Balancing lake

A balancing lake (also flood basin ) is a term used in the U.K. describing a retention basin used to control flooding by temporarily storing flood waters.^{[1]: Glossary} 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 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.^{[1]: Part E} Failure to do this, especially in older settlements without separate storm sewers and foul sewers, can cause serious pollution as well as flooding.