«1.0 WATER HARVESTING AND STORAGE IN VALLEYS USING SMALL EARTH DAMS Training notes by Prof. Bancy Mati 1.1 What is a small earth dam? A dam is a structure ...»
h) Dead storage: the amount of water that remains in the dam at the lowest level. It also comprises the part of the reservoir that cannot be drained by an outlet or by pumping. The latter depends largely on the suction arrangements of the pumping set up. Note should be taken that it is not always wise to drain a dam completely, most especially if ‘cracking clays’ have been used in the embankment, core or reservoir floor.
1.4.2 Requirements of a good dam design The design of an earth dam considers the technical, social, economic and environmental data.
Preliminary designs and cost estimates are prepared and reviewed by hydrologic, hydraulic, geotechnical, and structural engineers, as well as geologists. Environmental quality of the water, ecological systems, and cultural data are also considered in the site-selection process. Factors that affect the type and size of structure include the topography, geology, foundation conditions, hydrology, possibility of earth movements, and availability of construction materials. The foundation of the dam should be as sound and free of faults as possible. All dams are designed
and constructed to meet certain basic requirements, which include:
(i) It should consider peak flood flows, and the design of spillways and other protective structures, (ii) The dam should remain stable under all conditions, i.e. during construction, while in operation, both at the normal reservoir operating level and under all flood and drought conditions.
(iii) The dam wall and its foundation must be watertight to control seepage and maintain the desired reservoir level.
(iv) The dam should have sufficient spillway and outlet capacity (v) A freeboard is usually included in the design to prevent floodwater overtopping.
(vi) Impacts of the dam on the water table of affected areas, and whether this is desirable or not.
(vii) Reservoir silting, which should be minimized or accounted for in the dam design, (viii) Environmental impacts on river aquatic life e.g. riparian vegetation, fish and fisher-folk, (ix) Impacts on human habitations, and resettlement There are costs associated with the compensation for land being flooded as well as population resettlement. This may also include the removal of toxic materials and buildings from the proposed reservoir area.
1.4.3 Site selection criteria The most suitable site for a small earth dam is where the valley will enable the construction of a straight embankment dam. Such sites are normally found at valley cross-sections where a natural deep gorge exists. Sometimes, a natural depression on sloping ground can provide a good site for a curved hillside dam. It is essential to select a site where the dam foundation will be watertight and without seepage, while it accords ease of construction and a stable structure can be assured. Thus,
small earth dams should be sited in areas which bear the following characteristics:
(i) The site should be located where surface runoff from rains on the catchment area, or other runoff flows, can fill the dam reservoirs at least once a year. The dam must have the potential to fill with runoff (most years) or store sufficient water between runoff events that fill the reservoir. It is essential that the dam and reservoir have sufficient depth and volume to last through extended periods of drought.
(ii) A topographical survey of the proposed dam site is normally done to determine features such as slope, width and height of dam, reservoir capacity, as well as to estimate costs, prepare necessary information for licensing and provide construction details.
(iii) The dam site should be selected on a natural valley which will provide a relatively high depth to surface area ratio (for a given design volume), to minimize evaporation losses.
A simple way of identifying such a site is where the valley is bounded by steep hillsides.
The dam can also be sited just below the confluence of two tributaries to gain more volume. One of the best sites for construction of a dam is a narrow part of a deep river valley; the valley sides then act as natural walls. The primary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The sites are usually those where the gap becomes a minimum for the required storage capacity. The current use of the land to be flooded should be dispensable.
(iv) Thorough site investigations are needed especially for the dam foundation to avoid cracked, loose soil or other weaknesses that may cause seepage or failure. The dam foundation must be solid impermeable rock with no soil pockets or fracture lines, while rock surfaces should not be fractured or cracked, to avoid causing leakage losses. In some cases, field pumping tests are performed to evaluate seepage potential.
(v) Soil conditions must be suitable for both compaction and the prevention of seepage losses through the dam. Pre-construction soil testing should be done at the proposed site. This testing can be accomplished by digging several test pits where the dam and reservoir is to be located. Soils should be checked to depths at least a metre below that of any proposed excavation for the dam or reservoir. T (vi) There should be no soil erosion in the catchment area, no anthills, pits, sewage outlets, saline or calcarous soils. An assessment of the hazard potential downstream should be done. Watershed activities that could affect the water quality or quantity of runoff are also assessed.
