Appraisal of design practices for river diversion structures

Contents

Abstract

1. INTRODUCTION
1.1. Problem Statement
1.2. Objectives
1.3. Methodology
1.4. Structure of the Thesis

2. LITERATURE REVIEW ON DESIGN OF RIVER DIVERSION STRUCTURES
2.1. Flow Diversion Structures
2.1.1. Diversion Weirs
2.1.2. Barrages
2.1.3. Intake Structures
2.2. Plan Form of Rivers
2.3. Bed Form of Rivers
2.4. Longitudinal Profiles of Rivers
2.5. Shape in Cross Section
2.6. River Hydraulics
2.6.1. Flow systems in natural channels
2.6.2. Flow theories and application

3. COLLECTION AND ANALYSIS OF DATA
3.1. Current Design Practice
3.2. Database Building
3.3. Investigation of Problems
3.4. Categorization
3.5. Selection of Prominent Problems

4. APPRAISAL OF DESIGN PRACTICES
4.1. Description of Major Problems
4.1.1. Main canal siltation
4.1.2. Downstream scouring
4.1.3. Damage on scouring sluice and intake gates & clogging of sluices
4.1.4. Damage on farmlands and canals
4.1.5. Damage on downstream apron
4.1.6 Excessive sedimentation of headwork
4.1.7. Upstream flooding
4.2. Analysis of the Design Practice
4.2.1. Design for siltation protection in main canals
4.2.2. Design provisions for reducing downstream scouring
4.2.3. Design of scouring sluices and gates
4.2.4. Design for protection of farmlands and canals
4.2.5. Design of downstream apron
4.2.6. Design for protection of head works from sedimentation
4.2.7. Design for protection of upstream flooding
4.3. Improvement of current design practice & alternative designs
4.3.1. Siltation in main canals
4.3.3. Scouring sluice and intake gates & clogging of sluices
4.3.4. Damage on farmlands and canals
4.3.5. Downstream apron
4.3.6. Sedimentation of head works
4.3.7. Upstream flooding
4.4. Issues for Further Research in Alternative Designs
4.4.1. Vortex Vanes
4.4.2. King’s Vane
4.4.3. Vortex Tubes
4.4.4. End sills
4.4.5. Downstream apron from natural materials
4.4.6. Inflatable dams
4.4.7. Chanoine wicket gate
4.5. Success Cases

5. CONCLUSIONS AND RECOMMENDATIONS

6. REFERENCES

7. APPENDICES

Abstract

Diversion structures are constructed to withdraw a portion of the stream flow for meeting the different water demands to the required place and quantity. These structures, though physically seem small piece of engineering work, the different considerations for hydrologic, hydraulic, sediment and structural analysis make their design complex. Improper consideration of these factors often ends up in consequential problems. Thus practicing engineers in the field are expected to make sure that this problem will not happen in the designs under very limited or no actual data in the field.

In this research work a case study of problems & comparative successes of 33 existing river diversion works in the Amhara region was assessed. It is tried to trace the causes of the major problems related to the design considerations and identify the knowledge gap between the current design practices and similar experiences in other countries. Besides, a GIS ArcView based database showing all features of the existing diversion structures is prepared for operational projects between the year 1971-2000. Among the many problems observed in the existing diversion structures problems of sedimentation at the headwork and main canal, downstream scouring, upstream flooding, damage on cut-off and apron, clogging of intake and scouring sluice outlets are found to be critical. The causes for these problems are attributed to the shortcomings in the hydraulic, hydrologic, and structural designs of diversion systems in addition to the improper operation and management of the schemes. These shortcomings in the design of the schemes are mainly problems in the delineation of backwater effect of the structure, the consideration for complete dissipation of energy of overflowing water, lack of knowledge for sediment consideration in fixing of sluice sizes & impact of sediment on the various components of the structure, the lack of consideration of the river morphology during the adoption of empirical formulae for design, etc. The major findings in the investigation and analysis of the observed problems revealed that the design of such systems is mostly adoptive, hardly standardised and do not consider local conditions. Different recommendations and alternative designs that are obtained from previous & recent research results are given with regard to reducing the observed problems & improving the current design practice.

1. INTRODUCTION

1.1. Problem Statement

Frequent drought coupled with ever increasing population & failure of rain fed agriculture is the main reason for the prevailing food shortage and extreme poverty in the country in general and the Amhara region in particular. Policy makers and practicing engineers who are engaged in food security activities have been looking for options to alleviate the problem.

Development of irrigation through river diversion and micro dams is considered as the most tenable solution for the growth of a country like Ethiopia with ample land & water resources. River diversion structures, owing to their relatively small investment cost, ease of construction, simple operation & maintenance are preferred to micro dams for irrigation. Based on this fact a number of such schemes are designed and constructed in the Amhara region in the previous years.

The diversion structures constructed so far, have suffered problems resulting from their siting ,hydrologic, hydraulic & sediment considerations in their designs and operation & management (Photograph 1&2). These complicated and interrelated technical and managerial problems have reduced their functional efficiency and increased maintenance cost. If this trend is allowed to continue, apart from the far reaching consequences of the problems, similar engineering solutions in the future will not be considered as dependable option by policy makers and beneficiaries despite its immense potential to alleviate the problem of food shortage and improve the quality of life of the people.

1.2. Objectives

This research is undertaken with a main objective of :

- Assessing problems on the existing river diversion structures and compiling set of recommendations for the design and operation of diversion structures in Amhara Region, based on local conditions.

And the specific objectives include:-

- Establish a systematic database on implemented diversion systems in the region and categorize them based on similar problems they face.
- Investigate the causes of failure for selected structures in the region with regard to their design from hydrologic, hydraulic, sediment transport, river morphologic and structural aspect and identify the most sensitive parameter for planning and designing of river diversion structures in the region & indicate future areas of research.
- Investigate selected successful structures in the region and draw lessons from their success.

1.3. Methodology

To undertake the research relevant data is collected from design documents, post implementation review reports and discussion with designers. The data sources are institutions like the Commission for Sustainable Agriculture & Environmental Rehabilitation ( Co-SAERAR), Ethiopian Social Rehabilitation & Development Fund (ESRDF) & Organization for Relief and Development in Amhara region (ORDA). From the design reports, the current design practice for river diversion structures is examined. The extent and frequency of problems on the structures is assessed from the post implementation review reports. Categorization of sites according to common problems they face is done using statistical analysis. This is done by type, basin wise and for each site. One scenario of problem interrelation is obtained from the categorization. Along with the interrelation the statistical analysis helped to come up with options for analyzing and prioritizing the problems. Few sites were visited to have a better insight on the severity of the problems (Photograph 1& 2).

1.4. Structure of the Thesis

The thesis is designed to comprise 5 chapters. Chapter 2 deals with literature review and experiences relevant to design of diversion. This is followed by summarizing the current design practice in the region, establishment of data base and statistical analysis in chapter 3. In Chapter 4 the current design practice is appraised in problem-by-problem approach technique and suitable recommendation for each problem is given. Issues requiring further research activity and the possible causes of success of some sites are also discussed in this chapter. The general findings of the research is given in the form of conclusions & recommendations in chapter 5.

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Photograph 1. Headwork & appurtenant structure sedimentation(Beteho, 1999)

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Photograph 2. Downstream scouring & sediment impact on weir body

(Gimbora, 1999)

2. LITERATURE REVIEW ON DESIGN OF RIVER DIVERSION STRUCTURES

2.1. Flow Diversion Structures

When water is diverted from rivers, depending upon the quantity of mean stream flow and demand for the purpose, a dam like structure with or without gates or water inlet mechanism directly off taking from the river may be built across the river where diversion is sought. The overall purpose is to ensure that all the flow can be diverted when water levels are low and also to ensure the evacuation of sediment from the conveyance system.

The operational aspects of river diversion works differ depending on the purpose. In hydropower and water supply development works, high stage flow abstraction is considerable and diversion for irrigation is limited to base flow abstraction. River diversion works and/or river intakes can be rather simple, such as bank/bottom intake or rather complex such as barrages or weirs. Different types of flow diversion structures exist, the choice among each type depends on the conditions of sediment, river morphology (plan, longitudinal profile, and cross section), topography, economy,etc.

Flow diversion structures can be grouped in to three main categories based on their hydraulic functions:

- Weirs
- Barrages; and
- intake structures without regulation works

2.1.1. Diversion Weirs

The diversion weirs differ from barrages by the mode of raising the water level to the off taking canals. In case of diversion weirs water the canal’s full supply level requirement is met by raising the height of the diversion structures itself whereas barrages utilize gates . Thus barrages are useful whenever variable head of water is needed to satisfy the demand and when the effects of backwater are pronounced in case of diversion weirs. Depending on the shape, the material and arrangement of appurtenant structures, there are different types of diversion structures.

- Rock fill weirs with downstream sloping aprons
- Vertical drop weirs
- Glacis type weirs

The selection among these different types of structures depends on suitability of river regime, hydraulic requirement, foundation, economy, ease of construction, etc.

Rock fill weirs: Rock fill weirs are made of dry stone rubbles and are the simplest type of construction and are suitable for fine sandy foundations (Figure 1). There are two approaches for the design of rock fill weirs. A theory developed by S.V. Isabch with the help of experiments on canals and natural streams to obtain information that are required to attack the problem of the design of this type of weir mathematically[Lelivskay S. 1959, Volume 4]. In contrary, Lorenz G. Straub states that by dipping large stone in fast flowing water, the weir can be built by making use of the hydraulic fill effect that contains particles of varying size [Lelivskay S. Volume 4].

Vertical drop weir: This control structure consists of a raised crest with vertical or slightly sloped downstream face, a horizontal floor and an end sill as energy dissipation mechanism (Figure 2).

Such types of structure have the following advantage over the other types

- The construction of such weirs is very simple
- Suitable for hard clay and consolidated foundations

The horizontal component of the flowing jet velocity is inexistent so that energy dissipation is mainly effected by impact. Due to these energy dissipation is efficient if the jump location is made to be stable. Their design and analysis can be done with the simplest as well as complicated methods available to date.

Glacis type weirs Though there exists different arrangements as the case may be, weir proper, scouring sluice, the divide wall, the impervious apron and protection works are considered to be common features of these type of structures (Figure 3).

Glacis weirs are suitable for permeable foundation and are usually designed with low crest and counterbalanced gates. The formation of hydraulic jump on the sloping glacis and the possibility of quantifying uplift pressure at key points of the structure are considered to be the main advantage of this weirs over the other type of structures.

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Figure 1 Typical section of rock fill weir

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Figure 2 Typical section of vertical drop weir with ponding by the crest

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Figure 3 Typical section of glacis weir

The analysis and design of vertical drop and glacis weirs involves two main activities, Hydraulic and structural.

a. Hydraulic design

Hydraulic design of weirs deals with the evaluation of surface and subsurface forces acting on the structure. This will assist in determination of the configuration of the structure and its various components that will be most economical with best functional efficiency.

