Effect of Inlet Turbulent Boundary Conditions on Scour Predictions of Coupled LES and Morphodynamics in a Field-Scale River: Bankfull Flow Conditions

Abstract

This paper presents a systematic numerical investigation to study the effects of inlet turbulent boundary conditions on the coupled hydrodynamics and morphodynamics computations of a natural river, the Feather River, located in northern California. A coupled flow (Eulerian) and sediment (Eulerian) dynamics numerical framework is employed to simulate fully coupled hydro-morphodynamics of a 600-m-long reach of the river in which there are several bridge foundations. The turbulent flow of the river is modeled using large-eddy simulation (LES). The considered inlet boundary conditions consist of (1) uniform flow, (2) instantaneously varying turbulent flow generated from a precursor straight-channel flow simulation, and (3) instantaneously varying turbulent flow produced from a precursor river flow simulation in a 900-m-long reach of the river located immediately upstream of the study area. The volumetric flow rate of the river in all cases is 1,250 m3/s, which corresponds to a high flow rate that lasted for about 24 h and led to the formation of a deep scour hole around the bridge foundations. The river bathymetry before and after the high river flow conditions are obtained using a series of field measurements. The latter bathymetry is compared with the simulated bed morphologies to assess the accuracy of the simulation results, and the former is utilized to produce the computational grid system of the river. The suitability of various inlet boundary conditions for coupled flow and morphodynamics simulations is evaluated by comparing the corresponding time-averaged flow field and riverbed elevation profiles after 24 h of actual time. The numerical study revealed that, unlike the flow field, the effect of the inlet turbulent boundary conditions on the riverbed morphodynamics is negligible. In addition, a validation study is presented that attempts to compare the numerical simulation results with those of experimentally measured data for flow and scour patterns around a skewed pier.