Monsoonally influenced circulation around coral reef islands and seasonal dynamics of reef island shorelines

Abstract

Synchronous measurements of reef flat waves and nearshore currents were made around the perimeter of three coral reef islands in the central Maldives. Nearshore current velocities were shown to be tidally modulated in most instances with maximum velocities associated with lower tidal stages. Peak currents were consistently observed on the windward and lateral flanks of island shorelines (0.3–0.4 ms − 1 ). The relatively low elevation of reef surfaces and medium to low energy incident wave energy ( Hs ~ 0.2–0.3 m) accounts for low tidal stage peaks in velocity as this corresponds to the period when wave setup-induced current processes are maximised. Significantly, all nearshore currents were unidirectional and alongshore. The generation of unidirectional alongshore flow patterns results from island configuration which deflects flow and variations in reef geometry that control: spatial variations in wave-setup around the reef platform edge and at the island shoreline, which can drive alongshore currents; the presence of a nearshore moat that channelizes flow in the nearshore: and variations in reef geometry where topographic low points act as preferential drainage points. Results show that changes in monsoon conditions, that modulate wind and wave patterns from the west to northeast, can force substantial changes in reef platform and nearshore current flows, but that the degree of change is dependent on reef platform shape and position in the atoll. The circular study island was found to exhibit a near-complete reversal in unidirectional flow from northeast flow in the westerly monsoon to southwest flow in the northeast monsoon. However, the elongate and triangular shaped reef platforms exhibited more localised changes in flow patterns. Significantly, those shorelines that exhibit the greatest changes in shoreline currents correspond to the sections of coast that exhibit the largest morphological adjustments between seasons. Results provide significant insights for the morphodynamics of reef platform islands. In particular, the nearshore process regime is dominated by alongshore processes. Furthermore, reef platform shape and the magnitude of change in direction of wave approach to reef platforms provide critical indicators of the morphodynamic behaviour of reef platform islands.