Water Quality Capture Volume for Storm Water BMP and LID Designs

This paper summarizes the methodology and procedure used in an optimization and statistics computer model developed for determining the water quality capture volume (WQCV) for storm water best management practices (BMP) and low-impact development (LID) facility designs. The WQCV is directly related to the local rainfall pattern, watershed imperviousness, and drain time applied to BMP/LID storage devices. Aided by a computer model, the performance of a LID/BMP basin can be predicted using the local rainfall-runoff continuous simulation that computes the long-term runoff volume-based and event-based capture ratios using the principle of water volume balance among rainfall amount, hydrologic losses, and runoff volume captured in and bypass flow overtopping the storage basin. For a regional study, this procedure can be applied to a range of basin sizes to produce the optimized design value for WQCV. The numerical algorithm used in the computer model offers both runoff volume capture and event capture ratios. Typically, but not always, the optimal runoff volume and event capture ratios lie between the 80 and 90th percentile of the local runoff volume population. The computer model used was developed as freeware for evaluating the performance of a BMP facility or producing regional design charts. The model accepts the standard hourly or 15-min rainfall data format provided by the National Climatic Data Center. Hourly data are typically available for major metro areas in the United States for a period of 20 to 60years.