  the canal along  the ground  slope unless heavy  earth  work  is done This will be unwise and  uneconomical.  Hence,  it becomes imperative that at suitable position,  the canal bed is given a vertical or inclined drop.
Location and Design Criteria
The Location depends upon the ground conditions of the area through which the canal is flowing and the type of canal.
o In main canals, which do not irrigate adjacent lands directly, the drop structures are located from the consideration of economy in cost of excavation the channel
o The drop structures are located in relation to ground level and channel bed level so as to balance cut and fill between reaches as far as possible.
o In secondary or tertiary canals, which irrigate surrounding lands, the drop structures are located in such a way that they don’t cause any loss of command, 
o The full supply level in the channel may be so marked that it covers all the command points and allows a minimum working head of 0.30m for off- taking regulators and 0.15m for outlets.
o The Froude Number (i.e. VNgd) is not allowed to excess 0.5 in approach channel, o The drop structures are designed to have strait drops and standard impact basin, o Dead load of cone, and soil, lateral earth pressures and earthquake forces (not
generally significant) are considered to decide component structures.
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o There are various types of falls constructed in the world such as ogee falls, rapid falls,  vertical  drop  falls,  glacis  falls,  modified  glacis  type  etc.  However  on extensive  research  carried  out  in  Poona  Research  Station  of  Central  Water Commission,  Govt,  of  India  un-flumed  vertical  falls  with  discharge  up  to  14 cusecs  and  fall  height  up  to  1.5  are  found  most  satisfactoiy.  Hence  un-flumed vertical falls (crest length equal to width of canal) are recommended in the present context. The cost of this type of fall is the lowest.
o  In  vertical  drop  fall  the  nappe  impinges  in  to  water  cushion  down  below  and dissipation of energy is affected by the turbulent diffusion. No standing waves are created
o The Cistern Length = 5( Hl* D )
The Cistern Depth = 0.25 (Hl * D)
Where:
Hl = Drop of fall (m )
D = Depth of crest below T E.L.(m)
The cistern may be constructed with side and floor thickness of 300mm thick in C.C. C- 30.  In  secondary  and  tertiary  canals drop  structures may  be constructed  with  stone masonry laid in 1:6 cement mortars The inner side of cistern may be plastered with 1:3 cement mortars.
g)   Water Measurement Structures
It is seldom that a discharge measuring structures are constructed separately in any canal system because the existing structures such as falls or head regulators are used to measure the discharge. Hydraulically discharge rating curves are developed for such structures and used for discharge measuring purposes. No separate structures (such as venturi flumes or standing wave flumes) are being proposed in the canal system.
h)  Cross-Drainage Structure
Cross drainage structures are required wherever the canal line crosses natural drainage channels. As far as possible, the canal should be carried above or below the channel, and “level crossing” should be avoided since they cause silt to enter the canal.
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o In every system, canals cross some natural drains along its alignment. Provisions are  required  to  be  made  to  cross  the  drain  across  the  canal  safely  without damaging the canal structure and interfering with its functions.
o The cross drainage works may be three types depending upon the level at which a drain crosses the canal. If the level of canal bed is higher than the full supply level  of  drain  the  drain  is  designed  to  cross  through  an  aqueduct  or  siphon aqueduct  or  culvert.  If  the  situation  is  other  way  round,  the  drain  will  cross through super passage or siphon. In a special situation when water level in drain and canal is approximately the same the structure in this category will be level crossing.
o Several drains descend from upland areas and cross into the proposed command areas  situated  on  right  side  of  river  course.  These  drains  have  well  defined courses and cross Primary Canal and Secondary canals at the levels which are considerably below the bed levels of canals. Therefore simple Pipe Culverts or Box Culverts of adequate size shall be provided at drainage crossing points, 
o  The  cross  drainage  culverts  shall  have  sufficient  size  to  pass  25-year  return period flood.
o The catchments area of these drains shall be demarcated on topographical sheets in 1:50,000 scale. US SCS method shall be used for determining 25-year return period flood discharges.
o  The  IDF-  curves  which  have  been  developed  from  data  of  self  recording  rain gauges situated in the region and surrounding area shall be used for finding out the peak discharges.
o The natural drains which cross the canals shall have adequate cross section to pass flood discharge of 10-year return period. If the existing section is smaller than the one required for passing 10-year return period flood the drain section shall be resection to the desired size.
