Modelling near-bank flow hydraulics

Abstract

River bank erosion models are a fundamental requirement for understanding the migration and evolution of river meanders, estimating the potential for land-loss and threat to floodplain infrastructure, and predicting the delivery of contaminated floodplain sediments to aquatic ecosystems. While progress has recently been made in understanding and modelling processes controlling large-scale mass failure, less attention has been paid to the role that fluvial erosion plays in bank retreat. This project aims to address this gap by developing a new fluvial erosion model. Recent developments in bank erosion monitoring technology, and in the quantification of the bank erodibility parameters using jet-testing devices, offer the means of determining fluvial erosion rates and bank erodibility. However, the missing link remains the need to obtain highresolution, spatially distributed, flow data to characterize the near-bank fluid shear stresses that drive bank erosion. One possible solution is to use Computational Fluid Dynamics (CFD) models as a substitute for empirical data.
Herein I evaluate a series of three-dimensional CFD simulations for a meander loop on the River Asker at Bridport in southern England. CFD models under specific steady peak flow conditions were developed using Fluent 6.2, with peak flow discharge estimates obtained from an adjacent gauging station. All the models obtained from the three examined flow events were successfully verified and validated using clearly defined and structured procedures. The modelling results indicated that the main qualitative features of the flow remain even as flow discharge varies. However, notable differences were observed between the examined flow events, such as, a general increasing of velocity and shear stress throughout the reach as flow stage is gradually increased, a slight reduction in the size and extent of separation zones at bank full stage, a movement of impingement points further downstream, and a continuation of the secondary flow within the fast streamtube further towards the bends exits. Bed/bank shear stress is mostly seen to decrease at shallow riffles as discharge approaches bankfull, while pools experience an increase in bed/bank shear stress with increase in discharge. Zones of higher bed/bank shear stress extend and combine, while marginal recirculation zones and areas of relatively low bed/bank shear stress generally reduce in area to form discrete locations for erosion and deposition phenomena. At bank full stage, the magnitudes of velocity and simulated shear stresses within the inner bank separation zones are found to be higher than those observed under low flow conditions and they may be sufficient to result in the removal of accumulated sediments into the main downstream flow. The presence of regions of high velocity in the form of a streamtube, especially along the outer banks, creates high shear stresses within these areas. As a result, outer bank migration rates are likely to be relatively high in bends with inner bank separation zones.

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ORCID for Stephen Darby: orcid.org/0000-0001-8778-4394

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