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Subsurface harvesting systems

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{{Language-box|english_link= Subsurface harvesting systems | french_link= Coming soon | spanish_link= Coming soon | hindi_link= Coming soon | malayalam_link= Coming soon | tamil_link= Coming soon | korean_link= Coming soon | chinese_link=地下集水系统 | indonesian_link= Coming soon | japanese_link= Coming soon }}__NOTOC__ [[Image:subsurface harvesting systems.png|right|80px]][[Image:SubsurfaceDamSite.JPG|thumb|right|400px200px|Subsurface dam being built out of clay soil across a sandy riverbed in Kitui, Kenya, 2007. They are working in the dry season; note the riverbank is much higher than they are working.Photo: [http://www.waterforaridland.com/ Water for Arid Land.]]]
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. 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 easier one 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.
===Suitable conditions=={{Climate_change|heading1=Drought |text1= '''Effects of drought''': Can dry up; Water yields low. <br>'''Underlying causes of effects''': Less recharge of aquifer due to less rainfall; Increasing population & water demand; Size of aquifers – e.g. limited sand volume; Too much silt accumulates behind dam; Wells not sunk deep enough into water table; Incorrect siting causes leaking dams. <br>'''To increase resiliency of WASH system''': Competent siting & construction; Build sand dam in stages to reduce silt; Soil & water conservation techniques in upper part of catchment; Dig abstraction wells/pipes deeper.|heading2=Drought effects on cement|text2='''Effects of drought''': Badly made concrete or cracked linings (e.g. in tanks, dams, waterways, wells, and other structures). <br>'''Underlying causes of effects''': Less water used for curing; Impure water used for mixing. <br>'''To increase resiliency of WASH system''': Ensure adequate mixing, ratios, purity of ingredients; Minimize water content in mixture; Ensure adequate curing.}} 
* Siting requirements are similar to the [[sand dam]].<br>
* Subsurface dams should be built where sand volume is already sufficient to store water.<br>
* Ideal where groundwater flow converges from a large catchment into a narrow passage.<br>
 {{procontable | proborder="1" cellpadding="5" cellspacing="0" align="center"|-! width="50%" style="background:#efefef;" | Advantages! style="background:#f0f8ff;" | Disadvantages|-| valign="top" | - Subsurface dams are not likely to deteriorate, with little danger of breaching <br>
- Losses from evaporation are very much less than the 2.0 metres lost annually from a free water surface in a dry tropical area<br>
- Construction has long lifespan and require minimal maintenance <br>
- The breeding of insects and parasites such as mosquitoes and bilharzia parasites is prevented<br>
- Contamination of stored water, by people and animals, is greatly reduced, particularly if well and handpump are provided to abstract water in a hygienic and controlled manner<br>
| convalign= "top" | - Leakage is often difficult to detect <br>
- Water losses may occur through cracks in the impermeable layer (it may be possible to drill a well in the fracture zone and utilize the dam as an artificial recharge structure)<br>
- Downwater flow will be impacted to communities downstream<br>
- the river may change course<br>
- the possible dam construction sites may be too far from the water users<br>
|}}  ===Resilience to changes in the environment=======Drought===='''Effects of drought''': Can dry up. <br>'''Underlying causes of effects''': Less recharge of aquifer due to less rainfall; Increasing population & water demand; Size of aquifers – e.g. limited sand volume; Too much silt accumulates behind dam; Wells not sunk deep enough into water table; Incorrect siting causes leaking dams. <br>'''To increase resiliency of WASH system''': Competent siting & construction; Build sand dam in stages to reduce silt; Soil & water conservation techniques in upper part of catchment; Dig abstraction wells/pipes deeper.
More information on managing drought: [[Resilient WASH systems in drought-prone areas]]. ===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:SubsurfaceDam.jpg|thumb|right|300px200px|Mechanics of a subsurface dam. Drawing: WHO.]]
The dam can be constructed of concrete, masonry, blockwork, stone-filled gabions with waterproof membranes such as plastic sheet or clay layer, or stabilised soil.