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• *
2.8.6 Drainage System
Drains are constructed with the objective of relieving excess water from agriculture lands and  disposing  of surplus water not required.  The type of drain  most suitable for a particular site will depend upon the purpose for which the drainage system is required.
a) Design Criteria of Field Drains
Following design criteria are used in design of field drains.
o The water from the furrows of the farm is collected into field drains which run laterally to the furrows.
o The design cross section of field drains should meet the combined requirements of capacity, erosion control, depths, side slopes, and maintenance and allowance for sedimentation.
o  Two  types  of  cross-section  can  be  distinguished;  the  triangular  and  trapezoidal. On account of easiness in construction triangular section with 6:1 side slope is preferred If the field drains are provided along the field roads trapezoidal section can be adopted. They will have usually side slopes of 6:1 to 10.1, a minimum depth of 0.25m, a minimum cross section area of 0.5m2 and a grade varying from
0.1 to 0.3%. The table below gives some recommended field drain dimensions. Table 2.14 Recommended Field Drain Dimensions
Type of Drain
Triangular
Depth (m)
Bottom Width (m)
Side Slopes            Utility
0.15 to 0.30      •
6.1 or flatter up to 10:1
Crossable and easily movable
Trapezoidal            0.25 to 0.50      2.50          6:1 or flatter up to 10:1
Crossable and easily movable
o Freeboard is not provided
o All the field drains have the same section because computed section on the basis of  discharge  is  smaller  than  the  section  provided  on  the  basis  of  practical considerations.
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S6o  The discharging  capacity  of field  drain  is checked  by  Manning  s formula where value of Manning’s ‘n’ is assumed  between  0.03  to  0.04  depending  on  weed growth and status of maintenance.
o  Earth  excavated  from  drain  section  is  utilized  in  formation  of  road  and  canal banks.
o Banks are not provided on field drains.
o  In  ordinary  situation  the  field  drains  always  run  in  excavation  and  seldom  in filling.
o  The  field  drains  are  proposed  to  be  designed  to  evacuate  superfluous  rain  and irrigation water from the command area in 24 hours as the dominant crop is maize which can not stand flooding for more than 24 hours. 24-hour rainfall having 2- year return period is considered in design of field drains. In a situation when a field is being irrigated and heavy rain occurs, the field drainage system should be capable of removing excess rain and irrigation water effectively during stipulated time otherwise it will result in impairment of crop growth and farm operations, 
o The period of evacuation and disposal of surplus rain is entirely dependant on the tolerance  of  individual  crop.  Based  on  the  experience  following  periods  of evacuation are recommended:
Rice
Maize and sorghum Sugarcane and banana Cotton
Vegetables
7 days
2 days
7 days
3 days 1 day
o The surplus rain water is found out by US SCS method. The amount of surplus rain  water  depends  mainly  upon  type  of  soil,  vegetative  cover,  land  use  and rainfall.
2
Q = (P- 0.2 S) /(P + 0.8 S) Where:
Q = Surplus Rain in mm
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P = Rainfall in mm
S = Retention Parameter in mm derived from SCS Curve Number
o It is assumed as 70% of field water supplied for irrigation per ha during the
months of maximum demand be evacuated along with surplus rain water.
b)         Parallel Field Drainage System
The parallel field  drain  system is the most effective method  of surface drainage.  The same is being recommended in this project. It is particularly appropriate in flat poorly drained areas with many irregularities. The success of the system depends on proper land forming  to  assure a proper slope of the rows.  These rows discharge their water in  to parallel field  drains constructed  at convenient places in  the field.  Such  field  drains consist of shallow graded channels with side slopes flat enough to allow farm machinery to cross The field drain dimensions are conditioned more by requirements of installation and maintenance than by hydraulic designs. They will have usually side slopes of 6:1 to 10:1, a minimum depth of 0.25m, a minimum cross section area of 0.5m2 and a grade varying from 0.1 to 0 3%.
c)  Drain Spacing
The drain spacing depends upon the hydraulic conductivity of soils, crops to be grown, topography and gradient of land after grading. In practice it varies from 100 to 200m on relatively flat land (slopes less than 0.5%) which, after grading, slopes in one direction. The crop  furrows directly  drain  in  to  field  drains and  should  have slope 0.1  to  0.2%. The land should be ploughed parallel to field drains and rest of farm works to be done perpendicular to the field drains. For turning the farm machinery a turn strip is located within the field away from the down slope field boundary.
d)   Tertiary Drains
The field drains discharge into tertiary- drains.(Table 2.16)
o All the tertiary drains shall follow drainage lines or depression ditches from the consideration of economy.
