Comparative Assessment of HEC-HMS 2D and HEC-RAS 2D Rain-on-Grid for Continuous Urban Watershed Modeling

Abstract

The application of two-dimensional (2D) hydrologic and hydraulic modeling tools is increasing for overland flow simulation as they represent spatial changes in depth, velocity, and flow conditions more accurately. Recently, the US Army Corps HEC-HMS (Hydrologic Engineering Center Hydrologic Modeling System) included the capability of unstructured 2D hydrologic-hydrodynamic modeling. In parallel, HEC-RAS 2D (Hydrologic Engineering Center – River Analysis System), initially applied for river flow simulation, was adapted to include rain-on-grid (RoG) features and infiltration algorithms, providing some hydrological modeling capabilities. HEC-RAS hydrological capabilities are limited to event-based simulations as there are no provisions to represent abstractions such as evapotranspiration or groundwater/baseflow contributions. By contrast, HEC-HMS supports continuous 2D surface modeling and includes baseflow options in both semi-distributed and fully distributed configurations, albeit with simpler hydrodynamic calculation capabilities. Studies performing a direct comparison between HEC-HMS RoG and HEC-RAS RoG approaches in representing urban hydrology remain scarce. This MS thesis aims to fill that gap by assessing their performance in Moore’s Mill Creek Watershed, in Lee County, Alabama, with a focus on long-term continuous rainfall-runoff modeling. Both models run on the same unstructured mesh and use identical rainfall, terrain, land‑use, and soil data. Model simulations are compared over an extended period to evaluate simulated depth, velocity, and flow hydrographs against field observations. The comparison shows HEC-HMS superior performance for extended simulation and provides practical guidance on parameter alignment, data needs, and tool selection.