Selecting potential riverbeds

The result of the desk study results in a first indication of which (parts of) catchments are suitable for constructing sand dams. Community involvement is crucial to the success of the sand storage dam implementation project from this step onward. Information on community involvement can be found here. Information from the community needs to be integrated with the probability map from the desk study, resulting in a more fine tuned map of the most potential (parts of) riverbeds. These will be visited together with the water committee to collect valuable data based on which building location(s) are chosen.

First draw a sketch of the riverbed and surrounding areas. The following information based on practicalities from Nissen-Peterson (2006) should be added to this sketch.

Location and types of water-indicating vegetation
Vegetation that indicate the presence of water, can be growing on the banks where the reservoir will be located, as proof of the riverbed capacity to store water. In the table below some names of trees are given which indicate water at a given depth below the surface.


Water indicating vegetation with root depth

Location of waterholes, their depth to the water table and quality of the water
Presence of waterholes especially after the rainy season) is an indication that the riverbed contains water and that it does not leak to deeper groundwater very fast. Pay special attention to those providing water the longest during the dry season. Also note the depth of the water table to the riverbed surface.
The water quality in the waterhole is an indication of the quality of water which can be harvested after building a sand dam. However, water quality can improve significantly by taking protective measures against animals can make significant improvements (see: RAIN water quality guidelines).

Location and types of rocks and boulders
If large boulders are present in the riverbed, care should be taken in choosing the sand dam location. The sand dam is preferably build on (and its wings attached to) hard rock or a compacted and strong soil. If a large boulder is mistaken for hard rock, water can leak from the sand dam reservoir, leading to unnecessary loss and potential undermining of the sand dam. Check whether hard rock is present in the riverbanks and –bed by looking for rock outcrops.

Pay special attention to the presence of halite near the riverbed, which is a salty whitish substance that turns water saline. If salty rocks (white and pink mineral rocks) are situated in the riverbanks upstream of a dam, then the water may be saline and therefore only useful for livestock. Local communities often know if there are any salty rocks, because livestock like to lick them for their salt content.

Coarseness of the sand in the riverbed
The material which is present in the riverbed is a good indication of the material which will fill up the sand dam reservoir after construction. Coarse sand is preferred, since it has a larger infiltration capacity and water can be abstracted more easily.

Height of the riverbanks
Suitable riverbeds have two high riverbanks. During flood events the river should not flow over the riverbanks, because this can cause erosion of the riverbanks, flooding of downstream located villages and it might cause the river to change its course. In this case, the sand dam will be left as a ruin. By using flood data and information from local water departments and local knowledge of the community, the maximum water height during a flood event can be determined. The minimal height of the riverbanks should be:

Minimum height riverbanks = Height of dam + Flood height + safety height

Width of the riverbed
Riverbed width preferably shouldn’t exceed 25 metres. The reinforcement required for such kind of long dam walls is too expensive; hence the sand dam will not be cost-effective. Other alternatives, such as subsurface dams, can be applied.

Bedrock or impermeable layer
To ensure storage of water within the sand dam aquifer, losses through leakage to deeper groundwater should be minimised. Therefore, the dam should be built on solid bedrock or an impermeable layer. This will also protect the sand dam from undermining through groundwater flow underneath the sand dam. This can be checked by using the geological map and outcrops in the area. Also, holes can be dug in the riverbed to find the depth of a consolidated layer or bedrock layer.

Construction material
The construction material locally available (such as sand, rock outcrops, bricks, etc.) can help to determine the type of sand dam (jconcrete, masonry etc) to construct. For example, a masonry dam is not the best choice if no stones are available in the area since transporting them from other areas is very expensive.

Riverbed crossings and roads
Rural roads often cross riverbeds. A sand dam is preferably located near these crossings, if physical conditions allow, because it will be easily reached through existing roads. Also, for transportation of materials it is important to know the most suitable roads or routes.

Houses, schools and shops
The presence of houses, schools and shops near the riverbed is useful for organising the project. Also, an assessment of the water demand can be made.

Land rights
To avoid conflicts, care should be taken in areas where the dam site is owned or used by two or more villages or individuals. The involved communities will be able to indicate possible problems.