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:

• DSN Report 3.1.2: Lowe R, Ghisalberti M: Sediment transport processes within coral reef and vegetated coastal ecosystems: 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