Design Criteria For Dam 4lf

Mekelle Water Supply Development Project Design Criteria for Dams and Appurtenant Structures

1. Introduction 1.1

Foreword

This document provides guidance and criteria for the design of Giba River Dam and its pertinent structures. There are no established and obligatory design criteria and standards for design of dams and appurtenant structures in Ethiopia,. Hence, the design criteria below are based on widely used international practices. In most cases the design criteria are adopted from the recommendations by the United States Army Corps of Engineers (USACE) and/or by the United States Bureau of Reclamation (USBR), two American Government agencies with vast practical experiences on design of dam projects. Where it was not possible to find design criteria recommendations by USACE or USBR, other international standards and publications have been considered. Previous design practices and experiences by other prominent designers have also been taken into . 1.2

Basic Data

1.2.1 General Preparatory and background basic data for design include the following: • Previous studies dealing with the subject matter. • Existing studies concerning the environmental and socio-economic impacts concerning the reservoir, the watershed and other development plans (if any), which have affect on the case. • Seismic data, regional and Horn of Africa. • Regional geological data. • Formerly existing geological/geotechnical data concerning the relevant area or its near proximity. • Climatologic, hydrometric and hydrologic data of the project area and its surroundings. • Existing surveys. • Aerial photos. • Existing maps: topography, geology (all available scales, governmental and others). • Any additional relevant data. 1.2.2 Geological and Geotechnical Study The main objective for these investigations is to provide sufficient data for the following: • Understanding the geological structure of the dam and reservoir site. • Dam foundation design. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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• Design of the associated structures. • Construction material.

Pertinent requirements include: • Geological map of the area, to a scale of 1:10,000. • Layout of the investigation elements such as core drillings and test pits description, with field and laboratory test results. • Geological/geotechnical sections along the dam axis and investigation profiles including location of boreholes, test pits, trenches, geophysical tests, water and ground water composition and levels, etc. s between defined geo-engineering units, (classified according to lithology, RQD, weathering, strength, permeability, etc.). • Estimated infiltration/percolation hazards and expected rates through the dam foundation and reservoir bottom and banks. • Determination of the between overburden and bedrock, weakness zones within bedrock, including t systems and their opening and infilling, discontinuities (lenses), karstic formations, etc. Encounter of local instabilities (creep, landslide). • Applied geological/geotechnical conditions concerning specific elements of the Works (Embankment, spillway, intake, diversions, etc.). • Photographs of special features on site and nearby, of all core boxes, of test pits and trenches, presented with the logs. • Final report of the site investigation, including recommendations for the design and for additional field and laboratory work, if required, to be carried out by the Contractor before and/or during the execution of the work. • Determination of possible construction materials (Quantity of the materials to be at least twice that actually required) and location of borrow areas inside the impounding area (bearing in mind to maintain adequate natural blanket cover as required) and its vicinity. 1.2.3 Hydrologic Study The objective for the meteorological/hydrologic studies be as follows: • Water ingress to the reservoir in of relevant stochastic phenomena. • Concerned climatologic/meteorological effects such as evaporation and direct rainfall. • Occurrence of exceptional and floods in of peak discharge, duration, volume, return period, etc. • Sediment transport and reservoir sedimentation. Pertinent requirements of the study include: • Long-range series of monthly inflows and rainfall at the reservoir. • Relevant daily/monthly evaporation rates from open water surface. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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• Study of expected peak floods for short-medium term return period (25 to 100 years) for determination of during-construction diversion works and of catastrophic events (PMF) for 24 and 48 hour storms. • Long range series of sediment arriving at the reservoir.

2. Dam Structure 2.1 Basic Requirements The following criteria shall be met to ensure satisfactory earth and rock-fill structures: • Under all conditions of construction, reservoir operation, and seismic activity, the embankment, foundation, and abutments shall remain stable. • Seepage through the embankment, foundation and abutments shall be properly controlled and collected to prevent excessive uplift pressures, piping, sloughing and removal of material by solution, or erosion of material by loss into cracks, ts, and cavities. The design shall consider seepage control measures such as foundation cutoffs, adequate and non-brittle impervious grouting, upstream impervious blankets, filter and transition zones, drainage blankets, relief wells, etc. • Sufficient freeboard shall be provided in order to prevent overtopping by waves. The freeboard shall also include allowance for the normal settlement of the foundation and embankment as well as for seismic effects as applicable. • Spillway and outlets of sufficient capacity shall be designed to prevent overtopping of the embankment at the design flood. 2.2

Selection of Embankment Type

The following major factors shall be considered to reach the most viable type of dam: Topography: The site topography of a relatively narrow valley with high, rocky walls suggests an erthfill or rockfill embankment. Irregular valley conditions might suggest a composite structure, partly earth and partly concrete. Composite sections might also be used to provide a concrete spillway while the rest of the dam is constructed as an embankment section. Topography may also influence the selection of appurtenant structures. Natural saddles may provide a spillway location if conditions in the adjacent watershed permit. If the reservoir rim is high and unbroken, a chute spillway may be considered. Geology and Foundation Conditions: The geology and foundation conditions at the dam site may dictate the type of dam suitable for that site. Competent rock foundations with relatively high shear strength and resistance to erosion and percolation offer few restrictions as to the type of dam that can be built at the site. Gravelly/silty foundations, if well compacted, are suitable for earth or rock-fill dams. Special precautions shall be taken to provide adequate seepage control and/or effective water cutoffs or seals. The \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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main problems may include measures to control settlement, piping, excessive percolation losses, and protection against erosion in the foundation at the downstream embankment toe. Non-dispersive clay foundations may be used for earth dams but require moderate embankment slopes because of relatively low foundation shear strength. Availability of Construction Materials: An economical type of dam will often be the one for which building materials can be found within a reasonable haul distance from the site, including material which is to be excavated for the dam foundation, spillway, outlet works and other appurtenant structures. Materials which may be available near or at the reservoir site include soils for embankments, rock for embankments and riprap, and for concrete aggregate (sand, gravel, and crushed stone). Construction scheduling, allowing direct use of such materials might prove cost saving. When nearby suitable building material is unavailable, the hauling distance will cause costs to rise. Spillway: The size, type, and restrictions on location of the spillway are important factors in the choice of the type of dam. When a large spillway is to be constructed, it may be considered to combine the spillway and dam into one structure, indicating a concrete overflow dam. In case that the required excavation from the spillway structure or the foundation can be utilised in the dam embankment, it may be advantageous. Environment: Environmental considerations have become very important in the design of dams and can have a major influence on the type of dam selected and/or the amount of water released. The principal influence of environmental concerns on selection of the dam is the need to consider protection requirements, location of the spillway and riparian/ bottom release facilities. Economic Considerations: The selection of the type of dam shall be made after careful analysis and comparison of possible alternatives, and after thorough economic analyses that include costs of spillway/freeboard balance, water abstraction and control structures, and foundation treatment. Climate: Construction of earthfill dam during wet weather will cause difficulties which should be taken into consideration. The use of concrete faced rockfill may be considered for better efficiency or shorter construction time, if long and persistent wet weather prevails. Time Available for Construction: This can be an important factor, especially if there is time restriction/shortage. This factor shall be considered in conjunction with climate. In a climate of well defined rainy seasons, it may be practicable to construct an earth and rockfill dam over more than one dry season. A concrete face rockfill dam may be time saving as it enables to place the rockfill in both seasons. For the anticipated size of embankment dam, the choice may be among types including: \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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• • • •

Earthfill with vertical/slanted and horizontal drains. Earth and rockfill with central core and filter and transition zones. Earth and rockfill with sloping upstream core and filter and transition zones. Concrete faced rockfill.

