Platform-top reef sand apron morphodynamics and the half-empty bucket

Abstract

In shallow water atop many isolated platforms and atolls, reef sand aprons largely consist of debris shed platformward from shelf-margin reefs towards the lagoon. Although their general geomorphology and sedimentology are broadly understood, quantitative details of the possible role of reef sand apron hydrodynamics on their geomorphic evolution remain less well constrained. To test the hypothesis that on-platform reef sand apron progradation is prone to completely infill adjacent lagoons, this study documents over 50 new hydrodynamic simulations that isolate and evaluate the relations among geomorphology, waves, and tides. The results show how deep-water waves hit the shelf margin and favor on-platform sediment transport, but highlight that tides modulate the influence of waves while generating currents on their own. Across the range of wave heights and tidal amplitudes, however, platform-directed bed shear stress on shallow reef flats decreases with increasing reef and reef sand apron width. Parts of broad reef sand aprons can even include off-platform-directed shear stress during incoming flood tide. These insights motivate a conceptual model for how the process of widening of a reef sand apron by lagoonward sediment transport decreases the magnitude of the very forces that drive the transport, inhibiting further progradation. Such an autogenic, self-limiting dynamic curbs the propensity of lagoons to fill with coarse sediment shed from the reef by reef sand apron expansion. Instead, many atolls are doomed to remain “half-empty buckets,” even in the absence of external change such as a relative change in sea level.