Multi-decadal gravel beach dynamics in the United Kingdom and Ireland: Insights from satellite-derived shorelines

Abstract

Gravel beach systems provide vital protection from coastal flooding and erosion. They are highly dynamic and exhibit complex responses to hydrodynamic forcing over a range of temporal (hourly centennial) and spatial scales (m to km). Yet gravel beach evolution, particularly at interannual to decadal scales, across the spectrum of coastal settings, remains poorly understood. We use four decades of Satellite-Derived Shoreline (SDS) data to explore the morphodynamic behaviour of 45 selected gravel beach systems around the United Kingdom and Ireland. We apply a site-specific SDS extraction methodology and derive shoreline trends along 1554 shore-normal transects. Our findings indicate significant variability in decadal trends between sites ranging from 0.60 m/year retreat to 2.24 m/year progradation, with 36% of sites showing significant long-term trends over the study period. Nesses and spits were by far the most dynamic systems exhibiting the largest changes at transect level (from 4.73 m/year retreat to 10.5 m/year progradation), and the most significant changes in planform shape, while most constrained and unconstrained sites remained stable. We classify the observed behaviours, providing a first inventory of morphodynamic behaviours across different gravel beach systems in the United Kingdom and Ireland. We find that leading regional winter-averaged atmospheric indices provide some insight into planform behaviour over the entire domain, with 16 sites (35.6%) showing at least moderate (R ≥ 0.4) statistically significant correlations (p ≤ 0.05). Our results provide a deeper understanding of the long-term behaviour of gravel beach systems that can inform more effective coastal management strategies.