A Method for the Determination of Design Discharges for Urban Stream Restoration Projects in Northern Virginia

Abstract

It is well documented that urbanization changes the hydrology of watersheds (Hammer 1972; Booth 1991; Rose and Peters 2001). Increases in runoff volume and velocity from urbanization result in stream channel degradation (Hammer 1972; Henshaw and Booth 2000; Walsh et al. 2005; Leopold et al. 2005a; Poff et al. 2006). While stormwater management measures may be implemented to reduce the impact of stormwater runoff on streams, these practices do not reverse stream channel degradation that has already occurred. Stream restoration utilizing Natural Channel Stream Design (NCD) techniques is an effective way to reverse the effects of urbanization and return natural function to a stream. The design (bankfull) discharge for an NCD stream restoration project is the cornerstone of a restoration design. Existing methodologies for determining design discharges, such as hydrologic modeling and bankfull identification, have not worked well for NCD stream restoration projects in urban watersheds. The use of hydraulic geometry relationships serves as an alternative method for determining design discharge, but the required information is not generally available for urban Northern Virginia streams. However, rural regional curves developed for the Maryland piedmont, adjusted for watershed impervious area, provide a means to determine design discharges for urban stream restoration projects in Northern Virginia.