Subsurface harvesting systems
The use of subsurface dams in naturally occurring alluvium creates groundwater storage upstream of the dam, raising the water table and preventing evaporation losses. These are structures that reduce or stop the flow of shallow groundwater, usually in seasonal riverbeds. A subsurface dam is constructed across a valley in a seasonally dry, sandy riverbed, by digging a trench down to the bedrock or other impervious layer (clay) at the base of the river. The dam, which is placed in the trench, may consist of a wall or screen and covered with excavated material until it is completely concealed. The refill material must be properly compacted. There are two types of groundwater dams: a subsurface dam (which is explained here) and a sand dam. A subsurface dam is the one easier to build of the two.
Contents
Suitable conditions
- Seasonal rivers in semi-arid regions with permeable sediments and an impervious layer at a shallow depth (maximum 3 to 4m deep).
- River valleys with gradients of between 1 and 2% usually enable the highest storage.
- Ideal where groundwater flow converges from a large catchment into a narrow passage.
| Advantages | Disadvantages/limitations |
|---|---|
| - Subsurface dams are not likely to deteriorate, with little danger of breaching - Construction has long lifespan and require minimal maintenance |
- Leakage is often difficult to detect - Downwater flow will be impacted to communities downstream |
Construction, operations and maintenance
Costs
- Materials costs and construction: US$1,400
- Feasibility study: US$200
- Technical support: US$275
- Operation and maintenance: low
Field experiences
A subsurface reservoir in Pernambuco, Brazil has an average depth of 4m, is 50m wide, and 500m long, storing about 4,000m3 of water.