The Influence of a Cross-Reef Channel on Wave Setup and Circulation over a Fringing Reef at Ipan, Guam, Revisited

Observations of currents, waves, and water levels are analyzed to assess the two-dimensional (2D) low-frequency circulation over a shallow, shore- attached, fringing reef cut by a deep, narrow channel at Ipan, Guam extending the work of Clark et al., POSTER ID B0876, AGU Ocean Sciences 2012. As incident wave breaking is inhibited by the channel, the resulting 2D spatial variability of the breaking wave setup induces an alongshore pressure gradient over the reef flat. Estimates of the observed wave setup are highly correlated at the sensors in the far-field of the channel, with a large reduction in setup at the sensor in the near-field of the channel. An analytical model for the 2D setup is presented that is consistent with the observations, and delineates the near-field of the channel in terms of the ratio of the width to length (aspect ratio) of the reef. Over the seven-week deployment, the average reef-flat currents reveal a circulation pattern with cross-shore (onshore) flow two kilometers to north of the channel and increasingly alongshore flow over the reef flat towards the channel. The along-channel current is significantly correlated with the reef-flat circulation, and with the offshore wave height and tidal level. Assuming a quadratic drag law, the alongshore reef-flat currents driven by the setup-induced pressure gradients yield a drag coefficient C_D=0.01 ±0.001, which is consistent with previous studies given the smooth reef at Ipan.