Huge internal waves cause benthic storms

Australian researchers have observed the generation of internal waves flowing hundreds of metres beneath the ocean surface on the North West Shelf. The research -carried out by The University of Western Australia and funded by the Western Australian Marine Science Institution (WAMSI) with support by Woodside Energy and the Australian Research Council- will benefit the design and safety of oil and gas infrastructure.

"Internal waves can affect the thousands of kilometres of oil and gas pipelines on the North West Shelf," research leader Professor Greg Ivey, UWA's Winthrop Professor of Geophysical Fluid Dynamics, said.

"They are similar to surface waves but instead of travelling along the top of the water they travel beneath the surface and occur when cold water from the bottom of the ocean is pushed up the continental slope by tides.

"They can be up to 80 metres high in water columns where the total water depth is only 120 metres. As they travel inshore they steepen, which results in large near-bottom currents and energetic turbulence - a benthic storm," he said.

The deep-sea experiment involved dropping a 30-metre mooring 400 metres to the ocean floor using a 600-kilogram anchor, about 200 kilometres offshore and 60 kilometres north-west of the North Rankin platform, which is 125 kilometres north-west of Karratha.

"Our first question was to try to understand where these large internal waves came from, and the second question was to determine their environmental impacts. We designed this experiment to take place in the 'generation' region where we believed the waves originated," he said.

The novel mooring system - carrying five current meters and 30 temperature loggers - was left at the bottom of the ocean where, each second for three weeks, it logged data on local currents and temperatures. The data set is being analysed by UWA PhD student, Cynthia Bluteau, who is co-supervised by Assistant Professor Nicole Jones and Professor Ivey.

"What we found is that the near-bottom region of the ocean is very 'energetic' because of the strong surging motions that can rush up the Continental Shelf's slope," Professor Jones said.

Professor Ivey was able to confirm that the internal waves originated at a depth of 400 metres and took 30 to 40 hours to travel the 70 kilometres to reach Woodside Energy's North Rankin platform.

"Experiments have taken place offshore from Ningaloo Reef and the new mooring instrument system is now being deployed in the Kimberley Browse Basin region," he said.

"We will be able to calculate the travel time, wave strength, turbulent response and other factors associated with the moving waves at different locations on the North West Shelf.

"It will be of huge interest to marine managers, the offshore industry, coastal developers, aquaculture business owners, fisheries managers and the tourism industry."

Contact:
Sue McKenna (WAMSI)
(+61 8) 6488 4574 or (+61 4) 24 196 771