Changes

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. The structure is built inside an existing riverbed, so that after a flooding event, for example, water is increased due to a new and higher water level within the sand.

* Siting requirements are similar to the [[sand dam]].<br>* Subsurface dams should be built where sand volume is already sufficient to store water.<br>* Seasonal rivers in semi-arid regions with permeable sediments and an impervious layer at a shallow depth (maximum 3 to 4m deep).<br>* River valleys with gradients of between 1 and 2% usually enable the highest storage.<br>* Ideal where groundwater flow converges from a large catchment into a narrow passage.<br>