Technical Guideline for Design of Headworks - FC2oida2014.web.fc2.com/photo_gallery/07headworks.pdf · Japan International Cooperation Agency (JICA) Oromia Irrigation Development

Japan International Cooperation Agency (JICA)

Oromia Irrigation Development Authority (OIDA)

Technical Guideline for Design of Headworks

May, 2014

The Project for Capacity Building in Irrigation Development (CBID)

Foreword Oromia Irrigation Development Authority (OIDA) is established on June, 2013, as a responsible body for all irrigation development activities in the Region, according to Oromia National Regional Government proclamation No. 180/2005. The major purposes of the establishment are to accelerate irrigation development in the Region, utilize limited resources efficiently, coordinate all irrigation development activities under one institution with more efficiency and effectiveness. To improve irrigation development activities in the Region, the previous Oromia Water Mineral and Energy Bureau entered into an agreement with Japan International Cooperation Agency (JICA) for The Project for Capacity Building in Irrigation Development (CBID) since June, 2009 until May, 2014. CBID put much effort to capacitate Irrigation experts in Oromia Region through several activities and finally made fruitful results for irrigation development. Accordingly, irrigation projects are constructed and rehabilitated based on that several Guidelines & Manuals and texts produced which can result in a radical change when implemented properly. Herewith this massage, I emphasize that from Now on, OIDA to make efforts to utilize all outputs of the project for all irrigation activities as a minimum standard, especially for the enhancement of irrigation technical capacity. I believe that all OIDA irrigation experts work very hard with their respective disciplines using CBID outputs to improve the life standard of all people. In addition, I encourage that all other Ethiopian regions to benefit from the outputs. Finally, I would like to thank the Japanese Government, JICA Ethiopia Office, and all Japanese and Ethiopian experts who made great effort to produce these outputs.

Feyisa Asefa Adugna General Manager

Oromia Irrigation Development Authority

Addis Ababa, Ethiopia May, 2014

Introductory Remarks Growth and Transformation Plan (GTP) from 2011 to 2015 intensifies use of the countrys water and other natural resources to promote multiple cropping, better adaptation to climate variability and ensure food security. Expansion of small scale irrigation schemes is given a priority, while attention is also given to medium and large scale irrigation.

In Oromia Region, it is estimated that there exists more than 1.7 million ha of land suitable for irrigation development. However, only 800,000 ha is under irrigation through Traditional and Modern irrigation technology. To accelerate speed of Irrigation Development, the Oromia National Regional State requested Japan International Cooperation Agency (JICA) for support on capacity building of Irrigation Experts under Irrigation Sector.

In response to the requests, JICA had conducted "Study on Meki Irrigation and Rural Development" (from September 2000 to January 2002) and Project for Irrigation Farming Improvement (IFI project) (from September 2005 to August 2008). After implementation of them there are needs to improve situation on irrigation sector in Oromia Region.

JICA and the Government of Ethiopia agreed to implement a new project, named The project for Capacity Building in Irrigation Development (CBID). The period of CBID is five years since June, 2009 to May, 2014 and main purpose is to enhance capacity of Irrigation Experts in Oromia Region focusing on the following three areas, 1) Water resources planning, 2) Study/Design/Construction management, 3) Scheme management through Training, On the Job Training at site level, Workshops, Field Visit and so on and to produce standard guidelines and manuals for Irrigaiton Development.

These guidelines and manuals (Total: fourteen (14) guidelines and manuals) are one of the most important outputs of CBID. They are produced as standards of Irrigation Development in Oromia Region through collecting different experiences and implementation of activities by CBID together with Oromia Irrigation Experts and Japanese Experts.

These guidelines and manuals are very useful to improve the Capacity of OIDA Experts to work more effectively and efficiently and also can accelerate Irrigation Development specially in Oromia Region and generally in the country.

Finally, I strongly demand all Irrigaiton Experts in the region to follow the guidelines and manuals for all steps of Irrigation Development for sustainable development of irrigation.

Adugna Jabessa Shuba D/General Manager & Head, Study, Design, Contract Administration & Construction Supervision

Oromia Irrigation Development Authority

Addis Ababa, Ethiopia May, 2014

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Table of Contents

1. GENERAL DESCRIPTION ................................................................. 1

1.1 Introduction ................................................................................ 1

1.2 Definitions .................................................................................. 1

1.3 Aim of the Guideline ................................................................... 1

1.4 Scope of the Guideline ................................................................ 2

1.5 Components of Headworks .......................................................... 3

1.6 Basic Considerations of Headworks ............................................. 7

2. BASIC DESIGN INPUT DATA ............................................................ 9

2.1 Data for River Conditions ............................................................ 9

2.1.1 River Flow Regime ................................................................ 9

(1) Discharge Condition ......................................................... 10

(2) Water Level and Discharge ............................................... 11

(3) Sediment Load (if data are available) ................................. 15

2.1.2 Condition of Riverbed ........................................................... 17

(1) Condition of thalweg ......................................................... 17

(2) Riverbed slope .................................................................. 18

(3) Riverbed Materials ............................................................ 18

2.2 Studies on the Influence of Flood Control and Water Use ............ 19

2.2.1 Flood Control Plans .............................................................. 19

2.2.2 Situation of Upstream and Downstream Drainage ................ 19

2.2.3 Dikes, Bridges and Other Structures .................................... 20

2.2.4 Present Condition of River Water Utilization ......................... 20

2.3 Geotechnical and Geological Investigations ................................. 21

2.3.1 Drilling ................................................................................ 21

2.3.2 Test Pitting .......................................................................... 22

2.3.3 Bearing Capacity Tests ........................................................ 23

(1) Standard Penetration Test (SPT) ....................................... 23

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(2) Loading Test ..................................................................... 26

2.3.4 Investigation on Riverbed Deposit ........................................ 26

2.3.5 Groundwater Investigation .................................................. 26

2.4 Assessment & Planning for Construction Works .......................... 27

2.4.1 Meteorology, Surface Water, Groundwater

and Riverbed Conditions ................... 27

2.4.2 Construction Equipment and Materials ............................... 28

2.4.3 Transportation of Equipment and Materials ......................... 28

2.4.4 Power Source for Construction ............................................ 28

2.4.5 Availability of Enough Labor for Construction ...................... 29

2.5 Topographical Survey .................................................................. 29

2.5.1 Topographic Survey ............................................................. 29

2.5.2 Longitudinal and Cross-Section Survey ............................... 30

2.5.3 Surveys for Other Temporary Works and Compensations ..... 30

2.5.4 Collection of Topographic and Other Related Maps .............. 31

2.6 Data for Temporary Works .......................................................... 31

2.6.1 Annual Maximum Daily Rainfall and

Annual Maximum Hourly Rainfall ................ 31

2.6.2 Annual Daily Rainfall .......................................................... 32

2.6.3 Environmental Impact Assessment .................................... 32

3. DESIGN OF HEADWORKS ............................................................... 33

3.1 Basic Design ............................................................................... 33

3.1.1 Design Conditions ............................................................... 33

(1) Design water intake discharge .......................................... 33

(2) Design intake water level .................................................. 35

(3) Design flood discharge ...................................................... 36

(4) Design flood level .............................................................. 38

