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«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 ...»

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Training notes by Prof. Bancy Mati

1.1 What is a small earth dam?

A dam is a structure or barrier constructed across a valley, river or stream to conserve, store or to

control the flow of water (figure 1.1) The water may be used for drinking water supplies, hydroelectric power generation, irrigation, or environmental conservation. There are many types of

dams, based on their use, construction material, size and shape. Depending on construction material, dams can be made of concrete, rock-fill, masonry or earth. Concrete dams may take various forms. These include gravity dams which are huge structures designed to use their own dead weight to resist the horizontal force of the water. An arch dam is built with its convex front facing the upstream side of the valley or reservoir, and derives its strength essentially to its shape.

Thus an arch dam uses less concrete than a gravity dam. Concrete-buttress dam, is a gravity dam reinforced by structural supports, thereby reducing material needed to construct the wall itself by using support buttresses around the outside base.

Figure 1.1 (a) Sketch of a three- (b) Earth dam and reservoir dimensional view of an earth dam (photo courtesy of Jean Muhinda) Earth dams utilize soil of good compaction quality to build up the embankment, and are variously known as earth-fill dams.

Earth dams rely on their weight to hold back the force of water, just like gravity dams. The cross-sectional profile of an earth-fill dam is a broad-based triangle. Thus, a small earth dam is one whose embankment is basically constructed using compacted earth.

A small earth dam has a crest height ranging 2 to 5 m from high, while the reservoir capacity is at least 5,000 m3 but less than 1 million m3 storage volume. They can be designed by local technicians, built and managed/maintained by user communities. The dams can be of uniform material, or have clay core for better seepage control. They also have spillways to protect them from overtopping excess runoff flows.

Small earth dams are usually constructed for rainwater harvesting or on small rivers to retain flood runoff during the rainy season, on a watercourse which may be a perennial river or a dry riverbed.

The dam wall has a clay core, while the outlet has a stone apron and spillway to discharge excess runoff. Sediment traps and delivery wells may help to improve water quality but, as with water from earthen dams, it is usually not suitable for drinking without being subject to treatment. Small earth dams can provide adequate water for irrigation projects as well as for livestock watering.

Figure 1.2.

Sketch of an earth dam cross-sectional profile (Source: Danida, 2006)

1.2 Why construct a dam?

Dams have various uses. These include:

Irrigation: Small earth dams are particularly useful for providing water for irrigation in dry areas, and from streams which have low or no flows during the dry season.

Water supply: Small earth dams are particularly useful for drinking water supplies to rural as well as urban communities. The multi-purpose nature of dams makes them ideal for community scale water supplies for irrigation, livestock watering and domestic water supplies Water diversion: small dam can be used to divert water for irrigation, power generation, or other uses. Sometimes, they are used to divert water to another drainage or reservoir to increase flow there and improve water use in that particular area.

Figure 1.3 Water diversion from small earth (b) Diversion chute from an earth dam dam using a weir (photos by B.

Mati) Stabilize water flow: Dams are often used to control and stabilize water flow, often for agricultural purposes and irrigation Hydro-power generation – Small earth dams can be used for hydropower generation. This is particularly possible in dams having a steady flow and built across a gorge where there is a relatively good head drop. Many countries that have rivers with adequate water flow, that can be dammed for power generation purposes.

Land reclamation: Dams are used for land reclamation and to prevent innudation of water to an area that would otherwise be submerged. This facilitates reclamation of such areas for other use.

Normally, dykes or levees are used for diverting the water Flood prevention: Dams are sometimes constructed to impound excess flows during the rainy season and prevent flooding of downstream areas or infrastructure. They help to stabilize river flow especially of ephemeral streams.

Recreation and aesthetics: Earth dams provide a water body that can be used for recreational activities such as swimming, fishing, or tourism. Other than the water itself, a dam allows for creation of greenery and ecosystem restoration which are added benefits to agricultural use.

1.3 Types of earth dams

1.3.1 Earth-fill dams Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are constructed as a simple embankment of well compacted earth. Earth dams are trapezoidal in shape. They are constructed where the foundation or the underlying material or rocks are weak to support the masonry dam or where the suitable competent rocks are at greater depth. Earthen dams are relatively smaller in height and broad at the base. Earth dams are mainly built with clay, sand and gravel. The uupstream face of an earth dam is usually protected from erosion by a surface layer of flat rock, called riprap.

