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, 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. 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, tufted duck, ringed and little ringed plover, common redshank and northern lapwing. Canada geese 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. See also Stormwater Surface runoff Urban runoff Sustainable urban drainage systems Retention basin References Further reading Wetlands, Industry & Wildlife: A manual of principles and practices. (1994) (The Wildfowl & Wetlands Trust, UK). Chapter 15. Hydrology Hydraulic engineering Stormwater management