In hydraulic design, the main features to be determined include

- The crest elevation/pond level
- The length of weir/waterway

Determining the crest elevation of the weir affects the water profile in two major ways:

- The height of the crest affects the discharge coefficient and consequently the head over the crest and the afflux (back water curve).
- The height of the weir affects the shape and location of the jump and the design of stilling basin.

The crest elevation of the weir proper has to be set higher than that of the under sluice section to attract a deep current in front of the canal intake and minimum flow may remain near the regulator. The elevation of the crest is usually decided with the requirement of the full supply level of the canal under minimum flow condition.

The following considerations should be given due attention while fixing the crest elevation of the weir [Baban R.,1995].

- If the entire flow of the river at low flows is diverted, the crest elevation must be set at a level so that the ponded water gives the required head to supply the canal with the design flow.
- If the minimum flow in the river exceeds the discharge of the off take canals, the crest level of the weir can be set lower than the river water level which is required to deliver the design flow in the canal to allow for downstream releases.
- The maximum/allowable upstream water surface elevation must also be considered in selecting the crest levels. The maximum allowable water level depends on the upstream river bank elevation and location of infrastructure. The upstream farmlands or settlements must be considered during maximum flood and the crest elevation to be chosen should ensure no risk of flooding to these properties.

The length of weir/ waterway should be designed from the consideration of the following factors:

- Afflux
- Upstream sedimentation
- Design flood, looseness factor1

The waterway and afflux are correlated. With increase of afflux waterway decreases and vice versa, hence a limit placed on maximum afflux shall limit the minimum waterway. A weir with a long crest gives a small linear discharge and hence the required energy dissipation per meter of the crest is smaller than what is needed for a shorter crest length.

For river regimes in a transporting zone, there are a number of approaches to determine the stable regime width of a river (table 3).Constructing a weir longer than the river width causes formation of islands/bars upstream. As a result the canal intakes will be cutoff from the river flow. The formation of the islands upstream of the weir reduces the effective length of the crest [Baban R,. 1995].

Hydraulic design of weirs/ barrages from subsurface flow consideration involves the determination of the uplift pressure and the effect of subsurface flow actions on piping or undermining the foundation.There are four widely known methods of sub-surface design theory (Table 1). The suitability of each method depends on the type of weir; the methods are listed as follows.

Table 1 Methods of hydraulic analysis

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* Flow net from trial and error is difficult for irregular geometry.

Bligh suggests the percolating water follows the base of the foundation of the hydraulic structures and the loss of head at any point is proportional to the length of the creep. This method that has been used for many years does not make any distinction between horizontal and vertical creep. Despite its inaccuracy in estimating the uplift pressure, its simplicity and high factor of safety make it popular to date.

On the other hand Lane improved Bligh’s theory by differentiating the vertical and horizontal creeps. According to Lane, the horizontal creep is less effective in reducing uplift than the vertical creep. Lane’s theory has never got any practical consideration and has never been used in any design and hence it is obsolete.

The Flow net method is similar to the previous two and assumes that the soil layer under the foundation is homogeneous. This method is a graphical solution of the Laplace equation for the steady state flow; the graphical solution is obtained by drawing flow and equipotential lines by trail and error or these days by the use of computer programs. Determining the uplift pressure in a foundation with an irregular base and several lines of cut off walls by this graphical method can be very time consuming and the flow net becomes complicated. The method represents the solution of the governing flow equation, the use of the method in designing small weirs is not common due to the inability to draw the flow net and the complications if irregular geometry is encountered.

The flow net analysis revealed that:

- The loss of head does not take place uniformly in direct proportion to creep length as stated by Bligh.
- Loss of head depends upon the shape of the foundations, depth of impervious boundary and level of upstream and downstream beds.
- When the equipotential lines are traced to be closer, the rate of loss of head will definitely be quicker and vice versa.

The modern theory of Khosla has the advantage of determining the uplift pressure as well as the exit gradient. The determination of the exit gradient is the peculiar characteristics of this method apart from its flexibility to apply it on irregular foundation.

Khosla’s theory has come up with the following points

- Safety against piping cannot be obtained by providing sufficient floor length as stated by Bligh, but can be obtained by keeping the exit gradient well below the critical value.
- Undermining starts from the downstream end of the impervious apron that progress upstream hence provision of downstream cutoff is indispensable.

To protect upstream scouring and undermining from the downstream end, the cutoffs with sufficient depths should be provided. The cutoff depths are determined from the maximum safe exit gradient or maximum scour depth considerations.

b. Structural design

Structural analysis consists of dimensioning of the various parts of the structure to enable it to resist safely all the forces acting on it. All external forces acting on a weir are the result of flowing water.

The structural analysis of vertical drop weirs consists of determining

- Static and dynamic water pressure
- Uplift pressure
- Soil reaction at the weir base
- Friction forces at the base which develop to balance the horizontal force
- Weight of the weir and water wedge

The analysis of the dynamic force that occurs at the first impact of the flow is not considered since the buildup of water is gradual. If a weir is designed to match the lower profile of a free water surface of the overflow, theoretically, the water pressure on the face of the weir should be nil, as it is the case in WES (ogee shape) weir. However in practice this is hardly the case since the water profile may not match the designed shape. Therefore the water curve downstream of the weir cannot be determined theoretically.

For structural design of glacis weir, the barrage slab can be designed as a reinforced concrete raft and utilizing the weight of the piers and groins to assist the glacis & apron slabs in resisting uplift. This design is known as the raft design, whenever adopted, care should be taken to provide some positive pressure or weight in excess of the total uplift pressure. Where the raft design is adopted, the reinforcement will be designed in accordance with reinforced concrete design practice. Where the gravity design is used, the slab is often reinforced to prevent temperature and shrinkage cracks.

2.1.2. Barrages

Seasonal rivers are characterized by sediment concentration of the flow and frequent overflowing of the banks. Design & construction of conventional type diversion structures with ponding by the crest on these rivers have the following problems:-

a. Silt accumulation upstream of the weir. The river bed eventually rises to the weir crest level and causes more frequent flooding and outflanking.
b. The river cross section becomes too small to dispose the river discharge in wet season
c. Severe impact of boulders on the crest of the structure

To avoid these problems and when variable discharge is required, a barrage (sometimes called an open weir) with movable gates of different nature and operational features can be constructed. The weir in this case can be constructed as a concrete downstream glacis or as a sharp crested weir (Figure 4).For the case of a sharp crested weir, it can be designed as [Bos, 1989]

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Where Pw is the height of the gate b is the clear span between the piers h is the head over the crest of intakes.

Control gates are part of these structures that enable ponding during dry weather flow. The control consists of different movable gates that can be operated either manually or automatically. Hydromechanical gates have operational & cost advantage over manually or electrically operated gates.Among the available types of hydromechanical gates, the chanoine weir gate has been commonly used in France where it was created [Lelivskay S., 1959]. The system has two alternative operations, a rapid one at an arrival of an unexpected flood and a slower but more comprehensive maneuver usually carried out before the annual flood. The first operation is entirely automatic assuring no danger of flood inundation. The principle of the automatic operation of this weir entirely relies on the level of water on the wicket (Figure 5 ). As long as the water level remains at the normal, the center of pressure is below the pivot, and the timber leaf that constitutes an element of the bearing curtain, remains in its upright position when the water level rises, the center of pressure moves upwards above the pivot, which causes then the wicket to turnover.

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Figure 4 Typical section of a barrage

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Figure 5 The chanoine wicket gate in barrage

In designing the chanoine weir, the main area of focus is to locate properly the center of rotation of the wickets. From the following free body diagram of a chanoine wicket gate The centroid of the hydraulic pressure that can determined.

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In this case, therefore the center of pressure is at one third of the height of the wicket a, for all other cases the position of the center of pressure varies between these two extreme points.

2.1.3. Intake Structures

Intake structures directly deliver the required flow to the conveyance system. These structures can be part of the diversion structure (weir or barrage) or can be constructed with out diversion dams as a lateral or bottom intake type. Siting of intakes requires attention with respect to the angle it makes with the axis of the flow & the plan form of the river. There are two types of intake structures:

a. Streambed intakes ( bottom intakes)
b. Side Weirs

a. Stream bed intakes

These types of intakes are suitable for steeply sloping mountainous rivers with irregular bed configuration and appreciable transport of boulders. This intake consists of an intake channel protected by trash racks on the bed of the stream (Figure 6 ). The racks have to carry the entire bed load including the passage of boulders over them. Trash rack maintenance will thus involve not only cleaning and declogging, but also periodic replacement of worn or damaged bars.

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Figure 6 Typical plan & section of bottom intakes

For a rectangular channel of depth y and width b, the spatially varied flow equation can be

modified for SVF decreasing discharge case as :

Abbildung in dieser Leseprobe nicht enthalten (12)

In case of bottom racks, the discharge through the rack depends up on the effective head. When the direction of flow through the rack opening is nearly vertical the energy loss in the process in negligible and thus the effective head on the rack is practically equal to the specific energy. Mostkow found that this is true for racks that are composed of parallel bars [Chow V.T., 1973 ]. In this case the rate of withdrawal through the length dx is expressed as :

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Where ε is the ratio of the opening area to the total area of the rack surface, Cd is the coefficient of discharge through the opening. For a rectangular channel, the energy equation yields

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Then the water surface slope

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Integrating this equation and setting initial conditions (i.e. at chainage, 1 x = 0, y = y)

yields the equation of the flow profile as:

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This equation is a very important equation for giving the knowledge of the length of rack for withdrawal of the main flow through the rack depending on the depth of flow y. at y=0, complete withdrawal is possible and length of rack will be:

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On the other hand, when the direction of flow through the rack-opening makes an appreciable angle with the vertical, the flow will impinge on the sides of the openings. This results a loss of energy and a change in direction of the flow from inclined eventually to vertical. According to Mostkow this is the characteristic of racks that are composed of perforated screen and that the corresponding energy loss is approximately equal to the velocity head of the flow over the rack. It may therefore be assumed that the effective head on the rack is equal to the static head or the depth of flow over the rack. The rate of withdrawal through the length dx of the rack may be expressed interms of the static head as :

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Accordingly the water surface profile after substitution and simplification will be:

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Integration of this equation and evaluation of the integration coefficient will yield two equations of the profile. The two integration coefficients C1 & C2 are related as:

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Evaluation of the two integration coefficients is based on the assumption that at equation for perforated racks is given as:

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The length of the rack can then be determined for the limit of withdrawal based on the depth y. For complete withdrawal, y = 0, the length of the rack can be given as (22) In both cases, the entrance to the reach of the rack may be considered as a broad crested weir thus the equation for broad crested weir will hold good.

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Thus the discharge of partial withdrawal from the main flow through the rack is

Qw =Q1-Q2

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Cd o varies considerably along the rack openings. Experimental values based on slope gives values of Cd as shown in table 2.