The selection of the dam type to be designed shall consider all factors. The governing consideration in the selection of the dam type shall be the design of an adequately safe dam that incurs the least cost. 2.3

Axis Alignment

The embankment axis shall comprise straight sections and of the most economical alignment fitting the topography and foundation conditions. Changes in the alignment shall be radially curved. 2.4

Abutments

Alignment: The alignment shall be adjusted to avoid tying into narrow ridges, or into abutments that diverge in the downstream direction. Zones of structurally weak materials in abutments, such as weathered overburden and talus deposits shall be taken into or avoided. Abutment Slopes: Where abutment slopes are steep, the core, filter, and transition zones of the embankment shall be widened at locations of possible tension zones resulting from different settlements. The possibility of changing abutments steep slope to more moderate or stepped ones shall also be considered. Settlement: Large differential settlement near the abutments may result in transverse cracking within the embankment. Considerations shall be made to use higher placement water contents combined with flared sections and filters. 2.5

Freeboard

One of the requirements for design of an embankment dam is to ensure safety against overtopping due to inadequate freeboard. Normal freeboard is the difference in elevation between the crest of the dam and the normal reservoir water level (NWL). Minimum freeboard is the difference in elevation between the crest of the dam and the maximum reservoir water surface (MWL) due to the design flood. The difference between normal and minimum freeboard represents the surcharge head. • MWL plus 1:100 years wind event effect shall be considered for determining the minimum freeboard. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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• NWL plus 1:1,000 year wind event effect shall be considered for determining the Normal freeboard. The freeboard shall be established taking into the following factors: • Seiche effects. • Wind set up of the water surface. • Wave action. • Run-up of waves on the dam. • Malfunction of spillway and/or outlet for a moderate wind. • Hydrologic uncertainties resulting from inadequate database. • Consideration shall be given to landslide-generated water waves and/or displacement of reservoir volume, or to prevent them. Comparison shall be made between the most critical combinations to be used for determining the normal freeboard. The methods available for freeboard calculation such as the Savilles equation, Zuider Zee formula, Stevenson formula, etc shall be used. The wind speed to be used in wave calculations shall not be less than 30 m/s. Wind speeds are greater over water than the measured over land. The normal ratio between wind speeds over water and over land is shown in Table 1. Table 1: Ratio between Wind Speeds Over Water and Over Land. Effective fetch, km 1 2 4 8 Wind speed ratio 1.1 1.16 1.23 1.29

12 1.31

In accordance with USBR recommendations, normal and minimum freeboard requirements shall be evaluated. The freeboard that results in the highest top-of-dam elevation shall be adopted. 2.6

Camber

In addition to the freeboard, a sufficient camber shall be provided to allow for settlement of the foundation and embankment. Crest camber shall be determined by the anticipated magnitude of foundation and embankment settlement. The USBR recommendation for a camber of 1% of the embankment height shall be adopted, if no other consideration prevails. 2.7

Crest Width

The top width of earth or rock-fill dam within conventional limits has little effect on stability. The crest width is often governed by construction procedure and the access required. Depending upon the height of the dam, the minimum top width according to USACE is between about 7.5 and 12 m (25 to 40 ft).

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Reducing the number of the embankment zones near the top shall be considered, to reduce the overall width needed for the dam construction. Widely accepted empirical methods for determining the crest width shall be used; e.g., the Japanese code 1957 specifies crest width (W) in of dam height (H) as follows: W = 3.6 × 3 H − 3 (metres). 2.8

Slope Protection

Riprap: Adequate slope protection shall be provided for anticipated earth and rock-fill dam structures to protect upstream slope against wind and wave erosion. Dumped riprap is the preferred type of upstream slope protection. The rip-rap shall satisfy the following requirements: • The rip-rap shall be composed of solid (unckracked) homogeneous rock not containing weak zones with only up to 3% voids. • The riprap shall have the shape and weight to dissipate wave energy without being displaced. • The riprap shall be strong enough to perform without degrading or breaking down to smaller pieces. • It must be durable enough to withstand, without loosing strength, effects of longterm exposure to alternating weather conditions, water composition, varying inundation and saturation periods. Depending on the minimum operating water level of the dam, the use of less or no riprap in the lower portion of the dam shall be considered below the minimum operating level, for a length along the slope of double the wave height. A ledge (berm) of sufficient width at this level, is necessary to provide to the rip-rap. The ledge shall be slanting down 2 – 3% (in upstream direction). In the calculation of stone weights, the wave height and wave period shall be taken into . USBR Design Standard No. 13, Chap. 5 Protective Filters, and Chap. 7, Riprap Slope Protection as well as USACE Publication 1110-2-2300 App. C are among the widely used methods in the design of the upstream slope protection of embankment dams. Bedding Layers: The gradation of the bedding material shall be calculated to provide retention of bedding particles by water motion through the overlying riprap layer and retention of the material underlying the bedding layer. If the underlying material has low plasticity, the gradation of the bedding material shall be established to conform to the following filter criteria: D15 B > 5 D15 E D15 B > 5 D85 E D85 B > 5 D15 R 5 \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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Where: D15B = the 15 percent ing the size of the bedding. D85B = the 85 percent ing the size of the bedding. D15E = the 15 percent ing the size of the material to be protected. D85E = the 85 percent ing the size of the material to be protected. D15R = the 15 percent ing the size of the riprap. An intermediate filter layer may be required between the bedding and riprap. The above mentioned shall also conform with the same USBR and USACE publications. Downstream Slope Protection: Since the downstream faces of earth and rockfill, and concrete faced rockfill dams have a rockfill zone on the downstream slope, erosion is not a major issue. A uniform surface within the specified tolerance should suffice. For downstream slopes of earthfill or shell material dams the following shall be taken into : • Covering the surface with a layer of rockfill, gravel over geotextile, or by establishing grass cover. • Providing berms at a maximum vertical interval of 10 meters to limit the vertical distance of runoff travel, the berms shall be slanted (2 – 4%) towards the dam and downwards. Measures shall be considered to prevent blockage of the outlets. • Lined drains on the berms shall convey the runoff and carry it to the abutments. Providing open lined drains shall be considered at the of the dam with the abutments. Alternatively, the drains on the berms may be extended. • The drained water shall be released away from the dam and dam/abutment . 2.9

Embankment Zoning

The embankment dam shall be zoned to provide an adequate impervious zone, transition zones between the core and the shells, adequate filters for seepage control, and shells for stability. The embankment zoning shall use as much material as possible from required excavation and from borrow areas with the shortest haul distances and the least waste, first considering the flooded area for borrow places (for additional storage volume, but without increasing seepage). Gradation of the materials in the transition zones shall meet the filter criteria presented in Section 3.13. 2.10 Embankment Materials 2.8.1

Earthfill or Core Material

Most soils can be used for earth-fill or core construction as long as they are impermeable, insoluble, non reactive to the water chemical composition and substantially inorganic. Rock flours and clays with liquid limits above 80% shall be avoided. The lower limit of the plasticity index shall be 5 to 10 %. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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Fine-grained soil shall be used for embankment or core construction only with water contents suitable for compaction and for operation of construction equipment. Well-graded soils are preferable to soils having relatively uniform particle sizes. Embankment soils that undergo considerable shrinkage upon drying shall be protected by adequate thicknesses of non-shrinking fine-grained soils to reduce evaporation. The use of clayey soils as backfill in with concrete or masonry structures shall be avoided (if possible), except in the impervious zone of an embankment. If fine-grained material is in short supply resulting in a thin core, it should have a low permeability. The use of wider filter zones shall be considered. 2.8.2 Rockfill Material Rockfill material shall be composed of sound uncracked fresh rock, which is not affected by the water chemical composition, with a specific weight of at least 2.65 ton/m³ and not more than 3% voids. Sound rock is ideal for compacted rockfill. Some weathered or weaker rocks may be suitable, including sandstones and cemented shales (but not clay shales). Rocks that break down to sizes smaller than specified during excavation, placement, or compaction are unsuitable as rockfill, and such materials shall be treated according to it's final properties as soils, or be rejected. Rock is unsuitable if it splits easily, crushes, or shatters into dust and/or small fragments, or reacts to water composition. The suitability of rock shall be judged by examination of the effects of weathering action in outcrops and by on-site and laboratory tests. Rockfill composed of a relatively wide gradation of angular, bulk fragment might settle less or slower than if composed of different shaped stones or grade. Uni-graded riprap protects the embankment better (depending on correct size). 2.11 Impervious Core There is no definite rule for determining the safe thickness of the core. The width of a central impervious core shall be established using seepage and piping considerations, types of material available for the core and shells, the filter design, and seismic considerations. According to common practice, a core width at the base, or cutoff, amounts to at least 25% of the difference between the maximum reservoir and minimum tailwater \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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elevations. Cores with a width of 30% to 50% of the water head, (at that particular section), have proved satisfactory. Core thickness of about one-half of the dam height, (at that particular section), is also accepted. A core top width of about 3 m is the minimum for construction purposes. The permeability of the compacted core material shall not exceed 10-5 cm/s. The water moisture content shall be carefully determined according to laboratory test results and geotechnical considerations, (see also the subject of cracks). 2.12 Shoulder or Shell of the Dam In a common type of earthfill or earth and rockfill embankment, a central impervious core is flanked by more pervious shells that the core. The upstream shell shall be designed to provide stability against end of and during construction, rapid drawdown, earthquake, and other loading conditions, and protected against waves. The downstream shell acts as a drain that controls the seepage and provides stability under high reservoir levels, and shall be designed to provide stability against steady state seepage, during construction, end of construction and during earthquakes. Control of seepage through the embankment shall be provided by internal filter drains. 2.13 Filter Design The filter design for drainage layers and internal zoning of a dam is a critical part of the embankment design. It is essential that the individual particles in the foundation and embankment/core are held in place and do not move as a result of seepage/erosion forces. The zones of material shall meet “filter criteria” with respect to adjacent materials. These criteria are satisfactory for use with filters of either natural sand and gravel or crushed rock and for filter gradations that are either uniform or graded. The types of material to be protected by filters shall comprise: Category % finer than #200 sieve 1 > 85 2 40-85 3 15-39 4 <15 The filter criteria shall be calculated for the 4 categories according to USBR Design Standard No. 13, Embankment Dams, Chap. 5: Protective Filters, as well as by USACE publication EM 1110-2-2300 App. B. It shall have the following parameters: • Filter material produced from rock origin shall conform with the requirements of Section 3.10b. • The gradation curve of the filter should have roughly the same shape as the gradation curve of the protected soil. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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• Filters shall not contain more than 5% of fines ing the No. 200 sieve (0.075 mm), and the fines should be cohesion-less. • Filters shall not contain organic material, nor any other material differing from the approved filter material itself.