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o The tertiary drains shall have drop structures at regular interval to control erosion, 
o Small drop structures at frequent intervals shall be preferred over few large
structures because in doing so, the earthwork will be economized.
o The depth of tertiary drain shall be in the range of 1.0 to 2.0 m. and minimum bed width 1.30m from construction point of view
o  The  drains  up  to  1.0m  shall  have  side  slopes  1:1,  1.5  m  depth  1.5:1  and  2.0m depth 2:1 from stability considerations. But it is impracticable to have channel of varying sections and therefore side slopes of tertiary drains shall be kept as 3 :1 in entire reach.
o No tertiary drain shall have depth more than 4.0 m.
o Tertiary drains normally discharge into existing drains or gullies. The sites where
the tertiary* drains join existing gullies shall be protected to avoid erosion 
o The cross sections of tertiary drain shall be trapezoidal with side slope 3:1 and minimum bed width 1.30 m and shall be designed by Manning’s formula with 
N=0.030, 0.035. The velocity shall be kept in non-silting and non-scouring range
o If the existing gradient along the drain alignment is higher than required for maintaining the safe velocity, drop structures having straight fall of 1-2 m height
are recommended to the provided at suitable locations.
o  The  depth  of  tertiary  drains  and  secondary  drains  are  governed  by  the  depth  of field  drains  and  the depth  of main  drain  depends  upon  the  depth  of secondary drains  out  falling  in  to  it  in  order  to  have  sufficient  head  of  water to  out fall without creating back flow in to the discharging drains.
o As far as possible natural drainage channels are used as secondary or main drains as far as  possible.  If the existing x-section of natural channel is insufficient to accommodate the design discharge the section is remodeled.
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Table 2.15 Recommended Dimensions of Tertiary Drains
Recommended Side
Type of Dr
ain
Depth
(m)
Slopes
Minimum
Slopes
Triangular
0.3 to 0.6
6:1
3:1
Triangular
>0.6
4:1
3:1
Trapezoidal
0.3 to 1.0
4:1
2:1
Trapezoidal
>1.0
1.5:1
1:1
e)  Main Drain and Outfall Drain
Alignment
o These drains should follow drainage lines that are lowest valley lines as far as
possible.
o These should not pass through depressions, ponds or marshy areas.
o These should not pass through village habitation areas.
o The alignment should be such that the full supply level always remains below
natural surface level.
Capacity
o Normally cut section should accommodate the design discharge when drain 
follows natural valley line.
o  When  cut  section  is  inadequate  its  resection  should  be  done  and  excavated material should be deposited on either side of the embankments with large gaps between spoils so that over flow spills over the area returns to the channel when recharge recedes.
o The drains are never designed for worst conditions. The drains of a bigger size or catering  to  infrequent  rainfalls  prove  costly.  In  other  words  the  occurrence  of damages at periodical interval is to be accepted.
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o The experience indicates that drains of bigger size tend to deteriorate fast as these are  tot  required  to  carry  design  discharge  frequently.  Consequently  in  carrying smaller discharge drains tend to silt up fast. On the other hand drains of smaller size remain in better condition and can take occasionally higher discharge with marginal scour of bed or sides and encroachment on free board.
o Big drains need more land resulting in loss of cultivated land permanently 
o In Ethiopia the duration of storm is about 2 days. The drains are proposed to be designed for two days rainfall of 5- year return period assuming period of disposal
of two days as the dominant crop in command area is maize which can stand flooding of two days.
Cross Section
o The side slopes of the drains are kept as 1.5:1
o The cross section may be selected in such away that it is hydraulically efficient and economical in excavation.
o Whenever there is a likelihood of back water effect on account of flooding in the river in to which the drain outfalls the top of embankment should be so designed the flood levels on account of backwater conditions are accommodated within the section over which the minimum free board is provided
f)        Drainage Structures
(i)      Outfall Structures
o  At  the  junction  of  drains  out  fall  structures  are  provided.  These  structures  are designed to discharge freely in to the receiving drain
o The water level of discharging drain is always kept at high level than the water level of receiving drain to avoid the chances of backflow.
o These structures are positioned at the outfall points of field drain, out falling in to tertiary drains, tertiary drains out falling in to secondary drains and secondary drains out falling into the main drain and main drain out falling in to the river, 
o When main drain outfalls into river a cross regulator needs to be provided to preclude the chances of flooding of irrigated area from back flow from river.