Table 2 Experimental values of coefficient of discharge for the given slope

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In general the value is higher for racks of perforated screens than for racks of parallel bars. The value is higher for horizontal slopes than inclined. The local value increases as the flow depth on the rack increases if the bars are parallel to the direction of the main flow, but decrease with the depth if the bars are in transverse direction. It is shown that Cd= F (D/ s ,SL, hE, flowtype) [Subramanya K.,1979] Where D- diameter of the rack bar; s –clear spacing of the bars in the rack SL- longitudinal slope of the rack

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b. Side intakes

Side intakes as a flow diversion structures are used when there is abundance in stream flow for the given demand. A side weir is an overflow structure formed in the side of a channel that allows lateral flow. The flow pattern over a side weir is fundamentally different from other weirs that are aligned normal to the approach flow. The flow over a side weir is a typical case of spatially varied flow and the main channel velocity affects the discharge over the side weir, even if this velocity is small.

The most important parameters to be determined are

- The discharge to be withdrawn
- The length & height of the weir

Raising the water level without obstruction to these weirs can be achieved by two means:

i. Concave-Convex Guide Banks: A concave convex guide bank can be constructed upstream of the intake so that water may be withdrawn from an artificially convex bend in the stream. This takes advantage of secondary flow effects that encourage the natural tendency for sediment to deposit on the concave bank. The height of the side weir can be determined from the consideration of sediment exclusion to the conveyance system. The crest length is fixed from the consideration of discharge requirement. For all discharges, the super elevation water profile should be drawn. From this the weir height can also be fixed to meet the lowest profile and the maximum demand.

ii. Channel Constriction: The response of free flow system to small disturbances is markedly different depending on the flow situation in the stream, i.e. subcritical or supercritical. In supercritical flow state, small disturbances go downstream considerably. In case of subcritical flow, energy considerations may suffice for analyzing changes. In contrary, supercritical state of flow requires additional consideration for flow analysis.

Channel constriction in supercritical state: Determination of the depth of flow after the constriction is the primary interest. The section of the channel in contraction depends on the method of contraction (Sudden vs. Smooth). In smooth contraction of channels with supercritical flow state, normal, transition & contracted sections are formed. Obtaining straight reaches in natural rivers is not always possible, hence for locating and designing side weirs using the experiments done for rectangular reaches needs to assume the channel as a rectangular one or contracting the channel in a rectangular fashion. De Marchi obtained the equation for side weirs located in a rectangular channel assuming the energy remains constant along the weir[ASCE, 1979].

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Where h is the height of the weir E is the specific energy y the depth of flow , and suffixes 1 & 2 refer to the upstream and downstream ends of the weir. Experiments by Subramanya & Awashty [ Subramanya K,.1979] have shown that CM = f( F1) and gave the following values fro CM For subcritical flow

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For supercritical flow with F1 > 0 CM = 0.36-0.008F1 (32)

The study and design of river diversion structures requires the knowledge of the following river features:-

1) Plan form
2) bed form
3) Longitudinal profile
4) Shape in cross section
5) River hydraulics

2.2. Plan Form of Rivers

The plan form of rivers is the most important parameter in the study and design of diversion structures especially in deciding the location and alignment of the structures. Besides, the proper design of river training & protection works that could be constructed along with the diversion system will be significantly influenced by the plan form of the river. The knowledge of rivers plan form helps in undertaking hydrologic and hydraulic computations while designing structures in the river under consideration. Design inputs for hydrologic and hydraulic computations can also be obtained from the information on the plan form of a river. Information regarding extent of meander & sinuosity can be obtained from detail investigation of the plan form of the river for use in determination of adjusted Manning’s roughness as suggested by W.L Cowan (table 12) and hydrologic computation. Locating side intakes or off take points of a diversion weir could also be accomplished by detail investigation of the plan form of the river and undertake further hydraulic computation for design of the various appurtenant structures.

2.3. Bed Form of Rivers

In the study and design of hydraulic structures, knowledge of bed form of rivers is significant in that: -

- The velocity distribution, resistance to flow, bed load and suspended load transport are intimately connected with bed forms [Garde etal ]
- The form roughness influences the stage discharge relationship and the depth for a given discharge [ Kinori etal Volume II, 1984]
- Resistance to flow in alluvial channels is largely determined by bed configuration

For sand bed streams, various types of bed forms are identified so as to appreciate the significance of each type of bed form in the foregoing hydraulic variables.

2.4. Longitudinal Profiles of Rivers

The dominant water-sediment process along a river system, which is the key element in selecting and designing hydraulic structures such as diversion weirs, can be assessed from an examination of river profiles. Longitudinal profile along the main stream of the watershed will often provide valuable information about its properties and the extension of its various parts with respect to elevation.

Examination of the longitudinal profile of rivers gives information on the nature of slope, physical characteristics of sediment and erosion & deposition phenomenon..

2.5. Shape in Cross Section

Stream channel cross sections, together with the boundary roughness data, the area and the shape of the sections determine what water levels will be attained with the given discharges. The preparation of the rating curve before and after construction of the structure will be significantly influenced by the cross sectional shape of the river. From these the necessary protection measure for backwater effect upstream and also tail water situations downstream could be undertaken.

The estimation of stable or regime width of alluvial streams attracted many researchers with several approaches to analyze the problem, To date none of them are widely accepted as a correct and universal approach. The researchers have tried to obtain relationships between hydraulic conditions and the cross sectional shape of alluvial streams.

Of the known empirical formulas, the regime theory approach for prediction of width is given in the following table

Table 3 Prediction of waterway regime channels

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Dominant discharge

Unlike regime channels, alluvial streams carry extremely varying discharges and sediment loads. Thus this wide variation in stream flow makes it difficult to choose a representative discharge in studying the stream characteristics.

Dominant discharge is that discharge & gradient to which a channel returns annually, at this discharge equilibrium is most closely approached & the tendency to change is the least[Garde etal 2000 ]. It is this discharge that determines the average channel dimension. For the same dominant discharge, a channel can assume different shapes and surface widths depending on the size of the sediment. The coarser the size of the sediment, the flatter is the semi-ellipse and greater is the water surface width.

Several authors tried to quantify this discharge in different areas; their summarized research result is outlined below

- For north Indian rivers, the dominant discharge was found to be equal to that of the bank full discharge
- Blench designated that the discharge that is equaled or exceeded for fifty percent of the time as the dominant discharge.
- The USBR defines the dominant discharge as the discharge that will carry the greatest sediment load of material coarser than 0.0625mm with respect to time.
- Ackers & Charlton define the dominant discharge as that constant discharge which would produce the same meander length as produced by the varying discharge
- Leopold & Madock prefer to use a discharge of a particular frequency of occurrence for comparing the hydraulic geometry of streams as dominant discharge.

Bed generative discharge

Another important concept with respect to influencing the channel cross section is the concept of bed generative discharge. According to Schaffernak the bed generative discharge is the discharge that transports the largest volume of coarse material [Garde etal 2000 ].

2.6. River Hydraulics

2.6.1. Flow systems in natural channels

With sufficient investigation of flow system in natural channels determining the different water surface stage and discharge conditions, effects on the river due to any form of channel constrictions, determination of abstraction volume for bottom and side intake structures will be possible .

Knowledge of the hydrodynamic process in bends is of significant practical importance especially from the point of view of prevention of silting, the choice of location for intake structures, dynamic stabilization of river morphology,etc.

The hydraulics of flow around bends is characterized by:

- Super elevation of the water at the outside of the bend due to the centrifugal force acting on the flow around the bend
- A strong downward current at the outside of the bend which will cause erosion
- A tendency that, at any location within the bend a water or sand particle at the bed moves usually stronger toward the center of curvature than a water or sand particle at the top
- An extremely distorted and complicated velocity distribution throughout the bend. The velocity increases greatly at the convex side owing to the centrifugal action of the flow.

2.6.2. Flow theories and application

The uniform flow, gradually, spatially and rapidly varied flow theories has a wide application for the design and analysis of diversion structures.

Gradually varied flow theory

The concept of gradually varied flow theory has a popular application in examining the response of the river system for obstruction to their natural flow conditions. The response of the river system for various obstructions can be examined through the help of the various types of flow profiles. Using the general dynamic equation for gradually varied flows, flow profiles for different discharges can be computed. Investigation of the effect of the backwater as a result of river obstruction has a wide application and importance in delineating flood areas and designing protective works such as retaining walls, dykes, levees,etc. The following methods are widely used for flow profile computation :

- Graphical integration
- Method of direct integration
- Direct step method
- Standard step method

Among the various methods of flow profile computation, the standard step method of flow profile computation gives practical and precise solution for natural channels.

Computer based models

These days, to avoid the routine computation of flow profiles, various models have been developed. Among these models, HEC-2 a general case computer model, as developed by the US Army Corps of Engineers is widely used.

This model was developed to calculate water surface profiles for steady and gradually varied flows in any form of a channel and subcritical or supercritical flow conditions.

Both subcritical and supercritical flow profiles can be estimated and the effects of various hydraulic and other structures in the over bank region are considered.

Spatially varied flow

There are two broad cases of SVF i.e. the increasing discharge and decreasing discharge. The salient principles that govern the increasing and decreasing discharge cases are different and the analysis of flow profile is different. The decreasing discharge type of spatially varied flow is exemplified by side weirs (side intakes) & bottom racks (bottom intakes) and the increasing discharge type is common in side channel spillways, road side and drainage ditches .

The decreasing discharge case of spatially varied flow can be considered as a flow diversion where the diverted water doesn’t affect the energy head. The use of energy equation is more convenient for solving the problem of SVF in decreasing discharge case.

Hydraulic Jump

Another important application of flow theories is the analysis of hydraulic jump. It plays a significant role in dissipating the excess energy of flow resulted from change of flow from supercritical state to subcritical state. The excess energy of water, unless dissipated properly will cause downstream scouring of beds and banks of channels, damage of river system and related consequences.

In designing a stilling basin for dissipating the excess energy of overflowing water using hydraulic jump, the following prominent features of the jump should be strongly considered:-

- Position of the jump
- Tail water condition
- The type of jump

The location of the jump is solely determined by the relative magnitude of the tail water depth with that of the sequent depth. In this respect there are three distinct cases worth considering.

Case I. Tail water depth y2’ = y2 (Sequent depth to y1) this condition satisfies the jump equation and jump is stable.

Case II. Tail water depth y2’ < y2 (Sequent depth to y1) the tail water depth is decreased as a result the jump will move downstream to a point where the hydraulic jump equation is satisfied.

Case III. Tail water depth y2’ > y2 (Sequent depth to y1). In contrary to case II, the jump will be forced upstream and may be finally drowned at the source to become a submerged jump. A rating curve that relate discharge Vs. tail water depth( tail water rating curve) and discharge Vs. Sequent depth ( jump curve) can be drawn and based on the relative position of jump and tail water rating curves, the location of the jump can be decided and the remedial measures could be taken accordingly.

