Researchers in the WAMSI Dredging Science Node are developing new automated scientific instruments to measure sediment deposition in the field with prototypes deployed and tested.

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.

This pressure parameter is incorporated into sediment transport models by proponents during the Environmental Impact Assessment (EIA) process to make predictions of the likely extent, severity and duration of their impacts on the environment.

However, according to Dr James Whinney from James Cook University there are no scientific instruments that can actually measure net deposition in the field.

“Measuring net sediment deposition is quite challenging” Dr Whinney said. “When sediment falls out of the water column and deposits on the seafloor, not all of it will stay there with the energy from waves and currents causing some of those sediments to resuspend and be transported elsewhere. This is what we mean by net deposition and is the actual pressure that will be exerted on an organism.

Deposition sensor deployed in laboratory experiments alongside traditional sediment traps and sedpods.

“The most common technique currently used to measure deposition is to use a sediment trap which typically comprise of vertical PVC tubes that are closed at the bottom and open at the top and mounted on a frame on the seafloor. These are generally deployed for months at a time before they are retrieved and the amount and characteristics of the sediment in the tube can be measured.” Dr Whinney said.

“The problem with this is that once sediments have settled in the tube, they get trapped there and can’t be resuspended by wave energy. So while they can accurately tell you the total amount of sediment that settles, they can’t tell you what the net deposition would be after some of this has been resuspended and transported elsewhere. “Another problem is that they can’t tell you how much settles each day. Just averaging the total amount of sediment by the number of days it was deployed is not accurate as on calm days more sediment would settle out of the water column whereas on rough and windy days less would settle.

This is important to know for organisms like corals as they can actively remove sediment that has deposited on them but only up to a point. If the rate of deposition becomes too great, they will not be able to keep up and begin to get buried by that sediment.

To address this problem Dr Whinney and his colleagues have been developing an automated deposition sensor that can measure net deposition as well as provide accurate information of the amount settling each day.

“The deposition sensor we are developing uses optical back scatter allowing readings to be taken every few minutes and uses a perforated plate on top of the sensors to mimic the surface of a coral so that deposition occurring on the instrument is similar to that on a coral.”

“During the development of this sensor it has been tested at AIMS, used in experiments at SEASIM and also deployed off Townsville and on the site of the Wheatstone Project, off Onslow. The data from the sensor is promising, measuring expected levels of sedimentation in the laboratory and gathering interesting data from the field which compares well with wave and turbidity data.”

Calibrating deposition sensor in laboratory by adding known quantity of sediment.

Dr Ray Masini from the Office of the Environmental Protection Authority and the Node Leader – Policy, said developing an instrument that can accurately measure net sediment deposition will be a major step forward and improve confidence during the EIA process.

“While proponents use sediment transport models to make predictions of the amount of sediment deposition that will occur, because we can’t actually measure net sediment deposition in the field, there is no way to verify and refine the sediment deposition rates that were predicted by those models” Dr Masini said. “This reduces confidence for both the Environmental Protection Authority and project proponents in the prediction of impacts during the EIA process.”

“It is also important for scientists conducting experiments to establish thresholds of response to dredging-related pressures.  To accurately define the sediment deposition rates that led to mortality, sub-lethal impact and first observable effect, you have to be able to accurately measure this in the first place.”

“Therefore the development of this deposition sensor will be an important step forward and provide both the Environmental Protection Authority and project proponents a greater level of confidence in the prediction of impacts during the EIA process,” Dr Masini said. “Which is what the Dredging Science Node is designed to do – to 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