Mesoscale shoreline evolution on a carbonate sand island: Anegada, British Virgin Islands

Abstract: Anegada, the easternmost island of the Virgin Islands group (Caribbean 12 Sea) is a low Pleistocene carbonate platform surrounded by Horseshoe Reef, the world’s 13 third largest fringing reef. The western part of the island consists of an extensive 14 beachridge plain (>40 ridges). The sandy carbonate shoreline exists in three morpho-15 dynamic domains that exhibit distinctive behaviour over the 59-year study period (1953 16 to 2012). The northern shore is dominated by westerly longshore drift under fairweather 17 conditions and cross-shore sediment transport during high energy events. Storm wave 18 runup and high nearshore sediment availability contributes to construction of shore-par-19 allel beachridges. The western end of the island is affected by refracted waves that 20 drive strong erosion and sediment transport. This is reflected in a succession of alter-21 nating rapid shoreline recession and progradation phases over the study period. The 22 south-central shoreline is exposed to low wave energy and is stable and colonised by 23 mangroves. The fringing reef plays a dominant role in mesoscale shoreline morpho-24 dynamics, both as a sediment source and in wave energy dissipation. Quasi-stable points 25 and embayments suggest a strong influence of the reef framework in controlling shoreline 26 morphology and position. Sediment transfer from the reef to the shoreline appears to 27 take place via shore-oblique, linear sediment transport pathways that develop across the 28 lagoon in response to modification of incoming waves. Cannibalisation of the shoreline 29 sediment over the past 50 years is leading to straightening of the shoreline planform. 30 This is counter to the long-term (Holocene) development of beachridges and suggests a 31 change from a strongly positive to negative sediment budget.