Stormwater harvesting and management/Groundwater recharge/Leaky dams



A floodwater harvesting technique, these are permeable structures built across seasonal riverbeds which retain flash flood water that has a high silt load. The idea is to retain the high-energy floods and stimulate settlement of suspended sediment behind the dam. Water with a lower sediment load is then available to leak through the dam and infiltrate the downstream riverbed which is not blocked by sediment deposits. They have been proven to be able to recharge local aquifers. Leaky dams are excellent in semi-arid areas since they greatly reduce evaporation, in regards to storing water.

Implementation of this technology must be accompanied by other interventions. These include the strict protection and complete control of grazing along with extensive watershed management in the catchments to reduce the sedimentation loads in the reservoirs as well as enhancing the natural recharge of precipitation. Shrubs and local plants may be planted to help break up the soil (with their root systems) so that infiltration is easier.

Suitable conditions
Good practice is similar to that for a sand dam.
 * Ensure they are not built in an area where water will bypass the structure. Riverbanks should be equal in height and tall enough (height of dam + height of flood +10%), and dam should not be constructed near the bend in a river.
 * Site where river is narrower, so that construction is cheaper.

Drought
Effects of drought: Less water stored behind dam. Underlying causes of effects: Less recharge. To increase resiliency of WASH system: Reduce water loss behind by using adjustable sheets filled with small size gravel on the upstream side of the dam.

More information on managing drought: Resilient WASH systems in drought-prone areas.

Floods
Leaky dams are meant to handle flash flood rain events. Very intense rain events could provide faster erosion or carry sediment loads beyond the dam (contrary to the dam's purpose of filtering silt). Or if the rain is slow and long, greater water amounts could add greater silt behind the dam in a shorter amount of time, which may clog the ability of the dam to do direct groundwater recharge. In these cases, more attentive maintenance is required to check for erosion and silt build ups (or escapes).

Construction, operations and maintenance
General advice on cement: A common cause of cracks in structures and linings (e.g. in tanks, dams, waterways, wells) is errors in mixing and applying the cement. First of all, it is important that only pure ingredients are used: clean water, clean sand, clean rocks. The materials have to be mixed very thoroughly. Secondly, the amount of water during mixing needs to minimal: the concrete or cement needs to be just workable, on the dry side even, and not fluid. Thirdly, it is essential that during curing the cement or concrete is kept moist at all times, for at least a week. Structures should be covered with plastic, large leaves or other materials during the curing period, and kept wet regularly.

Specific advice: Similar construction as dam san dams:
 * Construct wing walls to avoid erosion around edges.
 * Spillway is designed for river flow, and therefore varies according to the site.
 * Avoid downstream erosion of dam base by making protective slab made from more rock-filled gabions.
 * Timing is important: dams should be built during the dry season, but don’t build dams too close to the rains in order to avoid dam being washed away.

The dam can be made from rocks of various sizes (boulders, cobbles, stones, large gravel) that are found in and around the riverbed. In Pakistan, rocks of 200mm diameter were used. In Balochistan, the rocks are held within wire mesh nets that are built in 5 steps to total height of 4.9 metres. The exact height of the dam will vary and depend on the riverbed topography. A reinforced concrete cut-off 1.5 metres deep with 2 metre toes on both sides of the dam ensure stability. To prevent the water behind the dam percolating too quickly, it helps to put adjustable sheets filled with small size gravel on the upstream side of the dam body (helps in cases of drought).

Water extraction

 * Water that infiltrates is used as soil moisture for crops cultivated after a rainfall event.
 * In Balochistan, Pakistan, pipes have been added on top of the 2nd and 4th steps which discharge surplus water downstream.
 * Water can be used via shallow wells and boreholes in the area.

Costs
Costs vary, but a 5-step dam, 4.9 metres high, could cost around US$26,000 to construct.

Field experiences
A leaky dam constructed in Balochistan has a catchment area of 1.79 km2 with a storage capacity of 11,000m3.

Manuals, videos and links

 * Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas. Excellent study of an arid area in Pakistan that used leaky dams.

Acknowledgements

 * CARE Nederland, Desk Study: Resilient WASH systems in drought-prone areas. October 2010.


 * Gale, Ian, Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas. UNESCO's International Hydrological Programme (IHP), 2005.


 * Original Akvopedia.org article and more images: akvopedia.org/wiki/Leaky_dams