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< *
When the stage in river is high the cross regulator will be kept closed. However when the stage in river is low the cross regulator shall be left open.
(h) Culverts
Cone,  pipe culverts shall be provided  for roads crossings the drains.  In  case pipes of required dimension are not available box culverts of suitable size shall be provided. The culverts are located where roads cross the drainage alignments.
(iii)Drop Structures
Similar to irrigation canals the drop structures shall be provided to prevent erosion due to excessive flow velocity in drains. Normally vertical drop structures are provided having drop heights ranging from 1.0 to 2.0m with suitable energy dissipation structures. The location  of culverts and  drop  structures shall be decided  on  availability  of final topographical map.
2.9 Financial and Economic Analysis
2.9.1  Estimation of Cost & Benefit Stream
The assessment of value of all cost and benefit shall be at constant price and for the economic analysis shadow price shall be used. Complete financial & economic analysis requires financial data to be presented cost and benefit stream in detail. Benefits and cost analysis enables to trace movements of income and expenditure or outlay on the project. This includes direct and indirect cost and benefits stream valued at constant price.
2.9.2   Methodology'
The analysis and evaluation of ERER Irrigation Project follows the conventional profit analysis. Thus the financial analysis uses the market prices and the economic analysis uses shadow prices. In addition, the analysis identifies and compares the cost and benefit
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incurred with and without the project. In this analysis opportunity cost and the discount ratio are taken at current inflation rate
The financial prices of farm inputs and outputs are based on the current prices of the local markets, which in turn will be based on market saving and field level. To reflect the opportunity cost to the society, the financial prices are adjusted to economic price
a)Benefit Stream
There can  be tangible and  intangible benefit from the proposed  irrigation  project. Tangible benefits of this agricultural project will be appraised  at market price or economic value These tangible benefits can  arise either from an  increased  value of production or from reduced cost of production.
(i)Direct Measurable Benefits
Increased Crop Production: Availability  of water for irrigation  will allow full utilization of land for agriculture in the project area especially during dry season. At present,  rain  fed/traditionally  irrigated  agriculture is practiced  in  the area,  the development of the irrigation project will provide additional water for more production. The increased production of crops/vegetables/fruits by the local farmers would be the main factor to increase farmers income and it is also hoped to motivate the farmers to participate in the project
Increased Employment: The implementation of the Erer irrigation project will create additional employment opportunities in the project area. This increased employment
which is associated with the irrigation farm labor, would also supplemented by higher cropping intensity, which in turn creates employment opportunities even for farmers outside of the command areas.
Increased  Government  Revenue:  Through  income  taxes  the  project  has  its  own contribution as source of government income.
(ii)Indirect Measurable Benefits
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Food Security: Improved  crop  production  through  the irrigation  project will increase farm income in  the project area.  Food  supplies will increase and  there will be cash income available for food or other purchases
Risk Minimization to  Farmers; Farmers’ risks are considerably  reduced  through irrigation and the higher cropping intensities will protect them against shortfalls in food production.  Reliable water supplies will also  encourage farmers to  diversify  their cropping patterns to take advantage of market conditions.
Technology Transfer; Improved technology and farming practices will be available to farmers as a result of the support and extension services included in the irrigation project. This will directly  benefit farmers in  the direct project area but will also  serve as a demonstration to other farmers in the region.
Improve Nutritional Status,  Increased  cash  income resulting  from increased  crop production will result in higher family consumption of a greater variety of foods. This in turn will result in improved diets and nutrition levels in the project area.
(b)Cost Stream
According  to  the guide line by  Ministry  Of Finance & Economic Development (MOFED), there exist four major cost components to be considered in project financial and economic analyses. This includes:
•    Investment cost
•    Operating cost
•    Working capital
For agricultural project in general and irrigation project in particular, the investment cost and  operating  costs are important.  Thus,  these two  cost components are given  due consideration.
Investment Cost
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Investment cost to be considered here in the Erer irrigation project includes:
•    Land preparation,
•    Dam & associated cost
•    Catchments treatment cost
•    Spill way
•    Canals roads
•    Machinery & equipment cost
•    Other Irrigation infrastructure development cost
•    etc
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Operating Cost
These cost components includes all cost stream which will be incurred annually through out the operational period  of the project,  such  as production  cost,  repair/maintenance cost, raw materials cost, labor and utilities cost and other expenses.
