Hebgen Dam, a concrete-core earthen embankment dam of 85 ft tall and 721 feet long, is located in the Madison River in the State of Montana.  This dam has a unique curved spillway in which transverse waves were observed during a water spill event, which raises concerns on weather side walls of the spillway are sufficiently high to contain water during a PMF flood event. A computational fluid dynamics (CFD) model was developed using ANSYS FLUENT, a state-of-the-art CFD software package, for the curved spillway, which includes a portion of reservoir, the spillway, and tailrace water area. The volume-of-fluid (VOF) model was used to predict transverse waves and water surface profile in the spillway as well water levels along two side walls of the spillway for a PMF event.  Comparison of CFD results with field observations, HEC-RAS modeling results and theoretical wave height calculation demonstrates that CFD modeling can accurately predict wave heights and can be used for assessing overtopping risks of flood water and provide quantitative flow data for the dam owner and FEMA to make informative decisions.