A balancing lake (also flood basin ) is a term used in the UK to describe a retention basin used to control by temporarily storing flood . The term balancing pond is also used, though typically for smaller storage facilities for and stream.
In open countryside, heavy rainfall soaks into the ground and is released relatively slowly into watercourses (ditches, streams, ). In an urban area, the extent of hard surfaces (roofs, roads) means that the rainfall is dumped immediately into the drainage system. If left unchecked, this has the potential to cause flooding downstream. The function of a balancing lake as part of a sustainable urban drainage scheme is to contain this surge and release it slowly. Failure to do this, especially in older settlements without separate and Sanitary sewer, can cause serious pollution as well as flooding.
More advanced systems are computer-controlled such that the entire flow of a river can be diverted into a holding lake, perhaps to reduce the impact of a large scale rainstorm in the catchment on communities downriver.
For aesthetic and safety reasons, the system can be designed so that there is a permanent lake. A lake with an equivalent area of 1,000 by 1,000 metres will hold a million cubic metres of water for each metre of depth. Typically such a lake would have an outer earth bank of 1 metre, then a leisure path, then a 10 cm inner bank to the steady-state level.
As well as local storm drains, the lake's primary purpose is to intercept the river Ouzel, a tributary of the river Great Ouse. The catchment area is Oxford Clay that tends to get saturated easily, so field run-off has always been a problem.
The South Basin is designed for recreational use, mainly dinghy sailing and wind surfing, with a circumference path and banks as described above. It is linked to the North (Wildlife) Basin and can be drawn on to manage the level of the latter more finely. The North Basin has a large, undisturbed, central island. The extensive shallows support a good crop of aquatic plants and invertebrates. Very quickly, it became a key wildfowl site.
In winter, it attracts up to 2,500 wild birds, with a wide variety of migrating waders in spring and autumn. Common tern, tufted duck, Ringed plover and little ringed plover, common redshank and northern lapwing. Canada geese have become naturalised and they are permanent residents. Both basins have deep ponds to maintain the fish population during droughts. The lake is managed as a public open space, receiving up to a million visits each year.
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