2023 CSDMS meeting-106

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Identifying sediment transport pathways on a heavily managed and chronically eroding, mixed sediment beach

Dominique Townsend, (she/her),University of Southampton Hasting , United Kingdom. D.Townsend@soton.ac.uk
Julian Leyland, University of Southampton Southampton , United Kingdom. J.Leyland@soton.ac.uk
Hachem Kassem, University of Southampton Southampton , United Kingdom. H.Kassem@soton.ac.uk
Charlie Thompson, University of Southampton Southampton , United Kingdom. celt1@soton.ac.uk
Ian Townend, University of Southampton Southampton , United Kingdom. I.Townend@soton.ac.uk

Mixed sediment beaches are common across the globe, yet despite this, they have not been as extensively studied as sandy coasts. In the UK, these gravel-rich ‘shingle’ beaches are used as a first line of defence against flooding, limiting the amount of overtopping on heavily developed coasts such as those found on the South East coast. The shingle beach at Pevensey Bay, in East Sussex is the UK’s largest natural flood defence and is maintained through publicly funded, long-term beach management activities, recharge, recycling and by-passing of material. Monthly beach surveys, carried out to inform these works, revealed that the foreshore was experiencing chronic erosion with the loss of approximately 8,000 m3 of sediment each year. Whilst changes to the upper beach are constantly tracked and managed changes occurring just below the waterline were practically unknown. Identifying the pathways for sediment movement across the nearshore zone was a key objective for this study to help understand the continued erosion of the foreshore. Analysis of multibeam bathymetry and X-Band radar reflectance data revealed the presence of transverse finger bars in the nearshore zone. The bars extended up to 700m offshore and were approximately 80-120m wide, with a maximum amplitude of 0.5m. Weekly-averaged reflectance imagery showed the position of the bars, which are orientated at 45o to the coastline, as the surface roughness of the sea was moderated by the seabed. These high temporal frequency roughness signatures showed that the bars were permanent features on the seabed and that they were migrating at a rate of approximately one wavelength a year. The movement of the bars was triggered by excess wave energy; peak migration rates were reached in the 2020 November to December period, whilst virtually no movement occurred between April and September 2021. Similar spatio-temporal patterns have been observed in erosive and accretive pulses in the upper beach and the link between bar movement and beach response is examined. Gaining a clearer understanding of the movement of sediment within the nearshore zone on beaches such as Pevensey will improve our understanding of how mixed sediment beaches function and brings into question whether active upper beach management is the most sustainable long-term option