Reporting a Case Study on Computational Fluid Dynamics (CFD) Modelling to Estimate Fluvial Bank Erosion

River bank erosion models are an important prerequisite for understanding the development of river
meanders and for estimating likely land-loss and potential danger to floodplain infrastructure. Although
bank erosion models have been developed that consider large-scale mass failure, the contribution of
fluvial erosion (the process of particle-by-particle erosion due to the shearing action of the river flow)
to bank retreat has not received as much consideration. In principle, such fluvial bank erosion rates
can be quantified using excess shear stress formulations, but in practice, it has proven difficult to
estimate the parameters involved. In this study, a series of three-dimensional Computational Fluid
Dynamics (CFD) simulations for a meander loop on the River Asker (200 m long) at Bridport in
southern England were undertaken to elucidate the overall flow structures and in particular to provide
estimates of the applied fluid shear stress exerted on the riverbanks. The CFD models, which
simulated relatively low and relatively high flow conditions, were established using Fluent 6.2
software. The modelling outcomes show that the key qualitative features of the flow endure even as
flow discharge varies. At bank full, the degrees of velocity and simulated shear stresses within the
inner bank separation zones are shown to be higher than those observed under low flow conditions,
and that these elevated shear stresses may be sufficient to result in the removal of accumulated
sediments into the main downstream flow. Regions of greater bed/bank shear stress spread and
combine, whereas recirculation regions and zones of moderately small bed/bank shear stress
decrease in extent. At high flow, the extent of velocity and shear stresses over the areas of inner bank
separation were found to be greater compared to those simulated at low flow and may be adequate to
inhibit the movement of sediment into the main downstream flow. These patterns of modelled flow
have consequences for the sediment dynamics, bank erosion and meander migration in the studied
reach.