A combined GIS and HEC model for the analysis of the effect of urbanization on flooding; the Swan Creek watershed, Ohio

Abstract

Abstract This study investigates the effects of changes in land-use on the hydrology of the Swan Creek watershed in Lucas County, Ohio, over a thirty-year period. The study presents the GIS based runoff models and the HEC-RAS Flood analysis for the watershed. The runoff modeling was performed using a modification of the rational formula first proposed by Rossmiller in 1980. The C factor (land use) in the rational equation (Q = CIA) is modified to account for land use, soil type and slope factors. Four model runs for each of the land-use coverages were created based on the statistical precipitation recurrence intervals: P 10 , P 25 , P 50 , P 100 (a total of 12 model runs). The models differed only in the basic land-use classifications for each decade of interest. The runoff analysis predicts that increased runoff volumes in the watershed are directly related to increasing urbanization. The study showed that runoff volumes of a P 100 storm in 1973 of certain sub-basins has the equivalent runoff of a P 10 storm in 1995. The flood modeling was performed in the Army Corps of Engineer's Hydrologic Engineering Center's River Analysis System (HEC-RAS). Twelve model runs were also created in HEC-RAS to evaluate the effect of the land-use changes in each of the time periods for each of the storms. The study showed that flood extent of a P 50 storm in 1973 across the watershed has the equivalent flood extent of a P 10 storm in 1995. The flood modeling predicts that the Swan Creek hydrologic system responds more quickly and severely today to events that would have been smaller in the past.