Water level and currents of tidal and infragravity periods at Tague Reef, St. Croix (USVI)

A two-week study, at Tague Reef, St. Croix, USVI investigated the magnitude and spatial variation of tides, sea level differences, infragravity waves, and unidirectional cross-reef currents on a modern coral reef. Infragravity oscillations of water level (∼ 27 min period) of 1-2 cm height correlate with a quarter wavelength resonance over the shelf. Particle displacements associated with these waves may be important to the dispersive characteristics of the reef environment. Estimates of cross-reef mass transport per unit width ranged from 0.058 to 0.032 m²s ^-1. Sea level differences across the reef (1-4 cm) varied at diurnal and infragravity periods with contributions from wave set-up, and a small contribution from cross-shelf wind stress to the observed sea level differences. The quadratic bottom friction coefficient over the reef was estimated at 0.06-0.2, 20-70 times greater than on open shelves, reflecting the reef's extreme bottom roughness.