Wave energy gradients and shoreline change on Vabbinfaru platform, Maldives

Abstract

This study examined coral reef platform wave processes and shoreline dynamics over short timescales. Wave energy gradients and shoreline change were measured over a 3-week period on Vabbinfaru platform, North Malé Atoll, Maldives, during the westerly monsoon in June 2010. Wave processes were measured using nine pressure sensors recording near continuous data for 19days around the reef and shoreline of a small circular sand cay. Toe of beach position was surveyed before, during and after the deployment to map changes in shoreline configuration. Results show that wave height and direction on the windward reef are closely controlled by local wind activity inside the atoll lagoon. Wave transformation across the platform was found to exhibit strong tidal modulation and results in distinct cross-reef energy zonation. Results are presented by comparing two contrasting boundary wind conditions: the first 2weeks characterised by moderate southwest winds, and the third week characterised by stronger northwest winds. Wave data was interpolated to platform scale and used to show spatial variations in energy exposure during the different boundary conditions. Under southwest winds, greatest wave energy was present on the western and leeward (eastern) reefs, driving an energy gradient towards the sheltered northern and southern shorelines. A net 4.5% increase in beach area was measured during this period. During stronger northwest winds, higher wave energy impacted the reef and was concentrated on the western reef, northern reef and northwest shoreline. An energy gradient formed around the island towards a low energy zone located at the southeast shoreline. Significant shoreline change occurred during this period with the toe of beach retreating landward by more than 10m on the northwest, northeast and southwest of Vabbinfaru Island. Beach area was reduced by 3761m2 (17%) as the shoreline was forced to adjust in response to a new hydrodynamic regime. Results show that the coral reef platform is able to modify and filter the incident wave climate, resulting in marked spatial differences in wave spectra. Spatial differences in wave energy are sensitive to local wind activity, resulting in rapid alteration in wave energy distribution around the platform. Results highlight that reef island shorelines are morphologically sensitive to short-term changes in boundary wind and wave conditions. Wave energy gradients driven by local wind activity have the potential to drive alongshore fluxes of sediment around island shorelines. Therefore, changes to the process regime can result in disequilibrium of the shoreline, forcing rapid adjustment of island sediment.