Using PCSWMM to simulate first flush and assess performance of extended dry detention ponds as structural stormwater BMPs in a large polluted urban watershed

Urbanization impacts stormwater runoff and pollutes receiving waters. Total suspended solids (TSS) are of particular concern as they can act as a transport agent for other pollutants. Moreover, the existence of the first flush phenomenon (FF), whereby the first stage of storm runoff is the most polluted, can also have profound effects. This research is concerned with the study of a framework for designing structural best management practices (BMPs) that mitigate stormwater harm in a large watershed based on comprehensive analysis of pollutants, rainfall parameters of influence, and the existence of FF. The framework was examined in St Anthony Park watershed, a large urban watershed in St Paul, Minnesota that outlets directly into the Mississippi River via a storm tunnel. The use of the Personal Computer Storm Water Management Model (PCSWMM) to simulate FF and to evaluate the effectiveness of structural BMPs has not been previously investigated for an urban setting with seriously polluted stormwater runoff like St Anthony Park watershed.

St. Anthony Park watershed stormwater was found to be highly contaminated, and most pollutants correlated positively with TSS. Subsequently, TSS were used to represent pollutants in PCSWMM. The model was built based on the existing drainage system of the watershed, and was calibrated and validated using recorded storm and runoff data. FF was numerically examined using various numerical methods and was found to exist. Subsequently, extended dry detention ponds (EDDPs) were examined as a potential BMP option that could efficiently control both water quantity (by diverting initial volumes of stormwater, thus addressing FF) and quality (by reducing TSS). EDDPs are basins that detain stormwater runoff for a calculated time to allow particles and associated pollutants to settle. Two location-based designs were examined: either a central EDDP at the main outfall on the Mississippi River, or a set of seven smaller EDDPs upstream. Distributed EDDPs were more efficient at reducing peak and total TSS loads. However, distributed EDDPs failed to completely eliminate FF, which was attributed to the long duration of time required for TSS to settle. Nonetheless, the high efficiencies seen when examining the other parameters indicate that distributed EDDPs were still successful at reducing stormwater pollution and should be considered for implementation. A cost-effective, rapid, and accurate method to simulate FF and study the optimal BMP design was thus implemented for a large urban watershed through the PCSWMM model. The results of the research study should better inform legislators and decision makers on optimal stormwater management at the St. Anthony Park watershed.