Abstract

I will discuss observations of tidally varying wave-forced flows in the reef system on Ofu, American Samoa, a barrier reef and lagoon system that appears “open” at low tide and “closed” at high tide. At high tide, the free surface pressure gradient nearly balances the radiation stress gradient in the depth-integrated momentum equation. At depth, there is an imbalance between these two forces, generating an undertow and flows that turn alongshore and, for some of the time, offshore, behavior similar to rip currents observed on beaches. At low tides, the wave forcing drives purely onshore flows. In general, wave transport (including assumed roller behavior) is important to determining the total net transport. In both cases, the vertical structure of this flow can be predicted with some accuracy using the surf-zone model of Svendsen, albeit with an eddy viscosity that is proportional to the rms wave velocity. While the dynamically closed nature of the lagoon mostly suppresses cross-reef transport, there is always some flow through the lagoon, with the strongest  flows occurring at high tides and when the wave forcing is strongest.