Quantifying and modelling the transport and fate of debris from dredging operations is essential to predicting the environmental impact of large-scale port and coastal developments. Now a ground-breaking project to deliver one of the largest single-issue marine research programs in Australia has brought together the world knowledge on modelling the size, extent and behaviour of dredging debris to determine the most reliable science-based results achieved to date.

In this Western Australian Marine Science Institution (WAMSI) Dredging Science Node (DSN) report, modelling exercises to simulate dredge plume dispersal conducted for Environmental Impact Assessments (EIAs) were reviewed for the tropical Australian marine environment.

Relatively speaking, protocols for hydrodynamic and wave modelling have been well established, however this is not the case for sediment transport modelling due to the more complex nature of sediment transport processes and lack of data, which is a huge obstacle for improving model predictions. The report found the following need to be considered:

• Lack of suitable data for model calibration and validation;
• Three dimensional models are more expensive but preferable to two dimensional models as the vertical structure of the plumes can be simulated; and
• If the dredge plume is likely to travel to areas that are under the influence of large-scale currents, it is recommended to either set up nested models or appropriate open boundary conditions.

Compared to hydrodynamic and wave modelling, sediment transport modelling has many challenges. Some major issues  reported by this review include:

• Modelling non-cohesive sediments (sand) and cohesive sediments (mud) requires different approaches;
• Sediment transport models are often not calibrated or validated due to the lack of relevant field data;
• Parameters used in sediment transport models are often not reported in EIA documentation leaving considerable uncertainty in assessing the model performance;
• Currently sediment transport modelling in EIA documents does not include ambient sediments. Their dynamics could be important in determining the overall impact of dredging; and
• Assessing the long-term impact of dredging requires the modelling of extreme events such as tropical cyclones and floods.

One of the main objectives of the WAMSI DSN is to promote the use of best practices in numerical modelling. This latest review of current knowledge in EIA modelling, led by CSIRO researcher Dr Chaojiao Sun, is a first step towards establishing detailed and specific guidance with regard to modelling physical and sediment transport processes in tropical and subtropical Australia, including model calibration and set up, and potential ranges of suitable parameters when possible.

Links:

• DSN Report 3.1.3: Chaojiao Sun, Kenji Shimizu, Graham Symonds (CSIRO) Numerical modelling of dredge plumes: a review
• All Dredging Science Node Reports can be found at: www.wamsi.org.au/dredging-science-node/dsn-reports

 

Background:

The Western Australian Marine Science Institution is delivering one of the largest single-issue marine research programs in Australia. It will vastly improve the planning and regulation of major dredging operations in our precious marine environment.

This world-class marine research is enhancing capacity within government and the private sector to predict and manage the environmental impacts of dredging in Western Australia. The outcomes will increase the confidence, timeliness and efficiency of the assessment, approval and regulatory processes associated with dredging projects.

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.

Dredging Science