Understanding the dynamics of a mixed sand and gravel coastline: a multi-method approach.

Abstract

A multi-method approach was developed to characterise the behaviour of a mixed sand and gravel (MSG) coast at Thorpeness, Suffolk, UK (52.1823°N, 1.6130°E). The field site is highly complex due interactions of the underlying geology, bimodal sediment size distribution and great alongshore variability in the physical settings even within relatively short distances (~300 m) and has been historically impacted by short-term, focused erosion events occurring over 100-200 m of the beach frontage. The research employed a range of observational and numerical modelling techniques to understand the beach and coastal processes at the site. Analysis undertaken in this study provided insights of differing shoreline response to the same offshore drivers. Results show a combination of variable geomorphology, a bi-modal wave climate and active nearshore processes causes large differences in shoreline behaviour along short (less than 300 m) lengths of the beach frontage. Novel sediment analysis has highlighted the need to assess the vertical and surficial proportion of sand and gravel to gain better understanding of the spatial variability in mixed beach behaviour. Use of X- band radar data demonstrated that periods of prolonged northerly or southerly waves resulted in two differing offshore bathymetric configurations, these act to modify the incident wave climate and the local beach behaviour. The analysis output informed a 10-year wave hindcast model used to simulate longshore sediment transport (LST) for these two bathymetric states. The model identified a persistent long-term convergence of LST at the Ness irrespective of wave and nearshore conditions, which supports the hypothesis that sediment is recycled offshore to form a bar-like feature in moderate southerly conditions and is eroded during storms. The outputs of these analyses informed coastal management decisions, specifically that in the short-term the most sustainable and least detrimental option to reduce the impact of erosion is small-scale beach recharge using the Ness as a source of material.