Rainfall-Runoff Modeling of the Chapel Branch Creek Watershed using GIS-based Rational and SCS-CN Methods

Abstract

Chapel Branch Creek (CBC), located within the Town of Santee adjacent to Lake Marion in Orangeburg County, SC, is listed on the SC 2004 303(d) list of impaired waterbodies due to elevated levels of nitrogen (N), phosphorus (P), chlorophyll-a, and pH. In this study, using a GIS-based approach, two runoff modeling methods, the Rational and SCS-CN methods, have been applied to the (~1600 ha) CBC watershed to estimate both event storm runoff and peak runoff rates. Rainfall intensities from five observed storms were used in the models together with the runoff coefficients and curve numbers (CN) for various land use categories obtained by digitizing (1:5000 scale) 2005 NAIP aerial photos of 1-m resolution. In order to test the models, results were compared to the observed peak flow rates and runoff for storm events recorded at sampling location 7 (SL-7), which drains an area of ~583 ha within the watershed. Results of this study show that a weighted low C-value (Rational Method) and a weighted Q (SCS-CN method) yield the most accurate estimates of peak runoff rates. The peak rates estimated by the SCS-CN method were in better agreement with the observed data than the Rational Method. In addition, both the estimated runoff depths and the peak flow rates were more accurate when aggregated by sub-unit within the catchment than when calculated for the whole drainage catchment using the SCS-CN method.