Chasing Cobbles: A Regional Exploration of Composite Beach Morphodynamics

The dynamic nature of coastal hazards has drawn interest in developing, holistic, nature-based, sea defence strategies. Composite beaches are a type of mixed sand-gravel beach regarded as excellent natural coastal defence due to their dynamic stability in changing hydrodynamic conditions (Blenkinsopp et al., 2022a; Bayle et al., 2021).These beaches are distinguished by a dissipative sandy lower foreshore which is backed by a reflective gravel or cobble berm close to the mean high-water level. Wave energy is dissipated along the sandy foreshore, and the steep, porous cobble berm drives swash asymmetry, stabilizing the upper beach and protecting the hinterland by minimizing overtopping (Bayle et al., 2021). Essentially, composite beaches embody the two most stable end-members of the morphodynamic continuum (Blenkinsopp et al., 2022). Recent developments have sought to exploit the morphodynamic stability composite beaches offer by installing a ‘dynamic cobble berm revetment’. These revetments are intended to mimic the cobble berm found naturally on a composite beach. Prototype-scale flume experiments and trial installations along vulnerable sections of the US West Coast have shown promising results in the face of rising sea levels and energetic wave conditions (Blenkinsopp et al., 2022b; Bayle et al., 2021). However, our understanding of composite beach behaviour (processes, responses to storms and longer-term evolution) is distinctly lacking due to the absence of dedicated studies. Therefore, our current definition of composite beaches may not adequately encapsulate the range of sub-morphotypes of composite beaches.

This research tackles our lack of knowledge by conducting one of the first detailed studies of composite beach behaviour on a regional scale. Currently, the term ‘composite beach’ covers a broad variety of different sand-gravel beach morphologies. By analysing a wide range of different composite beach types in a range of locations we will develop a more robust definition of composite beaches and their sub-types. Analysing historic topographic data of UK composite beaches enables us to gain new insights into the general behaviour of these beaches. Initial results indicate that natural cobble berms demonstrate morphological variations in constituting cobble size ranges, crest elevations, slope angles and berm width. These berms undergo relatively minor morphological changes when runup is confined to the seaward slope. In energetic conditions, when overtopping happens, larger changes can occur, but the berm remains dynamically stable rarely losing  cobble volume.

 

• References

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Blenkinsopp, C.E., Bayle, P.M., Martins, K., Foss, O.W., Almeida, L.-P., Kaminsky, G.M., Schimmels, S. and Matsumoto, H., 2022b. Wave runup on composite beaches and dynamic cobble berm revetments. Coastal Engineering [Online], 176, p.104148. Available from: https://doi.org/10.1016/j.coastaleng.2022.104148.

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