Ocean-shelf exchange through the Berau barrier reef, Indonesia

The present study investigates the freshwater spreading on the Berau Continental Shelf, Indonesia, including the ocean-shelf exchange through a barrier reef located at the shelf edge. Moored and shipboard measurements on currents and turbulence were taken as part of the multidisciplinary East Kalimantan Research Programme. These measurements, and collected data on sea levels, currents, wind speed and bathymetry, were used to setup and calibrate a three-dimensional hydrodynamic model in the ECOMSED environment, which is derived from the Princeton Ocean Model. The data and model results were first used to study the tidal propagation and mean circulation patterns on the entire Berau Shelf. The diurnal and semidiurnal tides propagate across the isobaths towards the coast, where amplitudes increase. Tide-induced mean currents dominate over monsoon-driven currents, and feature a southward transport pattern close to the coast and a northward transport patterns at 10 to 20 meters depth. Next, the river plume behaviour is studied. Key factors controlling the river plume behavior include advection of stratified waters by the subtidal motion and mixing, which inhibits the stratified region to extend beyond the reef region. The tides drive freshwater in northeastern direction, towards the reef area. The model is subsequently refined and used to study the freshwater transport and exchange of water via the reef gaps and over the reef flats in detail. Moored ADCP data reveal extremely large roughness heights in the reef passages and reef flats. These limit the exchange of tidal energy to some degree, acting as a control on sealevel gradients over the reefs. The spatial structure of velocity exhibits tidal eddies generated by irregularities of reef gaps. The flow in the center of the reef passage is often opposed to the flow near the reef boundaries. The mean mass transport in the passages that were studied were found to be caused by Eulerian residual currents generated by gradients in the sea surface topography, and by Stokes transport. Freshwater fluxes show similar patterns as the mass transport. A main factor controlling the freshwater transport was established to be tidal pumping. The exchange of water across the barrier reef is concentrated in the reef gaps, while the flow over the reef flats is insignificant for the overall spreading of freshwater in the region.