Bank Erosion Processes, Trends and Impacts in a Hypertidal Estuarine System

Estuarine systems represent the dynamic transition zone between fluvial and marine systems and as such they are sensitive to changes in both domains resulting from impacts of climate change and human activities related to coastal and water-flow management especially in densely inhabited areas. Further, these tidally influenced systems are subject to a unique set of driving conditions linked to bidirectional flow processes. The potential growing risks of shoreline erosion in coastal, estuarine and inter-tidal environments have been identified by a number of studies in recent years. However, bank erosion processes in tidal settings remain poorly understood, especially when compared to the large volume of research concerning fluvial bank erosion. In general, the well-established fluvial bank erosion literature suggests that bankline erosion involves two main sets of processes: hydraulic erosion and gravitational collapse. Given the additional complexity of the process mechanics involved in tidal settings, arising mainly from the presence of bi-directional flows, process insights gained from studies of fluvial bank erosion might not be appropriately applied in a tidal context.

The present study aims to improve our understanding of estuarine bank mobility dynamics through investigation of the evolution and rates of bank retreat/accretion acting in the Severn Estuary (UK). The Severn Estuary has one of the highest semidiurnal tidal ranges in the world (about 15 m in the outer estuary, up to 8-9 m in the middle parts of the system, and 2 to 3 m in the inner river-dominated sector). Here we estimate bank mobility throughout the estuary from the river-dominated to the tidal-dominated zones during the last 119 years, via analysis of historical maps and recent satellite images. We use the findings from this analysis coupled with recent data collection to propose an empirical model of bank mobility throughout the entire estuary, highlighting the characteristics and the differences between riverine and coastal erosive processes. The model indicates that (i) the highest bank mobility (both in term of erosion and deposition) is located in the mid part of the estuary, close to the bedload convergence zone (BLZ), with other ‘hot spots’ of change linked to major anthropogenic disturbances either in the outer and inner estuary, and (ii) that the erosive mechanics associated to severe lateral land losses in the estuary are mainly driven by impulses in energy delivery to the bank surface in occasion of very high tidal oscillations (particularly in spring overbank tides) and severe storms triggering mass wasting in form of toppling and rotational failures.