Comparing SWMM and HEC-RAS Hydrological Modeling Performance in Semi-Urbanized Watershed

One of the most common applications of hydrological models is in studying urban watersheds, with distributed and semi-distributed models being used more frequently. The choice of modeling tools and the selection of hydrological processes to represent is dependent on the modeling objectives, available resources, and the scale of the study. Certain modeling setup parameters can have important effects on model accuracy, such as the representation of aquifer components, detailed surface storage, and discretization, among others. The effects of these parameters are often unknown since most hydrological studies do not systematically alter these parameters nor consider different modeling tools. This study performs a comparative analysis between SWMM and HEC-RAS in describing the hydrological response of a semi-urbanized headwater watershed in Alabama. Within each model, different modeling parameters were varied and the effects on the model accuracy were assessed with the stream depth and velocity collected in the field. The results showed that the parameters most impactful in SWMM modeling results were the existence of an aquifer component as well as the careful representation of surface storage. Conversely, HEC-RAS results were comparable with SWMM results when an aquifer was not present, but the runoff was highly overpredicted when infiltration was not considered. These results can indicate the applications and limitations of rain-on-grid model results that neglect interactions with shallow groundwater.