LES- and RANS-based Modeling of Coupled Flow & Mobile bed Evolution in a Natural River Under Various Flood-induced Turbulent Boundary Layers*

Flow field and sediment transport in rivers under flood conditions are dominated by high-energy, large-scale, coherent structures. High-resolution numerical modeling provides a reliable tool to better understand the flow and bed morphodynamics interaction. The inlet turbulent boundary layers (TBL) used to prescribe the incoming turbulence in natural rivers can have crucial effects on the flow and sediment dynamics. We employ an in-house CURVIB code, the VFS-Geophysics, to systematically investigate the effect of various TBLs on the LES and unsteady RANS computed flow and morphodynamics of a 0.6km-long area of the Feather River in California under flood condition. We employ three different TBLs, including: a uniform flow, a precursor straight-channel flow simulation, and a precursor river-flow simulation in a 1km-long area of the river immediately upstream of the study area. The pre-flood bathymetry is utilized to produce the computational grid system of the river. While, the post-flood bed bathymetry served as a benchmark to validate the computed bed morphodynamcis calculations.