(5) Study of riverbed evolution ............................................... 39

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3.1.2 Position of Headworks ......................................................... 46

(1) Points of selecting Headworks position ............................. 46

(2) Process of selecting Headworks position ........................... 47

3.1.3 Method, Location and Type of Water Intake ......................... 48

(1) Method of water intake ..................................................... 48

(2) Location of intake ............................................................. 49

(3) Type of weir and direction of weir axis .............................. 50

3.1.4 Design Dimensions .............................................................. 54

(1) Design water intake level .................................................. 54

(2) Elevation of crest height of weir ........................................ 55

(3) Ensuring creep length ...................................................... 56

(4) Study of possible effect on the river control of upstream ... 61

3.2 Detail Design ............................................................................. 65

3.2.1 Movable Weir ....................................................................... 65

(1) Sill elevation of movable weir ............................................ 65

(2) Spillway by movable weir .................................................. 66

(3) Scouring sluice ................................................................. 68

(4) Pier .................................................................................. 81

3.2.2 Fixed Weir ........................................................................... 91

(1) Section shape ................................................................... 91

(2) Type of fixed weir .............................................................. 92

(3) External forces ................................................................. 92

(4) Determination of section (Stability analysis) ...................... 97

(5) Correction of trapezoidal section ....................................... 98

(6) Apron ............................................................................... 100

3.2.3 Riprap ............................................................................... 101

(1) Basic concept for engineering of riprap work ..................... 101

(2) Conditions required for riprap .......................................... 102

(3) The shape of riprap .......................................................... 102

(4) The length of riprap of upstream side ............................... 103

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(5) The length of riprap A of downstream side ........................ 103

(6) Riprap B of downstream side ............................................ 107

(7) Use of Bligh's formula (Reference) ..................................... 107

(8) Structural engineering of riprap ........................................ 108

3.2.4 Foundation Work ................................................................ 110

(1) Functions of foundation work ........................................... 110

(2) Types of foundation work ................................................. 110

3.2.5 Upstream and Downstream Cut-off Walls ........................... 119

(1) Upstream cut-off wall ....................................................... 119

(2) Downstream cut-off wall ................................................... 122

3.2.6 Inlet .................................................................................... 123

(1) Function of Inlet ............................................................... 123

(2) Location of inlet ................................................................ 124

(3) Features of Inlet Design .................................................... 124

(4) Flow Discharge at Inlet ..................................................... 126

(5) Water Level Calculation for Inlet ....................................... 128

3.2.7 Gate .................................................................................... 141

(1) Selection of type of gate .................................................... 141

(2) Lifting Height.................................................................... 149

(3) Material ............................................................................ 149

(4) Dimension of gate for Slide gate and Stop-log ................... 150

3.2.8 Related Structures .............................................................. 151

(1) Settling Basin ................................................................... 151

(2) Protection of bank and major bed ..................................... 165

3.2.9 Control Facilities ................................................................. 168

(1) Operation equipment ........................................................ 168

(2) Power receiving and distributing facilities ......................... 168

(3) Operation and maintenance bridge ................................... 168

(4) Other operation facilities .................................................. 170

(5) Operation ......................................................................... 170

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4. DATA SHEET, CHECK LIST AND OTHERS ....................................... 175

4.1 Data Sheet .................................................................................. 175

4.2 Check List ................................................................................... 178

4.3 Coefficients of Roughness ............................................................ 184

5. EXAMPLE OF DESIGN FOR HEADWORKS ....................................... 187

5.1 Basic Design Input Data ............................................................. 187

5.1.1 Discharge through Float Measurement Method .................... 187

5.1.2 Riverbed Slope..................................................................... 189

5.2 Basic Design ............................................................................... 191

5.2.1 Design Water Intake Discharge ............................................ 191

(1) In case of getting discharge data in or near river basin

of project site ................................................................... 191

(2) In case of getting discharge data by actual measurement .. 192

5.2.2 Design Intake Water Level ................................................... 193

(1) Water level of the field at the highest elevation of

the irrigation area ............................................................ 193

(2) Water level at the starting point of the main canal ............ 193

(3) The hydraulic loss between the intake and

the starting point of the main canal ................................. 193

(4) Other structural losses at the intake

(hydraulic loss of entrance) .............................................. 194

(5) Calculation result of design water intake level .................. 196

5.2.3 Design Flood Discharge ....................................................... 197

(1) In case of getting past flood discharge data in or

near river basin of project site .......................................... 197

(2) In case of using the maximum flood in the past based

on flood mark or discharge capacity of the river

by slope area method ....................................................... 198

5.2.4 Design Flood Level ............................................................... 203

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5.2.5 Elevation of Crest Height and Length of Weir ....................... 203

5.2.6 Possible Effect on the River Control of Upstream.................. 203

(1) Water depth of the river where the place of headworks

before construction as design flood discharge

(Tail water depth) ............................................................. 203

(2) Water depth on the crest as design flood discharge ........... 203

5.3 Detail Design .............................................................................. 206

5.3.1 Fixed Weir ........................................................................... 206

(1) Section shape ................................................................... 206

(2) Determination of section (Stability analysis) ...................... 206

(3) Apron ............................................................................... 215

5.3.2 Riprap ................................................................................. 219

(1) Calculation of water depth at the weir toe

as design flood discharge ................................................. 219

(2) Calculation of water depth at the beginning point of

hydraulic jump ................................................................ 221

(3) Comparison with h1a and h1b ............................................ 221

(4) Calculation of supercritical flow length ............................. 221

(5) The length of hydraulic jump ............................................ 221

(6) Necessary Length of riprap A ............................................ 221

(7) Length of riprap B ............................................................ 221

(8) Length of upstream riprap ................................................ 221

5.3.3 Foundation Work ................................................................ 222

5.3.4 Upstream and Downstream Cut-off Walls ............................ 222

5.3.5 Inlet .................................................................................... 222

5.3.6 Gate .................................................................................... 222

5.3.7 Settling Basin ...................................................................... 223

(1) Width and depth of sedimentation ditch ........................... 223

(2) Length of sedimentation ditch .......................................... 224

5.3.8 Protection of Bank and Major Bed ....................................... 224

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5.3.9 Scouring Sluice ................................................................... 224

(1) Diameter of riverbed materials .......................................... 225

(2) Design of scouring sluice intake ....................................... 225

(3) Engineering of upstream portion of scouring sluice ........... 225

(4) Cannel width of scouring sluice ........................................ 226

(5) Design of upstream slope of cannel ................................... 227

(6) Design of downstream cannel ........................................... 231

5.4 How to use Goal seek ............................................................... 232

References ........................................................................................... 234

List of Authors/Experts/Editors/Coordinators ..................................... 235

1. GENERAL DESCRIPTION

1 .1 Introduct ion

Irrigation development, irrespective of scale i.e. small, medium or large, needs detail study and design. The study and design needs to be conductuted rigorously with minimum standard quality. Unless it results loss of money, wrong construction that inturn can result negative environmental impact (water loss, salinity, gully, conflict,etc.) and in general unsustainable development. The study and design phase plays decisive role in irrigation development. To enhance the quality of the study and design of irrigation projects, it is necessasry to attain minimum quality standard. For this reasson, it become necessary to prepare standard terms of refrence, guidelines and manuals.

