Thresholds of Wave Forcing: Implications for Atoll Reef Dynamics Under Sea Level Rise

We investigate the response of shallow reef flow to tidal variability and wave exposure during a 4-month field campaign in southern Huvadhu Atoll, Maldives. Incident waves breaking on steep fore reefs and reef crests generate a setup proportional to offshore wave height that drives a cross-reef flow. We emphasize a critical threshold—where the depth on the reef flat equals the depth at wave breaking—that separates two distinct reef flow regimes: one dominated by strong, unidirectional flow from the ocean into the lagoon and another where wave breaking ceases, flow rates decrease, and occasionally reverse direction. Recognizing the relative importance of water depth and wave energy, we develop a framework for interpreting shallow reef hydrodynamics in a combined tide-wave parameter space. This framework allows us to project how rising sea levels may alter reef flows—potentially leading to prolonged and more frequent periods of limited wave breaking and a decline in wave-driven transport.