Hydrologic vs Hydraulic Rainfall-Runoff Modeling

Rainfall-runoff modeling is commonly employed by hydrologic engineers for the purpose of determining a hydrograph. Historically, hydrologic methods utilizing unit hydrograph theory and basin averaged parameters have been used to obtain results. Of the available unit hydrograph methods, the National Resources Conservation Services (NRCS) procedure has gained widespread popularity and is most commonly used in the professional community. Recent advancements in both computing power and accessibility to high resolution digital elevation models (DEMs) has made 2D hydraulic rainfall-runoff modeling feasible. Version 6 of the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) allows users to input a precipitation boundary condition used in combination with both Curve number (losses) and Land use (Manning’s n) layers for the purpose of modeling the rainfall-runoff relationship over a 2D terrain using hydrodynamic equations. The goal of this study was to simulate the rainfall-runoff relationship using both a hydrologic method (NRCS, via HEC-HMS) and hydraulic method (via HEC-RAS) and then compare results to actual values recorded at a United States Geologic Survey (USGS) gaging station. A total of 38 watersheds with 59 historical storm events were analyzed for this study. Analysis showed that neither technique produced consistently accurate results when compared to historical data. The hydraulic (HEC-RAS) simulations produced an average (peak flow) percent error of 100%, with 28 out of 59 simulations overestimating the peak flow. The hydrologic (HEC-HMS) simulations had an average (peak flow) percent error of 161%, with 41 out of 59 simulations overestimating the peak flow. In general, results tended to improve for simulations involving larger watershed areas as well as larger precipitation events.