Bank erosion processes in cohesive hypertidal systems: inferences from the Severn Estuary (UK)

Abstract:

Estuaries are dynamic transition zones located between fluvial and marine systems and, as such, they are sensitive to climate change and human activities. The potential risks of shoreline erosion in estuarine and inter-tidal environments have been highlighted by recent studies, but bank erosion processes driven by bi-directional flows remain poorly understood. This study investigates bank mobility dynamics in hypertidal cohesive settings via analysis of the evolution and rates of bank retreat/accretion within the Severn Estuary (UK). In particular, we simulated how tidal variations on a range of cohesive sedimentary materials and across a spectrum of bank geometries (broadly representing the bank types encountered between the outer to inner estuary) affect the seepage dynamics and consequently variations in pore-water pressures that influence the potential for the banks to collapse via mass-wasting. The results highlight the important relationship between destabilising force due to tidal waves increasing the pore-water pressure within the bank deposits, and the presence (during low water stages) or absence (during high tides) of the hydrostatic confining pressures exerted by the water on the bank face. The results also highlight how the exfiltration of water from the bank during the ebb tide causes potential seepage surfaces along the bank faces that are able to exacerbate the destabilisation of the bank. This process is well documented in a series of bank collapses observed at a monitoring site located in the middle of the estuary during March 2019. Overall, the work highlights the significant role of tidally-modulated seepage driving bank instability.