Experts say a debris flow at Awatarariki is more likely to cause deaths than an earthquake in Wellington and it is “extraordinarily lucky” that no one died in the 2005 event.
The plan change hearings that will determine the fate of residents living on the Awatarariki fanhead at Matata continued at Eastbay REAP in Whakatane yesterday with submissions from a further 12 witnesses appearing on behalf of the Whakatane District Council.
Several Awatarariki residents were present along with district councillors Lesley Immink, Gerard van Beek, John Pullar and Gavin Dennis.
Tonkin and Taylor technical director Kevin Hind told the independent commissioners that no single debris flow was ever the same and it would be impossible to predict when the next would strike.
He said through climate change the region would most likely see an increase in rainfall, which was what triggered the original event, however, this might not necessarily trigger another debris flow.
There would need to be some time before rainfall events for debris to build-up in the catchment.
The commissioners, barrister Trevor Robinson, consulting engineer Fraser Campbell, consultant Rob van Voorthuysen and consultant Rauru Kirikiri flew over the catchment on Monday with Commissioner Campbell commenting that the scars from 2005 were still obvious.
“Is there enough debris available now for a similar event?” he asked.
Mr Hind said a lot of the original debris was not just from the stream; a lot came from landslides triggered by the rainfall.
“There is no reason why rainfall wouldn’t continue to trigger landslides, the hillside is highly erodible,” he said.
“I have been investigating landslides something in the order of 15 years. I have seen fatalities occur in sites that suffered far less damage than Matata. I have seen people killed by rocks the size of rugby balls, never mind huge boulders going through houses. It is my view that the people of Matata were extraordinarily lucky that day.”
Mr Hind said looking at the 2005 debris flow, he would have estimated that three people would have died near the railway and at least one in Clem Elliott Drive. He said just because a home wasn’t touched by the debris flow then didn’t mean residents would experience the same luck in potential future flows.
Follow-up evidence from Tonkin and Taylor water resources engineer Tom Bassett clarified that he estimated there was a one percent chance every year that Awatarariki would experience the same amount of triggering rainfall and a 63 percent chance it would occur in the next 100 years.
In response to questioning, he said if this was, hypothetically, to occur again tomorrow, he would be forced to conclude that it could occur more frequently and these estimated percentages would rise.
After their site visit, the commissioners were curious as to whether the large boulders that remained between Clem Elliott Drive and State Highway 2 would protect homes from a future debris flow.
University of Canterbury professor Tim Davies said the boulders were not a reliable barrier and could be remobilised and used as “ammo” by a future debris flow.
Professor Davies said it would not be acceptable to permit homes within the tree and boulder deposit envelope and he was confident the high-risk zone was in the right place.
When asked by Commissioner Campbell if he preferred an “overly cautious” approach, Professor Davies said “yes”.
“But we won’t know if it is overly cautious for another 5000 years when we have had more events to base our models on,” he said. “The risk to life is very, very serious.”
GNS Science principal scientist Chris Massey spoke on risk analysis methods and the risk Awatarariki residents faced compared to the risks people faced in other locations as well as the unsuitability of an early warning system on the fanhead.
Dr Massey said an early warning system would be unlikely to give people the time to evacuate before a debris flow hit.
He said he knew of a pa site near Little Waihi in which 150 people were killed by a debris flow in 1780. Another debris flow in 1846 killed 64 people. The next in 1910 only killed one because people had moved from the site after recognising the risk.
Commissioner Robinson asked what fatality risk had been deemed unacceptable by the Government noting “skydivers are at high risk but that doesn’t stop them jumping out of planes with or without working parachutes.”
Dr Massey said the risk to life from riding a jet boat was the same as walking to Fox Glacier and, while families were happy to do the walk with their children, they would never ride a jet boat with their children.
He provided a graph showing the relative risk to Awatarariki residents compared with other risks across New Zealand.
It showed residents in Wellington were less likely to die from an earthquake and residents in Whakatane were less likely to die from a tsunami.
“It’s about exposure to the risk,” he said.
Other witnesses spoke about how the council arrived at its managed retreat option and the robustness of its valuation process during this process, including the unusual step of needing two peer reviewers which was above and beyond anything valuers had had to comply with before.
The commissioners also heard from resource management consultant Gerard Willis who said it would be “impractical and unenforceable” to zone each home on the fanhead according to its individual risk.
Individual risk could be profiled by the size and construction of each home as well as who was living in it, whether they were able-bodied and if they worked from home or full-time off the fanhead.
“This type of micro-regulating would have a huge intrusion on people’s lives as they would have to notify whether their living conditions had changed,” he said.
Mr Willis also said it would not account for any visitors such as grandchildren coming to stay at the homes. “That planning intervention would be very intrusive.”
Yesterday’s hearing concluded with Boffa Miskell planner Craig Batchelar answering some questions he had been unable to answer ad hoc on Monday.
These related to the location of the high-risk area and what would happen to homes that were half in the high-risk area and half in the medium-risk area.
He said the existing home would be permitted but homeowners would not be able to extend its footprint through renovations although they could renovate the inside.
The hearing resumes at 8.30am today with one more council witness before Awatarariki residents take the stand. Whakatane District Council counsel will then have right of reply in the afternoon.