A Massey University expert says erosion a major risk water
A wind farm in the Turitea Reserve could ruin the city water supply. Erosion could be a problem that would be difficult to overcome, a Massey University geography professor says. The $1 million a year the Palmerston North City Council is hoping to get for its wind farm might not be enough to pay for the damage it does, John Flenley says.
The problem is the removal of vegetation ““ native bush or scrub, whatever ““ to install the turbines themselves, as well as the road construction needed to the site.
And it could take 100 years for all that vegetation to grow back.
Once the site is cleared, Manawatu rainfall will do its worst, releasing sediment that will be washed down the streams and will end up in the reservoir, he says.
“They (the council) may say the sediment will be trapped in the streams on the way down, and some might be caught this way, but it is such a steep catchment that I think most of it will end up, in a few years, in the reservoir.” Dr Flenley’s calculations show that in 10 years, the eroded material could have increased the level of sediment behind the dam by 13 metres, consequently shortening the useful life of the dam.
“I’m not saying this will happen, but it could, especially if there is another rain event (like the 2004 floods) during construction or shortly afterwards.” If his calculations are correct, the council will have to raise the level of the dam or build a new one, which would be very expensive, he says.
“I’m saying the council should be careful and have proper measurements made,” Dr Flenley says. “I have friends on this council and I don’t want them to go down in history as the council that destroyed the water supply and gave us enormous rate increases so a new dam can be built.” The average rate of erosion in land covered with bush or scrub is 0.01 tons per hectare per year, he says.
“But the expected increase in the erosion rate, after an area is disturbed and vegetation removed, is 1000 times greater.” As soon as the area is replanted, the rate drops to 100 times greater than before the disturbance. An article in a Landcare Soil Horizons magazine says new data shows forest reduces landslides by 90 percent and scrub by 80 percent. That is compared with land covered in pasture.
“There could be massive landslides if they are not careful,” Dr Flenley says. He says he likes the eco- park idea and is in favour of wind farms, but changing the Turitea Management Plan is morally wrong.
“I’ve never protested about anything in my life before … I’ve never even written to the newspaper, but this seems too much. It’s over the top on both counts. It’s a nature reserve and water catchment.” In the September 17 issue of The Tribune, Mayor Heather Tanguay said no native vegetation will be destroyed, but that’s impossible, Dr Flenley says.
Dr Flenley recently went for a drive up South Range Road and wondered why the ridge line is covered in scrub, not forest, because there are some native trees on the ridge line.
“It looks as if there was forest once. I think that the scrub is 100 years old, from when the reserve was set up in 1906/07. This (scrub) is only about 4 metres tall ““ that’s what you get after 100 years. “The developers say they are going to replace that instantaneously. It can’t be done. Regeneration under the extremely windy conditions can be very slow indeed.” Dr Flenley says he talked to a plant ecologist colleague at Massey University, who says the council would just get weeds, grass and maybe even gorse.
“They won’t be able to get back what they have destroyed quickly. I put to them you will put back the ecological succession back to forest by 100 years.” Farmland that has been used for the existing wind farms has all been back in good condition after a year ““ but that is grass, he says.
Getting back to scrub or forest is an entirely different matter.
By Helen Harvey
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