Comparison of Basin Performance Modeling Techniques

A comparison of methods used for analyzing the long‐term pollutant removal efficiency in a stormwater detention basin points out some fundamental differences between simulation, using EPA's Storm Water Management Model, and a statistical technique advanced by Hydroscience, Inc. For the 24.6‐yr record of Atlanta, Georgia hourly rainfall, the mean runoff event volume predicted by each method is similar. However, the inability of the statistical method to account for the reduction in the number of events (due to indigenous catchment storage) results in an estimate of total runoff 30% greater than the simulation results. The statistical technique employs a pollutant removal expression applied to the total flow captured in the detention basin, as contrasted to the simulation's utilization of intra‐event pollutant kinetics for removal within the basin. Solution surfaces of runoff flow capture and pollutant removal efficiencies are developed for each method as a function of basin volume and drawdown rate. The differences in performance estimates between the two methods increase as the basin volume and drawdown rate increase, with the statistical technique yielding the lower estimate of flow capture. An optimal combination of basin volume and drawdown rate was determined for each technique by incorporating a cost constraint. For a long‐term pollutant removal efficiency of 70%, the annual cost associated with a basin designed by the statistical technique is approximately twice that of a basin designed by simulation. The program SYNOP generates rainfall event statistics which may be incorporated with simulation, providing a coordinated approach for analyzing the performance of stormwater detention basins.