Scheme designed to withstand earthquake

The Wairau Fault could produce a magnitude 7.6 earthquake, causing fault displacement of five to seven metres, an environmental hearing into TrustPower's proposed hydro-electricity scheme was told yesterday.

But earthquake geologist Kelvin Berryman did not accept residents' concern that the proposed scheme placed them at risk if there was an earthquake. He said the scheme was designed to withstand a 13m displacement along the fault, when recorded movement had been only 5m to 7m.

The Environment Court in Blenheim heard experts assess earthquake and land instability risks and describe how the scheme was designed to withstand these risks.

The hearing is being held to consider whether TrustPower should be granted resource consent to build the scheme.

An earlier decision by a Marlborough District Council-appointed panel was appealed.

Experts called yesterday included senior water resources engineer Robin Dawson and geotechnical engineer Bruce Symmans, of Tonkin & Taylor, and Dr Berryman, of the Institute of Geological and Nuclear Sciences.

The proposed scheme crosses the Wairau section of the 110- kilometre alpine fault 11 times.

Dr Berryman said earthquakes had ruptured the Wairau faultline "approximately every 1200 years, plus or minus 600 years", he said.

Save the Wairau lawyer Mike Hardy-Jones said this meant the valley was already 1400 years past the last 1200-year event.

Mr Dawson said canals carrying water between power stations would be cut into the ground, with a low embankment wherever possible. There was no risk of these "in-cut" canals losing stored water in an earthquake, because the water was contained by the surrounding high ground. The alternative was "in-fill" canals with earth embankments, which could spill over if breached.

Wairau Valley landowner Alison Parr asked Mr Dawson what proportion of the canal was in-cut. She was especially interested in the section of canal eight, above Wairau Valley township.

Mr Dawson replied that 35 per cent of the canal length and 40 per cent of canal eight had a bank on the river (downhill) side which was higher than three metres.

She put it to Mr Dawson that the canal crossed the fault above the Wairau Valley township in three places. The embankment at one of these crossings was 6.4m to 6.7m. "This is not a low embankment. It is a high one," she said.

The only reason that collapses of the Ruahihi canal in the Bay of Plenty in 1981 and South Canterbury's Opuha dam in 1997 caused huge damage but no loss of life was because there was no-one in the path of the floodwater at the time, she suggested.

David Clark, lawyer for affected landowners John and Joan McLauchlan, suggested that an alternative route yesterday proposed for the final section of the canal and power station five might not be suitable because of soil instability.

This might mean building a headpond in close proximity to a faultline. Asked whether having a headpond above the landowners' house posed a greater risk to their safety than a canal, Mr Symmans said the consequences of failure would be greater because of the amount of water involved. However, good design would ensure that the risk remained the same.

- The Marlborough Express