Dispersion in the Lee of a Headland: A Case Study of Circulation off Coffs Harbour

Efficient dispersion of pollutants discharged into coastal environments is predicated on a detailed understanding of the interactions of flow and local bathymetry. The purpose of our study was to investigate such interactions in order to assess the dispersion of effluent from the Coflfs Harbour Sewagerage Strategy. Times series and transect Acoustic Doppler Current Profiles (ADCP) were used together with local wind data, 3DD hydrodynamic modelling and CORMEX modelling to evaluate dispersion characteristics in coastal waters south of Corambirra Point just beyond the southern limit of the Solitary Islands Marine Park. Based on the good correlation between local winds and observed surface currents we found that our study period (September to November 2000) favoured southward flows compared to long-term wind data which indicated that wind driven along-shore flows were typically equally distributed northward and southward. Current meter records indicated wake effects and phase eddies (transient recirculation) in the lee of Corambirra Point. The resulting shear zone was located inshore of the 30m isobath, where surface flows were 2-3 times less than those offshore. Retarded and variable flows within this sheltered zone in the lee of Corambirra Point limit the potential for dispersion and increase potential for re-entrainment of plume waters compared to offshore waters. ADCP transects suggested that the shear zone may be relatively discrete with little evidence of increasing along-shore current gradient offshore from the 30m isobath. Offshore flows included a lower proportion of northward currents compared to inshore locations. Observations together with plume modelling (CORMEX) indicate potential benefits in discharging beyond the shear zone. These factors together with cost considerations provide a basis to optimise discharge designs and subsequent monitoring strategies.