(iii)Labour
The economic cost of unskilled labour or the opportunity cost of such labour has been valued as 75 percent of the market wage rate.
(rv) Farm Inputs
The financial and  economic prices of fertilizer are based  on  current market price.  In Ethiopia, all fertilizers which are imported are made available to small farmers without taxation. Thus, in the project area the current market prices are used to calculate the price of DAP and  Urea,  which  are the most common  fertilizer in  the country.  However, chemicals for crop protection are taxed Thus, the economic costs of these chemicals are computed  by  deducting  custom tax  (20%) from market price Additionally,  most of improved seeds are imported and are taxed. Thus, the economy price of them are also calculated in the same manner
(iv)Machinery and Equipment
The cost of tractors, other machinery and equipment, are taxed if they are purchased from import -export trade agent or others Therefore to get the economic cost we will subtract both custom duty(30%) and VAT(15%) taxes from their market costs
(v) Farm Outputs
The economic price calculations are based  on  either import substitution  or export, depending  on  whether Ethiopia is expected  to  be a net importer or an  exporter of that product during  the project period  To  calculate the economic price of the product this study  considers their CIF export prices in  Djibouti from Ethiopia (Study of Agricultural input and  output marketing  agency  2007).  For those,  which  are not traded 
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internationally, the study considers financial price as economic price .The financial price of each product will be taken at Addis Ababa market or project area market.
(vi) Construction Costs
Construction  unit  rates  arc  based  on  data  base  of  current  construction  under  taken  by local and international contractors.
2.9.3   Crop Budgets
Comparative crop budgets are useful in assessing the relative profitability of alternative crops at a given point in time. The crop budgets are based on current yields and farm input levels without the effects of any of the proposed irrigation developments.
2.9.4 Financial Viability Criteria
Financial & economic analysis as important tools of viability  indicators presents its results to enable feasible project design and used as a ground step for decision making about the project.
2.9.5   Expected Output
A through assessment will be made with regards to project likely impacts and viability. Financial  analysis  will  reflect  the  formulation  and  assessment  of  how  the  project  is profitable The purpose of financial analysis is not just limited to project liquidity , credit worthiness  ,  financial  income  &  profitability,  but  also  part  of  the  process  of  project design   itself   in   that   its   results   recommend   selection   of   the   design   options   by demonstrating benefit & cost of the project stream and investment capital out lay.
2.9.6  Project  Viability  Indicators
a) Discounted Cash Flow/NPF
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Discounted cash flow measures the future income to its present value.
b)  Net Present Value (NPV)
Net Present Value, which is the present worth of the incremental net benefit stream, is used for project worth analysis in Erer irrigation project. The net present worth should be positive or acceptable for implementation of a project
c)         Internal Rate of Return (IRR)
The internal rate of return is the discount rate which makes the net present value of the incremental net benefit stream equal to  zero  Projects are accepted  if the calculated internal rate of return exceeds the cut-off rate or opportunity cost of capital.
d)Benefit/Cost Ratio
The benefit-ratio is the ratio of the present worth of the benefit stream to the present worth of the cost stream. If the benefit- cost ratio exceeds one, the project should be accepted for implementation. NPV gives an indication of absolute scale of the net benefit & cost. IRR provides the relative profitability of the project in relation to general level interest rate or opportunity cost of capital. Therefore, a positive NPV or IRR which exceeds the opportunity cost of capital or B/C ratio greater than one is a pre-condition for project viability
e)         Sensitivity Test
To check the effect of variation in selected cost and benefit on project rate of return sensitivity test analysis is carried out. Important variables, which are expected to influence the viability of the project, will be identified and checked.
The approach to be used for identifying sensitivity range is to follow switching value to estimate the effects of major variables on the viability of the projects to determine at which level to acceptable or reject the project. This enables to judge which variables is critical and important for viability of the project.
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f)  Economic Viability Criteria
The objective of the economic analysis is to  evaluate the projects contribution  to the project area in particular and to the national economy as a whole. Its aim is to apprising economic cost and benefits of the project and asses the alternative impact on the local area economy. It uses estimation of cost & benefits in terms of shadow pricing measuring economic profitability of the project in terms of economic value and justify allocation of scarce resource to the project. Financial analysis data base mainly used for economic analysis with some adjustment in cost and benefit stream of the project. The economic analysis uses conversion factor or shadow price for some cost & benefit stream.
