Measuring Sediment Deposition

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.

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.”

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.

 

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