THE EFFICIENCY FUNCTION OF DETENTION RESERVOIRS IN URBAN DRAINAGE SYSTEMS

Detention reservoirs in urban drainage systems are provided for reducing the peak flows downstream of the reservoir. The flow reduction is due to the storage volume of the reservoir in which the incoming flow is temporarily stored. The relationship between the peak reduction ratio (the peak outflow discharge to the peak inflow discharge) and the relative storage ratio (maximum volume stored in the reservoir to the volume of the inflow hydrograph) is evaluated in this study. This relationship, named the efficiency function, is derived by a numerical solution of the dimensionless routing equation. Using the three-parameter gamma distribution to represent the inflow to the reservoir, the efficiency functions can be expressed by simple linear equations. Each equation depends on a single parameter whose value is determined by the reservoir configuration (depth-storage relationship) and the type of the outlet device (e.g., spillway or culvert). The implementation of the efficiency function is most suitable for the preliminary design stage where a large number of alternative designs can be easily tested without performing the conventional routing procedure.