DSN Report 3.1.3: Numerical modelling of dredge plumes: a review

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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:

 

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.

Category: 

Dredging Science

DSN Report 3.1.2: Sediment transport processes within coral reef and vegetated coastal ecosystems: a review

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As part of one of Australia’s biggest scientific efforts towards minimising the impact of dredging related operations Dredging Science Node researchers have, for the first time, reviewed the current state of knowledge and gaps required to predict sediment transport within coral reef and vegetated coastal ecosystems.

Coral reefs, seagrass meadows and mangrove forests are ecologically-important and prominent features along Australia’s coastline, and considered to be particularly sensitive to dredging-related pressures. The potential for adverse impacts from exposure to suspended sediment plumes is well known; in turbid waters, a higher concentration of suspended sediment reduces light availability for photosynthesis and can clog feeding mechanisms. Indeed, the ecological function of marine biological communities exposed to sediment plumes from dredging or other activity (e.g. river inputs, cyclones, shipping and trawling) can be greatly compromised. As a result, these ecosystems are often prioritised in dredging monitoring and management programs.

These coastal ecosystems contain large and complex bottom roughness (or canopies) on the seafloor that can dramatically influence both near-bed water movement, and in turn, how sediment is transported.  However, modern sediment transport theory and models, including those used to predict the impact of dredging plumes, are still based entirely on the mechanics of how sediment is transported over open (bare) sediment beds.


(Source: Lowe R, Ghisalberti M: Sediment transport processes within coral reef and vegetated coastal ecosystems: a review)

Although many knowledge gaps still remain, the review looked at the existing framework for predicting flows within the canopies present in coastal ecosystems and how this can serve as a foundation for developing new sediment transport models that are applicable to these environments. Specifically it reviewed:

  • The traditional approaches and models used to predict near-bed sediment transport in the coastal ocean;
  • The unique hydrodynamic interactions of currents and waves with submerged canopies, and why traditional engineering approaches fail
  • Existing observations of sediment transport within aquatic vegetation and over coral reefs;
  • Measurement techniques for quantifying and monitoring near-bed sediment changes in coastal canopies; and
  • Prospects for improving predictions for the fate and transport of natural and dredging-derived sediments in these environments.


(Source: Lowe R, Ghisalberti M: Sediment transport processes within coral reef and vegetated coastal ecosystems: a review)

Based on this review, the following issues were considered important when making predictions of dredging impacts in these environments:

  • Detailed habitat maps prior to dredging projects should be used to assess potential biological impacts and the role that habitat type may have on sediment transport and where it deposits.
  • Within benthic ecosystems such as coral reefs and seagrasses, sediments are usually biogenic-derived (comprised of calcium carbonate) with physical characteristics that differ substantially from traditional siliciclastic sediments in coastal systems. Suspended sediment monitoring instrumentation may show very different responses to carbonate sediments and instrument calibration should be conducted with in situ water samples obtained on a site-by-site basis.
  • At present there are still major knowledge gaps in how the varied bottom roughness of natural coastal ecosystems controls sediment transport rates.
  • Due to this large uncertainty in sediment transport rates over benthic (seabed) canopies, model predictions over areas that include coral reefs and aquatic vegetation should be treated with extreme caution.

The report concludes, it is clear that new observations of sediment transport within environments such as coral reefs and seagrass meadows are needed to:

  1. provide the missing quantitative insight needed to better understand these processes;
  2. incorporate these dynamics into new predictive sediment transport formulations applicable to these environments; and
  3. finally embed these dynamics in process-based numerical models that can eventually be applied within predictive models.

Links:

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.

Category:

Dredging Science

DSN Report 2.1: Generation and release of sediments by hydraulic dredging: a review

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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:

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.

Category:

Dredging Science

WA Marine Science Strategy achieves unprecedented collaboration

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Western Australia’s marine science, government, industry, conservation and community sectors met at the inaugural Blueprint for Marine Science Forum in Perth to begin building an Australian-first collaboration around marine science.

For more information, click the following link: http://www.marinescienceblueprint.org.au/latest-progress/wa-marine-science-strategy-achieves-unprecedented-collaboration

Karratha and Exmouth consulted on decommissioning

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Adele found to host one of the more unique reef habitats on the planet

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A geomorphological and geophysical investigation of Adele Island has revealed new insights into the long-term evolutionary history not only of Adele Reef but also of Australia’s North West Shelf.

The mid-shelf Adele platform reef sits between the atoll type Scott Reefs, which are located on the very edge of the North West Shelf, and the highly atypical inshore fringing reefs of the Bucaneer Archipelago.

This reef is of scientific interest because it provides an environmental link between the Kimberley’s oceanic and inshore reef systems.

 

The findings by Curtin University graduate students Tubagus Solihuddin and Giada Bufarale have been published in the Journal Geo-Marine Letters as part of the Western Australian Marine Science Kimberley Marine Research Program.

Geophysical surveys showed that the Adele reef structure did not form exclusively during the Holocene, which is the current period of geological time that started some 11,500 years ago when the glaciers began to retreat. It is in fact made up of layers of different aged reef packages that grew during previous sea level highstands (intervals where sea level lies above the continental shelf edge).

