Balancing lake

A balancing lake (also "flood basin") is a term used in the U.K. describing an element of an urban drainage system used to control flooding by temporarily storing flood waters. The term balancing pond is also used, though typically for smaller storage facilities for streams and brooks.

In open countryside, heavy rainfall soaks into the ground and is released relatively slowly into watercourses (ditches, streams, rivers). 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 and foul sewers, can cause serious pollution as well as flooding.

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 rain storm in the catchment on communities down river.

For aesthetic and safety reasons, the system can be designed so that there is a permanent lake. A lake with an equivalent area of 1000 by 1000 metres rises by one metre will hold a million cubic metres of water. 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.

A permanent lake can provide useful recreation facilities such as sailing, windsurfing, or of wildlife. These uses 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.

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, 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.

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