Importance of Eddy Viscosity and Advection in Hydrodynamical Models for Simulating Flash Floods on Steep Sloped Watersheds

Flash floods in steeply sloped watersheds pose significant human life and infrastructure. Accurate prediction of these events relies on key parameters such as peak flow, time to peak flow, and the total overland flow volume. Numerical models are highly effective tools for predicting flash floods. The accuracy of hydrodynamic models is determined mainly by the solver equations used. Depth-integrated models offer various equation sets, with the full hydrodynamic equation providing the most detailed, though computationally intensive, solution. Eddy viscosity is another critical factor in simulating turbulent overland flow. Still, increased equation complexity leads to longer computational times and the need for smaller time steps to maintain model stability. Simulating turbulent overland flow in steep watersheds is particularly challenging because maintaining stability in these conditions is difficult. This study examined overland flow using artificial watersheds and model rainfall events, testing multiple solvers within the Hydrologic Engineering Center – River Analysis System (HEC-RAS). By keeping geometry, mesh, and rainfall inputs consistent, the study compared solver performance, identifying potential errors that arise under different conditions. Nonlinear advection, rather than gravity and roughness, was found to govern flow around obstructions. These findings are critical for improving the reliability of models that simulate the complex dynamics of flash floods, ultimately aiding in the reduction of risks posed by these hazardous events.