Water Quality Capture Volume for Stormwater BMP and LID Designs

This paper summarizes the methodology and procedure used in the computer model, Water Quality Capture Optimization and Statistics Model (WQ-COSM), developed for determining the Water Quality Capture Volume (WQCV) for stormwater BMP and LID facility designs. WQCV is directly related to the local rainfall pattern, watershed imperviousness, and drain time applied to BMP/LID storage devices. Aided with the computer model, WQ-COSM, 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, 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 WQ-COSM offers both runoff volume capture and event capture ratios. Typically, but not always, the optimal runoff volume and event capture ratios lie in between 80 and 90 percentile of the local runoff volume population. This WQCV design and analysis procedure is more robust than the one used to estimate the WQCV's using the regression equations outlined in ASCE Manual of Practice No. 87 and WEF Manual of Practice No. 23. The computer model, WQ-COSM, was developed as a freeware for evaluating the performance of a BMP facility or producing regional design charts. The model accepts the standard hourly or 15-minute 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 60 years.