Distinct shoreline behaviour along storm-dominated and geologically controlled coastal barriers

Abstract

Contemporary shoreline change is driven by a complex combination of factors, and as such is often highly variable along the coast. While differences in beach morphology can explain some of the variability in shoreline change, the geological constraints imposed by coastal geology are often overlooked. This work examines the influence of foreshore configurations with varying degrees of non-dynamic geological control, which are analysed in combination with hydrodynamic forcing to investigate seasonal to multiannual shoreline evolution along five coastal barriers in the Outer Hebrides, Western Scotland. These barriers are characterised by strongly geologically constrained evolution and are exposed to a storm-dominated wave climate. Due to the variable temporal interval between available cloud-free Planet Scope images, monthly averaged vegetation lines from 2016 to 2023 were derived from satellite imagery as a shoreline position indicator using an automated approach validated by visual inspection. The satellite-derived vegetation lines have a sub-pixel accuracy with a root mean square error of 3 m. Changes in monthly averaged shoreline position are statistically correlated with monthly extreme storm conditions, characterised by both extreme water levels and wave conditions. However, the control exerted by the variable geological configuration along the barriers results in distinct inter- and intra-site shoreline change behaviour, with lower shoreline variability observed in barrier sectors fronted by rocky foreshores, compared to sediment-rich foreshores. The observed multiannual shoreline change from 2016 to 2023 is characterised by a small but statistically significant accreting trend (mean 0.4 m/yr), likely representing the recovery of the barriers from extreme winter storms that impacted northern European coasts from 2013 to 2015. The results demonstrate that considering variable geological controls in shoreline change assessments improves the understanding of shoreline variability along coastal barriers, allowing to identify distinct storm-driven shoreline behaviour according to the degree of geological control.