1.2 Def in i t ions

In this guideline the following words and phrases are defined as follows: Headworks is a structure constructed across the river to effect local storage and rise water level locally to divert part or all the supply in to a channel. The height, quantity and period for which the supply is stored make it different from dam. In this guideline, headworks is defined as the facilities which divert water from a river (lake and marsh areas, excluding reservoir) into a canal for irrigation use.

[Reference] In the Multilingual Technical Dictionary of Irrigation and Drainage issued by the International Commission on Irrigation and Drainage (ICID), headworks is defined as "A collective term for all works (weirs- or diversion dams, head regulators, upstream and downstream river training works and their related structures) required at intakes of main or principal canals to divert and control river flows and to regulate water supplies into the main canal (s). "

1.3 Aim of the Guide l ine

The aim of this guideline is to show the basic and important way of design concepts and approaches of headworks for small and medium


Japan International Cooperation Agency (JICA) & Oromia Irrigation Development Authority (OIDA) The Project for Capacity Building in Irrigation Development (CBID)

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scale irrigation. Irrigation engineers are expected to design technically efficient, socially acceptable, economically viable and environmentale friendly headworks through this guideline for implementation. The guideline assists the engineer to follow a specific design procedure country wise to save time and materials.

1.4 Scope o f the Guide l ine

This guideline discusses the general procedures to be considered in the design and construction of small and medium scale irrigation diversion headworks. In Ethiopian, irrigation schemes are classified in to three types based on the area. Small scale irrigation scheme is less than 200ha,medium scale irrigation scheme is 200-3,000 ha and large scale irrigation scheme is greater than 3000ha (In Oromia 0-2.5 ha micro irrigation, 2.5-200ha small and the rest has the same classification with country level). Some of the concepts in this guideline need specific data. Material and equipment that are difficult to get at present situation of the country requires time, qualified and experienced human resource for accomplishement. In such cases, supplementary description is given in box on what to do at least in current condtion, how to estimate and design with available data without significant impact on the quality of project. The guideline takes into account all internal and external forces acting on a weir, how they affect weir and make sure the structure functions normal throughout its life time the purpose intended to serve by providing appropriate dimension to all elements. Technically, the guideline deals with all components of headworks including intake facilities, diversion weir for maintaining water level at the intake (except weirs with storage function), their related structures and operation and maintenance (O & M) facilities. The manual is also applicable for simple intakes without detail weir body.



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Fig.1.1 Layout of headworks

1.5 Components o f Headworks

Components of headworks are shown in Fig. 1.2. Standard figures of headworks plans and cross-sections used in the figure are shown in Fig. 1.3 and Fig. 1.4.

Fig.1.2 Components of headworks



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Fig.1.3 General Plan of headworks

Fig.1.4 (a) Standardized layout of headworks

Settl ing basin

Inlet / Intake Scouring

sluice gate

Devide wall (Guide wall )

Riprap (D/S)

Apron (D/S)

Apron (U/S)

Riprap (U/S) Wing

wall

Weir

Settl ing basin

Canal

Wing wall

Apron (U/S)

Riprap (U/S)

Inlet /Intake

Scouring sluice gate

Devide wall(Guide wall )

Weir

Riprap(D/S)

Apron(D/S)

Wing wall

River Protection (Gabion)

Hirna River River Flow



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Cross section

Fig.1.4 (b) Standardized cross section view of headworks

Wing wall

Wing wall


Intake gate

Weir

Devide wall (Guide wall )

Apron (D/S)

Settl ing basin



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Longitudinal section

Fig.1.4 (c) Standardized longitudinal cross section view of headworks

Apron (U/S)

Riprap (U/S)

Cut-off wall

Weir Apron (U/S)

Riprap (U/S)

Apron (U/S)

Riprap (U/S)

Apron (U/S)

Riprap (U/S)

Cut-off wall

Weir

Devide Wall




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1.6 Basic Considerations of Headworks In principle, headworks should have the necessary water intake facilities, maintain safety from any external forces, harmonize with flood control and water use of the river and be basically economical structure. The structures of headworks should be designed to secure the intake with enough irrigation water at all times, to release flood flow smoothly, to be safe from any external forces during flood and to maintain the original river functions identical to previous functions. The design of headworks must comply with relevant laws and regulations of the country.The design of headworks should be carried out by appropriate procedures with special attention to interrelated aspects as mentioned previously. At the basic design stage (Section 3.1), a general layout of each component of headworks is determined. Then a detailed design of each facility follows as Section 3.2. However through out the whole design procedures, feedback of intermediate results should be made if necessary to make each component harmonized as a whole. General design procedures of headworks are shown in Fig. 1.5



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Fig. 1.5 Flowchart for designing headworks



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2. BASIC DESIGN INPUT DATA Design of headworks needs input data that have good quality in different stages of the study. The design of headworks has to identify kinds of data, availability of data, quality of data in each stage. Headwork design not only identifies input data but also assesses impacts that have to be tested like effects of flood, upstream and downstream effects and others issues. In this section basic design input data and the reasons for the requirement of these data are briefly explained. This section helps the engineer to understand what they are designing, what to observe including impacts and other issues. The necessary basic primary and secondary data have to be identified and then to be collected. The collected data have to be examined. Planning is the primary work in designing i.e. what data to collect, how to collect, when to collect and where to collect. Data collection and examination has to be considered not only for design and construction phase but also for operation and maintenance aspects after construction. This is very helpful in securing the headworks to function properly, to economize the project during planning, design, construction and operation stages. Keeping this in mind, it is important to carry out efficient & effective planning considering the relationship between the basic design input data & design, construction and O & M. When basic design input data is not possible to be accessed with the same method as explained in guideline, it is necessary to get the information and data in another method. It is not recommendable to design without ungrounded information and data. 2.1 Data for River Conditions River conditions are the major factors to select design approach and parameters. These data shows the target river morphology before the design is done. This data includes river flow regime, river bed conditions, and saltwater intrusions of tidal rivers. 2.1.1 River Flow Regime River flow regime is concerned with the river geometry. This includes:

1. Discharge condition,



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2. Water level and discharge, 3. Sedimentations

(1) Discharge Condition Discharge condition is to understand and evaluate the river water flow situation. It is important to design intake discharge, design intake level and design flood discharge in Design Conditions explained in section 3.1.1. A flow duration table (or curve) shall be prepared based on the observation records of river discharge: 95-day discharge (probable discharge occurring more than 95 days in a year), Ordinary discharge (probable discharge occurring more than 185 days in a year), Low discharge (probable discharge occurring more than 275 days in a year), Base flow (probable discharge occurring more than 355 days in a year), Yearly average discharge(average of yearly daily mean discharge), Average discharge of irrigation period(average of daily mean discharge during irrigation period), Seasonable discharge variation(dry season discharge, wet season and rainy season discharge) When there are no records at the proposed site, a flow duration table (curve) should be prepared using records obtained from the stations close to the site on the same river or by transferring data from close to similar gauged catchments. < Supplementary explanation > Data is required for planning. For example, Irrigation engineers have to address the intended benefit of the area in consideration of the proposed river. In our country current condition, most of the rivers dont have recorded data for such condition. This will lead to collect information from local community (elders). This approach has a limitation because of the memory of the person with whom we discuss. This has to be corrected after identification, pre-feasibility and detail study made after measuring the flow at least weekly and monthly if possible daily by district and



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kebele experts. This data has to be supported by rainfall - run-off analysis (when there is rainfall data of the area is available). Please refer to Manual for Runoff Analysis in detail.

