Mixed sand and gravel beach morphodynamics: observations and modelling across the nearshore

Abstract

The nearshore zones of mixed sediment beaches have received relatively little attention until now. This is surprising considering that mixed sediment beaches have a widespread global distribution, intrinsic ecological value and increasingly an important role in flood protection. This thesis set out to expand the existing body of knowledge on mixed sediment beaches by examining, both observations from and modelling of, a mixed composite beach study site located on the South East coast of the UK (Pevensey Bay, East Sussex). The overarching aim of the research was to build a conceptual model of a mixed sediment beach system with a focus on nearshore behaviour which had previously been neglected. Taking a holistic approach, three research questions were posed to achieve this: 1) What is the cross-shore extent of the geomorphologically active zone within a mixed sediment environment? 2) How does sediment move through and within a mixed sediment system contribute to short-term and long-term change? 3) How does cross-shore sediment exchange vary under a variety of hydrodynamic, morphodynamic and sedimentological conditions?

To answer these questions, novel techniques, namely the use of X-band radar and Autonomous Surface Vessels, were used to capture field data at both high frequency temporal and spatial resolution. The depth of closure was examined for a mixed sediment coast and a conceptual model was developed to assist with the interpretation of local variations in the geomorphological active zone. A series of transverse finger bars were found in the shallow nearshore, and their mobility was linked to wave driven processes. Modelling was carried out in the process-based XBeach-X which used Pevensey as a test bed for mixed sediment beach responses to storm conditions, under varying hydrodynamic, bed slope and sediment composition scenarios. Together, the results showed that the nearshore was a highly dynamic zone, and that long-term losses of sediment in the subaerial beach were also being seen below the waterline at a larger scale. It is thought that this ongoing loss is the result of both human intervention and natural processes. Whilst the gravel-rich upper beach and sandy foreshore seem distinct from each other, notably from the discovery of a null point of change between them, the volume of the nearshore ultimately controls the amount of wave power that reaches the shore. The study paves the way for future research, opening the floor for numerous questions, such as ‘how resilient are these systems to climate change and ongoing human intervention?’

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Identifiers

ORCID for Dominique Townsend: orcid.org/0000-0002-0502-6061

ORCID for Julian Leyland: orcid.org/0000-0002-3419-9949

ORCID for Hachem Kassem: orcid.org/0000-0002-5936-6037

ORCID for Charlie Thompson: orcid.org/0000-0003-1105-6838

ORCID for Ian Townend: orcid.org/0000-0003-2101-3858

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