Estimation of Average Annual Removal Efficiencies of Wet Detention Ponds Using Continuous Simulation

Abstract

A curve-number-based, planning level, macrospreadsheet model wet pond annual efficiency simulation model (WEANES) has been developed to predict the average annual removal efficiencies (AARE) of wet detention ponds based on mass pollutant loadings. The model can be used as a design, planning, and permitting tool. WEANES uses the following three processes: (1) converts daily rainfall hyetographs to hydrographs using the curve-number (CN)—based Santa Barbara urban hydrograph method; (2) converts the inflow hydrograph to an outflow hydrograph from the wet pond outlet structure through routing; and (3) calculates annual inflow and outflow pollutant loads and pond efficiency. The model uses historical, site-specific, multiyear, rainfall data available from National Oceanic and Atmospheric Administration climatological stations. Required input parameters are: (1) watershed parameters (drainage area, pervious CN, directly connected impervious area, and time of concentration); (2) pond parameters (control and overflow elevations, pond side slopes, and surface areas at control elevation and pond bottom); (3) outlet structure parameters; (4) pollutant event mean concentrations; and (5) pond loss rate, which is defined as the net loss due to evaporation, infiltration, and water reuse. This paper provides an example of the model calibration and validation process for an existing wet pond. The effect of watershed and pond parameters on AARE is examined through sensitivity analysis. The WEANES-computed AARE values for total suspended solids ranged from 75.9 to 84%, whereas the U.S. EPA model predicted removal efficiencies of greater than 99%. The WEANES model can provide the user with greater flexibility while maintaining the simplicity of the U.S. EPA model.