Shoreline and beach volume change between 1967 and 2007 at Raine Island, Great Barrier Reef, Australia

Raine Island is a vegetated coral cay located on the far northern outer Great Barrier Reef (GBR), recognised as a globally significant turtle rookery. Cay geomorphology, specifically the morphology of the beach and swale, dictate the availability of nesting sites and influence nesting success. Understanding short and long-term shoreline change is critical for managers charged with protecting the nesting habitat, particularly as climate change progresses. Historical topographic surveys, a simple numerical model and geographic information system (GIS) techniques were used to reconstruct a 40-year (1967–2007) shoreline history of Raine Island. Results show that significant shoreline change has occurred on 78% of the island's shoreline between 1967 and 2007; 34% experienced net retreat and 44% net progradation during the study interval. Shoreline retreat is mainly concentrated on the east–southeast section of the shoreline (average annual rate of −0.3±0.3m/yr), while the shore on the western side of the island prograded at a similar rate (0.4±0.2m/yr). A seasonal signal was detected relating to oscillations in wind direction and intensity, with the southeast and west–southwest shorelines migrating an average of ∼17m from season to season. The volume of sediment deposited on Raine Island between 1967 and 2007 increased by ∼68,000m3 net, but accretion rates varied significantly seasonally and from year to year. The largest volumetric changes have typically occurred over the last 23years (1984–2007). Despite the recent concern that Raine Island is rapidly eroding, our data demonstrate net island growth (6% area, 4% volume) between 1967 and 2007. Perceptions of erosion probably reflect large morphological changes arising from seasonal, inter-annual and inter-decadal patterns of sediment redistribution rather than net loss from the island's sediment budget.