The potential for adverse impacts on seabed habitats from exposure to suspended sediment plumes is now a world-wide concern. This report presents a review of available knowledge relating to Western Australian waters in order to improve the ability to estimate and predict the characteristics of dredge generated sediments.

More sepcifically, the review compared knowledge in three areas: the generation of particle size characteristics when soil or rock material is subjected to dredging processes; the rates and distributions of dredge-induced sediment resuspension; and the early stages of dredge plume development.

When soil and rock material is disturbed by marine dredging activities some of it is released as particles into the water column and transported away from the source by currents, giving rise to suspended sediment plumes. These plumes are characterised by above-normal concentrations of sediment both suspended in the water column and settled on the seabed.

In more turbid waters, the higher concentration of suspended sediment reduces visibility and light penetration through the water column. Marine biological communities that are exposed to sediment plumes from dredging activity may therefore experience ecological impacts. It’s important to recognise however, that there are other factors (e.g. river inputs, cyclones, shipping and trawling activities) in addition to dredging which have the potential to resuspend sediments and increase turbidity.

Benthic primary  producer  habitats  are  seabed communities  within which algae (e.g. macroalgae, turf and benthic microalgae), seagrass, mangroves, corals or mixtures  of  these  groups  are  prominent.* Tropical seagrasses, for example, are important habitats for marine turtles and dugongs who use them for both a direct and indirect food source.

Sediment transport models have been used in recent years to predict the trajectory, extent and intensity of dredge plumes and to support ecological impact prediction and proactive management of dredging projects.

These dredge plume models require the input of suspended sediment source terms which specify the rates and settling characteristics of sediment particles introduced to the water column by dredging activities. The extent and intensity of the dredge plumes predicted by these models is significantly influenced by the source term specification.

The estimation or prediction of these source terms in advance of dredging has been challenging and a significant cause of uncertainty in applying dredge plume models in the context of environmental impact assessment, particularly for large capital dredging projects at locations with little or no previous dredging history.

This review focuses primarily on the generation and release of sediments by hydraulic dredgers, in particular the trailing suction hopper dredger and the cutter suction dredger. These are the two most common types of hydraulic dredgers used for major capital dredging projects in Australia.

The source term may vary greatly from one dredging case to another, since it depends on many factors, including: the nature of the in situ material to be dredged; the type and specifications of the dredging equipment; the dredging work method and dredge operating parameters; the site conditions (including bathymetry, currents and waves).

Figure 10 from Mills D, Kemps H, Generation and release of sediments by hydraulic dredging: a review – Structure of the TASS model for TSHDs (version 4.0) showing the sub-modules and underlying processes (from HR Wallingford 2013a, with kind permission from HR Wallingford and Ecoshape).

The report highlights that it is important that dredge-induced sediment suspension data sets are collected according to agreed protocols and methods (several of which are referenced in the report) so that calculations from these data sets can be reliably ranked and compared.

Overall, the number of these data sets has increased significantly in recent years. However many of these are not publicly available and their availability (and potential use) is restricted. Also, there are some relatively common dredging situations (e.g. trailing suction hopper dredging with low under keel clearance) that are not well represented by the available data sets.

The acquisition of high quality data sets, from both full-scale dredging operations and laboratory experiments, also leads to an improved understanding of the physical processes involved in the generation and release of dredged material particles and the early stages of plume formation. This enables the development of process-based source models as an additional means of estimating source terms, and this report reviews these models.

This report responds to Task 2.1 of the WAMSI Dredging Science Node Science Plan (Masini et al. 2011) which is to review  the  state  of  knowledge  regarding  the characteristics of  dredge-generated sediments,  considering  the  application  of  that  knowledge  to  Western  Australian settings.

*Environmental Assessment Guideline No. 3 (December 2009) Protection of Benthic Primary Producer Habitat

Links:

• DSN Report 2.1: Mills D, Kemps H, Generation and release of sediments by hydraulic dredging: 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