Turbulence in the bottom boundary layer of Moreton Bay, Queensland, Australia

The hydrodynamic and water quality modelling of the complex coastal water environments requires detailed knowledge of the forcing and boundary conditions. It also requires knowledge of how the bed influences the water column structure, which, amongst other variables, relies on the bottom drag coefficient. In many cases the drag coefficient is merely assumed and adjusted to achieve the desired flow conditions, with the actual value being unknown. The physical properties affecting this bottom drag have been examined using a new microscale turbulence profiler and an ADCP within a channel of Moreton Bay, Queensland, Australia. From the data collected it is shown how the water column was highly dynamic and dominated by a strong bottom boundary layer, which ensured strong vertical mixing rates near the bed. The level of turbulence was observed to be a function of the current speed. Importantly, the data collected by the turbulence profiler allowed for the estimation of the bottom drag coefficient. The drag coefficient for this site was estimated at 1.4±0.8×10-3. Significantly, the outcomes of this work revealed that through the use of turbulence profiling instruments it should be a relatively easy task to map the drag coefficient values over large coastal areas. This will permit the use of boundary conditions for numerical models which more closely represent real conditions