I. Theoretical Background
Flooding in urban areas is an unavoidable problem in many cities, and it causes huge costs to the society in structural and non-structural damage. The impact of urban flooding is often very difficult to evaluation precisely. The damages in urban flooding may be divided into three groups; direct damage, indirect damage, and social consequences.
A highly developed urban place results to an increase of runoff volumes and flow rates, which may cause more frequent flooding. To abate the adverse downstream effects of urban stormwater runoff most communities use flow retardation structures. Detention basins, retention basins, infiltration basins, infiltration basins, and roof top storage are some examples of flow retardation structures. A detention basin provides protection in floodplains by containing floodwater for a brief period of time. It can be constructed by damming a channel or by excavating a pond into the existing ground. The excavation of a detention basin is constructed by a combination of cut and fill.
A detention basin is an artificial flow control structure that is used to contain flood water for a limited period of time. Detention basins are more widely used for stormwater management than any other type of control. However, at present, its primary application has been for drainage control, i.e. peak-flow attenuation, rather than the water quality control. Detention basins designed for peak flow attenuation can be given an effective water quality control function at little added cost. (The Urban Water Resources Research Council of the American Society of Civil Engineers and the Water Environment Federation)
In designing a multipurpose detention pond there are certain guidelines that must be followed to ensure that the pond is properly maintained and do not become hazardous. 1.) Side slopes should not be greater than 3m horizontal to 1m vertical. 2.) An underdrain system should be constructed to minimize the wetness of the pond bottom. 3.) An alternative to the underdrain is the sloping of the pond bottom with at least 2% grade form inlet and outlet. Another means of elimination the wetness is to slope the pond to a gutter that flows through the pond. 4.) the pond bottom and the side slope should be finished with at least four inches of topsoil and seeded. This seed or sod should be capable of withstanding periodic flooding conditions. 5.) Outlets should be provided with rip-rap protection to prohibit erosion if soil is weak. 6.) An overflow spillway or weir should be placed at the high-water elevations and a minimum of 8m (3in) of freeboard should be provided. (NIPC 1986)