Sand Dams and Subsurface Dams

Decentralised storage of water is an important strategy in drylands like semi-arid and arid regions outside the reach of perennial rivers, springs, deep groundwater or other water sources.According to ED (n.y.) 40% of the world’s land surface is classed as ‘drylands’, which sustains 80% of the world’s poorest people.

Storage of water from the rainy season to the dry season, or even from wet years to dry years is highly important in such areas. Sand dams and subsurface dams (more general called groundwater dam, VSF 2006) can store sufficient quantities of water for livestock and minor irrigation as well as for domestic use. If sited and built properly, dams can give an appropriate answer to the water need. Unfortunately there are also many examples of groundwater dams, which have not been successful due to insufficient technical and social assessment and design.

A sand dam is a small dam built above ground and into the riverbed of a seasonal sand river, conditions that are found across the world’s drylands. Its purpose is to capture and store water beneath sand (ED n.y.): upstream of a sand dam sand accumulates, resulting in additional groundwater storage capacity of riverbed and riverbanks. This reservoir fills during the wet season, preventing quick runoff of valuable rainwater out of the catchment and out of the reach of the community. Water availability during dry seasons is prolonged and generally guaranteed. The quality of the water is protected against evaporation and contamination. Water quality is even improved through natural filtration in the soil (similar to what happens in a biosand filter). The technique of sand storage dams is not new: storage of rainfall and runoff, including subsurface storage, for beneficial use has been applied since 9,000 BC (HOOGMOED 2009).

Basic design principles

Before starting a sand dam project in an area, the community must be intensively involved to create a feeling of ownership, which has proven to be the key factor in successful construction and maintenance of sand dams (see also RAIN n.y.; decision making, demand creation, stakeholder analysis and visioning). Different types of sand dams can be distinguished (adapted from RAIN n.y.):

• Stone-masonry dam: A dam built of concrete blocks or stones. Local artisan can easily construct this type of dam. A stone-masonry dam is durable and suitable for any dam height. The dam is cheap when construction materials are available within the dam area.
• Reinforced concrete dam: A dam consisting of a thin wall made of reinforced concrete. It is a durable structure, relatively expensive but suitable for any dam height.
• Earth dam: A dam consisting of impermeable soil material (mostly clay or clayey soils, or black soils). This type of dam is relatively expensive to construct and it requires special skills for its design and construction. An earth dam can easily be damaged and even destroyed by underground flow. Earth dams are not popular and are seldom used (only for minor works).

Site selection and construction

(Adapted from PRACTICAL ACTION n.y.)

The first step is to carry out a site survey, which involves analysing the geological and physical characteristics of the site, especially the underlying rock structures and soil properties. Riverbeds with crystalline rocks and coarse sand have higher yield compared with volcanic rocks. Similarly, river valleys and regions sloping between 1 and 2% are ideal sites for sand dams as they normally give the highest water storage. Knowledge of hydrological data is important for estimating the total stream flow, size of river transportation thereby influencing the thickness and height if the wall. Information on geological and topographical characteristics and even hydrological data can all be sourced from relevant Government departments.

The second step is to obtain the required tools and materials. Essential materials include waterproof Portland cement, nails, binding wire, timber, sand and stones, which are sometimes readily available along the riverbeds. Other equipment includes spade, sledgehammer, shovels, wheelbarrows, power mixer and water containers. Where stones are readily available, masonry sand dams are recommended; otherwise concrete walls are equally strong and durable. To construct sand dam a deep trench is first dug across the valley wall or stream, reaching the bedrock or other stable layer like clay. To cut costs, local labour should be mobilised and involved in this process (community involvement).

A concrete or masonry wall is then built on the underlying rock bars across the river channels so that it can trap and hold back the sand brought by the river during flooding. The height may range between 2 to 5m high depending on the depth of the underlying rock or other stable layer.

At either end of the dam especially where the valley sides are flat wing walls may be added at an angle to the main dam to direct and confine the flows of channel as the sand stores water in its spores. Since the natural sorting and deposition of sediments in streams is a function of channel slope and the shape of channel cross section, channel geometry is quite important in sighting the prospective sand dam. While channel slopes may vary in different valleys and regions a slope of between 1 and 2% normally gives the highest water storage. The specific storage normally increases at the lower slopes than the higher ones.

