What is a Sand Storage Dam?

A sand storage dam (or sand dam) is a small dam build on and into the riverbed of a seasonal sand river. The functioning of a sand dam is based on sedimentation of coarse sand upstream of the structure, by which the natural storage capacity of the riverbed aquifer is enlarged. The aquifer fills with water during the wet season, resulting from surface runoff and groundwater recharge within the catchment. When the riverbed aquifer is full, usually within one or two large rainfall events, the river starts to flow as it does in the absence of the dam. However, the groundwater flow through the riverbed is now obstructed by the sand storage dam, creating additional groundwater storage for the community.

Typical sand storage dam during the dry season (S.A. de Haas, 2006)

During the dry season, water levels will drop due to abstraction of water, minor evaporation and possibly by leakage through the dam or vertical leakage into the bedrock. Meanwhile, subsurface flow (base flow) from the riverbanks towards the riverbed and through the riverbed itself will slowly and partly refill the riverbed aquifer. Because of the larger storage volume and he slower depletion, the river provides water throughout the dry season (when built under appropriate conditions), whereas otherwise the riverbed would have dried up long before the beginning of the new rains.

Schematic cross section of a typical sand storage dam (Borst & de Haas, 2006)

The volume of water available for abstraction is considerably larger than just the volume present in the riverbed sands. This is because a large quantity of the water is additionally stored in the riverbanks, recharging the sand dam reservoir in the dry season (Borst & de Haas, 2006; Hoogmoed, 2007).
Sand dams effectively increase the volume of groundwater available for abstraction as well as prolonging the period in which groundwater is available.