There are three sub-types of earth dams; (i) homogeneous type dams are constructed with a single type of soil throughout the cross-section, (ii) a zoned type dam, has an impervious core zone surrounded by a relatively pervious zone, and (iii) a diaphragm type dam, whereby a tall impervious wall of less than 10 m thickness replaces the impervious zone. Modern zoned-earth embankments employ filter and drain zones to collect and remove seep water and preserve the integrity of the downstream shell zone. Because earthen dams can be constructed from materials found on-site or nearby, they can be very cost-effective in regions where the cost of producing or bringing in concrete would be prohibitive.

1.3.2 Rock-fill dams Rock-fill dams are a variation of earth dams, whose embankments are constructed using loose rocks and boulders instead of soil. However, an impervious zone is created on the upstream face of the dam, made of masonry, concrete, plastic membrane, steel sheet piles, timber or other material. The impervious zone may also be constructed as a central cross-sectional pillar within embankment in which case it is referred to as a core. Rock-fill dams can be made with a steeper slope hence narrower than earth dams. When suitable rock material is available at site, transportation is minimized leading to cost savings during construction.

In cases where clay is utilized as the impervious material, the dam is referred to as a composite dam. To prevent internal erosion of clay into the rock fill due to seepage forces, the core is separated using a filter. Filters are specifically graded soil designed to prevent the movement of fine grained soil particles into the rock fill. Rock-fill dams are stable and more resistant to earthquakes due to the fact that the embankment structure contains loose particles which can vibrate independently. However, proper compaction and good quality control must be ensured during construction to prevent poor failure and seepage problems.

1.3.3 Regulating dam A regulating dam is one that is designed with a capacity to store the flash floods from a single day’s rainfall, and then release it slowly thus reducing the danger such a flood would have posed e.g.

causing erosion downstream. The reservoir therefore has a permanent water outlet that releases the stored water at a flow rate of minimum risk. The stored water drains away continuously until the reservoir is dry in a day or two, ready to receive the next flash floods. An adequate spillway must be provided to guard against the collapse of the dam. The dam embankment can be earthen, concrete or packed stone gabions.

1.3.4 Dry dam A dry dam also known as a flood retarding structure. It is a dam designed to control flooding. It normally does not hold back any water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream, that time, it stores water temporarily. A dry dam is a kind of regulating dam, but without water storage during the dry season.

1.3.5 Silt trap dams Silt trap dams are made across water courses/rivers to protect downstream structures from sedimentation. They are designed like ordinary earth dams, but the spillway is raised to enable sediments to sit in the dam. A simple technique, which can be adopted by smallholder farmers involves using old gunny bags packed with soil packed on each other (Figure 1.4). The soil is scooped form adjacent areas. It is a very low cost technique. This type of structure is temporary and should not exceed 2 m high since it is not very strong. The embankment made of soil-packed bags can be re-built each season to improve its storage capacity. As the soil accumulates, the embankment is grassed to improve its stability.

Figure 1.4 (a) Sketch of a silt trap dam (b) Silt trap dam made of soil-packed gunny bags made of soil-packed sacks (photo by B.

Mati) 1.3.6 Valley dams Valley dams are the dams normally built across valleys and small seasonal water courses, which, may be on the boundaries between two or more landowners. Valley dams are normally made of earth, and are thus earth fill dams (Figure 1.5). However, they are shorter and thus have smaller reservoirs. Since valley dams can collapse during exceptionally heavy rainfall due to poor maintenance, incorrect design or poor construction work, this could endanger people and structures downstream.

Figure 1.5 (a) Valley dam for water (b) Valley dam with rock toe harvesting (photos by B.

Mati) 1.3.7 Hillside dam A hillside dam is an off-stream storage reservoir, constructed on sloping land or on a hillside.

The system comprises a small dam, a collection area, a reservoir, a dyke, a spillway and a water draw-off device. It is constructed in hilly areas to capture runoff from catchments of area 100 to 2,000 ha. The water may enter the dam from surface runoff harvesting from the catchment above it, or it is brought by pipe or canal to the dam (pumped or by gravity). The main advantage of a hillside dam is that water may be taken down for use by gravity for irrigation or other purposes.

