Flow Over Acropora on a Barrier Reef in Koror, Palau

Lighthouse reef located in Koror State, Palau is a barrier reef next to a channel that connects Malakal Harbor to the open Pacific Ocean. Acoustic Doppler Profilers, RBR pressure loggers, thermistors, and tilting current meters were deployed to study the channel’s influence on the strength and behavior of the flow and how roughness and the complex topography of the reef affect turbulence in the water column. We use the momentum balance using the pressure gradient and the depth averaged velocity, as well as log fits to the law of the wall to estimate the drag coefficient over this reef that is dominated by table acropora. This analysis finds that the velocity profiles at the site location nearest to the channel obey the law of the wall with greater certainty than sites further from the channel outlet. We show that the channel behaves as a jet during ebb tide and as a sink during flood tide, resulting in a strong asymmetry in the strength of the currents in the alongshore direction. Additionally, the water column is more mixed during flood tide and more stratified during ebb tide due to the influence of the channel.