Interactions between coarse sediment transfer, channel change, river engineering and flood risk in an upland gravel-bed river

Abstract

Coarse sediment transfer in upland gravel-bed river systems is often neglected in the design and operation of river management schemes. Yet, it is increasingly attributed to problems within upland environments including bank erosion and enhanced flood risk. Developing a sufficient understanding of coarse sediment transfer and channel change requires strategic field monitoring. Predicting future channel change and flood risk under varying management options requires numerical modelling. This thesis employs a combined field monitoring and numerical modelling approach to explore the relationship between coarse sediment transfer, lateral channel change, river engineering and flood risk. Intensive field monitoring is used to understand sediment transfer and channel change. Methods include repeat cross-sectional resurvey, bank erosion pins and bank-top surveys, and sediment impact sensors. These data are used to illustrate the spatial and temporal variability of in-channel sedimentation and rates and mechanisms of bank erosion. When analysed further, these data explain patterns of sedimentation and demonstrate implications of coarse sediment accumulation for flood risk. The data are then used in the development and application of a quasi two-dimensional model of channel change. The model couples a one-dimensional sediment routing model with a lateral adjustment component to simulate patterns of downstream fining alongside vertical and lateral channel changes. By using a split channel approach, asymmetrical width adjustments are simulated based on critical shear stress thresholds. Lateral differences in bed elevation and curvature are used to distribute shear stress across the channel. Simulations are run to explore scenarios including changing the flow regime, removing bank protection and implementing a river engineering scheme on a reach with high sedimentation and bank erosion problems. The results highlight the potential impact that poor river management can have on upland rivers, demonstrating enhanced bank erosion upstream and greater sedimentation downstream of the engineered reach. The accumulation of sediment results in increased flood risk.