Identification of ‘energetic’ swell waves in a tidal strait

• A review of historical swell events in the English Channel.
• Identification of swell waves within the wave spectrum from wave buoy data.
• Identification of swell waves with the wave spectrum during a storm.

Energetic swell waves present a significant coastal hazard. Overtopping is particularly sensitive to these longer period waves, potentially resulting in coastal flooding. In narrow tidal seas which are linked to large oceans, such as the English Channel, the importance of swell for design considerations is not immediately obvious. However, historical experience shows occasional severe impacts from long period waves, such as on 6th February 1904, 13th February 1979 and most recently two events in February 2014. When wave buoy measurements are analyzed using spectral partitioning methods, it is possible to identify swell wave systems with significant energy within the overall spectrum. This often coincides with the occurrence of storm waves, which might mask the presence of the swell component without such analysis. Two case studies where flooding occurred during storm events are examined in this paper. It has been possible to associate individual spectral components with the occurrence of overtopping of coastal defences at a particular location. Further investigation of the characteristics and frequency of energetic swell waves are recommended to enhance prediction and the design of coastal defences, and hence reduce their consequences.