A sand dam is a reinforced concrete wall built across a seasonal river to hold underground water in sand (see above photo of Nzaaya Muisyo sand dam, Eastern province, Kenya). It is initially built one meter high and up to 90 meters across. During the heavy and erratic seasonal rains, the water and silt flow over the dam while the heavier sand settles to the bottom. Over one to three seasons of rain, the dam fills up with sand which acts as a storage tank for water. In good quality sand, the sand dam volume is approximately 35% water (Beimers et al., 2001). Most of this water does not evaporate as it is protected by the sand. Evaporation decreases by 90% at 60 cm below the surface (Borst et al., 2006).
The sand dam is always built on bed rock. A natural aquifer is formed under the sand as water accumulates. Often there is already an aquifer present and the sand dam simply increases the water in it. Over time, the aquifer increases in size and the water table of the surrounding area rises.
- What is a Sand Dam?
- Where Does the Water Come From?
- Where and When is it Best to Build?
- What Permits do I Need?
- How is a Sand Dam Designed, Constructed, and Maintained?
- Terracing around Sand Dams