An analysis of the unit hydrograph peaking factor: A case study in Goose Creek Watershed, Virginia

Abstract

Study regionGoose Creek Watershed, Loudoun County, Virginia, USA. Study focusExisting U.S. Army Corps of Engineers (USACE) policy suggests that the unit hydrograph peaking factor (UHPF) – taken as the unit hydrograph peak of any flood runoff, including the probable maximum flood, over unit hydrograph peak of the inflow design flood (IDF) – range between 1.25 and 1.50 to ensure dam safety. It is pertinent to investigate the impact of extreme flood events on the validity of this range through physically based rainfall-runoff models not available during the planning and design of most USACE dams. The UHPF range was analyzed by deploying the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model. New hydrological insightsThis study concludes that design events with return periods greater than 5-years are required for the UHPF to fall within the guidance range and that UHPF becomes less sensitive to rainfall intensity with increasing accumulation time. An effective rainfall factor (ERF) is introduced to validate existing UHPF guidance as well as provide a nonlinear UHPF scaling relation when effective rainfall does not match that of the UH design event. Finally, a method for quantifying the effect of hydrologic parameter and precipitation magnitude uncertainty on UHPF are demonstrated. The Goose Creek facility is shown to maintain dam safety given current UHPF guidance as it was designed using 25-year return-period rainfall.