Stinking mountains of rotting seagrass have made life miserable for hundreds of Busselton residents for many years.
UWA research has found the solution is to replace three groynes.
Port Geographe, a canal development a few kilometres out of central Busselton, promised a resort lifestyle to its residents before the development was built more than a decade ago.
Every winter, piles of seagrass, known as wrack, accumulate on most beaches after a storm. Usually the next storm surge will wash it back into the ocean. But at Port Geographe, the mounds of wrack stay where they are, becoming bigger with every bout of rough weather.
The massive piles decompose, emitting hydrogen sulphide, at times causing headaches and nausea among local residents.
Professor Carolyn Oldham, at the School of Environmental Systems Engineering, was asked by the Department of Transport and the Busselton Shire Council to lead a team of researchers to help them understand the wrack dynamics so they could find a solution to the problem.
Meanwhile, the council has been sending in bulldozers every day between June and September, to scoop up the piles of wrack that can get up to 2.5 metres high. It is transported to nearby Wonnerup Beach, where it is returned to the ecosystem.
"Three groynes that were built for the Port Geographe development have been causing seagrass accumulation," Professor Oldham said. "The rotten egg smell, the visual pollution of the mountains of wrack and the noise of the heavy machinery removing it has been very difficult for some residents."
She explained that wrack comes from the upper leaves of seagrass that are broken off in rough weather. "The groynes are stopping the natural circulation of the wrack in the bay, so the wrack is not being returned to the ocean," she said.
The two-year research project, in collaboration with Edith Cowan University and DHI, an environmental consulting company, looked at the cycle of wrack: when it comes onto the beach, how it decomposes and when and how it comes off.
Professor Oldham was joined by Professor Paul Lavery and Dr Kathryn McMahon from ECU, a big group of graduate students and research assistants; Tony Chiffing from DHI and Winthrop Professor Chari Pattiaratchi.
"We also had a lot of help from people in the local community. Some took photographs for us every day to document the sites," she said.
The group took their new knowledge of the origin and movement of seagrass wrack particles and incorporated it into a hydrodynamic model; nobody had done that before.
"Chari Pattiaratchi did the modelling for the project and, after extensive testing of the computer model, we found that the problems would be solved by removing two of the three groynes and replacing the third one with a groyne that follows the curve of the land, rather than jutting straight out. Some further sand and water quality modelling needs to be done now, to finalise the recommended re-configuration.
"We're still not sure of the exact role of wrack in our coastal ecosystems, but our hypothesis is that the decomposing wrack provides essential nutrients that keep our reefs and seagrass meadows flourishing."
Published in UWA News, 14 June 2010.
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