The dam embankment can be made of compacted earth, although concrete can also be used. The storage capacity of the reservoir can be about 10,000 to 400,000 m3. Since it lies on a hill, water is easily withdrawn by gravity through a pipe. Hill reservoirs tend to be more expensive than other conventional water harvesting systems because of their size and location, but they have many advantages as the water can reach larger downstream areas due to the height difference. The difference with ordinary pans and ponds is that a hill reservoir may not be located in a valley, thus making it possible to harvest large volumes of water from hills (Figure 1.6).

Figure 1.6 (a) Illustration of a hill reservoir (b) Hill reservoir lined with concrete system with irrigated area (photo by B.

Mati) The storage to excavation ratio of hillside dams can be quite low (1.5 is common) and increasing the value is desirable. The site should be as flat as possible, and with good soil suitability and catchment yield. If necessary, the dam can be covered with concrete or clay grouting to prevent seepage. Although hillside dams are less prone to failure than gully dams, they still require sound design and construction to be successful.

1.3.8 Hafir dams “Hafir” dams are usually small earthen reservoirs dug into the ground in gently sloping areas that receive runoff either from flood flow diverted from streams or from large catchment areas.

Generally, they have a volume ranging from 500-10,000 m3 and are used to store water for human and livestock consumption. Hafirs are located in natural depressions and the excavated soil is used to form an embankment around the reservoir to increase its capacity. Wing walls and improvements to the catchment apron may help to increase runoff into the reservoir, but seepage and evaporation are often high in the dry season. Hafirs differ from water pans in that they are generally bigger in size, and also have good sedimentation basins. In hafirs, watering areas are well allocated, the site is securely fenced and the reservoir is de-silted every season.

The major drawback with hafirs is the requirement of periodic cleaning to remove silt, which is not an easy task. Sediment traps and delivery wells may help to improve water quality but, as with water from earth dams, it is not usually suitable for drinking without some form of treatment.

1.3.9 Gully dams Gully dams are also off-stream dams since they tend to be located on an artificial watercourse which is on a hillside. Most gullies are formed from severe soil erosion due to uncontrolled surface runoff emanating from catchments above the gully or from road drains. Thus, a gully dam combines water harvesting with soil conservation (Figure 1.7). The method takes advantage of only those gullies or depressions which are stable enough and suitable for water storage.

A gully dam is designed just like any other small earth dam. However, care is taken to anchor the dam properly to the sides. Also, a large spillway is usually recommended at the centre of the structure, to act as a weir and thus prevent excess water from undermining the structure. The banks of the dam are built from soil material dug from an excavation contained within the storage area (see Figure 1). Gully dams are commonly used for storages ranging from 1,000 to 50,000 m3.

They normally offer a low capital cost per unit of the stored water capacity.

Figure 1.7 (a): Sketch of a plan view of (b) Gully dam in an active gully a gully dam (photo by B.

Mati) Water yield from gully dams can be delivered by gravity downstream for use in irrigation or livestock watering. All gully dams have a significant risk of failure either due to insufficient water, excess water, spillway erosion, bank overtopping, bank failure or excessive seepage. The demands for quality design and construction increase as banks increase in height. It is preferred to make the banks as low as possible to reduce chances of failure. In all cases considerable attention must be paid to compaction of the bank and consequently, the moisture content of the material when being packed into the bank. Gully dams generally have larger water flows moving into the storage than hillside dams. This is good for replenishment of dam water but it also demands careful design and construction of the spillway (and freeboard) to safely cope with flood flows. Ultimately, a gully dam may fill with sediment, thus it is also a silt trap dam.

1.4 Design of small earth dams

1.4.1 Components of a dam A small earth dam comprises a water impoundment zone or reservoir, the dam embankment itself, water outlet and delivery works, spillway and control facilities. The inflow of water into the reservoir must be monitored continuously and the outflow should be controlled to achieve optimum benefits. Under normal operating conditions, the reservoir is controlled by the outlet works, consisting of a channel or conduit at stream level with control gates. The main parts of a

dam include:

a) Heel: contact with the ground on the upstream side

b) Toe: contact on the downstream side

c) Abutment: Sides of the valley on which the structure of the dam rest

d) Galleries: small rooms like structure left within the dam for checking operations.

e) Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry.

f) Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side

g) Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.

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