Physical Processes in a Fore-Reef Shelf Environment.

Wave and current measurements were made across a rough-bottomed fore-reef shelf along the south coast of Grand Cayman Island. Wave heights attenuated 20 and current speeds 30 from the shelf margin approx 22-meter depth to a depth of approximately 8 meters, a distance of approx 0.4 km. Strong, rectilinear tidal currents dominated the deep shelf margin, but weak, directionally variable currents were characteristic of the shallow shelf. Attenuation of wave heights and current speeds across the shelf is attributed to frictional effects resulting from strong interactions with the unique boundary conditions of the extremely rough bottom. A dye experiment illustrated that strong on-shelf flow is directed up the deep coral reef grooves at the shelf margin. Wave force-dominated versus current force-dominated portions of the fore-reef shelf were defined from in situ measurements. Variations in organic communities, growth forms, and reef structure are consistent with these zones.