Users may need to establish a local committee to manage issues, such as controlling or supervising water use, preventing water contamination, carrying out O&M activities, financing O&M, and monitoring how much stored water is still available (a piezometer or auger hole may be installed to allow a caretaker or watchman to estimate how much water is left and decide if rationing has to be introduced). Proper management may also help to prevent social conflict. For O&M tasks at the dam site, a person who lives or farms near this site could be appointed. This person could also be responsible for water alloca- tion and be involved in monitoring activities, if users obtain the water near to, or at, the site. His or her authority should be clear and accepted by all users.
===Costs===
'''Initial costs''': A 3500 m3 dam costs US$2.40/m3 in Kenya and US$3.90/m3 in Tanzania.
* 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. <br>{|style="border: 2px solid #e0e0e0; width: 40%; text-align: justify; background-color: #e9f5fd;" cellpadding="2"<!--rsr logo here-->|- style="vertical-align: top"|[[Image:akvorsr logo_lite.png|center|60px|link=http://akvo.org/products/rsr/]]<!--project blocks here-->|- style="vertical-align: bottom"|[[Image:project 393.jpg|thumb|center|140px|<font size="2"><center>[http://rsr.akvo.org/project/393/ RSR Project 393]<br>Dawa Eresa Subsurface and Sand Dam project</center></font>|link=http://rsr.akvo.org/project/393/ ]] |[[Image:project 404.jpg|thumb|center|140px|<font size="2"><center>[http://rsr.akvo.org/project/404/ RSR Project 404]<br>Feasibility Study for Rainwater Harvesting</center></font>|link=http://rsr.akvo.org/project/404/ ]] |} <br> ===Manuals, videos, and links===* [http://www.sswm.info/sites/default/files/reference_attachments/WATERAID%202008%20Technology%20Notes.pdf Technology Notes: Subsurface Dams] - Section 11. Water Aid.* [http://www.waterforaridland.com/Books/Subsurface%20dams%20for%20water%20storage.pdf Subsurface dams for water storage in dry riverbeds]. Waterforaridland.com. ===Acknowledgements===* Brikke, François, and Bredero, Maarten. [http://www.ircwash.org/resources/linking-technology-choice-operation-and-maintenance-context-community-water-supply-and Linking technology choice with operation and maintenance in the context of community water supply and sanitation: A reference document for planners and project staff]. World Health Organization and IRC Water and Sanitation Centre. Geneva, Switzerland 2003.
==Reference manuals, videos, and links==* [http://www.wateraidircwash.org/ukresources/what_we_do/sustainable_technologies/technology_notes/247smart-water-harvesting-solutions-examples-innovative-low-cost-technologies-rain-fog-runoff Smart Water Harvesting Solutions: Examples of innovative, low cost technologies for rain, fog, and runoff water and groundwater.asp Technology Notes: Subsurface Dams]. Water Aid.* (or [http://www.waterforaridlandarcworld.comorg/publicationsdownloads/Book8/book8chapter6smart%20water%20harvesting.asp Cahpter6: Community pdf alternative link]) Netherlands Water Projects]. Scroll down to the sectionPartnership, Aqua for All, Agromisa, 6.2 Subsurface Dams. Waterforaridlandet al.com2007.
==Acknowledgements==* Brikke, François, and Bredero, Maarten. [http://www.who.int/water_sanitation_health/hygiene/om/wsh9241562153/en ''Linking technology choice with operation and maintenance in the context of community water supply and sanitation: A reference document for planners and project staff'']. World Health Organization and IRC Water and Sanitation Centre. Geneva, Switzerland 2003.* [http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CGsQFjAI&url=http%3A%2F%2Fwww.arcworld.org%2Fdownloads%2Fsmart%2520water%2520harvesting.pdf&ei=NXNzT7CYEIiTiQKdxviYCw&usg=AFQjCNECS9O4gaRRQadp5dd4B5RDPxTwQA&sig2=QwWuUbra7my8Dpq5c_0baw ''Smart Water Harvesting Solutions: Examples of innovative, low cost technologies for rain, fog, and runoff water and groundwater.''] Netherlands Water Partnership, Aqua for All, Agromisa, et al. 2007.* CARE Nederland, ''Desk Study : [[Resilient WASH systems in drought -prone areas'']]. October 2010.
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