 
Rollolith habitat on the southern Adele Reef consisting of both coralline algae (rodoliths) and corals (coraliths)

 

By counting these packages back in time, researcher Giada Burfarale was able to calculate that coral growth at the site of Adele reef first began over 400,000 years ago. Interestingly this is also believed to be a time when coral growth first initiated on the Great Barrier Reef.

“Geophysical surveys also revealed a series of 90 metre deep channels around Adele reef that most likely represent ancient channels cut by the Fitzroy, Isdell or Prince Regent rivers which flowed across the continental shelf during sea level lowstands (times when sea levels were at their lowest),” Co-author Curtin University Dr Mick O’Leary said.

“Also, by comparing geophysical data with stratigraphic reef core data from Scott Reefs and the inner fringing reefs it was possible to calculate that the North West Shelf has subsided more that 30 metres over the last 100,000 years, making it one of the more tectonically mobile regions on the Australian continent,” Dr O’Leary said.

The modern Adele Reef is characterised by having one of the more unique reef habitats on the planet. Earlier investigations by the Western Australian Museum reported the discovery of a unique ‘rollolith’ habitat located along the SW side of the island. Rollolith is the common name given to benthic organisms that grow as unattached free-living spherical colonies enabling live tissue to cover the entire colony surface.
Percussion coring the Adele Island sand spit using an Atlas Copco post driver

 

Reef coring by Tubagus Solihuddin and the team found that this is a relatively recent habitat, forming only as the Adele reef platform reached present sea level, less than 1000 years ago. The resulting increase in surface currents are driven by seven metre tides and large waves which wash across the reef platforms at high tides.

“So what we have found is, rather than observing a style of reef geomorphology that gradually transitions between the atoll style oceanic Scott Reefs and the atypical high fringing reefs on the inner Kimberley, the mid-shelf Adele reef is characterised by its own unique structure, habitats and growth history,” Dr O’Leary said. “These findings provide yet more evidence of just how globally unique the Kimberley reefs are.”

 

Links:

Solihuddin, T.,  Bufarale, G., Blakeway, D., O’Leary, M.J., (2016) Geomorphology and Late Holocene Accretion History of Adele Reef: a Northwest Australian Mid-Shelf Platform Reef, Geo-Marine Letters, 2016, 1–15. DOI: 10.1007/s00367-016-0465-3

 

The $30 million Kimberley Marine Research Program is funded through major investment supported by $12 million from the Western Australian government’s Kimberley Science and Conservation Strategy co-invested by the WAMSI partners and supported by the Traditional Owners of the Kimberley.

Category: 

Kimberley Marine Research Program

Genomic insights into ecological connectivity in the Kimberley

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WAMSI Kimberley Research Program scientists, Dr Oliver Berry (CSIRO), Dr Jim Underwood (AIMS) and Dr Kathryn McMahon (ECU) presented their findings at the Department of Parks and Wildlife on a study looking into the movements of animals and plants among Kimberley reefs.

The research, part of a larger collaboration involving CSIRO, the Australian Institute of Marine Science, Department of Fisheries, Curtin University, Edith Cowan University and the WA Museum, investigated seven organisms that represent common species (two hard corals, two seagrasses, a mollusc and two fishes) to infer the routine distances of dispersal and patterns of connectivity among key populations within the Kimberley.

CSIRO researcher Dr Oliver Berry explained the team looked at fine scale and broad scale patterns, including movements between the Kimberley and other regions such as the Pilbara, using genomics to investigate connectivity.

“Many marine organisms are transported a long way by ocean currents when they are microscopic plankton,” Dr Berry said. “Their movements are hard to track and so we use genomics to measure the relatedness of organisms on different reefs, and from this we can indirectly estimate how far they have moved.”

From this research the team uncovered the location of barriers, important stepping stones and transitional zones for a number of the species investigated. The findings provide important information for the design of marine protected area networks in the Kimberley and the management of commercial fish stocks such as the Trochus.

“It has been very satisfying and exciting to work together as a team to synthesis our new knowledge on the genetic connectivity of marine organisms in the Kimberley,” co-researcher ECU’s Dr Kathyn McMahon said. “I look forward to seeing this information incorporated into future management and conservation of this special region.”

A copy of the presentation given by the team on 15 August 2016 and a project summary is available at http://www.wamsi.org.au/research-site/ecological-connectivity.

 

The $30 million Kimberley Marine Research Program is funded through major investment supported by $12 million from the Western Australian government’s Kimberley Science and Conservation Strategy co-invested by the WAMSI partners and supported by the Traditional Owners of the Kimberley.

Category: 

Kimberley Marine Research Program

Vale Steve Blake

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It is with great sadness that we mourn the passing of the Western Australian Marine Science Institution’s (WAMSI) inaugural Chief Executive Officer, Dr Steve Blake.

Steve graduated with first class honors in Geology from the University of Edinburgh before moving to Australia in 1987. He undertook his PhD in marine geology at James Cook University, studying the Great Barrier Reef. Steve worked in various positions in Canberra before moving to Western Australia to join WAMSI as its first Chief Executive Officer.  

Steve joined WAMSI in 2006 and managed the organisation from its inception until 2012.  Steve particular enjoyed working with Indigenous groups in the northwest of WA and eventually left WAMSI to work with Ninti One to further build opportunities for people in Indigenous communities in remote Australia.

On behalf of all of us at WAMSI, we thank Steve for his enduring contribution to marine science and offer our sincere condolences to Janine and their three lovely daughters, of whom Steve was so proud.