Control of Hydraulic Jump by end sills

Hydraulic jumps can be controlled by end sills. For sill controlled hydraulic jumps the exact position of the jump cannot be determined analytically hence model studies are employed to determine the following relation established from dimensional analysis. Where F is the Froude number of the incoming flow h is the height of the end sill y1 is the approaching depth y2 is the depth upstream of the end sill y3 is the depth downstream of the end sill X is the distance from the toe of jump to the end sill In the model study, this position can be represented by the ratio between X & y2. The ratio is taken as a constant in each test, having a magnitude sufficient to ensure a complete jump. While designing the stilling basin, its length should be taken at least to be equivalent to X, however for economic reasons; the length of the basin can be made to be less than X, provided that the high bottom velocities at the end of the basin have reached a value considered safe for the downstream channel condition. On the basis of experimental data and theoretical analysis, Forester & Skirnde [Chow V.T., 1973] developed different diagrams that permit an analysis of the effect of a given weir/sill/ for

- Sill as a sharp crested weir
- Sill as a broad crested weir
- Sill as an abrupt rise

Diagrams were also developed to analyze the effect of a stepped weir/sill/ on the control of hydraulic jumps [Lelivskay S., Volume 3,1959].

Table 4 Summary of the different types of diversion structure

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3. COLLECTION AND ANALYSIS OF DATA

3.1. Current Design Practice

The design and construction of modern diversion structures in the Amhara region began in the early 1970’s. Foreigners were the main actors in the design of the schemes. Apart from these, the then Irrigation Design Team of the Ministry of Agriculture Rural Infrastructure Development Department was active in designing and constructing diversion structures for a command area up to 200ha. Recently Co-SAERAR and NGO’s like ORDA have designed and constructed a number of diversion schemes.

There is no code of practice or standard guide line for the design and operation of diversion schemes. The overall hydraulic and hydrologic analysis is based on practices derived mainly from Indian books that are the only readily available reference materials on the subject. The data on the design practice for the diversion schemes is, therefore, collected from the design documents availed from the above two organizations. The design practice is reviewed based on following five main themes

a) Site Selection
b) Structure selection
c) Hydrologic analysis
d) Hydraulic analysis for the design of the diversion structure components.
e) Structural analysis

These five themes will be discussed in detail as follows:

a. Site selection

Using 1:50000 topographic map potential sites are identified. Sometimes site identification is done in-situ. In any case a site is selected from the following considerations:-

i) The design engineer selects the diversion site from the consideration of its proximity to the area to be irrigated.
ii) The geologic & geomorphologic investigation will give information regarding the nature of the river reach and its suitability for the construction of the proposed weir ( frequently the vertical drop weir).

b. Structure selection

- Usually the field investigation focuses on selecting sites that are suitable for construction of vertical drop weirs. In all cases the structure selection is limited to this type of weir.
- In some cases ease of construction is taken in to account.

c. Hydrologic analysis

In hydrologic analysis of the study, the tasks frequently carried out include:

- Determination of peak discharge at the weir site
- Determination of the minimum flow ( and demand supply analysis)

To obtain this information, flow records as many years as possible should be available.

But in the projects reviewed so far, no river is gauged at or near the project locations to carry out the usual hydrologic analysis practice. Hence various formulas and thumb rules are used to obtain relevant information for the design.

- The minimum flow of the river is estimated using surface measurements by floating techniques. Usually this technique is exercised during each field visit
- Peak discharge is computed usually for a return period of 50 years, this is accomplished by either Soil Conservation Service (SCS) curve number method or the complex hydrograph method. The maximum value obtained by these methods is checked against the flood marks observed from observation and local information using the slope-area method.

d. Hydraulic Analysis for the design of the components of the diversion structures

The hydraulic analysis consists of determining

i) The tail water level
ii) Length of water way
iii) The flood and energy levels
iv) The afflux
v) The scour depth
vi) Shape of the weir
vii) Fixing the various dimensions of the structure

- The inputs for undertaking the analysis include design discharge, longitudinal profile data, the characteristics of the river .
- The tail water depth is determined using the slope area method; in all cases Manning’s equation is used.
- The linear water way is fixed using the formula that is claimed to be Lacey’s formula L = 4.75 Q & L = 4.83 Q. Here Q is the peak discharge & L is the theoretically required water way length for the incoming flow.
- Regime scour depth using Lacey’s formula is calculated to fix the upstream and downstream cutoff depths and determination of velocity head.
- Afflux, in all cases is assumed to be unity
- The Downstream total energy level is obtained by adding the high flood level before construction to the velocity head. The upstream total energy level is fixed by adding the afflux to the downstream total energy level. The whole purpose of doing these is to fix the upstream high flood level due to the construction of the weir. The new upstream flood level is fixed by subtracting the velocity head from the upstream total energy level. Usually a retrogression of 0.5m is used to fix the downstream high flood level. Subtracting the retrogression from the previously computed downstream high flood level will give the new downstream high flood level.
- The results of the foregoing computation are then used to size the various dimensions of the structure ( crest level, upstream & downstream cutoff levels and height of guide walls )
- Two approaches are employed in the design of the under sluice:

i) The most common one is designing the under sluice or scouring sluice to safely dispose the maximum of

- Either 10% of the peak discharge
- Or 5 times the canal capacity

ii) Based on the degree of submergence of the weir the discharge over the weir proper is determined from the discharge formula for broad crested weir under submerged condition

- The top & bottom width of the weir in case of broad crested weirs is determined from the formula as given by Bligh. The bottom width of the weir is fixed on two conditions. One when the water is at the pond level and the other when water is overflowing. The maximum value of the two cases is used. The other consideration is the use of balanced moment for stability. The Overturning moment M0 is made to be equal to the resisting moment Mr for both overflow and ponded condition. The maximum value is taken.
- The length of the impervious apron is fixed using Bligh’s creep length theory for two conditions. The first is when water is at pond level and the other is overflowing. The maximum of the two considerations will be taken. The length obtained in this way will be checked using Khosla method. The thickness of the impervious apron is also fixed using Khosla method.
- Dimensions of both intake gate and scouring sluice gate are designed by considering free orifice flow condition for intake gate and submerged orifice condition for the scouring sluice gate.

e. Structural analysis

Once the dimensions of the structure are fixed using the hydraulic analysis, its stability is checked using structural analysis. The structural analysis focuses on

i. Stability analysis of the weir body: safety against sliding, overturning and tension is checked for two conditions, one when water is overflowing and the other at pond level. Of the two conditions, the maximum value of safety will be taken.
ii. Stability analysis of the guide walls: The stability of the guide walls will be analyzed against overturning, sliding & tension after height & depth is determined from hydraulic analysis.
iii. Stability analysis of the divide wall(s): The length of the divide wall (s) upstream or downstream is fixed from intuition. Once the length and height is fixed, a trial top width is assumed and its safety for overturning, sliding and tension is checked for the worst condition of loading, i.e. silt load at pond level from the weir side and no water on the sluice side.

3.2. Database Building

Obtaining organized data on existing diversion structures is a tedious task in the country in general and the Amhara region in particular. Data on existing structures may be lost as a result of placing the information in scattered way which was witnessed during data collection stage of this thesis work.

Systematic database building on the existing structures is considered as one output of this research. Apart from availing data for future research and development activity, establishment of systematic database enables one to:-

- Observe changes in design practice as a result of gain in experience
- have a brief overview on what has been done in the subject
- Assess the potential and status of utilization of the resources
- Undertake performance monitoring and evaluation of the schemes
- Undertake research on any specific issue on the subject

The systematic database developed on MS Access data base format ( Appendix 1) and the features in the database (Appendix 2) are placed in the GIS ArcView map of the region (Appendix 3). The GIS ArcView map consists of the attributes of seventy (70) existing river diversions (table 5& 6). Almost all diversion structures are used for irrigation development and all regardless of size are included in the database. The attributes are obtained from the design reports and other documents, which are made available during data collection. It is to be noted that the presence of incomplete attributes is common, mainly due to absence of design reports. It was very much time consuming and difficult to undertake inventory of the current status of all the available (70) projects. Due to these limitations, the inventory reports obtained for 28 diversion schemes is taken as the basis for the problem identification, categorization and analysis, apart from visiting five (5) sites physically that are located by the main road side.

The attributes for the existing structures and their current status are designed from the following consideration.

Table 5 Attributes and purposes for existing diversion structures Attribute

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Table 6 Attributes and purposes for status of existing projects

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3.3. Investigation of Problems

No scheme is presumed to operate as designed; hence occurrence of problems in hydraulic structures is not uncommon. Diversion structures are designed for a design period of 50 years to operate without major damage. Most of the inventoried sites are constructed with in 5-15 years. Hence the observed problems cannot be attributed to their design period. The irrigation practice is not completely halted by the observed problems. But problems that hinder ease of operation, efficient utilization of resources and that cause far reaching consequences are given due consideration in this research. The magnitude of each problem is described in the subsequent sections. The observed problems with the respective frequency of the 33 sites are listed in the following table.

Table 7 Observed problems and their frequency

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The above problems represent the most serious ones that reduce the performance of the scheme the least and total exclusion of utilization at most. From the data base it is learned that construction of end sills is not common. This seems the sole reason for having such a low frequency of damage on end sills.

All of the foregoing problems will pose difficulty on efficient operation of the schemes in the short term and overall failure in the long term.

The extent of damage in each site is listed in table 8. By putting some threshold value, lessons could be drawn from those sites with minimum percentage of problems. The designs of the sites will be reviewed to see if there was any peculiar design or site considerations to reduce the problems.

Table 8 Percentage of problems in each site

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3.4. Categorization

It is worth mentioning that minimizing problems, if not avoiding, is possible first by understanding the main causes.The complex interrelation between the problems is the main difficulty for making a clear demarcation among the broad categorization. Appraisal of the current design practice requires knowledge of state-of-the-art worldwide and the original research outcomes and their suitability for local conditions. Even if no exact similar conditions exist, research outputs and experiences can be inferred to local situations from general similarities.

Based on these assumptions, the state-of-the-art of diversion structure designs in the region is related to the following main problem categories.

I. Problems related to site selection
II. Problems related to structure selection
III. Problems related to hydrology & sediment consideration
IV. Problems related to hydraulic design of weir and components.
V. Problems related to structural design of components
VI. Problems related to scheme operation

Table 10 Problem categorization

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3.5. Selection of Prominent Problems

For selection and analysis of the most prominent problems, three distinct approaches were considered. The results in the foregoing tables are the basis for comparing each option.

Option 1. The prevalence of the various problems exhibited in each site

Option 2. The frequency of each problem in the sites located in each basin

Option 3. The frequency of the same problem in the sites under consideration

Option 1. This option would have been the best had selecting a site (s) consisting most of the listed problems be enough for analysis of the given problem. However, doing so may preclude the analysis of some problems that could have been important interms of efficient operation of the scheme and causing subsequent damage. Due to these limitations this option is not considered for selecting problems for analysis.

