Dredge Spoil Placement Stability Study, Mermaid Sound

RPS assessed the potential sediment mobilisation for the proposed maintenance dredging and disposal activities in Mermaid Sound, Western Australia.

The study examined the degree of sediment mobilisation and related effects from several placement strategies at two potential spoil grounds under cyclonic conditions. Inputting expected sediment distribution data provided by Rio Tinto Iron Ore and Woodside Energy, an integrated sediment transport model – including sub-models for the hydrodynamics, waves and sediment fields – was developed using the Delft3D model suite. Wave and wind data were derived from calibrated models of three tropical cyclone events, and the hydrodynamic predictions were validated using measured data provided by Woodside.

The results showed that sediment mobility within the spoil grounds and beyond was likely to be low under ambient conditions and would quickly return to low levels following a cyclone event, and that use of a particular spoil ground and placement strategy was likely to minimise sediment mass loss.

Key Details

Project name

Dredge Spoil Placement Stability Study, Mermaid Sound 



Rio Tinto Iron Ore
Woodside Energy

The Pilbara Port Authority


Mermaid Sound, WA, Australia

Services provided

Hydrodynamic modelling

Wave modelling

Tropical cyclone modelling

Sediment dispersion modelling


The Port of Dampier is a major commercial hub of national economic significance. Dredging is essential to the function of the Port, and operations must be planned with due regard of the potential for extreme weather caused by cyclones.

RPS coupled its deep expertise in atmospheric and ocean modelling to identify the most appropriate site for the disposal of dredging spoil material required.


RPS has decades of experience modelling the cyclone prone region of Australia’s North West. Our team identified a set of historic cyclones that had tracked near to the Port of Dampier. Through re-analysis modelling of those past cyclone events, we were able to build a hindcast wind data model that was needed to drive a coupled hydrodynamic, wave and sediment model.

Our in-house 3D model was applied to test different dredge spoil disposal options under different forcing from historic cyclones. Analysis of the model results revealed common patterns in the behaviour of different spoil grounds, revealing that certain locations were less susceptible to erosion under extreme forcing. 

RPS provided the technical information needed to make an informed decision between alternative spoil ground locations, thereby minimising the risk of sediment remobilisation under extreme weather conditions. Our clients were able to plan operations and seek regulatory approval with full confidence knowing that their proposed site was backed by the best science, even under extreme cyclone scenarios. 

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