Tidal and subtidal flow patterns on a tropical continental shelf semi-insulated by coral reefs

The present study sets out to describe the tidal and subtidal water motion at the Berau coastal shelf, which represents a tropical continental shelf area of variable width hosting a complex of barrier reefs along its oceanic edge. Moored and shipboard measurements on currents and turbulence were made as part of the multidisciplinary East Kalimantan Research Program. These results and collected data on sea levels, meteorology and bathymetry, were used to setup and calibrate a three-dimensional barotropic hydrodynamic model in the ECOMSED environment, which is derived from the Princeton Ocean Model. Concurrent profile measurements of flow velocity and Reynolds stress with an acoustic Doppler current profiler were used to infer values of the drag coefficient on locations on the shelf and within the barrier reef environment. The drag coefficients determined from shelf measurements differed less than 20% from optimal model settings. Diurnal and semidiurnal tides have the same character, propagating across the isobaths with amplitudes that increase toward the coast. The tidal amplification is captured in a one-dimensional model, revealing an analogy with the Amazon Shelf. Regarding the subtidal depth-mean flow, tidal Eulerian residual currents dominate over monsoon-driven currents. Lagrangian mean flows reveal freshwater pathways from the coast to the coral reef environment. At a regional scale, the reef complex is established to be semitransparent to tides. The barrier reef influences the tidal phases rather than the amplitudes, especially in the shelf region that is fringed by the reefs. This may explain why tidal phases predicted from hydrodynamic models of tropical continental shelves often feature substantial discrepancies from observations.