Reef flat flow dynamics for a nearly closed fringing reef-lagoon: Ofu,
American Samoa
Abstract
We 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 (1984), 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.