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The thickness of filter layer(s) shall be determined considering: • Filter thickness required for fine particles retaining, with a minimum width of 2.40 m. • Drainage layer required for draining the water to relieve pore pressure (where needed). • Minimum thickness required for compaction. • Its location in the dam (say, abutments, etc.) • Earthquake effects. 2.14 Compaction Requirements 2.14.1

Earthfill

The compacted density ratio for earthfill shall be 98% of the standard maximum dry density, with a water content between OWC – 1% and OWC + 1% or between OWC and OWC + 2%, where OWC is the standard compaction optimum water content. Standard Proctor procedures not modified shall be used, in order to ensure moist compaction which leads to low permeability flexible fills. For soils which are difficult to compact, the compaction requirement may be relaxed to as low as 95% density ratio, under controlled laboratory tests, provided that compaction is carried out above optimum water content. The layer thickness after compaction shall be 200 to 250 mm, provided that the density and water content requirements are satisfied, homogeneously to all the layer's depth . Earthfill can be placed by scrapers or dumping truck and spread with a grader or bulldozer. Oversized materials shall be removed before compaction. The surface of the previously compacted layer shall be scarified prior to placing the next layer of fill to ensure good bond. Addition of a small amount of water to the scarified surface shall be considered prior to placing the next fill. Water content adjustments shall be carried out at the borrow area, with only minor adjustments allowed on the embankment. Soils in the borrow area which are more dry or wet than the required water content shall be conditioned for several days before use in the embankment. Failure to do this is common cause of difficulty in achieving the specified compaction requirements. 2.14.2

Rockfill

Procedures to be used in compacting rock-fill materials, especially where rocks are soft, shall be selected on the basis of test fills, in which lift thicknesses, number of es, and types of compaction equipment (i.e., different vibratory rollers) are experimented. USACE recommends that rock-fill shall not be placed in layers thicker than 60 cm, unless the results of test fills show that adequate compaction can be obtained using thicker lifts. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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As the maximum particle size of rockfill decreases, the lift thickness shall be decreased. The maximum particle size shall not exceed 90% of the lift thickness. Smooth-wheeled vibratory rollers having static weights of 10 to 15 tons, the type of which shall be decided according to the results of the test fills, are effective in achieving high densities for hard durable rock if the speed, cycles per minute, amplitude, and number of es are correct. Quarry-run rock having an excess of fines can be ed over a grizzly, and the fines placed next to the core. Fine rock zones should be placed in 30 to 45 cm lift thicknesses. There is no need to scarify the surfaces of compacted lifts of hard rock-fill. 2.14.3

Filters

Filters shall preferably be placed ahead of earthfill or rockfill as shown in Fig. 1 in order to reduce risk of contamination of filter zone and allow good control of the filter width.

Fig. 1: Filter Zone Placement Ahead of Other Zones. Excessive breakdown of the filter materials by the compaction equipment should be avoided or the filter materials changed. 2.15 Earthfill to Concrete Structures Interface The between earthfill and concrete structures like spillway can be potential sources of cracking and piping failure. Using cutoff wall collars may be employed, but has the drawback of inadequate compaction adjacent the walls. The recommended and relatively simpler detailing employed at the concrete-earthfill interface consists of careful compaction of the fill at water content above the optimum. This practice shall be observed in the design and execution. 2.16 Cracking Cracking develops within zones of tensile stresses in earth dams due to differential settlement, filling of the reservoir and seismic action. The design shall include provisions to minimise adverse effects. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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Cracks are of four general types, namely: Shrinkage Cracks: Shrinkage cracks are generally shallow and can be treated from the surface. Transverse Cracks: Transverse cracking of the impervious core is of primary concern because it creates flow paths through the embankment, caused by tensile stresses related to differential embankment and/or foundation settlement which may occur at steep abutments, junction of a closure section, structures where compaction is difficult, old stream channels filled with compressible soils, etc. Horizontal Cracks: Horizontal cracking of the impervious core may occur when the core material is much more compressible than the adjacent transition or shell material. The lower portion of the core may separate out, resulting in a horizontal crack. Arching may also occur if the core rests on highly compressible foundation material. Longitudinal Cracks: Longitudinal cracking may result from settlement of upstream transition zone or shell due to initial saturation by the reservoir, due to rapid drawdown, due to differential settlement in adjacent materials or seismic action. They do not provide continuous open seepage paths across the core of the dam and therefore pose no threat with regard to piping through the embankment. Longitudinal cracks may reduce the overall embankment stability leading to slope failure, particularly if the cracks fill with water. Defensive Measures: The primary defence against a concentrated leak through the dam core is the downstream filter (Sherard, 1984). Other design measures to reduce the susceptibility to cracking are of secondary importance. The susceptibility to cracking can be reduced by: • Shaping the foundation and structural interfaces to reduce differential settlement. • Densely compacting the upstream shell to reduce settlement from saturation. • Compacting core materials at water contents sufficiently high so that stress/strain behaviour is relatively plastic, i.e. low deformation moduli and shear strength, so that cracks cannot remain open (pore pressure and stability must be considered). • Staged construction to lessen the settlement effects of foundation and lower parts of the embankment.

3. Dam Slope Stability Analysis The stability of an embankment depends on the characteristics of foundation and fill materials and also on the geometry of the embankment section, and additional factors such as presence of water, loading conditions, etc.


