Relative importance of impervious area, drainage density, width function, and subsurface storm drainage on flood runoff from an urbanized catchment

urbanized catchment, the 14.3 km 2 Dead Run watershed near Baltimore, Maryland, USA, and the physics-based gridded surface/subsurface hydrologic analysis (GSSHA) model to examine the relative effect of each of these factors on ood peaks, runoff volumes, and runoff production efciencies. GSSHA was used because the model explicitly includes the spatial variability of land-surface and hydrodynamic parameters, including subsurface storm drains. Results indicate that increases in drainage density, particularly increases in density from low values, produce signicant increases in the ood peaks. For axed land-use and rainfall input, the ood magnitude approaches an upper limit regardless of the increase in the channel drainage density. Changes in imperviousness can have a signicant effect on ood peaks for both moderately extreme and extreme storms. For an extreme rainfall event with a recurrence interval in excess of 100 years, imperviousness is relatively unimportant in terms of runoff efciency and volume, but can affect the peak ow depending on rainfall rate. Changes to the width function affect ood peaks much more than runoff efciency, primarily in the case of lower density drainage networks with less impermeable area. Storm drains increase ood peaks, but are overwhelmed during extreme rainfall events when they have a negligible effect. Runoff in urbanized watersheds with considerable impervious area shows a marked sensitivity to rainfall rate. This sensitivity explains some of the contradictoryndings in the literature.