Linking nearshore morphological change to long term observed sand loss from a mixed sediment beach.

Few studies focus on the changing morphology of the nearshore zone of mixed sediment beaches, despite the fact that these beaches are found across the world. In the UK, these beaches make up ~25% of the coastline, and are often utilised as a first line of defence against coastal flooding. In Pevensey Bay, East Sussex, active beach management (sediment recycling and recharge) maintains the mixed gravel barrier beach to protect around 10,000 properties, culturally significant landmarks and internationally important wildlife sites. During the past 25 years, this management approach has successfully maintained the volume of the upper shingle part of the beach. However, the sandy foreshore area is experiencing a continuing loss of 8000 m³ of sediment per annum.

This study seeks to understand the drivers behind the sustained loss of volume. Examination of multibeam bathymetry data revealed the presence of transverse finger bars with a wavelength of approximately 80 – 120 m, orientated at 45 degrees from the shoreline in the subtidal zone extending between the -3.0 to -6.0 mOD contours. Sediment grab samples taken perpendicular to the crests and troughs, revealed the surface sediments to be comprised of very well sorted fine sand, with D₅₀ ranging between 150 – 169mm. Strong tidal currents flowing over these bed features modulate the sea surface roughness which can be detected in the X-band radar reflectance imagery. Using weekly averages of X-band radar reflectance imagery we show that the bars were a permanent feature over the 18-month period of observation and provide an indicative migration rate of approximately one wavelength a year to the east, which was validated against monthly bathymetric data. This novel approach of studying mobile sea bed features revealed a steady migration rate during the winter months, and virtually no movement during the summer period, suggests that the movement of the bars is driven by relatively higher energy south westerly waves. It is thought that the movement of these bars may be linked to erosive and accretive pulses which move easterly across the bay on the upper beach face. Understanding the process dynamics and broader role within the bay-wide sediment budget of these features is essential in comprehending the loss of sediment from the bay and will contribute to the future sustainable management of the site, where the management strategy for the next 100 years is currently under review.