A groundswell event on the coast of the British Virgin Islands: spatial variability in morphological impact

ABSTRACT Cooper, J.A. G. Jackson, D.W.T. and Gore, S. 2013. A groundswell event on the coast of the British Virgin Islands: spatial variability in morphological impact. Extreme waves on Caribbean islands originate from hurricanes, tsunami and swell. The latter are most widespread and common but are the least studied. They are generated by slow-moving, low pressure systems in the western Atlantic from which swell propagates in all directions, some of it reaching the exposed shores of Caribbean Islands. Large swell waves with an approximate 1:20 year return interval reached 14–16 ft (>4.5 m) waves with a 17 second period during March 2008. Morphological impacts (coral reef damage, coral rubble deposition, beach erosion) were reported throughout the Eastern Caribbean from Guyana to Puerto Rico. In this paper we present observations of morphological impacts on sand, gravel and coral rubble beaches of the British Virgin Islands (BVI). The morphological response was highly variable according to coastal geologic setting, exposure to wave activity and nature of beach material. Exposed sand beaches with sandy shorefaces exhibited scarping, overwash and offshore sand transport, similar to that reported elsewhere for sand beaches. In some instances covering sand was stripped to expose underlying beachrock. Sand beaches with fronting reefs showed little change. Modification of coarse clastic beaches through erosion and deposition generally amounted to little more than surface modification of much larger hurricane and tsunami deposits.