Multi-scale assessment of shore platform erosion

Abstract

The morphology and erosion of shore platforms is a pivotal component of rocky coast evolution as these features control both wave transformation and sediment dynamics. Models that predict coastline evolution and efforts to reconstruct past cliff retreat rates from cosmogenic isotope concentrations are forced to simplify platform morphology and commonly treat erosion only implicitly. The lack of an explicit incorporation of platform dynamics into such models reflects a poor understanding of erosion processes that have conventionally been considered to operate at one of two scales: fine scale abrasion captured by sub-mm precision point measurements of vertical change, and step back-wearing and block removal at metre-scale. Neither approach is well suited to informing a generalised model of foreshore erosion that bridges these two scales or that can be applied more widely. As a result without understanding mechanisms of foreshore erosion models which use these data are limited in their utility to address future coastal change under changing sea level and storminess.
To address this a multi-scale study was undertaken along the North Yorkshire coast (UK) using high-resolution and high-precision monitoring data collected at the spatial and temporal scales relevant to the processes in action. A novel method was developed to monitor mm-scale platform erosion using Structure-from-Motion (SfM) photogrammetry. The average platform down-wearing rate of 0.528 mm yr-1 was calculated from 15 individual 0.5×0.5 m sites. The volume frequency and 3D-shape distributions of the detachments suggest that erosion occurs predominantly via detachment of fabric-defined platelets. The erosion rate is faster closer to the cliff toe and at those locations where the tide cycles more frequently. Erosion rates calculated from the 2.6 years of data from 22 km of shore platform using high-resolution airborne LiDAR was 3.45 mm yr-1 when derived from individual detachments, or 0.01 mm yr-1 when spatially averaged across the platform. Average lowering of the platform sections containing steps was 0.04 mm yr-1, while in areas with no steps 0.01 mm yr-1. Whilst erosion rate cannot be predicted with confidence for any discrete point on the foreshore, systematic trends in across-shore erosion can be shown, with a peak in rate at 10-18 m from the cliff toe, with erosion intensity gradually decreasing seawards.
This new understanding of foreshore erosion has then been used to predict exposure ages from cosmogenic 10Be concentrations at the Hartle Loup platform. This analysis shows that the cliff has been retreating at the steady rate of 0.05 m yr-1 cutting the 300 m wide shore platform in the last 6 kyr. This derives rates of retreat comparable to contemporary erosion monitoring. Platform morphology has been shown not to adjust to an equilibrium shape, but it is rather actively modified depending on the interplay between present morphology, sea level and tidal regime. Importantly, this study provides methods to monitor foreshore erosion, enhances our understanding of mechanisms and
controls upon it, whilst the results can be used in models to predict rocky coast evolution by providing an empirically-based assessment of foreshore erosion.