Storm detention basins

Storm water should be detained, to prevent floods.

After centuries of accelerated surface water runoff, it is now accepted that rainwater should be controlled 'at source'. Runoff should be detained where if falls and discharged from the area as slowly as possible. This will attenuate the flood hydrograph. Water authorities, if they have the power, can insist that when new development takes place the volume of storm water discharge must not increase. This policy is sometimes known as "zero runoff". They can also impose a drainage levy on landowners who discharge rainwater into public sewers. Any developer who installs paving, roofing or drainage which will cause an increase in the rate of runoff can be required either to pay a compensatory fee or to carry out works which detain water and prevent it from causing flooding in downstream areas. This technique is known as 'urban stormwater management' (Hall 1984). When significant volumes of water are detained on the land, flood levels and flood peaks will diminish. The practice has many advantages but should be more than an engineering exercise: The preferred approach should be to regard urban runoff as a resource for potential re-use rather than an inconvenience, and to undertake stormwater flow management design in a catchment context (CIRIA 1992:5). Water can be used to recharge aquifers, to irrigate vegetation, to create ponds, wetlands and water features. From a nature conservation standpoint, these features are highly desirable. The CIRIA report also points out that 'controlled surface flooding' leads to significant economies in drainage costs (CIRIA 1992a: 55). This is the point which McHarg made in 1975 (cross ref to Ch. on Urbanisation). Another CIRIA report observes that 'some planning authorities have accepted storage ponds as part of open space provision while others have rejected even landscape flood areas' (CIRIA 1992b:11). The problem results from a petty dispute between drainage authorities and parks departments over who should be responsible for the maintenance of vegetated flood storage areas. Would they be 'parks' or 'drains'? The principle of storm detention has been considered 'increasingly necessary' in Britain (Institute of Civil Engineers 1981: 2). One of the chief reasons for the massive flood relief programme in the Lea Valley was urbanisation. Its 1,500 sq. km. catchment has an average population density of 1,667/ sq.km. When the first instalment of the river improvement scheme was nearing completion, in 1958, it was believed that urban growth in the Lea Valley and the construction of new towns at Stevenage, and Welwyn Garden City 'made the extension of the Flood Alleviation Scheme up the valley into Hertfordshire even more necessary'. It was thought that 'new towns with their large areas of impermeable road, roof tops and shopping areas pose a major threat to hitherto natural streams' (Wyllie 1958). Since much of the urbanisation took place on permeable chalk and sand, the paved surfaces caused dramatic changes in the rate of runoff and needlessly prevented rainwater from recharging the aquifer. By 1982, ten dry basins had been built in Stevenage, and a 0.78 ha wet basin, known as the Black Fen Valley Lagoon, was used for storm detention (and model boating) in Welwyn Garden City. They have been planned as central features in the French new towns (Secretariat General du Groupe Central des Villes Nouvelles, 1977). At Reston in Virginia, which is a planned community with a target population of 35,000, three detention ponds were included in the layout. Since lake front property sells for three times the price of other land in Reston, the detention ponds are seen as an excellent commercial proposition (Robbins et al 1981). There are two wet basins and one dry basin in the Cray Valley which were treated as single purpose engineering projects (Thompson 1966: 135). The dry basin, at Hall Place, is a soggy area in a public park [Fig 9.14]. If it had been designed as a wildlife habitat, it would have improved the appearance of a vacant expanse of grass. The wet basins look marvellous when the flood waters gather to remind us of the swamps which once characterised the winter landscape of northern Europe. Most swamps have been drained and it is a tragedy that the principle of storm detention was not adopted at an earlier date. In the first edition of this book I commented that the Cray wet basins, at Ruxley, are cut off from the surrounding urban area by fences and only licensed fishermen were allowed to enter. Members of the Kent Trust for Nature Conservation are now allowed in but there is no public access. Detention basins can also be used to improve water quality, by filtration, sedimentation and biological assimilation (Startin and Lansdown, 1995). Reedbed systems have been shown to reduce oxygen demand and to remove phosphates, nitrates, suspended solids and bacterial loading. These advantages, which are considerable, are best achieved by integrating water management into the town and country planning system.