Seasonal variations in wave characteristics around a coral reef island, South Maalhosmadulu atoll, Maldives

Abstract

Synchronous measurements of waves were made at six locations around a small reef platform island in the Maldives during both westerly and northeast monsoon conditions. Summary statistics of wave records indicated similar Hs values propagating onto windward and leeward reef surfaces and a distinct growth in wave height as waves shoal toward island shorelines. Waves up to 0.78 m were recorded impacting the island shoreline. Wave energy on the reef platform was strongly modulated by seasonal winds with wave energy during the westerly monsoon an order of magnitude larger than wave conditions in the northeast monsoon. Spectral analysis of wave records highlighted spatial differences in wave energy around the island. Windward reefs and shorelines are dominated by waves at wind and swell frequencies, whereas wind-wave energy is absent from leeward shorelines. Consequently, windward shorelines have up to twice the gross energy input than leeward shorelines. Results have significant implications for geomorphic processes. Longer duration and greater incident wave energy during the westerly monsoon has promoted higher growth of the western reef flat (+ 0.2 m) and more energetic shoreline runup processes have produced markedly higher western island ridges (+ 0.7 m). Incident shoreline wave processes are sufficient to entrain beach sediments during both monsoon periods although periods of active sediment mobilisation are constrained to mid- to high-tide stages. The windward–leeward gradient in shoreline wave energy is likely to drive littoral sediment flux. Seasonal modulation of the shoreline wave energy gradients account for observed oscillations in beach position around island shorelines. Large differences in incident wave energy between monsoons is considered to affect the rate of beach change around island shorelines rather than the total volumetric movement of island beaches.