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Sunday, January 01, 2006

Third turbine causing erosion to riverbanks

The Mercury: Third turbine causing erosion to riverbanks [01jan06]

THE third turbine at the Gordon power station is now being fully utilised, says Hydro Tasmania water scientist Helen Locher.

Dr Locher said the impacts of the third turbine, previously unable to operate at full power, was now evident on the Gordon River.

"The third turbine came into operation later than the first two, and was not used fully due to transmission constraints, whereas it is fully utilised now," Dr Locher said.

"It is clear in examination of the riverbanks that the bank profile and vegetation in the upper zones of the banks (between 2.5m and 4m above low water mark) close to the power station is still responding to the impacts of the third turbine, and vegetation is likely to retreat over time up to four metres."

She said the third turbine started operation in 1988.

"As demand increased over the years there was increasing utilisation, and for several years, 1997-98, there were transmission constraints that limited its full utilisation," she said. "These constraints no longer exist."

Dr Locher said mitigation measures imposed by the Joint Advisory Panel reduced the impact of Basslink on the Gordon River.

"The impact on the Gordon of Basslink would be significant in the absence of any mitigation measures.

"The investigations undertaken for the DIIAS [Draft Integrated Impact Statement] established that, in the absence of mitigation, increased hydropeaking from Basslink would cause environmental concerns for the instream biota, fish and macroinvertebrates, riverbank stability, and the riparian or streamside vegetation.

"Approval for Basslink was on the condition that Hydro Tasmania commit to two significant mitigation measures -- a minimum environmental flow and a ramp-down rule -- as well as a nine-year monitoring program and adaptive management."

Dr Locher said the impact of Hydro operations declined along the 75km stretch below the power station.

"The impact of power station operations is very marked close to the power station and less so as one goes farther downstream.

"The effect of the power station has been to leave a plimsoll line-like mark on the banks in the zone affected by discharges, with an absence of vegetation below the line and dense vegetation above it.

"The water level range between low water mark and high water mark from power station discharges varies with distance downstream, and at its greatest is about four metres where there is channel narrowing, at about 5-7km downstream of the power station.

The Gordon power station has three turbines, brought into operation in 1977, 1979 and 1988.

"The level of usage of three machines depends on rainfall; during dry years we use Gordon a great deal and less so in wet years," Dr Locher said. "The last five years have been very dry and there has been considerable usage of all three machines."

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