In situ observations of wave transformation and infragravity bore development across reef flats of varying geomorphology

The character and energetics of waves over reef flats can enhance shoreline erosion or accretion. There is increasing evidence from modeling and laboratory studies that infragravity waves (25 s < T < 250 s) and their degree of irregularity may be important factors in cross-reef sediment transport. Here we use in situ pressure measurements collected along cross-reef transects at 7 sites on Pacific islands with varying reef geomorphologies to examine under what conditions these infragravity waves occur and what factors enhance their irregularity. In addition to bulk spectral techniques for evaluating asymmetry and skewness, we catalogued aspects of each individual infragravity waveform, such as duration, leading and trailing edge slope, as well as the size and number of high-frequency sea-swell (T < 25 s) waves “riding” on top of the infragravity wave. In general, a greater fraction of total wave energy was transferred to the infragravity band on reefs with steeper fore-reef slopes and shallower reef flats. The infragravity wave amplitudes scaled with increasing water levels, but it was primarily at lower water levels when these waves became pitched forward and peaked. The most asymmetric, bore-like, infragravity waves occurred on the wider reef flats. At higher water levels, the number and height of the sea-swell waves riding on the tail of the infragravity waves increased, and these secondary wave heights were further enhanced by the additional water level afforded by the infragravity wave itself. Our results underscore how small variations in reef type and geomorphology can lead to different cross-reef wave energetics, particularly for infragravity waves. As irregular infragravity waves may be strong drivers of cross-reef sediment transport, understanding the conditions that lead to low-frequency, energetic bores on reef flats is critical to forecasting how coral reef-lined coasts will respond to sea-level rise and climate change.