Modeling Stormwater Basins for Potential Retrofit Designs

Outflow structures for stormwater detention basins can vary widely within a single watershed. The designs are often left to the whim of the local engineer. Why should there be such differences? This research attempts to discern the performance and efficiency of various typical and atypical weir shapes. To accomplish this task, a scale model of a stormwater detention basin was constructed with removable outlet weirs. The physical model was calibrated using typical weir structures (e.g. rectangular, V-notch, submerged orifice, etc.) in which accepted numerical models exist to calculate outflow discharge. Empirical stage-discharge relationships were then calculated for the atypical weir structures (e.g. stepped notch, combinations of typical shapes, etc.). The atypical weirs simulated herein represent the range of outlet structure designs located in the Upper Mantua Creek watershed, Gloucester County, New Jersey. This area of New Jersey has experienced large urban growth in the past 40 years, and many of the existing basins were not designed for the current runoff volumes they experience. Within this 7.3 square mile watershed, there are currently 61 stormwater basins and more than 50 different outlet structure designs. The ability of each stormwater basin to pass or detain a given flow event can now be determined empirically. From these data, appropriate retrofit designs can be implemented.