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Water Portal / Rainwater Harvesting / Groundwater recharge / Controlled flooding and Spreading basins

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{{Language-box|english_link= Water Portal / Rainwater Harvesting / Groundwater recharge / Controlled flooding and Spreading basins | french_link= Coming soon | spanish_link= Coming soon | hindi_link= वाटर पोर्टल / वर्षाजल संचयन / भूजल पुनर्भरण / नियंत्रित बाढ़ और विपथन-फैलाव वाहिकाएं | malayalam_link= Coming soon | tamil_link= Coming soon | korean_link= Coming soon | chinese_link=洪水控制/疏导水池 | indonesian_link= Coming soon | japanese_link= Coming soon }}__NOTOC__ [[Image:controlled flooding icon.png|right|80px]][[Image:ControlledFlooding.jpg|thumb|right|400px200px|Groundwater recharge in spreading basins. Arizona, ArizonaUSA. SourcePhoto: CAP (2002)]] This is a floodwater harvesting technique called '''controlled flooding''', where the water diverted from a river, with the help of diversion structures and canals, is spread evenly over a large surface area , known as '''spreading basins''', where it is used for recharging groundwater, irrigation, filling ponds, and watering grazing land. The concept is that a thin sheet of water flows over the land but at minimum velocity in order to avoid disturbing the soil cover. This includes [[Irrigation_-_Spate_irrigation|spate irrigation]], but also standard channel irrigation which takes river water via channels to fields (covered more in-depth in the Surface Water or Spate Irrigation sections of Water Studio).

===Suitable conditions===

Site in areas of high volume and intensity river flows where conventional irrigation structures are not feasible (spate systems should probably be promoted in areas where the practice is already in use).

{{procontable | proborder="1" cellpadding="5" cellspacing="0" align="center"|-! width="50%" style="background:#efefef;" | Advantages! style="background:#f0f8ff;" | Disadvantages|-| valign="top" | - Minimum land preparation is needed, so is very cost-effective compared to other infiltration methods <br>

- Most of the sediment load in the river water will settle on the land, e.g. 70% – this can be advantageous for agriculture <br>

- Good for infiltrating runoff water – measurements in Iran showed that 83.5% of the river water was infiltrated<br>

| convalign= "top" | - Large surfaces of land have to be made available for it to work <br>

- River flows in ephemeral rivers can be unpredictable in time and volume, which can lead to unpredictable spread on the land and damage to

structures<br>

- High sediment loads can reduce recharge rates over time. Their deposition also means farmland level gets raised above village, which causes the village to flood. Also riverbanks can breach, as a result.<br>

- Labour intensive work to create embankments for conveying storm water (in Eritrea, banks are between 5-10 metres high)<br>

- Unintentional negative impacts on water downstream when not carefully designed. Especially when the floodwater never reaches the sea – in this case, all water is already allocated in the basin and withdrawal in one place will be more likely to negatively affect another.<br>|}}  ===Resilience to changes in the environment=======Drought===='''Effects of drought''': Lower crop yields.<br>'''Underlying causes of effects''': Less water to crops from flooding.<br>'''To increase resiliency of WASH system''': Drought-resistant & fast-growing crops; Diversify livelihoods of farmers. More information on managing drought: [[Resilient WASH systems in drought-prone areas]]. ====Floods====Spreading basins are perfect for areas with floods. They are built to transfer excess water away from rivers, so that uncontrollable floods don't happen in unwanted places. However, in the event of too much flooding, the spreading basin must be large enough to handle the increased volumes of rain. Plus, with intense rain events, the higher velocities might be too much for the soil of the basin and create a washed out effect or likely just runoff before infiltrating. In this case, additional vegetation enhancement (to help the soils absorb more water), wetland enhancement, levee protection, and/or additional water storage will be helpful. ===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''':

==Construction, operations, and maintenance==

* It is better to concentrate efforts on low-cost diversion structures and to avoid sophisticated technological solutions. Efforts should be based on improving existing intakes.

* If using controlled flooding for irrigation, farmers should be more involved in development of improved spate systems – the new system should build on existing practices and land rights, not undermine them as has happened in the past.

* Provision of bulldozers has been very popular and has enabled spate farmers to build or restore damaged structures more easily. Problems with that is that sometimes downstream effects become too great since farmers upstream can build much larger structures, and also that bulldozers cannot easily be run and maintained in a self-sustaining fashion due to high costs. Support is therefore too large for small farmer groups and is best organized on a regional basis through local government, or with subsidies to allow participation of the private sector.

==Costs=Field experiences===[[Image:FloodwaterSpreading.jpg|thumb|right|200px|Plan of how floodwaters will spread. Click image to zoom. <br> Drawing: [http://unesdoc.unesco.org/images/0014/001438/143819e.pdf ''Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas.''] UNESCO.]]

==Field experiences==

[[Image:FloodwaterSpreading.jpg|thumb|right|300px|]]

====Iran====

Overexploitation of groundwater has caused significant drawdown of the water table (1.5m/year) and deterioration of groundwater quality in the Dorz Sayban Plain, which is located 115 km to the southeast of Larestan, Iran. 3500 hectares of land are irrigated using groundwater in this plain. To decrease the rate of the drawdown of the water table, five floodwater spreading systems for recharge of groundwater were designed and constructed in the region 1983 and 2001.

More than 70% of the suspended load has settled in the system. This will inevitably lead to clogging and reduction in efficiency in the system, but also an improvement of the soil for agriculture purposes. Additionally, the managed aquifer recharge improves the quality of groundwater, as the EC of floodwater is much lower than that of the groundwater (0.3-0.4 versus 2.0-9.0 dS/m).

==Reference manuals=Manuals, operations and maintenance===* [http://wwwweb.googlearchive.comorg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0CGIQFjAI&url=http%3A%2F%2Fwww/20151106164515/https://www.iaea.org%2Ftechnicalcooperation%2Fdocuments%2FBrochures%2Fsust/technicalcooperation/documents/Brochures/sust-groundwater.pdf&ei=KeB8T8WnN6PiiAKS_4W7DQ&usg=AFQjCNH3fWzPOyaJjopuEoluwIh-veyHVw&sig2=q9gJHSqAZNGoIrVbuHa_qw ''Sustainable Development of Groundwater Resources in Southern and Eastern Africa'']. International Atomic Energy Agency. ===Acknowledgements===* CARE Nederland, Desk Study: [[Resilient WASH systems in drought-prone areas]]. October 2010.  * Gale, Ian, [http://unesdoc.unesco.org/images/0014/001438/143819e.pdf Strategies for Managed Aquifer Recharge (MAR) in semi-arid areas.] UNESCO's International Hydrological Programme (IHP), 2005.

==Acknowledgements==* CARE Nederland, ''Desk Study Resilient WASH systems in drought prone areas.'' October 2010.* Gale, Ian, [http://www.googlecolusarcd.comorg/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CDgQFjAB&url=http%3A%2F%2Funesdoc.unesco.org%2Fimages%2F0014%2F001438%2F143819e.pdf&ei=uqt8T9_sL6ibiQKTqdzFDQ&usg=AFQjCNFjJrN8OJHYuNSZrDZIrDtZxc_LRg&sig2=J7p3YnDub8Ixn2n4DU-uBw ''Strategies for Managed Aquifer Recharge (MAR) in semi-arid areasAddress unknown future effects of climate change.''] UNESCO's International Hydrological Programme (IHP)Colusa County Resource Conservation District, 2005USA.

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