Option 2. From the viewpoint of associating observed problems with basin characteristics and identification of issues that require further research activity, this option is quite good. However the data position did not allow the use of this option.The main reason is unequal distribution of the existing sites inventoried. Due to these limitations, this option is also found not to be suitable for the purpose of the research.

Option 3. The frequency of the problem in the sites under consideration tells the severity of the problem & the existence of knowledge gap either in the design, construction or operation of the schemes. The more the frequent the observed problem is, the greater emphasis it requires. Problems ranked in this manner will show the true magnitude.

Besides, the option will allow treating all the problems exhibited regardless of being observed in the most damaged site or not. As there in no single site where all the problems exist. Due to these advantages, this option is considered for selecting the most serious problems.

Based on the relative frequency of the problems in the 33 sites and their interrelation, the following problems are selected for analysis. This ensures analysis of most of observed problems than analyzing problems in the most damaged structure.

Table 11 Frequency and rank of selected problems

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4. APPRAISAL OF DESIGN PRACTICES

The current design practice is appraised by taking the designs & observed problems on the structures on one side to compare with design practices & research results worldwide. By doing so, shortcomings and knowledge gaps in the current design practice could be identified. Based on these, recommendations and alternative designs for improvement of the practice could be forwarded.

4.1. Description of Major Problems

4.1.1. Main canal siltation

This problem is observed in all of the surveyed schemes. It is categorized as a problem that may result from improper site selection and/or sediment consideration and/or scheme operation. The extent of main canal siltation is not quantified in the survey reports. However, in some sites main canals are abandoned from the immediate off take point to some distance downstream Beteho,Sewir[Co-SAERAR,2000 ]. Farmers in this area dug a new canal starting from upstream of the river and join it to the old canal. Due to severe siltation the water level in the canal rises and submerges the canal berms and later overtop the main canal Silala,Inchike [ORDA, 2003 ].

River water often carries sediment particles ranging in size from big boulders to minute suspension. The crest level of the intake sill is fixed to be higher than the riverbed level or the scouring sluice sill level. By doing so, a considerable volume of sediment will be excluded from entering to the main canal. Despite these, Main canal siltation could occur due to either one or the combination of the following factors

- Malfunctioning of scouring sluice
- Change in river course upstream and direct intermingling of river flow
- Negligence in dredging the desilting/settling basins
- Inadequate design

Due to these or other causes, main canal siltation is the most serious problem that may lead to the reduction of the capacity of the channel and abandoning the scheme.

4.1.2. Downstream scouring

This problem is categorized under improper structure selection for the site or hydraulic design or both. Downstream scouring, as observed in about 67% of the surveyed sites encompasses the scouring of the riverbed beyond the protections provided. In some sites, the riverbed is scoured for more than a longitudinal distance of one kilometer Gimbora, Gilgelmena, Grumbaba [Co-SAERAR, 2000] & Inchike [ORDA, 2003]. The main cause that may lead to downstream scouring with regard to structure selection is attributed to the suitability of the particular structure for the river slope and flow conditions. Design and construction of the conventional type of vertical drop weir (which is most common in the region) in steep reach of the river, will cause a hydraulic jump whose energy will not be dissipated with in the provided cistern. Improper hydraulic design results from errors in fixing the position of the hydraulic jump, arranging favorable conditions for the jump at the required/presumed reach, analyzing the governing flow condition for defining the property of the hydraulic jump, etc.

4.1.3. Damage on scouring sluice and intake gates & clogging of sluices

Damage on the sluice gate, intake gate and clogging of sluices is observed in 64%, 36% & 15% of the sites under consideration respectively. This figure possibly indicates how the schemes are performing under difficult condition. The under sluice and intake gates in all cases are vertical lift gate type. Whenever users are not aware of the importance of gates, they never operate it whenever floods come. As a result the impact of the sediment/flow may be beyond its structural capacity for its smooth operation. Damage of under sluices gate means problem in operation that will result in clogging of the under sluice pocket, indefinite siltation of main canals, upstream aggradation/bed level rise/, possibility of outflanking, change in river course and subsequent damages to farmlands, canals, etc. The cause for the problem of gates can be attributed to scheme operation or improper structural design or both. The clogging of sluices may also result in damage of both the intake and the scouring sluice gate; hence problems in this regard are interrelated. The problem of under sluice clogging is categorized under improper hydraulic design , hydrology and sediment consideration.

4.1.4. Damage on farmlands and canals

This problem is categorized under improper site selection and hydraulic designs of components. It is observed that 42% of the sites under investigation possess this problem. The farmlands downstream are susceptible to damage if the river changes its course or if the energy of the flow is not sufficiently dissipated. This is witnessed in case of Gimbora & Sewir diversion schemes whereby 48ha & 15ha of farmland is damaged as a result of the change in river course [Co-SAERAR, 2000]. Out of the five schemes that changed their course due to any reason, all are found to damage the farmlands and canals downstream. Besides, out of 23 cases of downstream scouring 13 (or 57%) caused damage on farmlands and main canal running along the bank of the river.

The driving force for change of river course and damage on farmlands is attributed to the incomplete energy dissipation of the flow resulted from improper hydraulic design of energy dissipation system. Damage on main canal occur if it runs along an erodible banks with insufficiently designed bank protection works.

Besides, damage on farmlands and canals will occur if the conventional types of diversion structures are located in steep slopes.

4.1.5. Damage on downstream apron

This problem is observed on 39% of the surveyed sites and it is attributed to improper hydraulic design. This arises from poor knowledge of the impact of sediment on the structure and the erosive power of overflowing water. Impervious floors are designed to reduce the surface flow action that causes scouring due to unbalanced pressure in the hydraulic jump trough and uplift pressure of the subsurface flow. In the diversion schemes where this problem prevails, abrasion of the impervious floor is commonly observed and reinforcement bars are exposed thus causing damage on the downstream cutoff. This is observed in Gimbora river diversion scheme where its concrete apron is damaged to a considerable thickness [ Co-SAERAR,2000]

4.1.6 Excessive sedimentation of headwork

Headwork sedimentation in the context of this investigation is to mean the overall submergence of the structure. Headwork sedimentation could be initiated by clogging of the under sluices or the constriction of the natural river cross section. Clogging of the sluices in turn could be resulted due to excessive sediment load for its size. In this investigation about 33% of the sites suffer from this problem. Overall submergence is occurred in some cases Beteho, Sewir [Co-SAERAR,2000].

The complete submergence of the structures doesn’t stop irrigation practice. In such circumstances even if the off take gate doesn’t work, farmers are able to easily divert the flow to the main canal. But main canal siltation is severe. This problem is attributed to improper structure selection for the given site. This problem will be worse if the conventional vertical drop weirs are constructed in the flat reach of rivers characterized by enormous amount of sediment transport.

4.1.7. Upstream flooding

As a result of obstruction in the river system, a tendency of backwater is developed upstream. In alluvial streams with flatter slope, this problem is common. When the backwater extends upstream to a considerable length and lower elevation is encountered along the length of the backwater, the possibility of change in river course is high. Due to this, damage on main canals, guide walls at the diversion section, sedimentation of the headwork could encounter. In this investigation it is learned that about 33% of the structures exhibit the problem of upstream flooding. The problem is attributed mainly due to improper hydraulic computation (delineating the extent of backwater) and structure selection (suitability of structure to the given site).

4.2. Analysis of the Design Practice

4.2.1. Design for siltation protection in main canals

The design reports of the sites were referred to check what provision has been considered to reduce main canal siltation. The scouring sluice or under sluice is considered as the main device for excluding sediment in the main canal. The scouring sluice is designed to allow the maximum of either 10-20% of the peak flood or 2 to 5 times the main canal full supply discharge. Provision of settling basins is also considered in some of the sites. The design of the settling basin, as referred from the documents has no clear procedure- rather the size is fixed from intuition.

In case of river diversion for irrigation purpose scouring sluices will be opened during the rainy season as there is no demand for water. Apparently the sediment will be excluded from entering to the main canal. During low flow periods, demand for water increases thereby requiring the full or partial closure of the scouring sluice. By doing so a deep pool in front of the intake will be created thus relatively sediment free water will be allowed to the main canal.

The scouring sluice may not be operational all times due to any of the causes as discussed. Under this condition, it is apparent that this device has no or little importance in assisting main canal siltation. Hence the designs should have considered additional sediment exclusion devices that could work under all operating conditions of the sluices. Thus the drawbacks of the current design practice in this regard is the inability to look for all options of sediment exclusion from the main canal apart from the scouring sluice which is closed for most of the times during operation periods.

4.2.2. Design provisions for reducing downstream scouring

In all of the designs unless the riverbed is rock, vertical drop weirs are designed and constructed. To ensure complete energy dissipation, the impervious apron whose length is fixed from seepage considerations will be checked whether it could accommodate the hydraulic jump with the calculated length. If the calculated length is equal or greater, no special provision is made. The length of the apron, as computed from seepage analysis may be sufficient for accommodating the hydraulic jump. Unless the relative magnitude of tail water and sequent depths are determined for the geometry, there is no guarantee of confining the jump with in the given impervious apron. From theoretical analysis, it is possible to identify the location of jumps for different discharges. But due to variability of discharge for the given time period and the probabilistic nature of this discharge the possibility of jump dislocation is high and cause downstream scouring. Thus the design practice in this regard lacks checking the relative magnitude of tail water and sequent depths for all discharges to design the impervious apron accordingly.

4.2.3. Design of scouring sluices and gates

Scouring sluice and intake gates are designed to regulate the flow of water or sediment in the canal respectively. The design of scouring sluice and intake gates focus on dimensioning the width and the height, that is crucial for regulation of flow. During the rainy season, the under sluice gate will be opened hence the danger of damage is minimal. However, if unexpected flood comes or gates are forgotten to be opened by the attendants it will be damaged by the sediment or by the flowing water. Similar to the scouring sluice gates, no structural designs is carried out for intake gates. As a result the spindles and the skin of the gate gets damaged due to excessive accumulation of sediment in the scouring sluice pocket. The gates are designed to be operated manually hence the operating skill of users matters to avoid damage. This scenario shows the importance of structural designs in gates that are equally important to their hydraulic design.

The design of the under sluice pockets considers - 10-20% of the peak discharge or 2- 5 times the canal capacity; and the maximum of the two is taken to fix the dimension of the sluice in the submerged weir formula [Baban R., 1995].

Scouring sluices are basically intended to exclude sediment from the system, however the two considerations by no means guarantee excluding of sediment or avoid clogging. Both approaches overlook the sediment transport phenomenon of the river in which the scouring sluices are meant for. Consideration of the diameter of the largest sediment plays a very important role in determining the sluice opening dimensions. However if the computation results in wider area of the scouring sluice reduction of the approach velocity and deposition will be apparent.

To avoid this problem, distinction should be made between the different size ranges of sediment where this approach should be applicable. If the larger size of the sediment is equal or slightly less than the opening obtained by this approach, the sluice opening will be incapable of passing the sediment through it. The current design practice of designing scouring sluice and intake gates lacks checking the ability of the gates to resist the impact loads under extreme condition (i.e. impact of flowing water and sediment during floods). The shortcoming of the current design practice for designing the sluice opening is its inability to incorporate sediment characteristics as a parameter.

