An Analysis of Swell and Bimodality Around the South and South-west Coastline of England

Abstract. This paper presents an analysis of wave recordings with particular attention to assessing bimodality of the incident wave energy spectra and the occurrence of swell along the south and south-west coasts of the United Kingdom, (UK). A procedure is developed to perform an intensive analysis of a new and large dataset of measured wave spectra. A storm during February 2014 is analysed in detail, highlighting the observed wave conditions leading up to and during the collapse of the sea wall at Dawlish, UK. The analysis reveals the prevalence of trapped-fetch conditions and long-period swell during the February 2014 storm. Bimodality and the presence of swell are compared at three locations along the south coast of the UK. Results highlight the increase in bimodality during the 2013/2014 storm period, especially at Dawlish. The analysis also provides evidence of bimodality and swell waves occurring far along the English Channel. Observed wave conditions at Dawlish are compared to the parametric limits of empirical formulae to estimate wave overtopping. There were numerous instances of peak wave periods or wave heights outside the limits of the formulae, showing that existing design formulae do not yet adequately account for the range of conditions experienced in coastal waters.