After the construction of a sand dam, a new channel cross section is created together with new gentler channel slope immediately upstream of the dam. The modified channel must safely pass the highest expected flood without overflowing the banks and threatening the bank abutments. In addition to rock outcrops for firm foundations, high riverbanks are another desirable feature. Where banks are low the dam has to be raised on either or both sides and wing walls extended beyond the banks in order to direct floodwater and prevent it from cutting around the dam.

NISSEN-PETERSEN (2006) promotes a different construction method, where the dam and the spillway is heightened sequently with each flooding, in order to catch only the coarse sand and to allow the finer material to be washed downstream (see training material for further information).

Water extraction from sand dam

(Adapted from RAIN n.y.)

Scoop hole: the most common way to abstract water from a sand storage dam is by means of a hand dug scoop hole in the riverbed. Water is scooped out at first, and the water that refills the scoop hole is abstracted. This method is susceptible for pollution, especially by animals.

Well with hand-or rope pump: a well is the better alternative for a scoop hole. It will protect the water quality because animals and river water cannot enter. Many different types of wells exist. A well need to be covered (to prevent contamination, resulting in higher water quality) and a hand pump should be used to extract water.

Outlet pipe with tap: An outlet can be installed as a perforated pipe at the bottom of the dam just above the impermeable layer. The pipe should be covered fully with filter material and geo-membrane to prevent entry of sand and silt. Disadvantages of an outlet, is that it can weaken the dam structure, maintenance is complicated and it is also found be an expensive option.

(Adapted from VSF 2006)

A subsurface dam obstructs the groundwater flow of an aquifer and stores water below ground level. It is built entirely under the ground.

Basic design principle, site selection and construction

Similarly, when a sand dam is constructed it is necessary to excavate a trench in the sand bed in order to reach bedrock, which can be used to create a subsurface dam too. Groundwater dams are built across streams or valleys. A trench is dug across the valley or stream, reaching to the bedrock or other stable layer like clay. An impervious wall is constructed in the trench, which is then refilled with the excavated material. A subsurface dam is constructed below ground level and arrests the flow in a natural aquifer. The best sites for construction of groundwater dams are those where the soil consists of sands and gravel, with rock or an impermeable layer at a few metres’ depth. Ideally, the dam should be built there where rainwater from a large catchment area flows through a narrow passage.

The optimum zone for constructing a subsurface dam is generally found on gentle slopes in the transition zone between hills and plains. Finding suitable places to build the dam is harder when the river is wider. In view of an efficient reservoir it is important that it is based upon impermeable beds or bedrock are underlying the reservoir. In general the topographical gradient of the construction sites is between 0.2 to 4%, but in extreme cases slopes of 10 to 16 % have been used (NILSSON 1988). Underground reservoirs in sand-rivers are principally recharged by rainwater from flash floods, which originate in catchment areas with higher elevation. A single and short-lived flash flood may fully recharge a reservoir with water. Upon full saturation of the reservoir, the remaining flash floods will pass over the dam without further infiltration, replenishing aquifers downstream. The water is extracted through a well, which may be placed in the reservoir or by a gravity pipeline if the topographical conditions are favourable.

(Adapted from VSF 2006)

As becomes clear from the figures, the general principle of sand storage dams and subsurface dams is similar. In general, sand dams are used when the topographical gradient is high and subsurface dams are used when the topographical gradient is low. If the impervious layer lies near the surface, the storage capacity of the basin will be low. A sand storage dam will enlarge the storage, while a subsurface dam only uses the storage capacity under the surface. When the river is narrow and has high embankments, the storage capacity of a sand storage dam can easily be enlarged. The embankments provide a good anchoring possibility.

The cost of an average sand dam with a minimum life span of 50 years and storage of at least 2.000 m³ is about US$ 7.500 (2-4 meters height and 20 meters length; example from Kenya; see case studies in RAIN n.y.). In the case study of Kenya the community covered about 40% of the overall construction costs by being involved in the construction of sand storage dams by provision of labour and raw materials through sand dam management groups (RAIN n.y.).

According to NISSEN-PETERSEN (2006), subsurface dams are less expensive and easier to maintain than sand dams, but do have less capacity. Subsurface dams, basically made out of clay, were constructed in Kenya, with a capacity of 425 to 1342 m³ and at a cost of 900 to USD1600 (VSF 2006).

According to PRACTICAL ACTION (n.y.)the water is clean and of good quality for consumption due to filtering effect of the sand. Concerning sand dams: if scoop holes are used for water extraction, attention should be given regarding pollution from animals. Animals and humans should use different scoop holes far apart from each other. And the human’s one should be located close to the dam (RAIN n.y.). Read more about water pollution and water borne diseases.