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3.1

General Considerations

The stability of the proposed dam shall be analysed using state of the art software, such as Slope/W from Geo-Slope International Ltd. of Canada, or similar. The stability analyses shall be conducted with the following aims: • To determine the factor of safety for various slip surfaces of upstream and downstream slopes under steady state seepage condition, with or without earthquake. • To determine the factor of safety for various slip surfaces of upstream slope under sudden drawdown condition. • To determine the factor of safety for various slip surfaces of upstream and downstream slopes under end of construction condition. The foundation effect in the various slope stability analyses and load conditions shall also be ed for. 3.2

Loading Condition

Table 2 summarises the loading conditions and corresponding minimum factor of safety requirements advised by USACE and used worldwide. The design shall meet these requirements. Table 2: Various Load Cases and Minimum Required Factor of Safety Case Loading Condition Critical Slope FOSmin End of construction Upstream 1.3 1 Downstream 1.3 During construction Upstream 1.3 2 Downstream 1.3 Sudden drawdown Upstream 1.3 3 Steady state seepage Upstream 1.5 4 Downstream 1.5 Steady state seepage with earthquake Upstream 1.1 5 Downstream 1.1 Analyses shall also be carried out to assess the sensitivity of the safety factor to variation in shear strength, pore pressure, and slip surface geometry. Safety shall be ensured for a wide range of assumptions with regard to these factors. 3.3

Method of Stability Analysis

The slope stability investigation of the proposed dam shall be carried out using Slope/W software program or similar, based on the limit equilibrium method. The limit equilibrium methods, which satisfy both force and moment equilibrium conditions shall be used in the analysis. Both Spencer and Morgenstern-Price methods satisfy both force


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and moment equilibrium conditions and shall be used to obtain the factors of safety. The results obtained using such methods shall be compared with other methods. The pore pressures that would develop within the body of the dam and in the foundation under steady state seepage shall be initially estimated with the help of SEEP/W software (based on the Finite Element Method) or similar. These pore pressures, in of head, shall then be incorporated in the slope stability analysis using Slope/W software program, or similar. 3.4

Shape of Slip Surface

Circular slip surfaces are common and reasonable for earthfill dams; the sliding surfaces may take other forms in rockfill dams. For zoned rockfill dams with distinct zones of different properties, multi linear sliding planes shall be studied. This is also called “sliding block” method and shall be carried out in Slope/W software, or similar, with several fully specified slip surfaces. 3.5

Seismic Design

Seismic activities in Ethiopia are generally said to be related with Afar and the main Ethiopian rift valley. Taking into the expected long life period of the dam and not ruling out the chance of the dam site being hit by a damaging earthquake, adequate seismic design considerations shall be taken. Fig. 2 shows the hazard map of Ethiopia prepared by the Institute of Geophysical Observatory at Addis Ababa University for a Design Base Earthquake (DBE) with a return period of 300 years. Like other recently designed dams, this hazard map shall be used to estimate the DBE for the embankment dam. The ground acceleration contours displayed on the map in Fig. 3 were produced based on a 33 years data (1960 to 1993). However, as indicated in the book “Earthquake History of Ethiopia and the Horn of Africa,” by Pierre Gouin (1997), significantly larger earthquakes had occurred in the country earlier, albeit in the absence of well established recording equipments at the time. The 12 February 1845 earthquake – described in Fig. 3 – is particularly of great interest and shall be considered in the seismic design of the dam. The estimated location of the epicentre of this earthquake was within a distance of about 175 to 275 km and the destruction induced by this earthquake was reported to be large. Based on the tremors reported in different parts of the country including Gondar, Wollo, parts of Gojam and parts of Shoa, this earthquake was estimated to be of magnitude 4.0 to 4.5 and its estimated epicentre location at 12.2° N and 37.6° E. The above mentioned and other probable earthquakes shall be examined during the Feasibility Study stage.


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Fig. 2: Seismic Hazard Map of Ethiopia and its Northern and Eastern Neighbouring Countries. The hazard map is for a probability of exceedence of 0.0033 (return period of 300 years). Contours indicate peak ground accelerations as a fraction of g.


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Fig. 3: Location Map of the 12 February 1845 Earthquake. Based on international practice, the following approach shall be used in the seismic design of the embankment dam: • Use of the pseudostatic method of stability analysis using the Slope/W software, or similar, for reasonably well-built dam on stable soil or rock foundations, if estimated peak ground accelerations are less than 0.2g. • Use of dynamic deformation analysis techniques using the finite element method based Quake/W and Slope/W software in case the peak ground accelerations may exceed about 0.2g and the dam is constructed of or on soils that do not lose strength as a result of earthquake effects. • Use of dynamic analysis for liquefaction potential, or strength reduction potential, (using Quake/W) if the dam involves embankment or foundation soils that may lose a significant fraction of their strengths under the effects of earthquake shaking. • Adopting ample freeboard, wide transition zones, adequate compaction of materials in foundations and embankment, and a high level of quality control.


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4. Dam Foundation 4.1

Basic Requirements

The essential requirements of a foundation for an embankment dam are: • The foundation should provide stable for the embankment under all conditions of saturation and loading. If the natural foundation at the site is incapable of ing an embankment with economical slopes, the deficient material shall be removed or improved. • It should provide sufficient resistance to seepage to prevent excessive loss of water and the exit gradient should be low enough not to cause piping problems. • Differential settlements due to varying compressibility characteristics in different sections of the dam foundation should be restricted in order to minimise the possibility of cracks in the embankment, which can lead to undesirable seepage conditions. All these criteria shall be observed during the design of the dam. 4.2

Seepage Control

4.2.1 Cutoff Trench Seepage through an embankment is controlled most effectively by a cutoff into an impervious foundation. This can be accomplished by excavating a trench and backfilling it with compacted impervious earth, which is in effect part of the embankment core. Such a cutoff shall be sufficiently wide to ensure an acceptably low seepage gradient. The excavated slopes should be flat enough to avoid excessive stress concentrations. If there is a possibility of piping in the backfilled material, the design shall consider placing a filter layer on the downstream face of the trench. If the cutoff trench would have to be extended to an uneconomical depth, a slurry trench might be considered as a feasible alternative, or other means be considered. 4.2.2 Upstream Impervious Blankets When a complete cutoff is not required or is too costly, an upstream impervious blanket tied into the impervious core of the dam may be considered to minimise under-seepage. It is however noted that upstream impervious blankets are not used when the reservoir head exceeds 60 m (200 ft) because the hydraulic gradient acting across the blanket may result in piping and serious leakage. If a natural upstream blanket is not available, an ample amount of suitable material will have to be exploited near at hand or other economical solutions sought.


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4.2.3 “Dental Treatment” The exposed rock foundation, after excavating the overburden, shall be cleaned and the ts/cracks filled by “dental concrete”. In case of cracked or highly ted rock, providing additional shotcrete layer shall be considered. 4.2.4 Grouting For rock foundations with ts and cracks, grouting is effective to control seepage through the discontinuities. The water tightness of rock is measured in Lugeon units. In sections of the dam foundation at which the head of water exceeds 30 m, leakage is normally required to be 1 Lugeon or less, whereas up to 3 Lugeons can be accepted where the head of water is less than 30 m. Both grout curtain and consolidation grouting shall be considered. Grout curtain is formed by injecting grout mixes through closely spaced deep grout holes drilled along lines (main curtain plus auxiliary ones). Consolidation grouting is carried out through a number of closely spaced, shallow holes on a grid pattern in order to make the upper portion of the bedrock less pervious.

5. Dam Instrumentation The primary purpose of dam instrumentation is to provide data useful for determining whether or not the embankment or foundation is behaving in accordance with engineering predictions, or whether conditions call for intervention such as drawdown or evacuation of downstream population, etc. All or some of the following facilities shall be installed in the dam to monitor its performance and to confirm its structural behaviour. • Facilities for measuring leakage. • Instruments for measuring pore pressure (piezometers). • Devices for measuring the phreatic line. • Internal instruments for measuring vertical settlement, horizontal movements and foundation settlement. • Surface movement for horizontal and vertical movement. • Earth pressure cells. • Accelerometer for measuring earthquake induced accelerations. • Connecting network, computers and software for processing data and communication system to processing centre for data updating, reporting, dam status evaluation, etc.


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6. Concrete Faced Rockfill Dam (CFRD) A Concrete Faced Rockfill Dam (CFRD) may be proposed as a possible dam. Therefore, this section provides general guidance and criteria to be used for the design of CFRD. Since there is little or no design and construction experience on this type of dam in Ethiopia, the design guidance stated below have intentionally been made to include some detail theoretical aspects of CFRD. 6.1

General Arrangement – Modern Practice

As shown in Fig. 4, a typical CFRD shall comprise:

Fig. 4: Some Details of CFRD Plinth: Reinforced concrete slab cast on sound, low permeability rock to the face slab to the foundation. Face Slab: Reinforced concrete slab, preferably between 0.25 m to 0.6 m thick, with vertical, some horizontal and boundary impermeable ts to accommodate deformation which occurs during construction and might occur later on and when the water load is applied. Zone 2D: Transition rockfill, processed rockfill or alluvium, graded from silt to coble size, or in more recent dams from silt to coarse gravel size. The transition provides uniform for the face slab and acts as semi-impervious layer to restrict flow through the dam in the event that cracking of the faceplate or opening of ts occur. Zone 2E: Fine rockfill, selected fine rock that acts as a transition layer between Zone 2D and Zone 3A in the event of leakage through the dam. Zone 3A: Rockfill, quarry run, free drainage rockfill placed in layers about 1 m thick. This zone provides the main for the face slab and is compacted to a high modulus to limit settlement of the face slab.


