An Alongshore Momentum Budget Over a Fringing Tropical Fore-Reef

Abstract

Existing momentum budgets over coral reefs have predominantly focused on cross-reef dynamics, lacking analysis of alongshore processes. To complement existing cross-reef research and enhance our understanding of forcing variability at the semidiurnal period, this study examines the σ–coordinate, depth-averaged alongshore momentum budget over a fore-reef as a function of tidal phase. The observations were gathered over a 3-week timespan, between the 12- and 20-m isobaths of a Hawaiian fringing reef system, focusing on two moorings on the 12-m isobath, where median drag coefficients estimated from log fits are CD=0.0080[−0.002,+0.004] and CD=0.0023[−0.0006,+0.0009]. Analysis at one location shows that the unsteadiness, barotropic pressure gradient, and bottom drag are equally important, and their combination is sufficient to close the momentum budget. However, bottom drag is less important at the second mooring; the difference between unsteadiness and pressure gradient suggests that advective acceleration plays a significant role.