(2) Water Level and Discharge Rating curve is prepared by using long-term records of water level and discharge obtained from the data collected or analyzed from the previous section (1). In principle, discharge measurement should be applied by the current-meter. When the current-meter is not available, float measurement method can be applied. 1) Current meter method The current meter method is a method of calculating discharge by multiplying the flow area and the mean flow velocity observed by current meter. The follows are noted on measuring the flow velocity. To increase the accuracy of measurement, both the water depth and the mean flow velocity should be measured more than twice at the same cross section if possible. If the results of measurements are quite different, another measurement is required. In principle, the measuring points should be set at equal intervals on a measuring line crossing the river. However, the intervals should be reduced in a complex sectional feature or uneven velocity distribution (Table 2.1). In case of shallow (less than 75 cm) water depth, the one point method, measuring at 60% of water depth, should be applied. When the water depth is deeper than above, the two point method, measuring at 20% and 80% water depth, shall be applied. The water depth measuring line should be set at each boundary of dead water zone as shown in Fig.2.1 The discharge is the total of the numerical value obtaining by multiplying the mean flow velocity in the measuring point by the small cross sectional area that the mean velocity represents.



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Table 2.1 Interval of measuring points for water depth and water velocity

Fig. 2.1 Velocity measuring point in the current meter method

Photo 2.1 Measuring by current meter method

2) Float measurement method The float measurement method is a method of calculating discharge by multiplying the flow area of planning point and the mean flow velocity measured by the float. The measurement of flow velocity is calculated by dividing the distance among two transversal lines by the time which a float flow the section. The follows are noted on measuring the mean flow velocity:

Width of water surface (B)m

Interval of measuring points for water depth

(M)m

Interval of measuring Points for water velocity

(N)m

Less than 10 10~15% width of water Surface N=M

10~20 1m 2m 20~40 2 4 40~60 3 6 60~80 4 8

80~100 5 10 100~150 6 12 150~200 10 20

More than 200 15 30



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The float measurement method is used when the current meter method cannot be used,

The transversal line is set more than two, and the interval is not less than 50m basically. But the case of small and medium river, the interval can be changed to not less than 10m,

There is the planning point between the transversal line of upstream and the transversal line of downstream,

The measuring line of velocity of float is set in the direction of the flow of the river from the transversal line of upstream,

The relation between width of surface water of river and the number of measuring line of velocity of float is expressed in Table 2.4,

The point which the float starts is about 30 m upper from the transversal line of upstream basically. But the case of small and medium river, it can be changed for small rivers 5-10m, for medium rivers 10-20m,

The mean flow velocity at the planning point is calculated by multiplying velocity coefficient (Refer to Table 2.3) and average flow velocity calculated based on the results of measurement in each measuring line of velocity of float with time record.

Fig. 2.2 Layout of float measurement method

Approaching

sectionabout5

30m

Measurement

section

notlessthan1050m

Startline

Measuringlineof

velocityoffloat

Transversallineof

upstream

Transversal

lineof

downstream

Planning



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Fig. 2.3 Float type and stick type for float method Table 2.2 Selection criteria of float number and submerged depth of flow

Waterdepth NumberSubmergeddepthoffloat

0.7m 0.7m1.3m 0.5m1.3m2.6m 1.0m2.6m5.2m 2.0m5.2m 4.0m

Table 2.3 Velocity coefficient for float method

Number

Waterdepth(m) 0.7 0.71.3 1.32.6 2.65.2 5.2

submergeddepthoffloat(m)

Float

type

Sticktype

0.50 1.00 2.00 4.00

velocitycoefficient 0.85 0.88 0.91 0.94 0.96



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Table 2.4 Width of river and number of measuring line of velocity of float

Widthofriver 20m 20m100m 100m200m 200m

Numberofmeasuringlineofvelocityoffloat

5 10 15 20

< Supplementary explanation > The flow data is used for planning and design of headworks. Discharge is the basic data for planning irrigation capacity. When Dam and reservoir is planned not only minimum discharge but also annual data is required for design. Design discharge is needed for the design of stability and safety of headworks. Data about the level of water throughout the year is required to decide the necessary design and construct weir that will not to bring unacceptable water level raise. When there are no measured data discussion from local community has to be done through checking the flood mark, by taking survey of cross section and river slope. The discharge can be calculated by Manning formula: Q = A V Where:, Q: Discharge (m3/s)

A: Cross-section area (m2) V: Mean velocity (m/s) I: Hydraulic gradient (River bed slope ) R: Hydraulic radius (m) n: Coefficient of roughness (refer to reference)

This method doesnt help us to know the probability year of the discharge because it is only one time data. If rainfall data can be found, then it is possible to support the result by rainfall run-off analysis. Please refer to Manual for Runoff Analysis in detail.

(3) Sediment Load (if data are available) River sediment must be measured in order to fix the sill elevation of the intake, to decide the necessity of a settling basin, to design the scale and

21321 IRn

V



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frequency of sand removal from the settling basin. Sediment load can be divided into two categories:

Suspended load Tractable load (bed load)

Suspended load is part of the sediment transport suspending in the river water without touching the riverbed for a certain period. Tractable load is part of the sediment transport that is jumping on the riverbed or moving along the riverbed. When the sill of an intake is designed it has to be high enough to avoid the suspended sand entry to a canal.

In general, the distribution of suspended sand within a flow section is not even, so the following sampling method should be applied to obtain reliable data: During small to medium floods, sampling should be done along several measuring lines (at right angles to river flow) near the proposed intake site, At each measuring line, the distribution of flow velocity is checked in the vertical direction. The vertical section is then divided into several sections with similar flow velocity. Samples should be taken at the center of each section, Furthermore, each measuring line should be divided horizontally into several sections by flow velocity and samples should be taken at the center of each section, The quantity and grain size of the samples taken by the above method should be recorded and analyzed. In addition, the tractable sand should be estimated at several sections at 50 to 200 m intervals up to about 1 km upstream from the proposed site of the headworks. < Supplementary explanation > The sediment load is crucial for design of Scouring sluice gate and settlement basin. When there are no data and no time for measuring, observation of the river and information have to be gathered from local community (elders). In addition catchment data & experience, research,

Fig. 2.4 Uniform velocity

16Japan International Cooperation Agency (JICA) & Oromia Irrigation Development Authority (OIDA)

The Project for Capacity Building in Irrigation Development (CBID)


model results and literature can be consulted to make the information collected from the community more viable.

