Wave Energy Gradients and Shoreline Circulation Processes on Vabbinfaru Platform, Maldives

Abstract

Wave energy gradients and shoreline currents were measured during a three week deployment on Vabbinfaru platform, North Male Atoll, Maldives during the high energy westerly monsoon in June, 2010. Wave processes were measured using nine pressure sensors resulting in a total of 7605 synchronised bursts, making this the most comprehensive examination of reef platform energy gradients to date. To understand wave processes at the platform scale, instruments were deployed using a 3-dimensional array, encompassing the windward reef edge; windward, lateral and leeward reef flats and eight points around the island shoreline. Consistent with wave measurements, shoreline currents were measured at eight locations, 10 m from the toe of beach. Wind conditions and tidal elevation during the deployment were also measured, synchronous to each hydrodynamic burst. Results identify low to moderate wave energy on all aspects of the platform, where conditions are controlled by local wind conditions. During calmer wind periods, dual boundary conditions were observed, where maximum wave height for the study was recorded on the windward (western) reef edge, but mean wave height remained greater on the leeward (eastern) reef flat. During high wind events wave energy gradients were characterised by a concentration of energy on the windward (western) reef and shoreline, with low energy waves leeward (east) of the island. Currents around the island were low in velocity (<0.3ms-1) and unidirectional, where flow was generally directed east (windward to leeward) on the lateral shorelines. Windward flow separation and leeward flow convergence around the island were controlled by wind direction. These comprehensive hydrodynamic and boundary measurements enabled isolation of the portion of each forcing mechanism driving shoreline circulation. Separate model simulations were used to isolate wave driven and wind driven circulation, where wave forcing and wind forcing accounted for 50 – 70 %, and 1 – 40 % of measured velocity, respectively. Tidally forced currents were isolated using harmonic analysis, where tidal oscillation accounted for 5 – 40 % of the measured current velocity. Significantly, this research is the first to present an investigation into the driving mechanisms of circulation on the scale of a reef platform.