Dynamics and dispersion in the coastal boundary layer off Coffs Harbour in Eastern Australia

Abstract

Time series analysis and model simulations defined dynamics of coastal boundary layer formation off Coffs Harbour based on four deployed acoustic Doppler current (ADCP) meters and wind data from Coffs Harbour airport. Variance preserving spectra revealed peak energies at 7.8, 3.9 and 2.5 days plus ~24 and ~12 hours consistent with dominant forcing by winds. At inshore sites the highest energy levels occurred at the surface and decrease uniformly with depth at all frequencies with local peaks centred at exactly 24 hours, corresponding to peak local wind energy. In contrast, offshore sites showed depth dependency in the peak spectral energy with evidence of regional influences and wave-guide effects due to density stratification. Hydrodynamic simulations using the 3-dimensional explicit finite difference model 3DD revealed local bathymetric controls on circulation. A coastal boundary layer, delineated by a shear zone ~2km offshore in the lee of Corambirra Point, south of Coffs Harbour, was associated with formation of transient eddies. Model simulations and independent ADCP data identified 3 dimensional flow structures typified by clockwise rotation of flows down through the water column at all sites except for the quiescent, shallow water site in the headland wake south of Corambirra Point. The area south of Corambirra Point was predisposed to clockwise eddy rotation while offshore flows were generally shore-parallel. Pollutant dispersal was shown to be significantly less within this coastal boundary layer thus highlighting the need to consider effects of coastal boundary layers when locating discharges such as ocean outfalls.