definition

Flood detention reservoir, also known as an attenuation or balancing or flood control reservoir or detention basin, collects water at times of very high rainfall and then releases it slowly over the course of the following weeks or months. Some of these reservoirs are constructed across the river line with the onward flow controlled by an orifice plate. When river flow exceeds the capacity of the orifice plate water builds behind the dam but as soon as the flow rate reduces the water behind the dam slowly releases until the reservoir is empty again. In some cases such reservoirs only function a few times in a decade and the land behind the reservoir may be developed as community or recreational land. This new generation of balancing dams are being developed to combat the climatic consequences of climate change. Because these reservoirs will remain dry for long periods, there may be a risk of the clay core drying out reducing its structural stability. Recent developments include the use of composite core fill made from recycled materials as an alternative to clay.
(Sensagent, Flood detention reservoir, accessed on Sept. 2016)
They also facilitate some settling of particulate pollutants. Detention basins are normally dry and in certain situations the land may also function as a recreational facility. However, basins can also be mixed, including both a permanently wet area for wildlife or treatment of the runoff and an area that is usually dry to cater for flood attenuation.
(Susdrain, Detention reservoir, accessed on Sept. 2016)

Co-benefits and impacts

Apart from the temporary storage of water and detention and decrease of peak flows at the downstream areas, the facilities of a flood detention reservoir reduce the costs of large stormwater drainage system by reducing the size required for such systems (REDAC, p. 1, accessed on Sept. 2016). The stored water will change the water level, and basins should be designed to function in both dry and wet weather. Quantity can also be influenced by the amount of water that can be allowed to infiltrate into the ground if there is no risk to groundwater quality. Basins treat runoff in a variety of ways such as settlement of solids in still or slow moving water, adsorption by the soil and biological activity. In addition, basins offer many opportunities for the landscape designer. Basins should not be built on, but can be used for sports and recreation and can be part of public open space. Surrounding pond can also be landscaped to enhance aesthetics and provide habitat for wildlife.
(Susdrain, Detention reservoir, accessed on Sept. 2016)

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Last modified: Sept. 21, 2016, 5:20 a.m.