(vii) Location must be convenient for the user group. This could be where people and livestock are in need for water and where the community has implemented soil conservation measures on any cleared land in the catchment area of the dam (viii) Land tenure and ownership also affect sire location. The dam should, wherever possible, be located on public land with access road for the community members and users of the dam.
(ix) Location of dam also considers the cultural and socio-economic conditions so as to serve a large population, without infringing on laws, customs and social structures of beneficiary communities. Whenever possible, local communities should be supportive of the dam.
1.4.4 Dam capacity and side slopes A typical design of a small earth-fill dam is shown in Figure 4. For stability, the upstream slope must be a minimum of 3:1. Erosion protection is required to protect the dam from wave action.
This protection can be achieved with a combination of smaller and larger rocks (or other suitable material) and, with smaller projects, a floating log boom. The downstream slope requires a minimum 2:1 slope, seeded with native grasses to prevent surface erosion. The top or crest of the dam should be a minimum of 3 m wide (preferably 5 m) to accommodate road traffic and minimize the potential for erosion. The crest elevation should be a minimum of 1 m above the full supply level (FSL) of the reservoir. The dam should be fenced to prevent livestock traffic, as this traffic can be a major cause of slope and crest degradation. The water storage capacity of a dam and
reservoir (shown in Figure 1.8) can be estimated as follows:
Dam capacity = [Reservoir Length x Width (at the dam) x (Max. depth of the Water)]/3 Design of earthen Bund
The various components of an earthen bund include:
(a) foundation including key trench or cut-off, (b) height of bund, (c) side slopes, (d) top width, (e) free board, and (f) settlement allowance.
(a) (b) Figure 1.8 (a)Sketch showing components of a small earth fill dam, and (b) the cross-sectional area It is possible to construct a stable and economical earthen bund on any foundation. Sites with foundation conditions requiring relatively expansive construction measures should be avoided.
The most satisfactory foundation is one that consists of, or is underlain at a shallow depth by a thick layer of relatively impervious consolidated material. Such foundations cause no stability problems. Where a suitable layer occurs at the surface no special measures are required. It is sufficient to remove the top soil (with vegetation and roots) and plough the area to provide a good bond with the new fill material of the bund.
Where the impervious layer is overlain by pervious material (sand), a compacted clay cut-off extending from the surface of the ground into the impervious is required to prevent excessive seepage and to prevent possible failure by piping.
Determining Top width The minimum allowable top width (W) of the embankment shall be the greater dimension of 3
m or W, as calculated by the following formula:
W = (0.67H + 3); where H is the height of the embankment (in metres) The sides of the dam embankment (Figure 5) should slope at an angle that will provide a stable structure depending on the type of fill materials (Table 1.1). The upstream slope of earth dams should be no steeper than 1 vertical on 3 horizontal. The downstream slope of earth dams without seepage control measures should be no steeper than 1 vertical on 3 horizontal. If seepage control measures are provided, the downstream slope should be no steeper than 1 vertical on 2 horizontal.
The upstream side should be covered with stone pitching (rip-rap) and filter with recommended depth of 20 to 30 cm.
Height of Bund The height of bund will depend upon the volume of runoff to be stored and topography of the reservoir area. The high of the bund should also be selected in such a way that its cost per unit of storage (cum volume) is minimum. While calculating the cost corresponding to any height some allowance for settlement and free board, and temporary flood storage may be added to give the actual bund height or in other words the actual quantity of earth work.
Top Width of Embankment Adequate top width is provided to the bund so that it can be used as road way and communication routes adjoining villages or watersheds. Simple formulae for top width (T.W.) as a function of height (H) may be used.
Side Slope of Bund Adequate upstream and downstream side slopes of the embankment must be provided to satisfy the stability requirements of reservoir filled with water, sudden drawdown to minimise the erosion, and to facilitate establishment of good sod forming grass. The maximum side slopes recommended in case of small earth dams are given below in Table 2.1.