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Zone 3B: Coarse rockfill, quarry run, free drainage rockfill placed in layers about 1.5 m to 2 m thick. Larger rock may be pushed to the downstream face. This zone is less affected by the water load than Zone 3A, so a lower modulus is acceptable. The thicker layers allow placement of larger rock. Zone 2F: Some modern dams include a Zone 2F filter zone beneath the boundary t. This serves two functions: (1) to act as a high modulus zone to limit deformation of the slab at the boundary t, and (2) to limit leakage flow in the event the t opens. Zone 2F usually comprises maximum of 19 mm to 37 mm, with some silty fines, placed in thin (200 mm) layers. Many variations to the above mentioned zoning exist. A suitable zoning shall be adopted to meet site conditions and the quality of construction materials available. 6.2

Site Suitability

CFRDs are suited for dam sites with a rock foundation and a source of suitable rockfill. A CFRD might be a lower cost alternative than earth and rockfill dam. Factors that may lead to CFRD being the most economic alternative, include: • Non-availability of suitable earth fill. • Climate: CFRDs are suited to wet climates. Periods in which earth fill can not be placed are no hindrance for rockfill. This can result in significant overall savings in construction schedule. • Grouting for CFRD can be carried out independently of embankment construction. This may result in saving of overall time for construction. • Total embankment fill quantities are likely to be smaller and side slopes steeper for CFRD than for earth and rock fill dams. This may lead to reductions in the cost of the dam. 6.3

Rockfill Zones and their Properties

6.3.1 Zone 2D Grading Special emphasis shall be given in the design of CFRD to the grading and placement of the rockfill zone immediately below the concrete face slab. The International Commission on Large Dams (ICOLD, 1989) recommends a Zone 2D grading as given in Table 3 below, which is virtually the same as that suggested by Sherard (1985). They indicate that a maximum of 10% to 12%, ing 0.075 mm is desirable, while giving 15% as the upper limit. ICOLD indicates that Zone 2D shall be 4 m to 5 m wide.


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Table 3: Recommended Zone 2D grading Sherard ICOLD Amaya & Marulanda (1985) (1989) (2000) Size (mm) % finer % finer % finer 75 90 – 100 90 – 100 90 – 100 37 70 – 95 70 – 100 70 – 100 19 55 – 80 55 – 80 65 – 100 2.76 35 – 55 35 – 55 40 – 55 0.6 8 – 30 8 – 30 10 – 22 0.075 2 – 12 5 – 15 4–8 6.3.2 Zones 2E, 3A and 3B – Fine Rockfill,Rrockfill and Coarse Rockfill The basic requirements for rockfill in CFRD are: • The rockfill shall be free draining to avoid build-up of pore pressure during construction, and to allow controlled drainage of water which might leak through the faceplate. • The rockfill shall have a high enough modulus after compaction in the dam to limit face slab deflections under water load to acceptable values. Creep of the rockfill shall also be small enough to avoid excessive long term settlements. • It shall be readily available as a quarry run product with a minimum of wastage oversize or undersize rock. Zones 3A and 3B shall be placed in layers of the order of 1 m and 1.5 m to 2 m thick, respectively. Rolling for good quality rockfill shall be by 4 (up to 8) es of a 10 ton vibratory steel drum roller and shall anyway be subject to laboratory controlled on-site results of test levees (built as part of the dam). Zones 3A and 3B shall not have significantly large difference moduli of compressibility, and shall have a near vertical interface between them. For Zone 3A, lower strength (<30 MPa) rockfill shall be compacted in thinner (0.6 m to 0.8 m) layers to encourage break down to give greater strength (a higher modulus). Zone 3B shall be compacted in 1.5 m layers to take large rock. The rockfill grading for Zones 3A and 3B shall be: • The maximum size shall be that which can be incorporated in the layer and provide a relatively smooth surface for compaction. • Not more than 50% shall 25 mm sieve. • Not more than 20% finer than 2.76 mm. • Not more than 10% finer than 0.075 mm. Zone 2E shall be designed to act as a filter to Zone 2D and shall be placed in relatively thin layers (0.4 m to 0.5 m, same as Zone 2D).


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6.4

Side Slopes and Analysis of Slope Stability

When a CFRD is constructed of hard, free draining rockfill, the upstream and downstream slopes shall be fixed at 1.3H to 1V or 1.4H to 1V, corresponding roughly to the angle of repose of loose dumped rockfill. When weak rockfill or gravel is used for the dam rockfill zones, flatter slopes - 1.5H to 1V or 1.6H to 1V - shall be used to prevent ravelling of the faceplate. If foundation strengths dictate, flatter slopes shall be used (e.g. 2.2H to 1V, was used for Winneke dam, USA, Cooke, 1999). The stability of the slopes in the dam usually is not analysed (Sherard and Cooke, 1987). However, if analysis is to be carried out, the shear strength parameters of the rockfill shall first be determined. 6.5

Concrete Face

6.3.1 Plinth In order to provide a watertight connection between the face slab and the dam foundation, a plinth shall be provided. The plinth shall be founded on strong, non erodible rock which is groutable, and which has been carefully excavated and cleaned up with a water jet to facilitate a low permeability cutoff. The plinth width shall be in the order of 4% to 5% of the water depth (ICOLD 1989, Sherard and Cooke, 1987). The minimum plinth width shall be 3 m. The minimum plinth thickness shall be 0.3 m to 0.4 m. Over-excavation and irregularities shall be filled with additional concrete/dental treatment. The plinth shall be anchored to the rock with grouted dowels of 25 mm to 35 mm diameter, reinforcing steel bars of 3 m to 5 m long, at 1.5 m to 2.5 m spacing, or as decided according to the feasibility stage geotechnical field and laboratory results and recommendations. The stability of the plinth against uplift, sliding and overturning shall be checked. The plinth shall be reinforced to control cracking. ICOLD (1989) and Sherard and Cooke (1987) recommend that a single layer of steel 100 mm to 150 mm clear of the upper surface with 0.3% steel in both directions is adequate. 6.3.2 Face Slab Face thickness: ICOLD (1989) and Sherard and Cooke (1987) recommend that the minimum thickness shall be: • For dams of low and moderate height (up to 100 m), use constant thickness = 0.25 m or 0.3 m. • For high and/or very important dams, use thickness = 0.3 m + 0.002H, where H = water head in metres.


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Reinforcement: reinforcement shall be provided to prevent cracking due to temperature and shrinkage. In general, the face slab is under compression. ICOLD (1989) and Sherard and Cooke (1987) recommend the use of 0.4% reinforcing steel in both horizontal and vertical directions, with possible reduction to 0.3% to 0.35% in areas of the slab that will definitely be in compression. They also recommend that within 20 m of the perimeter and near changes in the plinth slope, 0.5% reinforcing slab shall be provided in each direction. The reinforcing steel shall be provided as a single mat at or just above the centreline. Appropriate vertical, horizontal and boundary ts and water stops shall be provided based on established design procedures, such as those recommended by ICOLD (1989) and Sherard and Cooke (1987). 6.6

River Diversion

Due to large river flows, the cost of diversion by tunnel may be very expensive. Therefore, design of a CFRD shall consider allowing the rockfill to be overtopped, provided that protection measures to prevent the displacement/unravelling of the uppermost lifts or downstream face are taken. Protection measures such as steel reinforcement of the rockfill or gabions anchored into the rockfill shall be considered.

