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.

Schematic representation of the sedimentation process (Gijsbertsen, 2007)

However, the river also transports finer materials, like silt and clay. These sediments have a lower settling velocity compared to sand will largely stay in suspension and are transported over the dam. However, fine sediments can settle resulting in a silt layer directly upstream of the sand dam. Days after the rainfall event, base flow will dominate river discharge. Coarse sediments can no longer be transported due to the low flow velocities and are deposited. Without the coarse material, the base flow water has excess energy leading to erosion of the river bed. Fine sediments will be (re)taken into suspension and transported, leaving the coarser grained material in the riverbed. Once the runoff has stopped completely, residual silt layers on top will dry and crack. Animals and people walking on the riverbed will pulverize this dry silt layer, making it susceptible for wind erosion (Borst & de Haas, 2006). These processes limit the accumulation of silt and clayey material behind the sand dam.
Sedimentation will continue until the ‘delta’ reaches the height of the sand storage dam. The sand storage dam is mature and filled with coarse sand. This can take several wet seasons, depending on the availability of coarse sediments, height of the sand dam, river discharge, catchment slope and rainfall intensity.

In upstream parts of a catchment it is recommended that sand dams are built in stages, since the availability of coarse material is generally limited and base flow is small or absent. The optimum height of one stage is site specific. The first stage is typically is 50 cm. It is recommended to consult an expert on this matter.

Hydrological functioning of a sand storage dam

In most semi arid regions, rivers are only holding water during and short after rainfall events, especially in more upstream parts of catchments. Due to the often short and intense rainfall events in combinations with certain soil types like silt and clay, a large part of the rainfall will leave the catchment as surface runoff instead of infiltrating the soil and recharging the groundwater. Up to 70% of the rainfall is lost as runoff.

A sand dam obstructs groundwater flow through the permeable riverbed. It only takes one or two heavy rainfall events to fill the enlarged riverbed aquifer completely, and after this the river will start to flow as it would in absence of a sand storage dam. The raised water table in the riverbanks results in a groundwater flow from the riverbanks towards the river bed. This leads to replenishment of the created reservoir (see figure). Water will be available in the riverbed as long as the groundwater flow from the riverbanks continues. Therefore it can be highly profitable to take soil- and water conservation measures in the upstream areas within the catchment, to increase infiltration of rain water creating more base flow within the catchment.

Water balance components (Borst & de Haas, 2006)