2.1.2 Condition of Riverbed

Data has to be collected regarding river bed conditions on condition of thalweg, river bed slope and river bed materials. (1) Condition of thalweg The river flood never flow uniformly covering all width of the river. The water collection of flood flows fluctuates. The river thalweg, or stream centerline, is formed after the floods. Therefore, the location of the headworks should be selected at a site where the thalweg is stable and located near the river bank where the intake is installed in the scheme. The scouring sluices are set in the thalweg. Study has to be carried out on the magnitude and frequency of floods, which may cause the movement of bed materials of average grain size. The maximum flow capacity should be estimated based on survey data on width, depth and gradient of the existing thalweg. Rivers that dont have stable thalweg needs investigation of the moving condition of the thalweg. When headworks is planned without weir type or when the river channel at the proposed location of headworks is wide and meandering, the following studies should be carried out: (a) The past changes of stream centerline based on old topographic maps, river trail maps and so on, (b) The characteristics of meandering river upstream and downstream of the proposed site of headworks, (e) The condition of rock foundation on the river bank and effects of scouring on the riverbed and (d) The influence of river structures such as piers of bridges. In order to understand these characteristics, a river survey of reaches upstream and downstream of the proposed site of headworks is required. < Supplementary explanation > Condition of thalweg is required for design of location. Particularly, it is important for design without weir type. When there are no data and no time for measuring information from local community has to be gathered. According to river situation, if the discharge is not including flood and is small and thalweg change is big, then it is better to make weir or change weir location. If the discharge is big and thalweg change is not a problem,



17

then this modification is not desirable.

(2) Riverbed slope

The assumption of aggradations and degradation of the riverbed are vitally necessary in order to determine the sill elevation of intake and to design foundation of headworks and related structures. For this purpose, stability and tendency of aggradations or degradation of riverbed should be studied carefully. Riverbed slope can be calculated by following formula. Iavg = Havg / L .(F. 2.1 ) Where, Iavg: Average Riverbed slope Havg: = 2 A / L (m) A: Individual area An = (Hn + Hn+1) / 2 Ln (m2) H: Accumulative height H = ELn EL0 (m) L: Distance (m) EL: Elevation (m) Fig. 2.5 Layout for Riverbed slope calculation < Supplementary explanation > The riverbed slope data helps for design and to estimate the river bed change. When there are no data and no time for measuring information has to be gathered from local community (elders). Actually, it is difficult to estimate the river bed situation and change, even if there is data. (3) Riverbed Materials Riverbed materials are the channel bottom of a stream or river where the normal water flow occurs. Sampling should be done at several points at 200 to 500 m intervals upstream and downstream of the proposed

No.0 No.1 No.2 No.3

H1 H2 H3

EL(m)

L(m)

EL1 EL2

EL3

(A0) (A1)(A2)

L0 L1 L2

EL0



18

headworks in order to get grain size distribution, specific gravity, maximum grain size (90 percent passing by weight) and average grain size (60 percent passing by weight). All sampling points should be indicated on a plan. The results of riverbed materials analysis are used to estimate the roughness coefficient of the river section, to presume riverbed change, to decide on the necessity of a settling basin, to design of scouring sluice and detachability of the silt. < Supplementary explanation > When there are no data or difficult to investigate during the design study time due to flood in the river, the investigation should be done at construction stage. Until that time, design can be done by using assumed values. 2.2 Studies on the Influence of Flood Control and Water Use Studies on the Influence of flood control and water use shall include flood control plans determined by the basin/river authorities (like Awash basin Authority). The study need to investigate the condition of upstream and downstream drainage, condition of existing river structures and present condition of river water utilization. 2.2.1 Flood Control Plans Flood control plans are required to be designed by the basin/river authorities in the river where the new headworks is proposed. The design of high water discharge, high water level, cross section and the annual schedule of the river improvement works, etc. have to be investigated for sustainability of the scheme. < Supplementary explanation > When there are no flood controls plans, design can be done freely or the plan can be made by engineers responsible for design of headworks. It is necessary to consider the situation of upstream and downstream to avoid damage to those areas. 2.2.2 Situation of Upstream and Downstream Drainage The situation of upstream and downstream drainage discharges into rivers during the ordinary flow should be investigated in order to discover the influence of rise of water level by weir upon upstream and



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downstream drain ability. For this purpose, it is necessary to study the function, scale and capacity of existing drainage facilities and the situation of in-flow to the river. < Supplementary explanation > The upstream and downstream drainage investigation may be easy but consultation with local community is necessary. 2.2.3 Dikes, Bridges and Other Structures

The scale and dimensions of the structure of foot protection, span, etc. of dikes, bridges and other structures should be investigated to clarify the influence of flood. < Supplementary explanation > Investigation on dikes, bridges and other structures are easy. Additional information has to be gathered from local community and administration. The result needs to get strict attention.

2.2.4 Present Condition of River Water Utilization Present condition of river water utilization such as irrigation, hydropower, fishery and others need to be investigated: The customary agreement on the construction of a weir, water balance

of upstream, downstream and flow restriction in the downstream should be clarified. If it is necessary, then the present condition and the customary agreement should be compared and cheeked,

Fluctuation of water level due to power generation and customs of rotational irrigation in the present water utilization condition, etc,

In the river, where the conservation of fishes are necessary, kind of fishes, their quantity, migratory season of fishes etc. should be investigated in order to use for the design of fish ways,

Other data related to current condition of river water utilization, such as utilization of underflow, should be collected.

< Supplementary explanation > Present condition of river water utilization item is used for planning and design. The investigation may be easy however discussion with the local



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community and administration has to be done. (Give attention only on applicable condition which are in the context of Ethiopia) 2.3 Geotechnical and Geological Investigations Geological and geotechnical investigations shall include the type of foundation material (e.g. rocks), chemical and physical condition of foundation material, relevant geologic structures, the thickness, bearing capacity, compressibility and hydraulic conductivity of overburden, the ground water level and its condition. Investigation on the foundations shall be carried out so as to establish the suitability of the site for the structure, determine geotechnical design parameters to design suitable foundation structures according to the existing foundation material, and also to design superstructures rationally considering ground conditions. The methods of these investigations and tests are described hereunder. < Supplementary explanation > This information is important for stability of the structures. The following investigations shall be carried out in consideration of the cost and the scale of project..

2.3.1 Drilling The purpose of drilling is to identify the type of riverbed material and stratifications of the ground, to take samples for geotechnical analysis, and to execute in situ tests in boreholes. To supplement geological information obtained by drilling, geophysical techniques like electrical resistivity test or a seismic survey, particularly for large-scale headworks located on a wide river, is often applied effectively together with drilling. Drilling sites shall be located at proper intervals not only at the centerline of the diversion weir, but also at the apron

GT-04

Fig. 2.6 Drilling image

Suction hose Water for drilling

Water swivel

Engine

Delivery hose

Stage

Guard fence

Drilling pump

Drilling machine

Winch

Casing pipe or Drive pipe

Wire rope

Engine

Drilling rod

Core tube

Reamer

Metal crown or Diamond bit



21

downstream and at the scouring sluices upstream and downstream of the proposed structure. It is desirable that drilling sites be set in a rectangular pattern so as to enable a geological map to be drawn. To stop underflow of water, the area of investigation should be extended sufficiently towards the abutments. For example, even when a floating foundation is preferable, confirmation of foundation rock will be useful for comparison of construction methods. Furthermore, in case of well or caisson foundation, drilling at each location of base is effective in saving construction cost by over sinking or cutting. < Supplementary explanation > Laboratory test can be done by taking pit investigation since drilling test is more or less costly for SSIP projects. This item is very useful for foundation and bank design. When it is not possible to collect data using drilling, pit opening manually can give important useful data. The cost of this investigation method is relatively cheap. The design has to be done by laboratory result for undisturbed & disturbed samples or estimating value from information of test pitting and other information. Resistivity test also can be an option. 2.3.2 Test Pitting Test pits allow direct observation and appraisal of the soil strata and geology of the foundation. Furthermore they enable sample collection (both disturbed and undisturbed) and in situ bearing capacity tests. The number of test pits shall be at least 3 (three) points (located two at banks and one at river center). Additional pits upstream and downstream along the center of weir axes helps. < Supplementary explanation > This investigation gives very practical data. If a foundation and bank investigation was not done during the study time, the foundation information has to be collected by test pitting before construction. The designer has to take into account not only data collected from test pit but also information from secondary data sources such as discussion with the local community.



