Hydrodynamic processes and sediment erosion mechanisms in an open channel bend of strong curvature with deformed bathymetry

Most rivers exhibit regions of strong channel curvature that are characterized by more complex and variable flow and erosion patterns, compared to regions of lower curvature. Studies investigating high-curvature bends using eddy-resolving techniques have been limited, and the effect of bend angle on flow and erosion has rarely been investigated. This study investigates flow in a 135 degrees nonerodible bank open channel bend of high curvature: ratio of radius of curvature, R, to channel width, B, is 1.5. The bathymetry is obtained during the final stages of a clear water scour experiment. Large Eddy Simulation is used to investigate the effect of secondary flow on the redistribution of streamwise momentum, the details of coherent structures, and mechanisms leading to erosion within the bend. Results are compared with those from a similar numerical study of a 193 degrees sharply curved open channel bend with R/B=1.35. The angle of the 135 degrees bend is representative of typical regular meander geometry, while the larger angle of the 193 degrees bend is representative of a tortuous meander geometry. The different bathymetries induced important quantitative and qualitative differences in the vortical and turbulence structure within the open channel for the two cases. Inner bank streamwise-oriented vortical (SOV) cells formed in both cases, but the position and extent of shear layers forming between regions of fast and slow moving fluid differed, and flow did not separate at the inner bank in the 135 degrees bend. An outer-bank cell was observed in the 135 degrees bend, but not in the 193 degrees bend. Distributions of predicted boundary shear stresses indicated the capacity of the flow to erode the outer bank of a sharply curved bend under two representative regimes found in the field.