Intertidal cohesive foreshores: Erosion rates and processes illustrated by a shore platform at Warden Point, Kent, UK

Data are presented from a 9-year monitoring period, using a Traversing Erosion Beam (TEB), of erosional lowering of an inter-tidal shore platform developed in cohesive London Clay at Warden Point, Isle of Sheppey, Kent, UK. Over the monitoring period, average lowering decreased from >30 mm a⁻¹ at 30 m down-platform from the MHW line to <15 mm a⁻¹ at 75 m down-platform and <8.0 mm a⁻¹ at the lowest measured point 120 m seaward of MHW, giving an overall average 17–18.0 mm a⁻¹ across the measured section of platform. Differences in lowering rates recorded between the three down-platform measuring stations were statistically significant, as were differences between average annual rates and between seasons. It is suggested that these differences in recorded rates of lowering result from variation in the relative importance over space and time of sub-aerial versus marine processes of weathering and erosion. Adding the local relative sea-level rise of 2.23–2.36 mm a⁻¹ the average annual rise in mean water level over the platform surface was ~20 mm a⁻¹ and at the cliff-platform junction >32 mm a⁻¹. Backed by cliffs 48 m tall retreating at a long-term average > 1.0 m a⁻¹ the maximum contribution of platform lowering to the long-term erosional budget of the study site was ~7.4%. The significant differences in recorded annual and seasonal erosion rates indicate that extrapolation of average rates based on data collected for two years or less on such rapidly eroding platforms should be treated with caution because they may not be representative of the longer-term average rate either for the entire platform or for individual monitoring points.