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2.3.3 Bearing Capacity Tests The bearing capacity test is very important in design of headworks and when conditions allow the following methods can be applied, some of them described hereunder. (1) Standard Penetration Test (SPT) The standard penetration test is the simplest method to find the properties of ground. It is commonly executed together with rotary core drilling. This method drives a sampling tube (split-spoon sampler), outside diameter 51 mm and inside 35 mm, separable in two, driven at the head of the drilling rod with a weight of 6.35 Kg falling 75 cm. The number of blows required to penetrate 30 cm is counted, and called the N-value. A disturbed soil samples for mechanical analysis can also be collected in the tube during the test. The relationship between N-value and the relative density of the ground both for sand and clay by Terzaghi-Peck is shown in Table 2.5. In general, the standard penetration test gives accurate results for sandy ground. For clayey ground, the bearing capacity should be calculated using the results of an unconfined compressive strength test on samples.

Table 2.5 Judgment of N value

Sand ClayN value Consistency N value Consistency

0~4 4~10

10~30 30~50

More than 50

Very looseLoose

Medium Dense

Very dense

0~22~4 4~8

8~15 15~30

More than 30

Very soft soft

Medium stiff

Very stiff Hard

The relationship between the unconfined compressive strength qu and the N-value is empirically explained as follows.

)/(22.1 2mtNqu .... (F. 2.2)

Picture 2.2 Standard Penetration



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If the length of the drilling rod is long, this may influence the results of the standard penetration test. Correction of the measured N-value shall be made by the following formula: N= N (l20m)

N= (1.06 0.003l)N (l>20m) (F. 2.3)

where, N': corrected N-value

N: measured N-value

l : length of rod

In addition, the N-value is useable to estimate the internal frictional angle of sandy ground and so on which is useful for foundation design. Therefore, the N-value shall always be measured at the time of drilling. Long-term allowable bearing capacity in relation to N-value is shown in Section 3.2.4.



24

Fig. 2.7 Sample of boring log sheet



25

(2) Loading Test There are two types of loading tests: the plate bearing test and the pile head loading test. Both are to confirm artificial failure of the ground by loading up to ultimate bearing capacity. The loading methods include;

Direct loading, Jacking method using anchor frame, and Loading method using lever.

In the test, loading is gradually increased, and load intensity, time, and settling depth are observed. Then, a record diagram should be drawn to be analyzed. At the investigation of headworks, plate loading test is rare, but a pile head loading test is desirable after pile driving test in case of a pile foundation, especially of steel pipe piles. < Supplementary explanation > This is also very important for design. In this study, bearing capacity of the foundation can be known. There are two methods to do this standard penetration test and loading test. The later needs machine and it is expensive. When these tests are not practicable, design should be done using standard value of Table 3.11, other project data or text values. 2.3.4 Investigation on Riverbed Deposit Over a long period, the course of the main channel and thalweg of rivers may move due to fluctuation flood discharges. Riverbed deposits should be investigated based on geological data and information in order to prevent construction failure. Dewatering could prove to be impossible during excavation for the foundation because of the water path in the trail of the thalweg where stone and gravel layers exist, and/or the lower layer below the cofferdam is broken by piping. 2.3.5 Groundwater Investigation To assess the influence of construction of headworks on drainage of nearby farmland and the water level in wells, the ground water conditions should be investigated by a field permeability test, etc. < Supplementary explanation > This is very important for design and to select construction method. When these tests are not done but groundwater is observed, it is necessary to



26

consider dewatering at construction time.

2.4 Assessment & Planning for Construction Works

Assessment for construction works should be conducted on the following items, which are necessary for construction planning. Meteorology, surface water, groundwater, riverbed conditions Construction equipment and materials Transportation of equipment and materials (write down for materials

such as quality, quantity, location as well as its availability) Power source for construction Availability of adequate labor for construction Construction material quantity & quality

2.4.1 Meteorology, Surface Water, Groundwater and Riverbed Conditions Meteorology and surface water should be thoroughly investigated because of their importance in deciding the construction schedule. Based on the assessment on annual rainfall and river runoff, workable days and periods, water level and discharge of diversion channels, elevation of cofferdam, etc. should be determined. Flood runoff pattern should also be investigated in order to decide timing of interruption of works during flood. Data on runoff, water level and flow velocity of floods in the construction period of the cofferdam are essential to make a proper construction plan. For example, the construction method for a cofferdam will differ depending on the characteristics of water level changes whether they are almost constant or fluctuate widely, and if fluctuation is large, what the cycle period is. Rainfall and temperature, etc. should be fully investigated in order to estimate workable days and to make a construction plan. Especially in the cold areas, snowfall and temperature in winter season are important. Investigations on the condition of the riverbed deposit and groundwater which are helpful for construction are described in Section 2.3.4 & 2.3.5. < Supplementary explanation > This item is in consideration of construction schedule. These investigations may be easy. Construction time of headworks is better in dry season.



27

2.4.2 Construction Equipment and Materials Construction materials include ready mixed concrete, steel materials and timber, etc. The supply situation of factory products should be checked on the possibility to cope with sudden requirement for large quantities as well as urgent requirements by the change of design. Availability of construction equipment considerably influences the construction schedule and results. Investigation should be done on the availability of construction equipment with the function and capacity to suit the site conditions, availability of spare parts, and necessity of stand-by equipment during construction period. < Supplementary explanation > This item is in consideration of construction. In the present situation, it is easy to get equipment and materials for construction of headworks of small scale irrigation project. However, it is necessary to collect the information frequently, sometimes, price change happens, supply of material becomes difficult. Particularly industrial input. If it is necessary, supplying main industrial inputs like cement by the client should be considered. 2.4.3 Transportation of Equipment and Materials Local roads and bridges should be thoroughly checked in order to transport a lot of heavy and/or long equipment and materials including construction materials, factory products such as steel gates, construction machinery, etc. If necessary, roads or bridges have to be repaired or newly constructed. Otherwise, heavy and/or long equipment and materials may be transported disassembled. < Supplementary explanation > This makes construction material transportation easy. In the present situation, most of the site is far from asphalt road. Necessity of access road should be considered.

