To satisfy the full suite of stormwater management design objectives (e.g., flood protection, water quality treatment, onsite retention, and environmental flow maintenance), a wide range of runoff conditions should be investigated. A variety of statistically derived design storm events could be used to represent expected loading/operating conditions, however continuous hydrologic simulation is a better option as it applies a long-term record with a wide range of storm durations and rainfall intensities. Further, only continuous simulation can represent the dry weather processes that are critical to evaluating long-term runoff response characteristics to quantify the hydrologic water budget and to develop flow frequency or flow duration curves. The accuracy of runoff response simulation also varies with the spatial resolution of subcatchments. The most accurate approach would model each hydrologic response unit (HRU – an area with homogeneous surface cover, slope, and soil texture characteristics). A more practical approach would include an area-weighting of HRUs within larger subcatchments. This presentation highlights how EPA-SWMM5 can be used to model both approaches: runoff from individual HRUs (a set of 13 different surface cover types ranging from forest to impermeable); and runoff from composite HRUs (area-weighted in a development block that features mixed-use zoning and routed through a piped collection system). These approaches are applied at 10 sites across the U.S., using local meteorological input data (i.e., evaporation rates and 1-minute rainfall over a 20-year period of record). Results are presented to compare and contrast the water balance and flow frequency/duration curves that vary by location.
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