TABLE OF CONTENTS Detention and retention basins are widely used to control the increased runoff due to urbanization of undeveloped areas. These basins can also offer excellent water quality benefits since pollutants are removed from the stormwater runoff through sedimentation, degradation, and other mechanisms, as the runoff is temporarily stored in a basin. Detention basins are sometimes called dry ponds, because they store runoff only during wet weather. The outlet structures are designed to completely empty the basin after a storm event. Retention basins are sometimes called wet ponds since they retain a permanent pool.
The primary function of a detention basin is to control the quantity of stormwater runoff. Most stormwater management policies require that the postdevelopment peak flow rates be reduced to predevelopment peak flow rates for one or more specified design return periods such as 2, 10, and 25 years. Peak flow reduction is achieved by routing the postdevelopment runoff through a detention basin, that is by detaining the runoff temporarily in a basin. Figures 14.43 illustrates the effect of a detention basin on storm water runoff.
The schematic diagram given in Fig. 14.44 shows the basic elements of a detention basin. In addition, sediment forebays are often used for partial removal of sediments from the stormwater runoff before it enters the detention basin. Energy dissipating structures such as...