7. Spillway 7.1

General Design Considerations

It is essential to design a safe spillway with ample capacity to prevent overtopping of the dam. The spillway shall be designed for adequate hydraulic and structural conditions. Spillway discharges shall not erode or undermine the downstream toe of the dam. 7.2

Selection of Spillway Type

Open or Closed Type. Whenever an open channel spillway is possible, this design shall have preference over closed type spillway (shaft spillway, siphon spillway, etc.). Gates or No-gates. A non-control type of spillway shall be adopted for embankment dams. Gated spillways are feasible where very good maintenance of the spillway and smooth operation of its gates are assured. 7.3

Selection of Design Flood

7.3.1 USACE Categories USACE recommended guidelines for selecting spillway design flood are based on classifying dams by height and storage impoundment on the one hand, and by harm \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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hazards at the downstream areas in the event of dam failure on the other hand potentials. These provisions are given in Tables 4, 5 and 6. Table 4: Impounding Reservoir Size Classification Size Category Storage (m3)* Small 62,500 to 1,250,000 Intermediate 1,250,000 to 6,250,000 Large 6,250,000 and above 3 *10,000 m = 6.1 ac-ft, and 1 m = 3.28 ft. Table 5: Harm Hazard Classification Harm Loss of Life (extent of Category development) Low None expected (no permanent structures for human habitation) Significant

High

Few (no urban development and no more than a small number of inhabitable structures) More than few

Height (m)* 7.5 to 12 12 to 30 30 and above

Economic Loss Minimal (undeveloped to occasional structures or agriculture) Appreciable (notable agriculture, industry, or structures) Excessive (extensive community, industry, or agriculture)

Table 6: Recommended Spillway Design Flood (USACE) Harm Hazard Size Category Spillway Design Flood (SDF) Low Small 50 to 100 yr frequency Intermediate 100 yr to 1/2 PMF Large 1/2 PMF to PMF Significant Small 100 yr to 1/2 PMF Intermediate 1/2 PMF to PMF Large PMF High Small 1/2 PMF to PMF Intermediate PMF Large PMF 7.3.2 USDA Categories The Forest Service of U.S. Department of Agriculture recommends selecting the spillway design flood criteria as shown in Table 7 below.


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Table 7: Recommended Spillway Design Floods according to USDA Forest Service Hazard Potential Size Class Spillway Design Flood High A PMF B PMF C 1/2 PMF to PMF D 100 yr frequency to 1/2 PMF Moderate A PMF B 1/2 PMF to PMF C 100 yr frequency to 1/2 PMF Low A 1/2 PMF to PMF B 100 yr frequency to 1/2 PMF C 50 to 100 yr frequency Definitions for Table 7 are as follows: Hazard Potential: The classification of a dam based on the potential for loss of life or damage in the event of a structural failure under clear weather conditions with normal base inflow to the reservoir and the water surface at the elevation of the uncontrolled spillway crest. High-Hazard: Dam built in area where failure would likely result in loss of life or where economic loss would be excessive; generally areas of urban or per-urban community type developments that have more than a small number of habitable structures Moderate Hazard. Dam built in area where failure would result in appreciable economic loss, with damage limited to improvements such as commercial and industrial structures, public utilities, and transportation systems, and serious environmental damage. No urban development and no more than a small number of habitable structures are involved. Loss of life would be unlikely. Low-Hazard: Dam built in undeveloped area where failure would result in minor economic loss, damage would be limited to undeveloped or agricultural lands, and improvements are not planned in the foreseeable future. Loss of life would be unlikely. Size Class: The classification of a project for istrative purposes, based on height and storage, as follows: Class A projects: Dams that are 30 m (100 feet) high or more, or that impound 62.5 MCM (50,000 acre-feet) or more of water. Class B Projects; Dams that are 12 to 30 m (40 to 100 feet) high, or that impound 1.25 to 61.5 MCM (1,000 to 50,000 acre-feet) of water.


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Class C Projects: Dams that are 7.5 to 12 m (25 to 40 feet) high, or that impound 62,500 to 1,250,000 m3 (50 to 1,000 acre-feet) of water. Class D Projects: Dams that are less than 7.5 m (25 feet) high and that impound less than 62,500 m3 (50 acre-feet) of water. The inclusion of structures less than 1.80 m (6 feet) high or impounding less than 18,500 m3 (15 acre-feet) of water is optional by the approving officer. 7.3.3 Design Flood Based on the above tables and definitions, the design flood for the spillways of the proposed Giba River dam shall be ½ PMF of 24 or 48 hours (the larger of the two) storm. The effect of 48 hours rainstorms PMF on outgoing routed floods shall be examined as well. At this event the dam shall be capable to withstand also a full wave effect with sufficiently safe freeboard. In addition, the dam shall be checked to be able to contain a full PMF event with a minimum freeboard of 0.60 m. 7.4

Spillway Components

A spillway for embankment dams usually comprise: • Approach channel. • Control structure (overflow section or another type). • Control section. • Discharge channel. • Energy dissipater (stilling basin). • Outlet channel. • River outfall. The design shall make sure that each of the spillway components is given a suitable cross-section to the design flood discharge safely, and to suit the hydraulic needs, the site topography and geologic conditions. 7.5

Basic Hydraulic Design Considerations

The following conditions shall be considered in the hydraulic design of the spillway: • Sub-critical flow condition in the approach channel and low velocity. • Critical flow condition as the water es over the spillway weir crest (control section). • Critical flow at the end of control section (at side channel spillway, etc.). • Supercritical flow condition in the discharge channel (chute). • Transitional flow at or near the termination of the chute, where the flow transitions back to sub-critical conditions.


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• Adequate conditions at the outlet channel and river outfall, to prevent adverse settings which might cause damage. 7.6

Approach Channel

The approach channel conveys the water to the control structure. The guiding considerations in selecting the layout and design of the approach channel shall be: • The entrance velocity shall be maintained lower than critical, while channel curvatures and transitions made gradual to minimise the head loss through the channel. • Water seepage from the approach channel avoided, particularly to under the dam. 7.7

Spillway Control Structure

7.7.1 Overflow Type The overflow spillway shall be given a straight alignment in plan and a cross-sectional shape with the best possible efficiency (standard USBR ogee crest). 7.7.2 Side Channel Type When considered, a side channel for the same spillway crest as above shall be designed such that no submergence of the spillway crest shall occur during the design discharge as above. 7.8

Transition Conduit

The transition conduit of the spillway shall be designed such that no adverse backwater effect is created. The transition shall be designed to the given flow to the downstream without abrupt drop in water surface or excessive turbulence. 7.9

Discharge Channel

The discharge channel of the spillway (chute) shall be designed taking the following into consideration: • Minimal curvature. • Rectangular cross section. • Flatter slopes for the upstream portion of the channel, if possible/economic, steeper slopes for the downstream portion leading to energy dissipater. • Trajectory profile with or without aeration shall be designed where the slope changes from flat to steep to prevent separation of the flow from the channel bottom. • In the case of choosing a closed cross section (tunnel or culvert) for the discharge channel, full-flow condition shall be avoided as far as possible. The flow surface in


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the transition conduit, computed for the design discharge, shall be kept to depth that shall prevent full flow.


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7.10 Energy Dissipater (Stilling Basin) The energy of the flow from the discharge channel must be dissipated before reaching the river. For this purpose, USBR recommendations for hydraulic jump stilling basin shall be adopted. The type of energy dissipater for the spillway shall be selected taking the following into : • Topography, geology and hydraulic characteristics at and around the site of the energy dissipater (flow conditions, tail water level, etc.). • Hydraulic parameters of the energy dissipater under consideration. • Related location of dam embankment and energy dissipater (distance, elevation, etc.). • Location of downstream outlet channel and other restricting factors, such as agricultural farms, housing lots, appurtenant structures, etc. 7.11 Outlet Channel The outlet channel conveys the spillway flow from the stilling basin to the river downstream of the dam. The outlet channel shall have a mild slope to preserve the subcritical flow and to prevent riverbed erosion. Protective measures (riprap, etc.) shall be designed if the flow velocity and downstream conditions make it necessary. 7.12 Tailwater Rating Curve and Backwater Effect A tailwater rating curve that gives the elevation-discharge relationship shall be determined to study the capacity of the river channel downstream of the spillway outlet channel. If the flow conditions will necessitate, the river bed downstream from the outlet channel shall be regulated/protected to best flow conditions. 7.13 Structural Design of the Spillway Once the spillway hydraulic design requirements are satisfied, its structural design shall be undertaken. 7.13.1

Approach Channel

Structural considerations for the approach channel are primarily related to erosion control and slope stability. The approach channel of the spillway shall be protected or lined if its bottom and side slopes are unstable in presence of water or flow. Where seepage from the channel threatens the spillway structure, the dam, and/or the abutment, lining to reduce seepage and uplift underneath the control structure shall be designed.


