By Cranfield University

Water is trapped in the sand dam when the river flows during the rainy season. In the dry season, water can be evaporated from the sand (though of course this is less than if it was an open water reservoir) and it is abstracted by the community through scoopholes and hand dug wells. Water can also flow between the underlying rock and river banks and the trapped sand. This can help recharge the local groundwater. It can also leak under the dam wall.

In this study we attempted to quantify these flows and understand the water balance of the sand dams. We focussed on three sand dams near Mtiti Andei in Kenya for 3 months from May to July 2017. At each dam we installed a series of piezometers which allowed us to measure water levels in the trapped sand in the dry season. We also built lysimeters which were open-topped tanks buried in the sand which allowed us to monitor evaporation from the sand. We collected sand samples in profiles across the river bed to measure the sand grain size. We conducted tests in the trapped sand to see how fast the water can flow through it.

The water level monitoring showed that the water in two of the dams had nearly dried up after three months, whereas in one dam there was still quite a lot of water left. The results from the evaporation showed that in the top 0.4m it was 6mm/day, and below this is decreased to 1mm per day.

Combining all the measurements into a water balance, we found that one of the dams lost water consistently through the dry season, one gained water throughout the dry season, and one gained water at the start of the dry season and then lost it later on.

Despite the leakages, the communities still get benefits from the sand dams. They can extract water form hand pumps scoop holes, and other wells constructed near the dams, and even from the water ponded at the base of the dam wall after it has leaked from the dam. They use the water for drinking, cooking, watering livestock, fish culture, brick making, laundry and irrigation. The water that leaks into the underlying rock is also important as it will be recharging the surrounding groundwater.

The full paper is available to download here for free.