INVESTIGATING THE OCCURRENCE AND CHARACTERISTICS OF RIP CURRENTS AT PORTHTOWAN BEACH, CORNWALL, IN RELATION TO THE LOCAL WAVE CLIMATE

Images from the new ARGUS imaging system at Porthtowan, North Cornwall were analysed to identify the presence, number and size of rip currents. The occurrence and characteristics of the rip currents were related to the local wave climate. Images were analysed during daylight low tides, with 45 images analysed during May 2009. A total of 46 rip currents were observed in 20 images, 25 images had no rip currents. An average of 3 rip currents per image occurred when rips were present, with a maximum of 4 rips identified in the visible area covered by the image (2.1km2). Results from May 2009 indicated that rip currents did not occur when wave height (Hs) was less than 0.5m or greater than 2m. This result was tested with images from June 2009, with the same pattern identified. A number of significant relationships between rip characteristics and the local wave height (Hs) were identified: (i) Rip width increased linearly with Hs (P=0.01); (ii) Number of rips increased linearly with Hs (P=0.036); (iii) Rip spacing decreased linearly with number of rips (P=0.01); (iv) Cross-shore extent of the rip currents increased linearly with surf zone width (P=0.01). Previous published modelling results have suggested that the wave height at Porthtowan will be reduced by 5% due to Wave Hub. For May 2009 the effect of this on rip current occurrence would be minimal, resulting in just 2 less days with wave heights between the rip occurrence thresholds (0.5-2m). Dimensionless fall velocity (Ω) can be used to identify beach state. Over one year (2009) the predicted 5% reduction in wave height leads to changes in the beach state during the winter months (when average wave height over the month can be more than 2m), with more time anticipated to be spent in an intermediate state, in particular January-March altering the beach state from dissipative to long-shore bar-trough.Faculty of Scienc