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7.13.2

Control Structure

The foundation of the control structure of the spillway shall be treated with cutoff and/or curtain grouting in order to reduce seepage through the foundation. Where cracks/ted rock are encountered, the foundation shall be consolidated and grouted as necessary and/or rock anchors be provided for. The control structure shall be designed to withstand all appropriate dead, static live, pseudo-static, and where necessary dynamic loadings. The control structure shall be designed as a gravity structure (dam) with additional loads from the flow over the crest that shall occur under maximum reservoir level. Structural stability, especially sliding stability, is a primary concern here because of the high uplift pressure. The side walls of the control structure shall be designed to withstand all possible combinations of various loadings such as backfill, earth pressure, water pressure, uplift forces and seismic forces. 7.13.3

Discharge Channel

The discharge channel shall be lined with reinforced concrete. The structural design shall consider all acting forces/moments. The discharge channel shall be provided with cutoffs or keys, and ts to stabilise the lining. Drains shall be designed underneath the bottom slab of the discharge channel. Side drainage facilities shall also be provided for preventing runoff from infiltrating behind the side walls in order to safely drain seepage water. Where cracks/ted rock are encountered, the foundation shall be consolidated and grouted as necessary and/or rock anchors be provided for. 7.13.4

Stilling Basin

Energy dissipaters are subjected to dynamic loading in addition to static loading. Dynamic loadings will occur as result of water flow interaction on dented sills, floor blocks, end sills, side walls and floor slabs. These loadings may occur as direct impact loads, pulsating loads due to turbulence, fluctuating water surfaces, water/pore pressure, etc. Each of these potential loadings shall be examined.


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8. Water Abstraction Works 8.1

General

Water shall be abstracted from the impoundment reservoir by pumping, It shall be designed with intention to allow its becoming functional in two stages such that: • The civil engineering shall be implemented to full design capacity in one, initial, go. • Pumping equipment with staged installation of mechanical and electrical units. The intake works shall be designed with multi-intake openings feeding a main feeder section that may enable use of water below final sediment level at least during the first period of the dam life time. The use and quality of water and cost shall also affect the type of crane needed, and the functions of the intake works. The intake works dimensioning is also affected by the chosen operating policy and maintenance procedures. Water front(s) accommodating the inlet openings shall be provided with trash racks facing the reservoir body. The trash racks shall be designed such that: • They can be cleaned manually from the intake works operating platform. • The free opening width between bars shall be of 20 to 25 mm. • The flow velocity through the rack (clean condition) at the maximum design abstraction rate shall not exceed 0.5 m/s. Stoplog s for emergency closure of upstream conduits or water ways shall be suitable for handling by suitable manually operated lifting device provided through gear transmission bulkheads or other means. Intake gates or penstocks shall be of positive pressure type, rectangular with width (horizontal) to length (vertical) ratio of 1:2 and designed to flow velocity at maximum abstraction rate not exceeding 2 m/s. They shall be manually operated from above through gear driven spindle bulkheads. 8.2

Functional Design

8.2.1 Design Parameters a. Characteristic Levels/Elevations • Maximum Water Level (MWL): The actual elevation of the spillway + height of water over the spillway when the maximum flood flow rate is routed (surcharge). • Maximum Operation Level (MaxOL): The actual elevation of the spillway. • Minimum Operation Level (MinOL): The actual final sediment level (considering the dam’s life span) + 1.50 m (height of raw water access opening) + 1.00 m.


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b. Pump Station Floor Level (PSFL) This level, determining the station floor level for pump motors’ installation shall be: PSFL = MWL + 1.00 m c. Flow Rates The treated water flow rates to be as follows: • Stage 1: 44,155 m3/day. • Stage 2: 91,300 m3/day. Daily operating period: 20 hours. Hourly flow rates: • Stage 1: 2,208 m3/h. • Stage 2: 4,565 m3/h. The water treatment plant (WTP) overall efficiency shall be considered as 94%. Consequently, the characteristic flow rates at the Low Lift Pump Station (LLPS) shall be: • Stage 1: QLLPS St1 = 2,340 m3/h. • Stage 2: QLLPS St2 = 4,840 m3/h. 8.2.2 Basic Equipment a. Intake Structure The inlet structure shall be provided with 3 pairs of double grooved openings enabling the abstraction of raw water at three levels, two openings in each level. The proposed specific water entrance velocity for trash racks unprotected by mechanical cleaning devices shall be v = 0.5 m/s. Consequently, the area of each opening shall be: S = QLLPS St2 / (2 * v) = 4,840 / (3,600 * 2 * 0.5) = 1.35 m2 The proposed dimensions of the openings shall be: • Width: 1.00 m. • Height: 1.50 m. Each opening shall be provided with the following elements: • Upstream groove, accommodating: · Trash rack 1.00 x 1.50 m, having vertical bars at 50 mm opening. · Stoplog 1.00 x 1.50 m unique element. · Manoeuvring beam, enabling the operation of trash rack or stoplog under MWL, to be operated manually by means of gantry crane. \\Mekelle4\Mekellewatersupply\SURFACE WATER COMPONENT\FS for SW Dev\Conceptual Design\Design Criteria for Dam.doc

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• Downstream groove, accommodating: · Sluice gate 1.00 x 1.50 m. The gates shall be of seating pressure with three sides “musical note” type gasket and flat type gasket at the bottom. The gates shall be operated from upper level deck by means of manual actuator. The intake structure shall be provided with a gantry crane sweeping all the 6 x 2 grooves. The tentative characteristics of the crane shall be: • Capacity: 1 ton manual hoist, chain operated. • Distance between runways: 2 m. • Length of runways: 15 m. • Movement: · Along: Manual. · Across: Manual, the chain block (hoist) shall be provided with a manually operated trolley. A submersible pump shall be provided to drain the entire intake and wet well, in case of need. 8.2.3 Pumping Equipment The pumps shall be of deep well turbine type with semi-closed impeller, non-removable rotating element and discharge flange above floor. The pumps shall be operated by electric motors. The pumps characteristics shall be: • Number: · Stage 1: 2 + 1. · Stage 2: 4 + 1. • Capacity: Qp = 1,210 m3/h. • Head: To be calculated. Each pump shall be installed within its own cell defined by partition walls. The suction bell of the pump shall be installed at about 1.0 to 1.5 m below MinWL. The tentative diameter of the suction bell shall be: Dsb = 0.6 m Consequently, the wet well floor that has to be at about Dsb / 2 lower than the suction bell shall be determined at 1.3 to 1.8 m below MinWL. The pump station (including the intake) overall tentative dimensions shall be: • Length: 15 m. • Width: 8 m.


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The pump station shall be installed in a building with tentative dimensions of: • Length: 15 m. • Width: 6 m. • Height: to be calculated. A manual overhead crane shall be provided for the installation / dismantling of the pumps. Its characteristics shall be: • Capacity: 2 ton manual hoist, chain operated. • Length of runway: 15 m. • Span: 3.5 m. • Movement: · Along: manual, by means of chain. · Across: manual, by means of chain operated trolley. 8.2.4 Additional Equipment • Discharge branches: DN 400 mm equipped with swing type non-return valves, manual flanged butterfly valves, air valve, manometer, etc. • Discharge manifold: DN 800 mm, equipped with electromagnetic flow meter and manual flanged butterfly valve. 8.2.5 Standby Power Station • Diesel generating set for 100% of Stage 2 capacity. • Bulk fuel tank for 7 to 10 days of 20 hrs/day continuous operation. 8.3

Structural Design

The following loads on the intake works structure shall be taken into : • Self weight ± water, i.e. inside and outside and only outside/or outside only. • Payload. • Buoyancy when empty and water in the reservoir at MaxWL. • wind load. • Wave load. • Earthquake. The structure shall be ed on a slab foundation concept, with or without anchor bars / piles as shall be determined according to site geotechnical conditions. The slab dimensions and thickness shall be determined on the basis of the acting forces, bearing stress on the foundation rock, and shear and moment in the concrete section; earthquake loads shall be considered.