2.4.4 Power Source for Construction Considerable power demand may arise during the construction of headworks for a short period, and power source is generally required. Therefore, power source, location of distribution lines and diverting points should be investigated. When electric power cannot be obtained easily,



28

independent power generation or an internal-combustion engine may be used. < Supplementary explanation > In current Ethiopian condition, electric power can be generated from small or medium generators for some construction works. 2.4.5 Availability of Enough Labor for Construction Household survey to know the availability of productive forces; labor at a reasonable price, in which month and so on has to be studied for the intended work. Works also has to be classified for community participation. 2.5 Topographical Survey Topographical survey for headworks should be done on the following items.

Topographic survey Longitudinal and cross sectional survey at headworks Necessary survey for temporary works Collection of related topographic maps

2.5.1 Topographic Survey Topographic surveys are necessary not only for construction of the headworks itself but also for the planning and design of cofferdams and temporary facilities, etc. Curvature conditions of the river thalweg and elevation of the riverbed near the site should be surveyed as follows: Scope: 1.5 times upstream meander length

1.0 times downstream meander length

Scale: river upstream and downstream: 1/1,000~1/3,000 At the headworks site: 1/200~1/500 for detailed survey. The contour lines on the map should be drawn at 0.25-0.5 m intervals for the riverbed and 1.0 m interval for the other parts.



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2.5.2 Longitudinal and Cross-Section Survey Profile and cross-section maps of the river are used for the design headworks and for hydraulic calculations such as of backwater. Prior to the commencement of surveying, the benchmark elevation to be used for the planning of the headworks should be adjusted to the benchmark used by organization in charge of benchmark or basin/river administrator. During the cross-section surveys, the flood mark of both side of the river around headworks should be indicated and incorporated for hydraulic calculations. This can be done based on elderly flood mark and some safety factor. The extent of this surveying along the river courses at least has to be 500m to each side of upstream and downstream of the site of headworks but effects shall be seen to decide the effect of flood. Scale and interval: Three cross sections; one weir axis, two upstream and downstream at 20m upstream and downstream of the site of headworks has to be done. Every natural change useful for the structure design with its profile and cross section survey for the headworks has to be done. Drawing scale for cross section; horizontally 1/100~1/500 and vertically 1/100~1/200 is acceptable. Cross-section surveys of at upstream and downstream for 200 m interval and the drawing scale by 1/100~1/500 has to be done. Drawing scale for longitudinal profile; vertically 1/100 and horizontally 1/1,000~l/2,000 adjusting the scale of the topographic map. In addition, benchmarks are recommended to be set at both sides of the headworks and at least two in the range to be able to seen from headworks. Please confirm with EMA Bench marks located around it. 2.5.3 Surveys for Other Temporary Works and Compensations Surveying for topographic maps at a scale of 1/200-1/500 should be done for planning and design of cofferdams, temporary facilities for concrete works, construction roads, and excavation of aggregate materials. The contour interval should be the same as for the topographic survey mentioned above.

Picture 2.3 Cross-Section survey



30

Profiles and cross-sections required for estimation of earthwork volume are prepared following the standard longitudinal and cross section survey mentioned above. 2.5.4 Collection of Topographic and Other Related Maps Prior to field investigations and surveys, topographic maps and other related maps around the site of headworks should be collected in order to prepare schedule to make a preliminary design using a topographic map (scale 1/25,000~1/50,000) and present land use maps. However, the accuracy of these maps should be checked to minimize the mistakes. Map from Ethiopian Mapping Agency, topographic and other related maps issued by the national survey organization should be obtained. < Supplementary explanation > This item helps to for design consideration and construction. During construction, the bench mark used at survey time should be checked and strengthen or transferred to suitable place, make it concrete etc.

2.6 Data for Temporary Works The collection of hydro metrological data such as rainfall and river water level prior to construction is very important not only for the headworks construction but also for temporary constructions. 2.6.1 Annual Maximum Daily Rainfall and Annual Maximum Hourly Rainfall These data are necessary to estimate flood discharge and flood stage for planning of temporary works. Since the estimation will be done by statistical analysis, as much data as possible should be collected. The number of years required for the estimation varies depending on required provable value, but data more than 30 to 50 years is desirable. Further, the smaller the catchment area is, the more records on annual maximum hourly rainfall or annual maximum 10 minutes rainfall are necessary.

< Supplementary explanation > In our case this has to be done easily by area-velocity method for construction period months and the temporary diversion has to be decided in this context.



31

2.6.2 Annual Daily Rainfall Data of annual daily rainfall at least over 10 years are necessary to presume flow regime. When discharge is analyzed using a tank or other model (please refer to Manual for Runoff Analysis for the details of the tank model), data for the design year and 3 additional years are recommended to obtain stability of the model. 2.6.3 Environmental Impact Assessment The Construction of' headworks may give variety to the natural environment (scenery, ecosystem of animals and plants, etc.) and life of people living around. Therefore, the environment around the site should be investigated in advance. The results of the study from previous sections should be effectively utilized for planning, design, construction and maintenance of headworks. In addition, muddy water may flow into the river downstream during construction of the headworks and may cause adverse effects on inhabitants downstream and life in the water by mud increasing and siltation. Therefore, the degree of adverse effects on fish habitat in the river and other aspects (resting and feeding area of fish and birds) should also be investigated. Special care should be taken in selection of construction methods and periods which cause noise pollution and vibration, etc. to residents around the site. The routes for transportation of equipment and construction materials by large-sized dump trucks should be carefully studied to prevent noise and vibration effects.



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3. DESIGN OF HEADWORKS 3.1 Basic Design At this stage, appropriate basic design criteria must be established to ensure the structure that can perform the intended functions. 3.1.1. Design Conditions Design water intake discharge, design intake water levels, design flood discharge and design flood levels must be known for the design of headworks. The height of fixed structures and the future level at which the riverbed will be stable must be studied in order to set appropriate design conditions. (1) Design water intake discharge The design water intake discharge is the intake discharge at maximum design intake. The intake discharge that governs the weir design and the dimensions of the intake must be set considering the design maximum intake of the overall irrigation. In other words, Irrigation plan is decided according to beneficiary area, crop pattern, crop water requirment, the base flow and the amount of usable water from the river. Design water intake discharge is decided based on the following flow chart,



33

Fig. 3.1 Flowchart for determining design water intake discharge Please refer to Guideline for Irrigation Master Plan Study Preparation on Surface Water Resources. < Supplementary explanation > Base flow calculation at the point of intake can be conducted by watershed ratio method (Catchment area method) to design water intake discharge Based on the standard base flow, the base flow is calculated by watershed ratio method. The formula of this method is as follows, D = DsAp / Ar



34

D : Base flow (m3/s) Ds : Standard base flow in river basin of project site (m3/s) Ar : River basin (km2) Ap : Catchment area of the project (km2) Based on the base flow, the amount of usable water is calculated. The formula is as follows, W = D K Qdw W : The amount of usable water (m3/s), D : Base flow (m3/s), K : Coefficient of released flow for downstream ecology (0.70.9), Qdw : The existing design water intake discharge in the downstream of the

project (downstream demanded water). In case of using actual measurement data, it should be subtracted from base flow.

