Sand dams are constructed above ground in an non-perennial river bed. During periods of high flow, sand and gravel accumulates upstream of the dam. Runoff water can easily infiltrate these highly permeable deposits, creating a small scale artificial aquifer upstream of the dam. Sand Dams are a cost-effective (virtually no operation and maintenance costs,) sustainable solution to rural water supply. The life span of sand dams are  at least 30-50 years. Sand dams are commonly found in sub-saharan countries and a detailed description can be downloaded here.