PAIRED-WATERSHED COMPARISONS TO ASSESS EFFECTIVENESS OF TRADITIONAL STORMWATER MANAGEMENT

Traditional stormwater management (SWM) commonly involves construction of detention basins and other structures to attenuate peak flows, mitigate flood risk and reduce streambank erosion. If SWM is effective at watershed scale there should be a detectable difference in streamflow response to storms between otherwise similar watersheds with large differences in SWM coverage. In this study we employ empirical analysis of rainfall-runoff response for hydrographs recorded at stream gages in two sets of watersheds tributary to the Gwynns Falls in Baltimore County, Maryland. Each set of watersheds are located in close proximity to each other with similar drainage area, comparable percent impervious cover, and similar geology, but strongly different percent drainage area controlled by detention basins.

We compare hydrologic response variables for Scotts Level (drainage area 8.6 km², 32.9% impervious, 10.5% watershed area draining to SWM) and Gwynns Falls near Delight (drainage area 10.5 km², 28.5% impervious, 42.7% SWM), both located in the upstream portion of the Gwynns Falls watershed. We also compare three small headwater tributaries in the Dead Run watershed (DR1: 1.2 km², 67% impervious, 48.1% SWM; DR2: 1.9 km², 49.1% impervious, 30.5% SWM; DR5: 1.6 km², 45.9% impervious, 2.9% SWM), located in an inner-ring suburb that drains to lower Gwynns Falls. We assess a population of storms for which we have bias-corrected watershed-average HydroNEXRAD radar rainfall records with 15-minute time steps and streamflow data recorded at 5 or 15-minute intervals. Response metrics assessed include Richards-Baker flashiness index, volume to peak ratio, R(t) discharge-precipitation ratio, ratio of peak runoff intensity to peak rainfall intensity, runoff ratio, centroid lag time, and lag from rainfall center of mass to peak runoff. Metrics were calculated separately for single-pulse rainfall events and for longer, more complex storms. Responses within and between watersheds were also compared for storms grouped by total rainfall accumulation. For both sets of watershed comparisons, the group of metrics assessed failed to show a clear trend demonstrating a difference in hydrologic response attributable to SWM, suggesting that other factors are as important or more important in controlling rainfall-runoff response.