4.2.4. Design for protection of farmlands and canals

Farm lands located downstream of the structures and canals running along the bank of the river will get damaged if the energy of the flow is not completely dissipated or the river changes its course. The adequacy of the design consideration to avoid this problem is discussed in section 4.2.2.

4.2.5. Design of downstream apron

Impervious aprons are designed for resisting the uplift pressure & impact/abrasion that is caused by subsurface and surface flow actions respectively. The designs utilize the Bligh’s theory for fixing the length of the apron and the uplift pressure at key points to determine the thickness of the apron using the theory of Khosla. The length provided thus will be checked whether it is able to accommodate the hydraulic jump or not. This design consideration may be able to provide the required safety from uplift pressure and hydraulic jump points of view. But the impact of boulders and the limit of the erosive velocity on the apron couldn’t be quantified using this approach. Thus the need to protect the impervious apron from impact of boulders and abrasion is essential for highly boulder-laden rivers hence the current design practice lack this consideration.

4.2.6. Design for protection of head works from sedimentation

In all inventoried sites regardless of site condition, diversion is accomplished by raising the crest of the vertical drop structure. The design approaches for all components of this diversion weir is the same for all river morphology condition. The crest length of this weir is fixed using Lacey’s water way formula. The effect of scouring sluice on initiating sedimentation and determination of its size as discussed in sec. 4.2.3. To satisfy the waterway requirement of the incoming flood a crest length that may be greater than the natural width of the river will be constructed. This encourages the formation of point bars and islands upstream which leads to upstream sedimentation through time. In contrast, narrowing the cross-section will cause rise in upstream flood level that may lead to sedimentation and change in rivers course. This generalization in the design consideration is believed to be the gap of the practice and the main cause for the problem.

4.2.7. Design for protection of upstream flooding

To prevent upstream flooding and outflanking, the designs consider utilization of permissible afflux equivalent to one meter which is commonly recommended for diversion structures [ Baban R.,1995]. The waterway requirement of the incoming peak flood is also fixed in all cases using Lacey’s regime width formula. Moreover, some designs consider construction of levees beyond the retaining walls upstream based on the rating curves. In most cases, the slope area method is used to draw the rating curve and this will be checked with the flood marks. However the peak discharge (and the corresponding stage) obtained by the slope area method and the flood mark cross checking exhibit significant differences. In some instances, the stage of the peak discharge obtained by slope area method varies with that of the flood mark by as much as 200%. For such conditions there is no clear-cut solution to take which discharge for basis of design.

Due to construction of the diversion structure, the flow pattern in the river system will be changed. One significant change is the increase of upstream water level which results the development of backwater. The extent of backwater effect should be known by determining the water profile upstream of the weir. The current design practice lacks consideration of flow profile computation for marking the extent of flooding and designing flood protection works. Besides clearly specifying the waterway requirement of the peak flood is difficult under the current design practice since there are two quite different widths as obtained using Lacey formula and the available natural width.

Table 12 Summary of analysis

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4.3. Improvement of current design practice & alternative designs

4.3.1. Siltation in main canals

The current design practice can be improved if the forthcoming recommendations and alternative designs are used. The recommendations and alternative designs are forwarded for two cases

I. Complete closure of scouring sluice gate/ direct intake from rivers

Complete closure of scouring sluice may exist when

a. The clogging of under sluice due to sediment
b. When water in excess of the demand, it flows over the structure
c. In case of barrages, where ponding is achieved by gates

Under these situations and in case of simple intake structures main canal siltation can be reduced by providing sediment extractors in the main canal itself. Three types of sediment extractors are known to exist. Among the three; settling basins are designed and constructed in some of the sites where inventory is made hence no further discussion on it is made. The other type is the tunnel type extractor which is suitable only for large canals. For the problem under consideration, Vortex tubes; another type of sediment extractor can be installed in the main canal. Vortex tubes (Figure 8) are devices that are constructed at the main canal head regulator in a vortex platform. Since sediment load increases from surface to bottom, a vortex tube that consists of a pipe partially cut at its top and immersed in a platform slightly higher than the canal bed, will assist removing the sediment through the escape canal which is provided with gate upstream.

This device requires availability of sufficient head & slope to operate the vortex and transport the excluded sediment respectively. This devices were tested by HR Wallingford hydraulic laboratory in different small scale diversion systems in Tanzania and Asia and are found to be effective

The design parameters for this system are:

- Diameter & length of the tube and number of tubes
- The slit opening (the cut section of the pipe)
- Hydraulic condition on the vortex platform (Froude number, velocity, head loss)
- Location of the platform downstream of the intake
- Length of the platform
- Height of the platform
- Angle of vortex tube with canal axis
- Hydraulic properties of the escape canal (geometry, roughness and bed slope)

Based on the experiment done at the Hydraulics Research laboratory at Wallingford , Procedures for design of vortex vanes are developed. In this research, the procedures are converted in to flow charts (Figure 9) and a FORTRAN program( Appendix 4) is developed to make the design of vortex tubes easily. This program is tested for field problems and agreement found with the manually computed result.

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Figure 8 Vortex tube as sediment excluder

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Figure 9 Flow chart for design of vortex tubes as sediment excluder

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II. Partial closure of the scouring sluice gate

For partial closure of scouring sluice gate, the minimum flow should be substantial to provide the demand otherwise complete closure is a must. For this case sediment accumulation around the intake structure and the main canal is reduced. However, this depends on the efficiency of the scouring sluice on excluding the incoming sediment laden flow. In the designs of the sites under consideration, no special treatment for this condition is done.The sediment to be excluded using this arrangement will join the river system just downstream of the structure. The main assumption for the design and provision of these devices is the mode of sediment concentration from surface to bottom. Hence the following devices could be considered as sediment excluder devices in conjunction with the scouring sluice

1. Kings Vane

H.W. King [Baban R.,1995] has introduced this device in 1954 as a sediment extractor in irrigation feeder canals. Recent researches in the subject shows the adaptability of this device in diversion structures if steady state flow exists in front of the canal intake. The device consists parallel vertical walls in the shape of a circular arc constructed in the sluice bay ( Figure 10).

The following parameters needs to be determined for designing this device

- The optimum clearance of the vane walls from the intake and divide wall
- The radius of the arc
- The over all width ( number of vanes)
- Spacing, Height, thickness of vanes
- Range of discharges

Based on different widths of the off take canals, different dimensions (design parameters) of Kings vane is given as follows in table 13 [Baban R.,1995].

Table 13 Parameters of design for Kings vane

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Figure 10 Kings Vane as a sediment excluder

2. Vortex vanes

Edmund Atkinson at the HR Wallingford has conducted theoretical and laboratory analysis to assess the suitability of vortex vanes as a sediment exclusion device.

From the experiment and field verification undertaken in different intakes in Philippines [Atkinson E.,1993] it was found to be effective in significantly reducing sediment entering the canal intake. Vortex vanes are simple devices that consist a pair of diverging vanes situated in the river bed immediately upstream from an intake(Figure 11). The upstream vane is approximately at riverbed level and the downstream vane is set higher so that the sediment-laden flow near the riverbed is trapped between the vanes. The vanes are aligned at an angle to the approaching river flow. This ensures the momentum of the approaching flow both generates a vortex to prevent sediment deposition and transport the sediment along the vanes and away from the intake. Vortex vanes are suitable for off takes with no diversion weir.

Vortex vanes have three advantages over other excluders [Atkinson E.,1993]

- They are cheap to construct
- They can often be incorporated at an intake without the need to alter existing structures
- Do not require a head drop for their operation (due to this, vortex vanes can be sited at intakes with no weir across the river, where conventional techniques of hydraulic flushing is not possible)

The design parameters of the vortex vane are

- The elevation of the upstream vane
- The elevation of the downstream vane
- The vane spacing
- The length of the vanes
- The position of the axis of the vane
- Depth of scour between vanes

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Figure 11 Vortex vane as sediment excluder

Based on the theoretical and laboratory analysis, the following design considerations for vortex vanes are made [Atkinson E.,1993]

Table 14 Parameters of design for vortex vanes

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4.3.2. Downstream scouring

To reduce problems of excessive downstream scouring the following alternative designs & recommendations to the improvement of the design practice are forwarded.

a) Recommendations with regard to hydraulic calculation

- For the given specific type of diversion structure, owing to uncertainty and hydrologic variability, Hydraulic jump calculation should be done for all stages/discharges. Tail water & jump rating curve conditions be compared so that to obtain relative magnitudes of tail water and sequent depths (three distinct cases of jump). The most sensitive relation should be used for considering additional energy dissipation arrangements.
- End sills, and drops with the design features as discussed in sec. 2.6.2, should be provided to confine the jump with in the required length. Various types of sill (sharp crested & broad crested weirs, stepped weirs) abrupt drop and rise in the channel floor can be used for the control of hydraulic jumps. The function of the end sill is to ensure the formation of a jump and to control its position under all probable operating conditions. The provision of end sills as a broad crested weir has and stepped sills demonstrated its usefulness in the rehabilitation design of some schemes Gimbora & Gerado[Co-SERAR 2000 ].

b) Recommendations with regard to structure selection In most sites where the river slope is steep and. vertical drop weirs are constructed, downstream scouring problem is observed. Due to short downstream length of the structures, the hydraulic jump will be formed at the horizontal cistern. Stability problem of jumps & inefficient energy dissipation are the main characteristics of flows in vertical drop weirs. To avoid these problems of vertical drop weirs the following recommendations are given:-

- The design of weirs with substantial downstream slope ( glacis weir with slope of 1:5 to 1:10) will encourage the formation of the jump at the glacis and reduces the possibility of the jump traveling downstream.
- The design and construction of the conventional type of diversion weirs in steep sloped rivers ( S>10%) should be avoided. The design of bottom intakes, barrages with different type of gates (to be discussed later) should be practiced.

4.3.3. Scouring sluice and intake gates & clogging of sluices

The problem of gate damage is two-dimensional either resulting of improper structural design or improper management/operation. The operational problem can be resolved either by training and teaching the users. Designing simple type of gate for easy operation could also reduce the problem. The structural design should consider:-

- Making use of automatic gates where operation is undertaken without the help of manpower (the details is discussed later)
- In the design of skin plates and their associated stiffening members, sediment loads in addition to the hydrostatic load shall be considered for a height equivalent to the height of the bed form in case of sand bed rivers and maximum diameter of sediment otherwise[Lewin J.,1995].

To avoid clogging of sluices, three approaches can be considered during their design

i) Considering the diameter of the largest particle size to fix the dimensions
ii) If considering the largest particle size results in a wider area sluice and reduction of approach velocity, the type of structure should be changed and sediment extractors be constructed in the main canal
iii) Consider the bed generative discharge as it is the more appropriate parameter that relates sediment transport with flow rate.

