A balancing lake (also flood basin ) is a term used in the UK to describe a

retention basin A retention basin, sometimes called a retention pond, wet detention basin, or storm water management pond (SWMP), is an artificial pond with vegetation around the perimeter and a permanent pool of water in its design. It is used to manage ...

used to control

flood A flood is an overflow of water (list of non-water floods, 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 of significant con ...

ing by temporarily storing flood

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

s. The term balancing pond is also used, though typically for smaller storage facilities for

stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...

s and brooks. In open countryside, heavy

rainfall Rain is a form of precipitation where water droplets that have condensed from atmospheric water vapor 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. ...

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

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

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 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

storm sewer A storm drain, storm sewer (United Kingdom, United States, U.S. and Canada), highway drain, surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to Drainage, drain excess rain an ...

s 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 pass through the pipe without restriction, while heavy flows create a backup, causing water levels behind the dam to rise. 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 (also spelled esthetics) is the branch of philosophy concerned with the nature of beauty and taste (sociology), taste, which in a broad sense incorporates the philosophy of art.Slater, B. H.Aesthetics ''Internet Encyclopedia of Ph ...

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 Stormwater, also written storm water, is water that originates from precipitation (storm), including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil ( infiltrate) and become groundwater, be stored on depressed lan ...

balancing lakes in the UK. The lake is designed to take surface run-off from

Milton Keynes Milton Keynes ( ) is a city status in the United Kingdom, city in Buckinghamshire, England, about north-west of London. At the 2021 Census, the population of Milton Keynes urban area, its urban area was 264,349. The River Great Ouse forms t ...

, 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'', e ...

, a tributary of the

river Great Ouse The River Great Ouse ( ) is a river in England, the longest of several British rivers called "Ouse". From Syresham in Northamptonshire, the Great Ouse flows through Buckinghamshire, Bedfordshire, Cambridgeshire and Norfolk to drain into the ...

. 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 and as far north as Yorkshire. The Oxford Clay Formation dates to the Jurassic, specific ...

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 Temperateness, temperate and subarctic regions of Europe, Asia and North America. It is stron ...

tufted duck The tufted duck (or tufted pochard) (''Aythya fuligula'') is a small diving duck with a population of nearly one million birds, found in northern Eurasia. They are partially migratory. The scientific name is derived from Ancient Greek , an unide ...

, ringed and

little ringed plover The little ringed plover (''Charadrius dubius'') is a small plover. The genus name ''Charadrius'' is a Late Latin word for a yellowish bird mentioned in the fourth-century Vulgate. It derives from Ancient Greek ''kharadrios'' a bird found in river ...

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 hi ...

and

northern lapwing The northern lapwing (''Vanellus vanellus''), also known as the peewit or pewit, tuit or tewit, green plover, or (in Ireland and Great Britain) pyewipe or just lapwing, is a bird in the lapwing subfamily. It is common through temperate Palearcti ...

Canada geese The Canada goose (''Branta canadensis''), sometimes called Canadian goose, is a large species of 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 ...

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