Climate

Sand storage dams are constructed in (semi)arid areas which have seasonal rainfall.

Why of importance?

Borst and de Haas (2006) and Hoogmoed (2007) conclude that the efficiency of sand storage dams is quite insensitive to the amount of rainfall. Only when the amount of rainfall drops below a certain threshold (20 mm for the Kiindu catchment) the amount of abstractable water decreases. This threshold value is dependent on the size of the upstream part of the catchment, since a large catchment will provide more baseflow.

However, the infiltration of precipitation is partly determined by rainfall intensity, which is of importance to groundwater recharge and thus provision of baseflow (recharging the depleting riverbed aquifer) during the dry season.

Rainfall intensity should be high enough to detach coarse grained material needed in the riverbed . Also, high intensity rains resulting in flash floods are necessary to transport coarse sediments which will fill the sand storage dam.

Optimal conditions

Low intensity rains result in more infiltration of precipitation into the soil, since the infiltration capacity of the soil is not so fast exceeded. However, this parameter will not be the limiting factor since rainfall amounts and intensities show much variation in semi arid and arid areas and both high and low intensity rains will occur.

Risks

1. Although a sand storage dam will provide water much longer than there would be without a sand dam, the reservoir will eventually empty (the moment depending on the volume of the dams). Thus, rainfall must fall once in a while
2. High intensity rains leading to high flash floods occur. The dam should be dimensioned to be able to cope with this. It is very important that no water will flow over the riverbanks around the sand dam, which will cause erosion and thus collapse of the dam.

How to evaluate?

A first estimation of the suitability of a catchment can be made using GIS.
Also, information can be gathered by estimation based on information from the local communities. Questions to be asked are for example how much and when rain falls and if the river starts to flow (and after how many rainfall events).