During estimation of peak discharges, there is no clear consideration of sediment transport, hence using this discharge for fixing under sluice size which is entirely meant to exclude sediment seems not appropriate. If taking the peak discharge is a must for fixing the size of the sluice by the above approach, it will be logical to use the discharge that causes bed motion in the river, this discharge is the bed generative discharge as introduced by Schaffernak [Garde etal 2000]. The use of bed generative discharge as the sole criteria for this purpose could be justifiable. Schaffernak [Figure 12] prepared three curves to determine the bed generative discharge as follows

Plot 1- Discharge as ordinate & Frequency of occurrence of that discharge as abscissa

Plot 2- Discharge as ordinate & Rate of sediment transport as abscissa

Plot 3- Discharge as ordinate and product of Frequency of occurrence of that discharge & rate of sediment transport as abscissa.

The maximum value of plot 3 gives the bed generative discharge.

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Frequency(DF) Rate of sediment Transport(QT) DF*QT

Figure 12 Determination of bed generative discharge

The definition of dominant discharge by the USBR is similar with that of Schaffernak’s bed generative discharge except the later has no restriction on sediment size.

4.3.4. Damage on farmlands and canals

Incomplete dissipation of overflowing water is the main cause for downstream scouring , change in river course and damage of farmlands and canals. All recommendations towards reducing downstream scouring will hold good to ensure safety against damage of farmlands and canals. Besides, the following recommendation will assist in improving the rating curves which gives information about the stage downstream.

Theoretically the rating curve at the a control along the river is influenced by the hydraulic characteristics of all cross sections in the river downstream of the point to the point where rapids falls or similar controls occur. Actually the effect of the hydraulic characteristics diminishes for cross sections further downstream and gradually becomes negligible. Thus the zone of influence being that length of river channel below which the hydraulic properties of the channel will have a negligible influence on the rating curve at the stilling basin position. For a channel with known values of arbitrary constant depth D and bed slope S, this zone of influence (L) can be determined as the ratio of this depth and slope of the river.

This criterion may also be expressed by stating that the survey must extend downstream to the point where the bank elevation is lower than the average channel bottom elevation at the stilling basin site. The zone of influence for deep rivers with flat gradient is appreciable and for steep and shallow streams the zone of influence is lesser.

The spacing of the cross sections must be chosen in such a way that:

a. The most efficient use can be made of the number of cross sections taken
b. Whether the cross sections are representative

To meet the first condition it must be considered that cross sections farther downstream have a smaller effect on the rating curve. An equal spacing throughout the length L is thus not the most efficient way of obtaining the desired information. The following procedures are recommended for undertaking survey along the river[Vic Gallay,2003].

- The total length L is divided in to four sub reaches as 0.1L,0.15L,0.25L, and 0.5L. In each sub reach a sufficient cross section should be taken to describe the average conditions of the reach
- Cross sections should always be taken perpendicular to the direction of flow, this sometimes creates problem in meandering stream channel.

Provision of appropriate energy dissipation mechanism that is described in section 4.3.2 is also important for safety of farmlands downstream of the weir site and bank protection structures and canals running along with in addition to selecting appropriate site for constructing the structure.

4.3.5. Downstream apron

The prime recommendation for protecting the concrete apron from the impact of boulders is to locate the common type of diversion structures on the alluvial stage of the river. In this reach the slope is not substantial to increase the kinetic energy of the flow/boulders and impact on the impervious apron. If doing this is not possible other options should be looked to protect the apron from impact.

To protect the concrete apron from the impact of boulders, one recommendation could be to mimic the natural riverbed condition on the apron. This requires lowering the bed level of the apron so as to accommodate the natural bed materials on top of it. In natural streams of varied bed material sizes, a given value of the velocity, V, will move the smallest particles while causing the larger stone to arrange themselves in a stationary over layer protecting the underlying sediment. This phenomenon is considered to be adopted on the impervious apron. The resulting condition is known as armoring [Bouvard M.,1991]because the bed appears to be armored or paved with large stones. The flow moves the material whose critical entrainment discharge has been exceeded but leaves the larger material.

The phenomenon of armoring is most likely to occur if the bed material contains a wide range of sediment sizes and if the critical shear stress for the largest elements is seldom exceeded during the year.

For the armour to be destroyed the following relations should be valid

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Where S is the slope of the river in percent and q the linear discharge in m3/sec/m & d90 is the diameter corresponding to the larger boulder.

The material to be laid on the apron is fixed from this consideration. Doing this ensures any impact on the apron will be undertaken on the armored surface instead and problem minimized. Besides, this solution has the advantage of roughening the surface and energy dissipation of the flowing water.

4.3.6. Sedimentation of head works

To protect excessive sedimentation of headwork and appurtenant structures the following alternative design & recommendations to the improvement of the design could be considered.

Basically, any site is suitable for diversion of water; however each site has its relative merit for the structure to be constructed. Hence the site to be considered for diversion should be examined from the relation between sediment-water-river slope and suitability of the structure to be constructed.

Excessive headwork sedimentation is the problem of alluvial rivers where the slope is flat. This slope initiates deposition of sediment around the scouring sluice and the headwork even at peak flood condition.

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Two possible causes are described to result upstream sedimentation, thus from the relation in the above table, it can be learned that upstream sedimentation is initiated mainly by flatter river slopes than clogging of the under sluice .For river formations with substantial bed load, the design of two alternative structures could be recommended:

i) Barrages with movable gates
ii) Direct intake from rivers
In the different arrangements of the alternative structures, sediment control in canals could be accomplished by utilizing the vortex vane device as discussed in section 4.3.1.

I) Barrages with movable gates

Barrages with different types of gate can be considered for raising the level of water during low flow. Here the gates will be removed during peak flood condition and installed during low flow seasons where demand for water increases specifically for irrigation purpose. The visible problem with this recommendation is the operation of the gates that cause tremendous problem if left unopened during unexpected floods, as it has been common in the scouring sluice gates. For ease of operation, a barrage with the following two-control mechanism is recommended:

a. Use of Inflatable dams as a gate

This is a recent technology that can be adapted to reduce the problem of headwork sedimentation. . This technology has been used for irrigation, hydropower, water supply, recreation, etc and a number of inflatable diversion dams constructed in Indonesia, Japan, Australia, Kenya, Philippines and the USA since the mid 70’s. This alternative design utilizes the use of a dam consisting of flexible tube fixed along the foundation (barrage crest) across the river. The rubber can be inflated to the required height using air or water pressure and deflated when not required. The material of the dam is rubberized fabric reinforced rubber membrane similar to that used in car tires. The rubber can be securely anchored to concrete foundations. Such type of dams are available in length up to 185 m & height 3m. These dams have the following unique benefits

- Low cost (interms of operation and installation): they are low cost and can be used to span in excess of 100m crest length without piers/intermediate support & it has low installation and operation cost (quick interms of time and low power requirement to pump air or water)
- Low maintenance cost: They are corrosion resistant, the membrane is resistant to the effects of the weather, the only metal parts in contact with water are the anchor plates & bolts which can be supplied in stainless steel or carbon fiber for aggressive environment
- Abrasion resistant: Less susceptible to scouring from sediment compared to concrete or steel structures
- Impact resistant: Absorb shock and vibration with their inherent flexibility and energy absorbing properties. Steel gates are poor in resisting impacts.

The FIN structure allows the fabric to Lay-Flat when deflated. This prevents damage from debris or ice. The lay flat characteristic eliminates the bulge at the end of the deflated body, which is prone to serious vibration & abrasion. It also permits passage of debris. The utilization of this dam/gate requires the knowledge of the shape under hydrostatic and hydrodynamic condition (water is overflowing) along with the media of inflation (air/water).

b) Use of hydromechanically operated gate

Electromechanically operated gates are expensive and manual operation is also seen as a source of problem whenever attendants forget to open it during floods. Hence the use of hydro mechanical gates is indispensable. The chanoine weir gates as discussed in section 2.1.2 are ideal for use in the situations under investigation.

II) Direct intake from the river

In alluvial streams carrying big boulders and substantial flow condition, dam less intake from the river gives adequate solution to avoid sedimentation on the headwork. The side intake of Kulfo river in Arbaminch state farm could be a good demonstration for this recommendation. However a thorough investigation and analysis on stream flow variability and peak demand analysis is necessary. If the stream is characterized by bends, locating the intake at the bend will give better performance interms of sediment exclusion from the main canal system and providing dependable level of water at all times or artificial bends could also be created ( Sec. 2.1.3). In straight reaches, the river channel can be contracted to raise the water level so as to provide sufficient head for the intake. Side intake consists of weir with slightly raised crest from the river and analysis of flow is essential to determine the various design features of the intake. The design of side intakes along with different methods of raising the water level with out constructing regulation across the river is discussed in detail in Section 2.1.3.

4.3.7. Upstream flooding

The recommendation to avoid upstream flooding in the designs can be given with respect to three considerations.

a) Hydrologic consideration

The procedures followed for computing the peak flood stage using the slope area and flood mark observation is a widely accepted method and there is no comment. However during the use of Manning equation in the slope area method, the way of determining the roughness coefficient needs to be revised. This is considered to be one source of the discrepancies between the different stages obtained from flood mark observation and the computation using slope area method. In using the Manning formula for this purpose, the selection of the proper roughness coefficient, n, is a relatively difficult task and a slight change often results in a widely varying estimate.

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Table 15 Corrections for roughness coefficient

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These equations respectively provide information on the extent of meander &irregularity to be used for corrections for roughness coefficient in the above

b) Hydraulic consideration

The hydraulic computation involves fixing the waterway for the peak discharge and it is commonly done using Lacey formula which is developed for alluvial rivers in regime. The correct type of Lacey’s formula is not clear, the two different types of this formula that are widely observed in the literatures

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The use of the above formula alone yields a discrepancy of 0.08m per 1m3/sec. The other requirement of this formula is the channel should be in regime condition; however the equation is employed in all river morphologic conditions which deviates from the considerations made in the original theory. In addition, the hydraulic computation overlooks the computation of flow profiles to considerable distance before and after the construction of the structure.Due to this the correct stage of the flood upstream cannot be exactly marked and protection measures designed accordingly. This formula is based on computing the waterway requirement for the dominant discharge not the peak discharge as it is seen in the design reports. Hence even for regime channels the concept of dominant discharge should come in to figure for the design of the waterway.

Flow profile computation, is widely done using the standard step method manually and these days a software developed by the Hydrologic Engineering Center (HEC-2) could be used for easy computation of the backwater profile. Once the flow profile is computed, the necessary protection measure could be taken accordingly.

b. Structure selection consideration

Diversion weirs with major ponding by the crest are of little help in reducing upstream flooding. Since the original water level is raised by the height of the weir, water level rise by that much could cause substantial rise upstream especially for flatter river slopes. Hence barrages that are discussed in sec. 4.3.6 will also be applicable for this case. The design of bottom rack intake structures is useful to avoid this problem apart from avoiding problems on damage of weir proper.