(i) Economic Net Present Value (ENPV)
Economic Net Present Value is the most common project worth analysis to be used for this project analysis. It is the present worth of the incremental net benefit stream. The net present worth should be positive or acceptable for implementation.
(ii)        Economic Internal Rate of Return (EIRR)
The internal rate of return is the discount rate which makes the net present value of the incremental net benefit stream equal to  zero.  Projects are accepted  if the calculated internal rate of return exceeds the cut-off rate or opportunity cost of capital.
(iii)Economic Benefit/Cost Ratio
The benefit-ratio is the ratio of the present worth of the benefit stream to the present worth of the cost stream. If the benefit- cost ratio exceeds one, the project should be accepted for implementation
Economic net present value (ENPV) gives an  indication  of absolute scale of the net economic benefit & cost. Economic net present value greater than one or EIRR which exceeds the opportunity cost of capital or B/C ratio greater than one is a pre-condition and criteria for project viability.
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2.9.7 Key Evaluation Parameters
The evaluation is based on a number of key parameters indicated below.
a)  Discount Rate
The basic discount rate of opportunity cost or capital suggested for the study is 11% with sensitivity analyses conducted al 10 and 16 percent
b)  Price
All costs benefits are expressed in constant prices (i.e. excluding general escalation) at May 2008 price levels. This will also be the base date for all present value calculations. In general, local market prices will form the basis for both financial and economic prices.
c)   Planning Horizon
The costs and benefits are evaluated over a period of 30 years This period is expected to be long  enough  to  fully  include all benefits from the expected  economic life of the irrigation facilities.
d)  Interest during construction
Interest during construction is assumed to be a financial cost and is therefore excluded from all economic cost streams.
e)  Escalation
The analyses will be based on real costs expresses at current, may 2008 GC price levels omitting  general price inflation.  General inflation  is not relevant for the economic analysis as it does not alter the relative prices in the analysis.
f)   Sensitivity Cases
Sensitivity case will be developed on the basis of foreseeable fluctuations in operating costs, high and low flow rates and modifications in the anticipated prices (revenue); and fixed costs.
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101references
I.   Feasibility Reports (Volume 1 to 12) - Erer Irrigation Projects - Sep’07 by WWDSE (Ethiopia) in association with Synergies Hydro (India) Pvt. Ltd.
2 “Fundamentals of Irritation Engineering” by Dr. Bharat Singh,
3.   “Theory and Design of Irrigation Structure” by Dr. R.S Varshney, S.C Gupta and R.L Gupta,
4.   “Irrigation Engineering & Hydraulic Structure” by S K Garg
5.  “Design of Small Dams.” by United States Bureau of Reclamation
6 ‘The engineering of Large Dams’ by Henery H. Thomas
7.  “Earth and Rock fill dams” by Bharat singh and H R Sharma
8.  “Irrigation Theory & Practice” by A.M Michcal,
9   Ethiopia Building Code Standards(EBCS)- Structural use of Concrete , 1995. 10 IS 10635:1993 “Free Board Requirement in Embankment Dam - Guidelines.” II. IS 7894:1975, Code of Practice for Stability Analysis of Earth Dams.” 12. IS 8826:1978, “Guidelines for Design of Large Earth and Rock fill Dams.” 13. IS 9429: 1995, “Drainage System for Earth and rock fill Dams-Code of Practice.” 14. IS 5186: 1994, “Design of Chute and Side Channel Spillways Criteria” 15. IS 10430:2000, “Criteria for Design of Lined Canals and Guidance for Selection 
of type of Lining”
16. IS 5968:1987, “Planning and Layout of canal system for Irrigation system” 17. IS 6936:1992, “Hydraulic Design of Canal Escape”
18. IS 7114:1973, “Design ofCross Regulator”
19. IS 4839 :1992, “Guidelines for Maintenance of Canals”
20. IS 7784:1993, “Design ofCross Drainage Works”
21. Ministry of Water Resources,2002”Design Guide line on Irrigation Systems” 22. Ministry of Water Resources,2002”Dcsign Guide line on Diversion Structures” 23. Ministry of Water Resources,2002”Design Guide line on Dam Design”

Summery