Table 1.1: Side slopes of earth dams according to fill materials used
Foundation Cutoffs Usually a cut-off joining the impervious stratum in the foundation with the base of the dam is needed. The most common type of cutoff is one constructed of compacted or puddled clay material. A trench, also called key-trench, is cut parallel to the central line of the bund to a depth that extends well into the impervious layer. The trench should have a bottom width of not less than 1.5 meters but adequate to allow the use of mechanical equipment if necessary, to obtain proper compaction. The sides of the trench should be filled with puddled clay or with successive thin layers of relatively impervious material each layer being properly compacted.
Free Board It is the added height of the bund provided as a safety factor to prevent waves and flood runoff from over-topping the embankment.
(i) Minimum free board (F.B.) for length of pond up to 400 m 50 cm (ii) F.B. for length of pond up to 800 m 75 cm (iii) F.B. for length of pond more than 800 m 100 cm.
Settlement Allowance This includes the consolidation of the fill materials and the foundation materials due to the weight of the bund and increased moisture caused by the storage of water.
Hand compacted (manually constructed) fill = 10% of design height Machine compacted = 5% of design height.
1.4.5 Spillways A spillway is a conduit or channel made on a dam and designed to pass water from the upstream to the downstream side of a dam (Figure 1.9). Many spillways have floodgates so as to control the flow through the spillway. The spillway should be designed with a wide base and a gentle slope, which will reduce water velocity and spillway soil erosion. The spillway base and sides should also be seeded to grass. To prevent spillway erosion, riprap (a collection of loose stones) alone or in combination with geotextile material may be required if the base slope of the spillway is steep. Side slopes of the cut spillway should be no less than 2:1 (4:1 slopes are preferred).
(b) The completed spillway of a dam Figure 1.9 (a) Spillway under construction (photo by B. Mati) (photo by Jean Muhinda) The spillway should be located away from the dam fill, not through or directly adjacent to the fill.
This placement will reduce the risk of the dam washing out. Culverts are often used in spillway design, and if undersized, they can restrict spillway flow and result in project failure.
Types of spillways
There are several designed of spillways depending on operation. They include:
A fixed weir spillway – which is usually at the centre of the structure and allows the overspilling for the common floods.
A service spillway or primary spillway is one which allows the passage of normal flow.
An emergency spillway is designed for extreme conditions, such as a serious malfunction of the service spillway.
An auxiliary spillway releases flow in excess of the capacity of the service spillway.
A fuse plug spillway is a low embankment designed to be over topped and washed away in the event of a large flood.
Fusegate elements are independent free-standing block set side by side on the spillway so as to allow an increase in the normal pool of the dam without compromising the security of the dam because they are designed to be gradually evacuated for exceptional events.
1.5.1 Environmental impact assessment Before the construction of any dam, an environmental impact assessment (EIA) is usually done to ensure that there will be no adverse effects on the human livelihoods as well as ecosystems affected by the dam. Reservoirs hold large quantities of water which could affect many ecological aspects of a river, particularly flow to downstream water users. An EIA can take several scenarios, such as the benefits to human society arising from the dam (agriculture, water, damage prevention and power), the harm or benefit to nature and wildlife, impact on the geology of an area. It also considers whether the change to water flow and levels will increase or decrease stability, and the disruption to human lives. Water releases from a reservoir including that exiting a turbine usually contains very little suspended sediment, and this in turn can lead to scouring of river beds and loss of riverbanks; causing erosion. Reservoir may also host pests and other disease causing organisms which were not endemic to the area. Positive environmental attributes of a dam should also be factored such as improved tree cover, and recharge of ground water resources.
1.5.2 Handling existing flows An important consideration in dam construction is how to handle the stream flow around or through the dam site during construction works. Stream flow records provide the information for use in determining the largest flood to divert during the selected construction period. One common practice for diversion involves constructing the permanent outlet works, which may be a conduit or a tunnel in the abutment, along with portions of the dam adjacent to the abutments, in the first construction period. The stream is diverted into the outlet works by a cofferdam high enough to prevent overtopping during construction. A downstream cofferdam is also required to keep the dam site dry. See also Cofferdam.
A cofferdam is a temporary barrier, dam or embankment constructed to divert water from its normal course during the construction of a dam, bridge or such other structures. It can be made using concrete, steel sheet piling, or wood. When the construction is completed, the cofferdam may be demolished or removed. Sometimes, a coffer dam may be made as a closed or open channel, which is converted into a pipe shaft on completion of the project. The construction of
an earth dam involves several steps which include the following: