Unacceptable risk drives decision
Port Hills residents threatened by rockfall were up to 100 times more likely to be killed in an earthquake than in a car accident, a geotechnical engineer says.
Earthquake Recovery Minister Gerry Brownlee yesterday red-zoned 285 Christchurch properties at risk or destroyed by cliff collapse, rockfall or land slip.
The Port Hills zoning had been one of the most difficult decisions to make, he said.
"The complexities of damage on the flat land compared to complexities of the damage on the Port Hills is like one to 10, 10 being the Port Hills.
"The balance has been ensuring we've got lines that demarcate what might be acceptable risk and unacceptable risk."
Computer modelling and rock mapping was used to determine the "life risk" formula.
Canterbury Earthquake Recovery Authority chief geotechnical engineer Jan Kupec said the risk of death in some areas was between one in 100 to one in 1000.
The chance of being killed in a car accident in New Zealand was one in 10,000, which was the international life-risk standard for most activities.
"Essentially, the further down the slope you are the safer you are. That's where, to some degree, we actually drew the lines."
Coastal cliffs at Redcliffs, Clifton, Richmond Hill and Whitewash Head, which were about 60 metres to 80 metres high, had collapsed and some properties below the cliffs were in the "debris apron".
Cracking had been discovered 20 to 60 metres back from the cliff edge, Kupec said.
"The cliff collapse can be initiated by seismicity but also we have several instances several thousand cubic metres fell down without any apparent triggers."
The cause of rockfall risk was Banks Peninsula's many exposed volcanic bluffs, Kupec said.
In February 2011, about 2000 rocks fell in Avoca Valley and about 600 fell in Wakefield and Heberden avenues in Sumner.
"We had several houses that were hit numerous times - some of them penetrated - and rocks in very close vicinity.
"The 3D modelling actually showed us that was not a fluke and could occur again if another event like this happens," he said.
Blasting would expose more loose boulders and protective fences could be ineffective if they were hit "20 to 50 times", Kupec said.
Some rocks were up to 10 tonnes and travelled at up to 90 kilometres per hour.
"We had harvested a whole lot of rocks off in February  and guess what, the same amount came down again on June 13 [last year]," he said.
- © Fairfax NZ News