The use of bottom rack type diversion is common in hydropower schemes in the polar zones where ice is a problem and in the mountain regions of Nepal, Papua New Guinea and Canada [Vic Galay 2003] .

The design and analysis of bottom intake racks involve the determination of

- Bar clearance & area of trash rack opening
- Bar length
- Slope of trash racks
- Flow profile computation

The design of bottom racks can be done according to the following flow chart (Figure13).

A FORTRAN program is developed for undertaking the design easily ( Appendix 5).

Figure 13 Flow chart for the design of bottom racks

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4.4. Issues for Further Research in Alternative Designs

4.4.1. Vortex Vanes

- The determination of the optimum height of the platform above the canal bed level is still an outstanding issue. Platforms higher than the bed level causes accumulation of sediments and formation of new bed level that can extend back to the intake.
- Developing appropriate technique for prediction of the performance the devices through models is essential.

4.4.2. King’s Vane

Optimum clearance between the guide vanes shall be determined from consideration of sediment as well.

4.4.3. Vortex Tubes

The optimum grain size whereby the devices function becomes efficient should be determined so that adopting to local conditions will be apparent

4.4.4. End sills

Provision of end sills to confine jumps thereby reducing downstream scouring is important. However, the sediment deposition behind this sill will reduce its efficiency thus ways of increasing the efficiency of end sills under sediment should be assessed.

4.4.5. Downstream apron from natural materials

Model study on the extent of reducing the bed level of the impervious apron with respect to providing sufficient space for the stones/armour height.

4.4.6. Inflatable dams

- Cost comparison of the rubber with the steel/wooden gates
- Performance of the system under hydrodynamic condition
- Model study of its performance under sediment laden overflows

4.4.7. Chanoine wicket gate

- Model Verification for operation and structural design of gates.

4.5. Success Cases

Diversion structures are designed and constructed under so many natural uncertainties and the performance of the scheme may not be as designed. In the inventory, some structures are observed to have t minimum problems as compared to other sites where a number of problems are recorded.

Among the 33 diversion structures , there are about 4 sites where the number of problems exhibited is only one (1) out of the fifteen (15) major problems.

The design documents and procedures of these structures have been referred if there were any peculiar design consideration. However it is found that the design methods, procedures and other considerations are same with the others. In trying to investigate the causes of success in having minimum problems, the following factors could the main causes of successes of this structures as observed from field observation:

- The catchment area of the schemes is well protected, as a result stable river system is achieved. Thus among the formulas used for the design, those that are derived presuming the presence of this catchment condition may have suited well.
- Traditional diversion irrigation has been practiced at the weir site prior to the construction of the diversion structures. Hence the interest and experience of the beneficiaries helped the structure to resist the problem that could arise as a result of improper management and operation. Besides, local information obtained from these areas about the general features of sediment & hydrology will be genuine and relatively exact.
- In upgrading such traditional schemes, the overall design follows replacing the traditional structure with concrete/masonry without altering the site & the width of the traditional structure.
- One of the structures is simple side intake structures with ponding by heap of river material manually done during low flow periods. Due to this, it escaped problems that could have resulted as a result of obstructing the river flow system during peak flood seasons.

5. CONCLUSIONS AND RECOMMENDATIONS

This research has aimed to identify & analyze the prevailing knowledge gap in the design & operation of river diversion structures. The problems on 33 river diversion structures are used as the basis for analysis. The investigation and analysis of the problems has given a chance to identify the area of knowledge gap in the design of the schemes. As a result a number of issues are found to be critical in contributing to the prevailing problems of the structure. The shortcomings of the current design practice with regards to site & structure selection, hydrology, hydraulic and sediment consideration could be attributed to many factors. The absence of choices to design different type of structures for different site conditions pose problems related to site & structure selection. Inability in obtaining locally calibrated research results and guidelines result in problems related to hydraulic computation and sediment consideration. Lack of hydrologic and sediment data or regionalized formulas lead to the use of empirical formulas & procedures that are developed for areas with no similar feature, resulting in problems related to improper hydrologic and sediment consideration. Due to variation in topography, catchment and river morphologic features, it is difficult to come up with unique standard for the design and operation of diversion structures. But following somehow standard procedure is not impossible at all. Thus from the investigation and analysis of the problems resulted from the current design practice , the following major design considerations can be concluded as the prevailing knowledge gap. These can be taken as a springboard for future efforts to prepare standard design procedure and guidelines.

- Any site is suitable for river flow abstraction for various purposes, however the structures to be considered from site to site are different. Thus examining the suitability of the site for the proposed structure or the suitability of the structure for the given site is very much important.
- The design of diversion structures shouldn’t be limited to the widely used available vertical drop weirs. Designers and the available books/guidelines are biased towards this type of diversion structure.
- As long as hydrologic data availability remains scarce, problems related to hydrologic analysis will keep on unresolved. Even under data scarce conditions, hydrologic computations should exhaust all available approaches. For instance the deviation of stages of peak floods obtained from the slope area method with that of the flood mark is common in the current design practice. The roughness coefficient to be used for the slope area method, however, could have been adjusted according to W.L Cowan approach. Thus the influence of this approach in narrowing the deviation could have been checked.
- Lacey’s regime equation is developed for rivers in transporting zones, but extensive use of regime formula has been used regardless of adequate knowledge of the river regime. There is a significant variation ( up to 200%) between the length of the e natural water way and the computed water way using Lacey’s wetted perimeter formula. This is a good demonstration for the inapplicability of the regime theory for all rivers.
- The formula /procedure developed for scouring sluice is not applicable to all rivers. The sediment size must be the prime factor than the peak discharge to determine the sluice size for rivers carrying big boulders such as the rivers in the Awash basin of the Amhara region.
- In conjunction with this finding, for relatively small sized sediments, bed generative discharge could be appropriate parameter to be used for fixing the dimensions of the scouring sluice outlet.
- There are a number of empirical formulas developed for the determination of scouring depth. The one which all schemes are designed is that of Lacey’s, the reason for this is as explained above, the availability of this formula in the readily available literatures. Unless a model test is done and verification/calibration is made for this formula, all other formulae are also equally applicable for the determination of scour depth even in rivers in regime.
- Flow profile computation is the single most important factor for marking flood levels after the construction of the structures. The current design practice lacks this important activity.
- Proper structural design of gates ensures ease of operation and management in turn overall safety of the scheme. Structural design of gates is overlooked in all designs probably due to the absence of mechanical engineers in the design team. The irrigation/civil engineer is the one who is responsible for the design of the headwork and component structures.
- Upgrading traditional diversion structures to modern diversion system not only ensures proper operation and management, but also gives an important information regarding peak and lean flow, nature and type of sediment, the suitable type of structure and etc.
- Experiences and technology inventions in the areas of flow diversion will save time, rescue failure. The existence of rubber dams, the use of vortex vanes as a sediment excluder are the breakthrough in this regard.
- The existing structures can be used as a model cum prototype to verify the importance of some of the recommendations (such as the use of vortex tubes, vortex vanes) for local conditions.
- The use of inflatable dams, and barrages with hydro mechanically operated gates on heavily boulder carrying streams should be tested in laboratory and findings scaled up to be employed in the rivers of the region that are found in Awash, Danakil and eastern part of the Abay basin.
- Performance measure guideline should be prepared during the design of each of the schemes so that the effect of each problem on the efficiency of the structure could be quantified. Then caution on should then be taken for the most sensitive cause of the problem in the design of similar structures.

6. REFERENCES

1. ASCE,1967, Vol.93, HY3,1968 Vol.94, HY4, Journal of Hydraulic engineering, Hydraulics division

2. Atkinson E., 1993. Vortex vane sediment excluder field verification report OD 126, HR Wallingford Oxfordshire,UK

3. Baban R., 1995, Design of diversion weirs, John Wiley & Sons

4. Bayou Chane Ed.,1989 Proceedings of the seminar on Erosion and sedimentation, Stockholm

5. Bouvard M., 1991, Mobile barraged and intakes on sediment transporting rivers, A.A Balkema/Rotterdam

6. Chow V.T,. 1973,Open channel hydraulics, McGraw Hill Book company

7. Continental consultants,1997,Component IVA-5 Diversion structures, ESRDF Guide line for small scale irrigation, Addis Ababa

8. Co-SAERAR,1996, Headwork design report, Gimbora irrigation scheme, Bahir Dar

9. Co-SAERAR,1997, Headwork design report, Beteho irrigation scheme, Bahir Dar

10. Co-SAERAR,1997, Headwork design report, Tikin irrigation scheme, Bahir Dar

11. Co-SAERAR,1998, Headwork design report, Sewak irrigation scheme, Bahir Dar

12. Co-SAERAR,1998, Case study of problem of Gimbora Diversion, Bahir Dar

13. Co-SAERAR,2000, Headwork design report, Gilgelmena irrigation scheme, Bahir Dar.

14. Co-SAERAR,2000, Headwork design report, Tebtebta irrigation scheme, Bahir Dar

15. Co-SAERAR,2000, Inventory report on the status of 24 diversion schemes, Bahir Dar

16. Central Water and Power Commission, 2001, Manual on estimation of peak flood, New Delhi

17. Garde R.J & Ranga Raju K.J,2000, Mechanics of sediment transport, New Delhi.

18. Garg S.K.,1989, Irrigation engineering and Hydraulic structures, Khanna Publishers Delhi

19. General Directorate of State Hydraulic Works, 1997, Course notes on post graduate course in sediment transport technology, Volume I&II, Turkey,Ankara.

20. Girishin M.M,1982, Hydraulic structures, Volume I & II Mir publishers Moscow

21. Kinori B.Z & Mevorach J.,1984,Manual of surface drainage engineering Volume II, Elsevier edition Amsterdam

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23. Lewin J.,1995, Hydraulic gates and valves in free surface flow and submerged outlet,Thomas Telford publication London

24. ORDA,1998,Headwork design reports, Silala,Zequa diversion irrigation schemes, Bahir Dar

25. ORDA,1998,Headwork design report, Gunagunit diversion irrigation scheme, Bahir Dar

26. ORDA,1999,Headwork design reports, Kulanti ,Timbel & Inchike diversion irrigation schemes, Bahir Dar

27. ORDA,2002,Headwork design report, Asera,Arno,Garno,Genet Bahir diversion irrigation schemes, Bahir Dar

28. ORDA,2003,Post implementation review report on 16 diversion schemes, Bahir Dar

29. Subramanya K,1979 , Flow through open channels, 2nd edition

30. Valentine H.R.,1970,Applied Hydrodynamics, SI edition the English Language book society.

31. Vic Galay,2003,Training Course notes on river morphology,Volume I&II, NW hydraulic consultants, North Vancouver

7. APPENDICES

Appendix 1 MS-Access interface for Data Entry

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Appendix 2 Sample database obtained from the switchboard

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Appendix 3 Output of Existing database of diversion structures in GIS ARCView

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Appendix 4 FORTRAN Program for Design of Vortex Tube

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