An Approach to Analyze the Hydrologic Effects of Rain Gardens

This paper provides a methodology to analyze the hydrologic effects of rain gardens in urban catchments without requiring unsubstantiated assumptions about how small heterogeneities like rain gardens will affect empirical parameters lumped over larger catchment areas. The uses a physically-based, distributed model to simulate overland flows and unsaturated subsurface flows at a resolution finer than the size of the rain gardens. The model was calibrated to simulate observations from rain gardens monitored over a four-year period by the U.S. Geological Survey, providing calibrated topography and soil profiles describing the hydrology of the rain gardens. The model was then calibrated to simulate the runoff observed from a 5.3 hectare urban watershed with 86 residential lots. The calibrated rain gardens were then merged into the calibrated urban watershed model to simulate a variety of scenarios, including random layouts of varying density and size rain gardens. Each scenario was simulated using design storms ranging from 6 month to 100 year recurrence intervals. Results showed significant reduction (compared to the scenario with no rain gardens) in peak discharge and runoff volume and significant increases in time to peak among different scenarios and return period. Reductions for both volume and peak flow ranged from 10 to 50 percent, even for larger storms (10 year to 50 year events).