Coral reefs provide important protection for tropical coastlines against the impact of large waves and storm damage by energy dissipation through wave breaking and bottom friction. However, climate change and sea level rise have led to growing concern for how the hydrodynamics across these reefs will evolve and whether these changes will leave tropical coastlines more vulnerable to large wave events.
Because the majority of reef flats are depth-limited, offshore water levels (tides) strongly control the ability of waves to propagate across a reef and impact the shoreline. In addition to offshore water levels, wave breaking also drives waters onto reefs and thus contributes to reef water levels; as offshore waves break over shallow reef topography, the change in radiation stress leads to a water level increase (‘setup’) over the reefs. Setup is strongly governed by offshore wave height and wave period, but is also dependent on tidal stage in that setup generally increases with decreasing water levels over the reef.
Due to the complex bathymetry and wave dynamics characteristic of reef environments, the transformation of waves over reefs is distinctly different from that on sandy beaches. Waves propagating onto shallow reefs steepen and break, and while some of the breaking wave energy propagates shoreward as reformed short-period waves, the spectral wave energy shifts into lower frequencies and long-period waves often dominate. These low-frequency waves are thought to be generated in the surf zone by breakpoint forcing, where the time-varying oscillation of the shore-wave breakpoint produces free low-frequency waves.Direct field observations of wave dynamics across coral reefs and the resulting water levels are limited. Here we provide direct in situ measurements of waves and water levels across a number of coral reefs, extending from the fore reef, across the reef crest, and to the shoreline. These measurements are provided to help better understand the evolution of waves across coral reefs and provide calibration and validation data to test models.