How does a sand storage dam work?

Filling of the sand dam aquifer

Sedimentation upstream of the sand storage dam occurs during heavy rainfall events, when river discharge will be high, transporting large quantities of sediments. The grain size of the transported sediments is dependent on river flow velocity and the material comprising the riverbanks. Since most of the land is bare at the start of the rainy season, soils are poorly protected against soil erosion, resulting in a high silt and sand load in the water.
The sand dam will reduce the flow velocity of the river at some distance upstream of the structure. This drop in flow velocity results in sedimentation. The materials found in the river bed prior to construction are a good indication of the type of sediment that will be collected by the sand dam. These sediments form a ridge comparable to the formation of a delta. Upstream of the ‘delta’, flow velocity is higher and coarse sediments are transported. Where the ‘delta’ stops, a sudden drop in flow velocity occurs causing coarse sediments to settle, building the ‘delta’ further towards the sand dam (see figure). Continuous repetition of this process causes the ridge of sand to move towards the dam, eventually filling the total volume behind the dam.

The evolution of sand dams