Does Stormwater Management Make a Difference?

The fundamental purpose of stormwater management (SWM) is to 'bend' the flow duration curve, i.e. to redistribute the arrival times of runoff such that flashy peak flows induced by urban development will be reduced in magnitude and frequency, with corresponding augmentation of base flow. Because there are so many variables that affect watershed hydrologic response to precipitation events, it is difficult to determine the extent to which SWM is truly effective at the watershed scale. The approach of this study is to compare the hydrologic behavior of small urban watersheds that are located in close proximity to one another, with comparable drainage area, geology, and percent impervious cover and with significant differences in the percent of drainage area controlled by SWM facilities. The Gwynns Falls watershed in the Baltimore metropolitan area offers several opportunities for comparative analysis of gaged watersheds with accompanying precipitation records. Scotts Level (drainage area = 8.6 km2, 32.9% impervious, 10.5% SWM) and Gwynns Falls near Delight (drainage area = 10.5 km2, 28.5% impervious, 42.7% SWM) are located in close proximity with similar geology. 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) and two intermediate-scale tributaries (DR3: 5.8 km2, 55.2% impervious, 38.4% SWM; DR4: 5.1 km2, 47.8% impervious, 7.5% SWM). Analyses include paired-watershed comparisons of cumulative monthly runoff and precipitation; flow duration curves for unit runoff rates; and quickflow hydrographs associated with individual precipitation events, as well as characteristic shape of unit hydrograph response to short-duration pulse rainfall. Preliminary results suggest that runoff ratios are more sensitive to percent impervious cover than to percent control by SWM. Flow duration curves show greater separation between paired watersheds at low flow than at higher flow; some comparisons suggest that SWM may augment base flow but others do not. In some cases the differences at the high-flow end of the spectrum are consistent with the anticipated effects of SWM coverage but differences among flow duration curves at the 1% and 0.1% exceedence probability do not suggest that SWM exerts a controlling influence on watershed response. Timing and magnitude of peak flow response to precipitation events also do not appear highly sensitive to watershed percent SWM control. These findings may be specific to the more traditional types of SWM dominant in the study watersheds. The presentation will include comparison with other sites where newer, advanced SWM structures are used to divert most surface runoff to infiltration.