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9. Diversion and Water Release Works 9.1

Functional Design

Preferably, a combined element shall be designed to accommodate the following functions: • Diversion of river flow, including floods, during the dam construction period. • Riparian release. • Bottom release to drain the reservoir and/or achieve at least partial sediment sluicing. 9.2 Diversion Works 9.2.1 General The common practice for diverting streams during construction involves one or more of the following provisions: coffer dams to isolate the dam construction area (preferably, these may eventually be incorporated in the dam structure, an approach channel, a diversion tunnel driven through the abutment or a conduit through or under the dam, and a tail channel. In the design of diversion works the following general considerations shall be given careful attention: • Diversion tunnels/conduits are costly structures, and should be designed conservatively to avoid replacement, enlargement, or extensive modification. • Economic advantage can be obtained by incorporating the coffer dam into the main embankment, and also using the diversion tunnel/conduit later on as bottom outlet. 9.2.2 Selection of Diversion Flood Selection of diversion flood depends on how much risk is involved in the diversion scheme under consideration. In the case of an embankment dam, where considerable areas of foundation and structural excavation are exposed or where flooding/ overtopping of an embankment under construction may result in serious damage or loss of partially completed work, the importance of eliminating the risk is relatively high. Previous design experience in Ethiopia indicates that diversion works for embankment dams were designed for a flood of 50 years return period. Other designers recommend the consideration of a flood with 100 years return period. Careful examination of Tables 3, 4 and Section 6.3 of these Design Criteria and anomalous events like recent years rainy seasons were considered and taken into . Accordingly, it is recommended to adopt a 1:100 years diversion design flood.


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9.3

Riparian/Bottom Release

The aim of these works shall be as follows: • Release of water for riparian rights. • Intentional drain of the reservoir in case of emergency (drawdown – DD) or to allow for major operations in the reservoir area. The USACE recommends that these works be sized to allow evacuating 90% of the reservoir storage within 4 months (120 days). • Sluicing of sediments from the reservoir bottom. It is an important measure that should be activated regularly, for the following purposes: · Release as much as possible trapped sediments in order to extend the reservoir’s service period, and · Keep the entrance to the bottom release works free from sediments in order to allow for its functioning in case it may be deemed. Factors that shall be taken into in the design of the works shall include the following: • Nature of flowing fluid, being suspension of sediment laden water. • Sufficient conveyance capacity and acceptable flow velocity of this fluid ing through the conduit. • Energy dissipation facilities at the outlet side that can sustain the case in of head and fluid consistency. 9.4

Diversion/Release Tunnel/Conduit

9.2.1 Tunnel/Conduit Type A standard “horse-shoe” cross section which provides a flatter base for ease in construction equipment is more advantageous over a circular cross-section. Convenience in grouting work should also be considered in deciding the shape of the diversion tunnel/conduit. For open (non-tunnel/conduit), a trapezoidal section shall be considered, that shall be incorporated with the water outlet and bottom release works. As there might be additional considerations during the design, the final type, shape and cross section shall be decided during the Feasibility Study stage. 9.2.2 Location of Inlet and Outlet Portals In order to minimise the length of the diversion conduit, the inlet and outlet portals shall be located as close as possible to the upstream and downstream extremities of the dam profile.


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The invert level of the diversion works conduit at the inlet shall be placed at an elevation near the river bed level, about 0.50 m above the river bed level. 9.2.3 Conduit Alignment The following shall be considered in selecting the conduit alignment: • The conduit alignment should, as far as possible, be located in competent ground formation, avoiding faults and major shear zones. • The conduit should be aligned on a stable foundation which shall also provide impermeability. • There should be sufficient horizontal and vertical rock cover at a tunnel grade to withstand the internal water pressure. The vertical rock cover shall be adequate to arching action over tunnel cavity. Else, the design shall take such a liability into consideration. • The selected alignment shall be such that it shall allow adopting a simple scheme of works to convert the diversion conduit into bottom outlet after the requirement of construction stage river diversion is over. • The number of bends shall be kept at the minimum possible and the bend deflection be at the minimum possible angles under the given topography. If possible, the conduit alignment shall not interfere or be suitably incorporated with the grout curtain under dam foundation. • Tunnel/conduit portals shall be so located that the stability of ground slopes above the portals shall be ensured without the need for heavy ing measures. 9.5

Basic Design and Hydraulic Considerations

9.3.1 General The water outlet works, conduit/tunnel, inlet and outlet channels, should be able to convey from the reservoir to the downstream: • The design diversion flow (where the water abstraction works incorporated with the diversion facilities). • Riparian flow. • Bottom outlet operation, (flushing, rapid lowering of water surface elevation upon demand/emergency), etc. 9.3.2 Inlet and Outlet Channels In positioning the intake works, the lowest level required for reservoir evacuation, bottom of active storage, sediment deposition level, etc. shall be taken into . It shall be affected by the following factors: • The best use of topography. • Having a good foundation. • Making the diversion conduit shortest as possible.


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• Suitable hydraulic performance in operation.


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• Causing as few complications as possible for embankment construction. • Access bridge not longer than necessary. The inlet channel shall be so located that it shall not be clogged with transported sediment and material from unstable excavated and natural slopes. Cut-slopes near the intake shall be stabilised by riprap, soil-cement or other means, if the need arises. The outlet channel shall be designed with energy dissipater, downstream channel slope and river outfall head regulator, etc. to protect the works and downstream river against erosion or deposition of eroded material. 9.3.3 Conveyance Structure The hydraulic head losses for all components of the conveyance structure shall be calculated. These head losses added to those for the intake structure, gates, valves, and any other source of head loss shall be used as the basis for sizing of the relevant components. Careful attention shall be given to concrete finishes in order to minimise turbulence and possible areas of sub-atmospheric pressure. 9.4

Structural Design Considerations

9.4.1 Conduit Cut and cover type of diversion/outlet conduit shall be designed as a reinforced concrete rigid frame with ts and cutoff walls. All forces from vertical and lateral pressures due to the embankment fill material, seeping water, reservoir water, and the weight of the conduit itself computed bending moment, shear and axial forces, etc. shall be taken into in the structural design of the conduit. 9.4.2 Tunnel The concrete lining of diversion tunnel shall be designed by considering all forces from the rock pressure, external water pressure due to percolating water from the reservoir (for the section upstream of the dam axis) and internal submergence water head, shear and axial forces, bending moments and all other affecting factors. The worst possible combination of all these shall be taken into .


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References • BCEOM & Associates, Abbay Basin Integrated Master Plan Studies, Phase 3, Volume IV, Pre-feasibility study, Part 5 – Irrigation and Drainage – Ribb, 1997. • Fell, R., MacGregor, P. & Stapledon, D., Geotechnical Engineering of Embankment Dams, A.A. Balkema, Rotterdam, 1992. • FEMA, Federal Guidelines for Safety: Hazard Potential Classification System for Dams. FEMA 333,10/1988, U.S.A. • FFMA, Selecting and Accommodating Inflow Design Floods for Dams. Fema 94, 10/1988, U.S.A. • GeoStudio 2004, Geo-slope International, Ltd., Canada, 2002. • Gouin P., Earthquake History of Ethiopia and the Horn of Africa, 1977. • I.C.E., Floods and Reservoir Safety, 3rd Edition, Engineering Guide, 1996, U.K. • Kesem Dam Draft Final Design Report, Section 005 – Dam and Ancillary Structures, WWDSE in cooperation with WAPCOS (India), 2003. • Mott MacDonald, MCE, WWDSE, Koga Irrigation Project Dam Design Report (Draft) 6/2003. • Nakano, R., Design of Fill-Type Dams, Japan International Cooperation Agency (JICA), 1991. • National Academy Press, Safety of Dams: Flood and Earthquake Criteria, 1983. • Robert, B. Janson, editor, Advanced Dam Engineering, For Design, Construction and Rehabilitation, 1986. • Sherard, J. L., Woodward, R. J., Gizienski, S. F. & Clevenger, W. A., Earth and Earth-rock dams, Engineering Problems of Design and Construction, John Wiley and Sons, 1963. • Tendaho Dam and Sugar Project, Draft Final Report, Section 005 – Dam and Appurtenant Works, WWDSE in cooperation with WAPCOS (India), 2003. • The Norwegian Regulations for Planning, Construction and Operation of Dams, Norwegian University Press, 1981. • Thomas, H. H., The Engineering of Large Dams, Part 1 & 2, John Wiley and Sons, 1974. • U.S. Army Corps of Engineers, General Design and Construction Considerations for Earth and Rock-Fill Dams, EM 1110-2-2300, 30 July 2004, and other EM publications. • United States Bureau of Reclamation (USBR), Design of Small Dams, 1987. • United States Bureau of Reclamation, Design Standards No. 13, Embankment Dams.


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