A coral reef surfzone: The dynamics of wavy flows on a shallow reef flat

We will present results describing the dynamics of tidally varying flows on the low-relief, shallow reef flat that borders the coral lagoon on the southeast shore of Ofu, American Samoa. As part of a larger array of instruments, we deployed pressure loggers, ADVs and a TRDI VADCP, an ADCP intended for stream gauging that is capable of measuring very shallow flows such as are found on reef flats, along a line extending from the edge of the reef flat into the interior of the adjacent lagoon. On Ofu, incident waves generally break at the seawards end of the reef flat, although this varies tidally. Depending on tidal conditions, broken waves then propagate across the ca. 100 m wide reef flat and become weak undular bores in the shoreward lagoon. Counter to what is seen on most reef systems, the net pressure gradient due to free surface slope is almost always offshore, and generally acts to balance the radiation stress gradient associated with dissipation of the waves. The drag force associated with the wavy mean flow is small; using a quadratic drag law including waves and mean flows, we find C_D ≈ 0.02. Examination of the vertical structure of the reef flat flow following the approximate analysis derived by Svensson (1984) shows that the vertically sheared flow that we observe, which can include offshore flows near the bottom (undertow), comes about because the vertical structure of the radiation stress gradient and pressure gradient are different, with the imbalance being balanced by the vertical Reynolds stress gradient. The observed behavior of the Ofu reef appears to reflect the relatively small reef pass through which flows driven over the wave flat must return to the ocean.