An analysis of subaerial beach rotation and influences of environmental forcing adjacent to the proposed Swansea Bay Tidal Lagoon

Beach Profiles surveys and gale climate data were utilised to assess medium timescale beach rotation at four beaches located along the shores of a crenulated embayment within Swansea Bay, Southwest Wales. The proposed Tidal Lagoon is located within this Bay. Results identified a 7 year (1998-2005) record of cyclic summer/winter rotation and a 14 year (1999-2013) record of annual rotation within the subaerial zone on all four assessed beaches. In the absence of headlands to trap sediment it is asserted that the driving force for beach rotation is the presence of Swansea Dockland/Tawe dredged channel complex, Port Talbot Harbour and the Neath dredged channel which form surrogate headlands essentially creating four separate beach systems through restricting sediment by-pass. Seasonal averaged wind and wave variables showed differing correlation with volume changes and cross-correlation results showed that volume variation lagged behind forcing variables by up to six months (i.e. the resolution of the data). This was confirmed by the annually averaged results which showed only subtle correlation. Here volume change in most cases lagged forcing variables by less than one year. Based on correlations, wind direction variability follows closely with volume changes but wind speed, wave period and height are generally opposite. Initial results suggest that the proposed Swansea Bay Tidal Lagoon, located between sediment cells and surrogate headlands, would have little negative effect on subaerial coastal processes. These datasets will be used as a benchmark for monitoring prior to, during and post construction, with results being used to update and inform subsequent strategies.