We developed a sophisticated 2D HEC-RAS model to simulate flood dynamics across the Waccamaw River watershed in the coastal plain region of the Carolinas. The region covers more than 1,100 square miles and frequently experiences severe flooding from tropical storms.
After hurricanes Florence, Matthew and Joaquin, the USACE initiated a SMART (Specific, Measurable, Attainable, Risk Informed, Timely) Planning feasibility study to assess flood risk and evaluate potential mitigation strategies. Our approach centered on using rain-on-grid hydrology, allowing for direct integration of rainfall input with hydrodynamic flow processes across the watershed.
Our team selected the HEC-RAS platform because it can simulate complex, wide-area flood events with precise rainfall inputs. The 2D rain-on-grid approach combines hydrology and hydraulics within a single model to streamline calibration, improve data integration and allow for dynamic analysis. Synthetic rainfall events, informed by National Oceanic and Atmospheric Administration (NOAA) Atlas 14 data, were created to simulate multiple return period storms, allowing for scenario-based flood risk analysis. The model calibration used historic radar-corrected rainfall data and stream gauge measurements from previous hurricanes to accurately replicate observed flooding.
The innovative use of GIS techniques for model geometry creation and calibration — combined with sensitivity analyses, enhanced model accuracy and responsiveness to varied storm conditions — provided USACE with a robust tool for assessing flood risk in the watershed. The approach supported identification of critical flood risk areas and provided a baseline for potential flood risk mitigation strategies.
The Waccamaw River basin’s hydrologic and hydraulic characteristics presented complex challenges. The watershed includes extensive flood plain areas, slow-moving waters, tidal influences and sensitive wetland ecosystems. The watershed’s hydrology is also shaped by extreme hurricane events, which introduce substantial rainfall in short periods. The need for accurate simulation of water movement over flat, wide flood plains and within complex wetland systems required a highly refined model and specialized calibration techniques to handle the sensitivity to initial flow conditions, roughness values and bathymetric data.