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 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 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 and safety). The water level can rise substantially without a significant