A balancing lake (also flood basin or Sustainable urban drainage scheme) is a term used in the U.K. describing an element of an urban

drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...

system used to control

flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...

ing by temporarily storing flood

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

s. The term balancing pond is also used, though typically for smaller storage facilities for streams and brooks. In open countryside, heavy

rainfall Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

soaks into the ground and is released relatively slowly into watercourses (ditches, streams,

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...

s). 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 will cause widespread flooding downstream. The function of a balancing lake is to contain this surge and release it slowly. Failure to do this, especially in older settlements without separate storm sewers and foul sewers, can cause serious pollution as well as flooding.

Engineering

At its simplest, a balancing lake can be constructed by creating a dam across a drain or stream at a convenient valley, with a restricted diameter outlet pipe through the dam. Normal flows are carried happily through the pipe, but heavy flows back up and the water behind the dam is choked back. Over the following few days, the level subsides. This is often enough for a small housing development. 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.

Other benefits

A permanent lake can provide useful recreation facilities such as sailing, windsurfing, or of wildlife. Water sports and wildlife habitats do not mix well, though a scheme can have both in linked basins where the recreational basin fills first and the wildlife basin is only used in exceptional conditions. * A recreational use facility can have relatively steep banks (perhaps with a footpath inside the bank next to the permanent lake for

aesthetics Aesthetics, or esthetics, is a branch of philosophy that deals with the nature of beauty and taste, as well as the philosophy of art (its own area of philosophy that comes out of aesthetics). It examines aesthetic values, often expressed t ...

and safety). The water level can rise substantially without a significant increase in overall area. * A basin that is intended for use by wildlife and for visual amenity needs to be relatively shallow for maximum plant life. It must be designed with the assumption that it will be invoked very rarely, especially during the nesting season.

Case study: Willen Lake, Milton Keynes

Willen Lake () is one of the largest (400,000 m²) purpose-built stormwater balancing lakes in the UK. The lake is designed to take surface run-off from Milton Keynes, the largest of a number designed to do so. The lake has capacity for an additional level increase of 1.3 metres, equivalent to a once in 200 years event. Unlike most of the rest of the UK, the city has separate storm and foul sewers, so sewage pollution is not a significant problem. Additionally, there are facilities to prevent accidental oil spills and the like from reaching the lake. As well as local storm drains, the lake's primary purpose is to intercept the

river Ouzel The River Ouzel , also known as the River Lovat, is a river in England, and a tributary of the River Great Ouse. It rises in the Chiltern Hills and flows north to join the Ouse at Newport Pagnell. It is usually called the ''River Ouzel'' ...

, a tributary of the river Great Ouse. The catchment area is

Oxford Clay The Oxford Clay (or Oxford Clay Formation) is a Jurassic marine sedimentary rock formation underlying much of southeast England, from as far west as Dorset Dorset ( ; archaically: Dorsetshire , ) is a county in South West England on the ...

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 The common tern (''Sterna hirundo'') is a seabird in the family Laridae. This bird has a circumpolar distribution, its four subspecies breeding in temperate and subarctic regions of Europe, Asia and North America. It is strongly migrator ...

tufted duck The tufted duck or tufted pochard (''Aythya fuligula'') is a small diving duck with a population of close to one million birds, found in northern Eurasia. The scientific name is derived from Ancient Greek '' aithuia'', an unidentified seabird ment ...

, ringed and little ringed plover,

common redshank The common redshank or simply redshank (''Tringa totanus'') is a Eurasian wader in the large family Scolopacidae. Taxonomy The common redshank was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his ...

and northern lapwing.

Canada geese The Canada goose (''Branta canadensis''), or Canadian goose, is a large wild goose with a black head and neck, white cheeks, white under its chin, and a brown body. It is native to the arctic and temperate regions of North America, and it is o ...

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.

Engineering

Other benefits

Case study: Willen Lake, Milton Keynes

See also

References

Further reading