The design water intake discharge is decided based on the amount of usable water, The design water intake discharge should be within the amount of usable water and be considered from the relationship between beneficiary area, crop pattern and crop water requirement. (2) Design intake water level The absolute requirement for the design intake water level is given as the water level required to be secured at the start of the canal set in the irrigation plan. In case of intake on weir, the design intake water level must be the highest of the following: The total of the water level required to be secured at the canal start plus the total of the head loss between the intake and the canal start; or the total of the intake threshold height from the bottom the silt scouring sluice to prevent sediment inflow and the intake depth. In case of natural water intake, consideration is given to fluctuation of river water levels and base flow during the irrigation period occurring at the probability of once in every 10 years is adopted in Japan as the design intake level. Assuming that this base level flow coincides with the timing when the design intake volume is required, final design intake level is then determined so as to satisfy the requirements of and mentioned previously. As stated earlier, the choice of design intake level is not only related to the location of



35

the headworks, but it is also affected by loss of head that varies depending on the distance between the intake and the canal start, existence of a slit basin and the flow velocity of the intake (size of the intake). Therefore, the final decision must be made after a series of trial calculations and reviews of various factors. At the preliminary design stage, it is recommended to take 1.8 to 2.3 times of the intake velocity head (V2/2g) as the loss of head at the intake. < Supplementary explanation >

Design intake water level is used for consideration of the headworks position. The headworks position has to be located upper than this level.

(3) Design flood discharge The design flood discharge is decided in accordance with the river control plan. In other cases, the design flood discharge is decided on the basis of the known discharge capacity of the target river. If there is an experience of flood greater than discharge capacity of the target river in the past, then the design dischage is decided on the basis of the flood. 1) Decision of design flood discharge Headworks structures (the weir in particular being a structure across the river) need to be stable enough to withstand floods while at the same time not being a serious obstacle to disturb the flow of floods. The design flood discharge and the design flood level explained in the following sections are the basic values for calculations stability of external force and stress. They are also basic values in deciding the features of the structure. Design flood discharge has to be decided from the past flood discharge data, the maximum flood in the past based on flood mark or discharge capacity of the river by slope area method or flood (1/50 probability year) by run-off analysis.



36

F

ig.

3.2

Flo

wch

art

for

sele

ctin

g d

esig

n f

lood

dis

char

ge



37

Wherever the proposed siting of the headworks may form an obstacle to the normal flow of the river it is necessary to take measures to expand river sections in the vicinity of the proposed site. < Supplementary explanation >

There has to be sound Judgement on the basis of regionalization; condition of the site and in consultion with other design etc.

(4) Design flood level If the target river is under a river control plan or it is likely to be incorporated in such plan in the near future, then the design flood level is set in accordance with the target plan. In other cases, the design flood level should be the level at the proposed site when the design flood discharge flows. < Supplementary explanation >

Design flood level is an input for design of stability, safety and consideration for influence to upstream in the absence of river control plan. Design flood level is calculated from design discharge on condition of after headworks is constructed.

He = (Qd/CL)2/3 from Qd= CL He 3/2 He= Hd+Hav Hav= Va2/2g Va= Qd / L (h+Hd) He = Hd +(Qd)2/[L(h+Hd)]2 /(2g), Design flood level at weir = Hd + Elevation of weir crest

Where He: Total energy head (m) Hd:Design head (Water depth on the crest) (m) Hav: Approach velocity head (m) Va: Approach velocity (m/s)

Qd : Design flood discharge (m3/s) C : 1.7 Discharge coefficient L : Length of weir (m) h: weir height (m) g: 9.8m/s2 gravity acceleration



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Design flood levels at other place are calculated by Manning formula. However, it is possible to take some cross section and use non-uniform flow equation.

012 34

22

2

2

R

VnAdx

dg

Qdxdhi

Where i :Bed slope

h: Water depth (m)

X: Length towards the downstream along the canal bed (m)

Q: Discharge (m3/ s)

A: Cross-section area (m2)

g: Acceleration due to gravity (m/ s2)

: Coefficient of energy correction ( = 1.1, is used in general but

= 1.0 may be used for simple calculation )

n: Coefficient of Manning roughness

V: Mean velocity (m/s)

R: Hydraulic Mean Radius (m)

(5) Study of riverbed evolution To prevent the function of the headworks from deteriorating due to riverbed evolution, it is necessary to study future riverbed evolution in the vicinity of the proposed location of the headworks.

Sediment Weir

He Hd

h

Ha v=Va 2/2g

Qmax



39

1) Summary It is extremely important to predict the future stable riverbed to decide the height of the water intake sill.

First, it is necessary to investigate the existing condition whether there is balanced retrogression or aggrandization,

Second, it is necessary to assess the variation of the bed that would occur when other structures are constructed upstream or downstream or when river improvement is carried out.

2) Natural conditions of riverbed formation The natural condition of the riverbed is the result of the following three elements.

Horizontal distance and the head from the source to the estuary,

Topographical, geotechnological and botanical conditions and erosion characteristics of the river basin,

Characteristics of rainfall distribution, discharge, volume, wave pattern and its frequency.

Out of the above three conditions, condition is a significant restraint in that the surface level of the tributary river must always be higher than the parent river or sea surface level.

Fig. 3.3 Flow chart of riverbed formation downstream

The flow chart in Fig. 3.3 describes the conditions and above. Usually local characteristics such as those of rainfall distribution and collapse of sediment amount are extremely indefinite. But the effect of the occurrence probability of these phenomena is rather

Rainfall

Discharge

Channelerosion

Collapsedsedimentamount

Sedimentamount

Agroecologyorcharacteristicofthesourceofariver

Riverbedformationdownstream



40

significant in upstream areas. Due to the slow speed in the migration of sediment downstream, the amount of sediment below the point of collapse gradually attains an average value. The change in flow from supercritical flow to subcritical flow would also promote the balance of sediment amount, with the downstream riverbed formed by a sediment amount controlled by the flow strength. Furthermore, the variation of local tractional forces would cause variation in grain distribution of sediment downstream. In this respect, grain size distribution of erosion products as well as wear occurring to the material in the course of moving downstream must be taken into account. 3) Effect of river structures on the riverbed Construction of any structure in the river (such as headworks, check dam, or bed compaction work, the execution of river improvement work or quarrying of gravels) will cause secondary variation of the riverbed (not due to the natural phenomenon, but rather artificial causes). This results in the formation of a newly balanced bed in response to the new condtions of flow. Generally, such secondary formation of the riverbed has the following tendencies: Downstream of such a structures a lowering of the riverbed may

occur as a result of change in river hydraulics forming the riverbed. This trend will gradually extend to downstream,

The bed upstream of such a structure tends to rise, If the existing water intake weir downstream is demolished due to

integration of the water intake with other facilities, etc., then the settlement of the riverbed would occur. This settlement will gradually will be extended upstream,

If a large volume of gravel is quarried from the riverbed, then settlement of the riverbed will gradually occur both upstream and downstreams,

If the tractive force is changed due to shortening of the flow route by river improvement, then it will affect riverbed formation,

4) Points in investigating riverbed evolution(it has to be in Ethiopia condition and simple way) It is necessary to check the following points to understand the riverbed evolution:


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Technical Guideline for Design of Headworks - FC2oida2014.web.fc2.com/photo_gallery/07headworks.pdf · Japan International Cooperation Agency (JICA) Oromia Irrigation Development