Numerical Investigation of Bed Shear Stress and Roughness Coefficient Distribution in a Sharp Open Channel Bend

The helical flow leads to the redistribution of the bed shear stress and roughness coefficient in open-channel bends, influencing the transport of sediment and riverbed evolution process. To explore the mechanisms underlying this redistribution, a 3D numerical simulation of a 180 degrees sharp bend was performed by solving the Reynolds-averaged Navier-Stokes (RANS) equations using the Reynolds stress equation model (RSM) as the anisotropic turbulence closure approach. The results indicate that the high bed shear stress zone appeared in the inner bank region from the 50 degrees to 110 degrees sections, and the section maximum bed shear stress gradually shifted outwards, owing to the advective momentum transport by the circulation cells. The quantitative analyses of the terms in depthaveraged Navier-Stokes equations indicate that the contributions of the crossstream circulation and cross-flow to the downstream bed shear stress were of the same order of magnitude, and the overall contribution rate of the cross-stream circulation was 20%. The contribution rates of the cross-flow, cross-stream circulation, turbulence, and pressure gradient to the transverse bed shear stress were approximately 30%, 7%, 3%, and 60%, respectively, indicating that the pressure gradient term arising from the transverse water surface slope played a dominant role. The Chezy resistance coefficient showed an overall decreasing trend along the bend. Therefore, an effective expression considering the streamwise variation along the centreline and transverse variation was successfully established to predict the uneven distribution of the Chezy resistance coefficient.

Automated summary

The helical flow in open-channel bends leads to the redistribution of bed shear stress and roughness coefficient, affecting sediment transport and riverbed evolution. A 3D numerical simulation of a sharp bend was conducted, revealing the occurrence of high bed shear stress in the inner bank region, which gradually shifted outward. The contributions of crossstream circulation and cross-flow to downstream bed shear stress were found to be of the same magnitude, with an overall contribution rate of 20% for cross-stream circulation.