Sizing Storm‐Water Detention Basins for Pollutant Removal

Abstract

A statistical formulation for estimating the average time of detention within a pond for a captured runoff volume is presented. For a conservative estimator, it is assumed that mixing takes place during an event and that settling occurs over a period to empty the captured volume or the time between successive events, whichever is smaller. This analytically determined detention time is used in conjunction with a pollutant‐settling efficiency‐detention time curve to estimate the settling efficiency. This curve is generated from Stormwater Management Model (SWMM) program simulations and shown to be independent of runoff statistics, pond configuration, and arbitrary but constant influent concentration under complete mixing. The analytical detention time estimate, in combination with the settling efficiency curve provides a valuable desk top method for the planning‐level design of detention basins for pollutant removal. The method performs quite well compared to the results obtained from long‐term SWMM simulation runs.