Environmental guidelines associated with dredging in Western Australia encourages dredging proponents to make scientifically sound predictions of the likely areal extent of any environmental effects associated with any turbidity-generating activities (EPA 2016). Smothering of benthic organisms by high levels of sediment deposition is known to be one of the key cause-effect pathways associated with dredging activities (Jones et al. 2016). The problem is that there are currently no reliable techniques to measure sedimentation over appropriate scales (ie mg cm-2 d-1). In this study we assess an alternative approach–a statistical turbidity model–to estimate sediment deposition and the size of a sediment deposition zone around a dredging project. The underlying principle is based on the difference in the likely deposition rates or accumulation magnitude that would occur between turbidity caused by natural sediment resuspension events and turbidity caused by dredging activities. Natural resuspension in the shallow reef environment is typically caused by tidal currents, swell and especially winddriven waves. High suspended sediment concentrations (SSCs) can be generated but typically occur under energetic conditions where the water column hydrodynamics are sufficient to keep at least some of the sediments from settling. Sediment deposition will occur but during quiescent periods, and following entrainment and dilution, and over a period where energy in the water column is gradually decreasing. In contrast, dredging can create high SSCs in a low energy water column where the ambient hydrodynamics are often insufficient to keep the sediment load in suspension. The sediments can rapidly fall out of suspension according to particle specific settling velocities. Sediment deposition generated under such conditions could exceed those during natural resuspension events, and which nearby organisms are physiologically adapted to. Furthermore, the increased sedimentation in a low energy environment during dredging is more likely to remain in-situ with no wave activity to resuspend and remove the accumulation.

Turbidity can be predicted by a model (see below) based on the collection of only a few variables including pressure fluctuations at the seafloor induced by surface gravity waves. The model, developed by the James Cook University Marine Geophysics Lab, is an additive power model which predicts daily turbidity from water pressure measurements which have been transformed into estimates of the natural turbidity resuspension mechanismswaves and tides. Other elements can also be incorporated into the model, for example wind speed. Model inputs were calibrated against (daily averaged) turbidity measurements. The primary purpose of the model is to estimate natural turbidity levels, and these modelled values can be used during dredging to decouple the natural and dredge related turbidity ie the measured turbidity is a combination of natural+ dredged turbidity, therefore subtracting the modelled (natural) turbidity from the measured (natural+ dredged) turbidity provides an estimate of the turbidity caused solely by dredging. Furthermore, seafloor sediment deposition can also be estimated from the model-if the measured turbidity exceeds the estimate from the model—and in the absence of alternative explanations such as sediments within river plumes—then an assumption is made that the difference is caused by a dredging plume and could result in high levels of sediment accumulation. The greater the difference between the measured and the predicted (modelled) turbidity value, hereafter referred to as the overburden, then the greater …