Date of Award

1989

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

Harry H. Roberts

Abstract

This study of wave and current transformations across a modern coral reef was conducted on Tague Reef, St. Croix (U.S.V.I.), during late March and early April 1987. The project objective was to improve our understanding of the magnitudes, frequencies, and spatial and temporal variations of waves and currents as they interact with a modern reef. Data were collected in 4 experiments over a 2-week period. During the experiments, atmospheric and sea state conditions were typical of Caribbean-Atlantic trade wind dominated areas. Results verify that infragravity waves of 27.7 min period and diurnal tides modulated high frequency wave and current processes. Phase lags associated with these long-term motions cause sea level differences across the reef crest that influence transport of water and suspended sediment. Spectral analyses of wave data indicate that spectra shape was conserved between the forereef and reef crest, and spectral broadening at the crest was induced by energy redistribution and dissipation (average of 65%). Wave propagation into the backreef caused further spectral broadening and increased energy dissipation (average of 78%). Water depth reduction during falling tides increased wave energy dissipation by 15% and 20% from forereef to crest and from forereef to backreef, respectively. A steady wave energy model, including frictional and turbulent dissipation, gives reasonable estimates of wave heights across the reef. This model reveals that, in contrast to sandy beaches, bottom friction on coral reefs is comparable to turbulent energy dissipation. Reef crest currents occur on three important time scales: (a) short period (seconds), associated with wave activity, (b) infragravity fluctuations, and (c) diurnal variations, of uncertain, but probably tidal, origin. Vertical shear of these currents was found to be negligible. The reef crest mean and instantaneous flow fields suggest that seawater, and perhaps sediments and nutrients, are continuously transported lagoonwards. In contrast, wave related instantaneous currents over the forereef indicate off-reef transport. An unsteady, vertically integrated circulation model with bottom friction explains the observed current patterns. Under low trade wind wave conditions the volume flux at the reef crest is 0.024 m$\sp3$s$\sp{-1}$, which can flush the lagoon in 37 hours.

Pages

205

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