Water Level Modulation of Wave Transformation, Setup and Runup over La
Saline Fringing Reef
Abstract
Coral reefs represent an efficient natural mechanical coastal defense
against ocean waves. The focus of this study is the La Saline coral
reef, located in the West of La Reunion Island in the Indian Ocean. The
area is microtidal and frequently exposed to Southern Ocean swell as
well as cyclonic events. The objective of this paper is to provide a
better understanding of the coastal defense characteristics of the reef
system for a range of Southern Ocean swell events and tides. Pressure
sensors were placed across the reef to measure water level fluctuations
and to determine gravity wave and infragravity wave components and their
transformation across the coral reef. A numerical model (XBeach surf
beat), validated using field observations, was used to deepen
understanding of wave transformation, wave setup and runup. Field
measurements and model outputs show that as gravity waves break over the
reef, the reef acts as a low-pass filter. Study results also suggest
frequency-dependent dissipation of infragravity waves by
bottom-friction. The resulting wave-induced setup is found to be the
dominant hydrodynamic component. The setup and runup are each 95% and
71% driven by the significant wave height (HS) with which they
increase, and transfer functions relating incident wave characteristics
to reef system hydrodynamics are proposed. At a semi-diurnal tidal
timescale, the setup and runup are in anti-phase, as the runup is
highest in conditions of reduced wave dissipation on the reef flat,
corresponding with high tides. These conditions also result in a lower
wave setup.