Re-defining sediment transport models over coral reefs and seagrass meadows
Novel research within WAMSI’s Dredging Science Node will redefine how current dredged sediment transport models predict key pressure parameters such as sediment deposition rates within ecologically significant marine habitats.
Sediment deposition and subsequent smothering of marine habitats such as corals and seagrasses is one of the mechanisms by which dredging can impact on the environment.
However, according to Professor Ryan Lowe from The University of Western Australia, current sediment transport models are severely lacking in their ability to predict rates of sediment deposition and re-suspension over coral reefs and seagrass meadows with any degree of confidence.
|Canopies formed by seagrass meadows impose drag forces that can trap sediment. This is not accounted for in sediment transport models.|
“The first step in the Environmental Impact Assessment (EIA) process for proponents of new developments is to make predictions on the likely extent, severity and duration of their impacts on the environment,” Professor Lowe said. “To do this for projects involving dredging, proponents use sediment transport models that make predictions of where dredge plumes will go and what impacts they will have when they get there.”
“Current sediment transport models assume that the seafloor is essentially flat and that nothing is growing on it. However, in reality the large roughness, or canopies, formed by coral reefs, seagrass meadows and sponge gardens impose substantial drag forces that will alter turbulent flow structure over very small spatial scales and can trap sediment. As a consequence, current sediment transport models can grossly underestimate the rates of sediment deposition that occur in and around these important habitats.”
|Sawhorse instrument frame deployed at Ningaloo Reef with hydrodynamic and sediment transport instrumentation. Photo: Andrew Pomeroy|
Professor Lowe and UWA collaborator Dr. Marco Ghisalberti are leading a research program combining field and laboratory techniques to address this problem.
“In the field we are measuring turbulent flow structure and sediment concentrations above and within the coral reef and seagrass meadow canopies,” Professor Lowe said. “These direct measurements are compared with various conventional sediment transport models and highlight the major deficiencies.
“We are also conducting parallel and complementary laboratory experiments. The advantage of laboratory experiments is that we can examine in detail the mechanisms and processes in a controlled setting. We can control the densities and heights of canopies, and factors like whether they are completely submerged or not. In this way we can precisely measure transport rates, near bed turbulence, sheer stress and look at the effect of canopies on transport rates and subsequent deposition,” he explained.
|Laboratory experiments of sediment transport through artificial canopies|
The ultimate goal of this research is to develop new and improved transport formulations and algorithms that can more accurately predict rates of sediment deposition and the subsequent impacts to seabed communities.
“If we can achieve this, then both the Environmental Protection Authority and project proponents will have greater levels of confidence in the prediction of impacts during the EIA process,” Professor Lowe said. “And this is what the Dredging Science Node is all about.”
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.