629 - Flood Discharge Estimation Methods for Ungauged Basins in Michigan

Michigan and other Midwest states have experienced a number of severe floods in recent years, and there is evidence that extreme weather events are occurring more frequently in the region. To help ensure accurate estimation of design discharges, this study investigates current hydrologic calculation methods and suggests revisions using updated hydrometeorological and geospatial data sets. For ungauged basins smaller than 20 square miles, a modified Curve Number method is used, based on a previous study that concluded that the standard NRCS unit hydrograph overestimated peak discharges. In this study, analysis of runoff events in small gauged basins demonstrates the uncertainty associated with the modified Curve Number method and peak discharge equation used in Michigan. For watersheds larger than 20 square miles, peak discharge regression equations are updated using Bulletin 17C procedures and generalized least squares regression methods. Previously significant predictor variables and recently developed watershed and climate variables used by surrounding states are considered for use in the updated Michigan regression equations. This presentation will provide an overview of both small and large watershed methods, focusing on the Curve Number-based method applied to smaller watersheds.