A STOCHASTIC POLLUTANT LOAD MODEL FOR THE DESIGN OF STORMWATER DETENTION FACILITIES

A set of equations dependent on specified pollutant trap efficiency fo r computing storage capacity-treatment rate combinations of a stormwat er detention system is provided. The equations are expressed in terms of the statistics of the runoff event variables: mean depth, duration, and interevent time. The hydrologic random variables are assumed to b e independent and exponentially distributed. A mass balance equation i s derived for the detention unit based on a first-order pollutant load model. With the aid of statistical derived distribution techniques, a n expression for the expected pollutant washoff load is derived, which is further used to develop analytical storage/treatment isoquants for specified levels of pollutant trap efficiency. The isoquants are defi ned in terms of the design storage capacity, the treatment rate. the p ollutant trap efficiency, and the runoff process statistics. A further extension of the model allows the derivation of equations for the est imation of the settling efficiency of pollutants of stored stormwater. The estimations for storage capacity-treatment rate are obtained from closed form expressions that, in addition. yield lower and upper boun d estimates of the design variables. Illustrative examples using data for catchments in Minneapolis and Denver are provided.