ABSTRACT
Field observations of bank processes and bed material movement at a meander bend on the River Severn, U.K., are presented and discussed in the light of historical evidence for channel change in the last 150 years. Mechanisms of bank failure, in general dependent on bank structure and composition, are here dominated by fluvial undercutting and mechanical failure of cantilevers in the upper bank. Failed material accumulates at the bank foot from where it is removed by fluvial entrainment. Tracer experiments show that bank retreat rates are fluvially controlled though failure mechanisms are not. Measured retreat rates are around 0.5 m/yr comparing with up to 0.7 m/yr historically on the same reach.
Contrasting forms of planform development are apparent, including complex loop formation, neck and chute cut-offs, and the rapid abandonment of lengths of cut bank and deep channel. Upstream changes and large, but infrequent, discharge events have profound effects which are difficult to predict.
Channel change can be usefully regarded as a sediment transfer process involving bank failure, sediment entrainment, transport, and deposition. However, both field studies and documentary analysis are needed for a good understanding of the full range of contemporary channel change characteristics in space and time.
