Realtime turbidity monitoring and modelling for dredge impact assessment in Darwin Harbour

Dredging within Darwin Harbour for the Ichthys LNG Project requires near real-time monitoring of turbidity and daily analyses to determine whether prescribed trigger levels have been exceeded due to to the dredging activities. Turbidity monitoring stations have been established in the far field of the dredge plume to assess the long term effects of fine sediment mobilised during the dredging and spoil disposal operations. Darwin Harbour is a macro tidal environment with tidal range exceeding 7 m at the highest spring tides and typically around 2.5 m at neap tides. Turbidity variability within the harbour shows a strong correlation with the tidal range. An empirical model relating the average daily turbidity to average daily tidal range has been developed and applied to provide forward predictions of the tidally-driven turbidity signal i.e. background. During periods when mixing is dominated by the tidal currents the model provides a reasonable basis for assessing the magnitude of dredging-related turbidity. The empirical model and its application to telemetered turbidity data collected near ecologically sensitive habitats provides a sound basis for management of dredging activities in the Darwin East Arm.