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

• Published in: Journal of hydrology. Regional studies Vol. 15; no. C; pp. 31 - 48
• Main Authors: Pradhan, Nawa Raj, Loney, Drew
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2018; Elsevier
• Subjects: Dam safety; Nonlinear scaling relationship; Return period rainfall; Unit hydrograph peaking factor; Dam safety; Unit hydrograph peaking factor; Return period rainfall; Nonlinear scaling relationship
• ISSN: 2214-5818; 2214-5818
• DOI: 10.1016/j.ejrh.2017.11.007

•Design inflows of many dams is based on simplified unit hydrographs (UH) theory.•However, hydrographs behave non-linearly between various rainfall events.•UH simplifications present a safety risk if an improper design event is utilized.•Process-based hydrologic models take account of nonlinearity in peaking.•A nonlinear UH peaking factor scaling relation is proposed to ensure dam safety. Goose Creek Watershed, Loudoun County, Virginia, USA. Existing 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. This 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.