__NOTOC__[[Image:leaky {{Language-box|english_link= Water Portal / Rainwater Harvesting / Groundwater recharge / Leaky dams icon.png|right]][[Image:LeakyDam.jpgfrench_link= Coming soon |thumbspanish_link= Coming soon |righthindi_link= वाटर पोर्टल / वर्षाजल संचयन / भूजल पुनर्भरण / रिसाव बांध |350pxmalayalam_link= Coming soon |'''Leaky dam.''' Re-hydrating the earth by sustainable, small-scale sub-surface water retention techniques, Vietnam. Photo: Partners voor Water (2009).]]tamil_link= Coming soon | korean_link= Coming soon | chinese_link=漏水坝 | indonesian_link= Coming soon | japanese_link= Coming soon }}
[[Image:leaky dams icon.png|right|80px]][[Image:LeakyDam.jpg|thumb|right|200px|'''Leaky dam.''' Re-hydrating the earth by sustainable, small-scale sub-surface water retention techniques, Vietnam. Photo: Partners voor Water (2009).]] A floodwater harvesting technique, these '''leaky dams''' 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.
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! width="50%" style="background:#efefef;" | Advantages
! style="background:#ffdeadf0f8ff;" | Disadvantages
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| valign="top" | - Can assist recharge of shallow wells <br>
===Resilience to changes in the environment===
====Drought====
'''Effects of drought''': Less water stored behind dam. <br>
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''':
[[Image:LeakyDam2.jpg|thumb|right|350px200px|'''Leaky dam.''' Balochistan, Pakistan. Facing downstream. <br> Photo: [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDgQFjAB&url=http%3A%2F%2Funesdocunesdoc.unesco.org%2Fimages%2F0014%2F001438%2F143819e/images/0014/001438/143819e.pdf&ei=uqt8T9_sL6ibiQKTqdzFDQ&usg=AFQjCNFjJrN8OJHYuNSZrDZIrDtZxc_LRg&sig2=J7p3YnDub8Ixn2n4DU-uBw ''Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas.''] UNESCO.]]
Similar construction as [[sand dam]]s:
* Construct wing walls to avoid erosion around edges.
* 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===* [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDgQFjAB&url=http%3A%2F%2Funesdocunesdoc.unesco.org%2Fimages%2F0014%2F001438%2F143819e/images/0014/001438/143819e.pdf&ei=uqt8T9_sL6ibiQKTqdzFDQ&usg=AFQjCNFjJrN8OJHYuNSZrDZIrDtZxc_LRg&sig2=J7p3YnDub8Ixn2n4DU-uBw 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, [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDgQFjAB&url=http%3A%2F%2Funesdocunesdoc.unesco.org%2Fimages%2F0014%2F001438%2F143819e/images/0014/001438/143819e.pdf&ei=uqt8T9_sL6ibiQKTqdzFDQ&usg=AFQjCNFjJrN8OJHYuNSZrDZIrDtZxc_LRg&sig2=J7p3YnDub8Ixn2n4DU-uBw Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas.] UNESCO's International Hydrological Programme (IHP), 2005.
