A scientific probe into the stability of Mount Maunganui – including the area that collapsed on Thursday morning – had already revealed it was prone to “mass movement” slips after heavy rain.
As emergency workers, rescue teams and recovery dogs search for six people missing after a massive landslide on to a Mount Maunganui campground, it has been revealed there were up to seven slips on the Mount during this week’s massive storm – some of them prior to the tragedy.
That included one about 5am on Thursday – four-and-a-half hours before the much larger and more devastating one.
Those earlier signs caused one camper to get up and urge others to wake and move away from the slope behind the Mount Maunganui Beachside Holiday Park.
Tragically, that woman is now unaccounted for – believed to have been in the campground’s toilet block when the later and larger slip occurred about 9.30am on Thursday morning.
After the initial slips, including the one that came down near the campground around 5am, Tauranga City Council announced at 8.56am that it was closing the walking tracks around the popular tourist spot as severe weather had “significantly destabilised the maunga, creating an ongoing risk of further slips and falling debris”.
But the council made no call to evacuate either the Mount Maunganui Beachside Holiday Park or the neighbouring Mount Hot Pools.
Both facilities are council-owned, and the larger slip came down half an hour later.
Mount Maunganui local Alister McHardy told the Herald he had witnessed earlier slips, describing two areas of moving ground on the mountain as he went for a morning walk around 6am.
He said he had phoned 111 to report the danger and try to get the walking tracks and camping ground evacuated.
But he claims the 111 operator “said I had to talk to the council”.
Instead, he said he started waking up campers himself to warn them of the land movement nearby.
The police said they weren’t in a position to confirm if they received any 111 calls about landslides before the dangerous 9.31am event.
But McHardy said: “I didn’t sleep very well last night. I’m kicking myself. I should have kicked up more of a fuss.”
Tauranga Mayor Mahé Drysdale won’t be drawn on why the campground and hot pools weren’t evacuated when the walking tracks on the Mount were closed, initially telling NZME yesterday it was “too early to comment on that”.
“Our focus right now is on the rescue effort and supporting those affected, along with the emergency services and our staff working on the ground,” he said.
This afternoon, when questioned again on why people weren’t evacuated after the female camper raised the alarm, Drysdale said those questions would be answered after a “proper process”.
He confirmed there had been “reports of a small slip” after the initial event at 5am.
The mayor added: “Right now our focus is on emergency services, supporting them and supporting those families.”
Among the six unaccounted for under the rubble are two 15-year-olds.
A further three people are on a list of campers who had not checked out, but who may have left the area before the landslide.
As well as being popular for its majestic Bay of Plenty coastline views, Mount Maunganui is also known for regular slips.
That was made clear in the 8.56am press release issued by the council half an hour before Thursday’s shock event, warning of “ongoing risk of further slips and falling debris”.
The mountain’s potential for landslides in extreme weather was also studied in great scientific detail by engineering and geophysical experts from Canada and New Zealand in 2014.
The resulting research paper, Spatial-temporal distribution of mass movements on Mount Maunganui, New Zealand, was presented to the 2014 Canadian Geohazards conference in Ontario. It looked deeply into the history of “mass movement” slips on Mount Maunganui.
It included a graphic showing the locations of all the landslides on the mountain, dating back to 1943, including a massive slip in 1977 in the same area where Thursday’s slip came down.
The map showed that 1977 slip, occurring behind the campground and hot pools, was the largest in width of the dozens that had been recorded.
The co-authors said that while the slopes of Mount Maunganui were generally “stable”, their physical and geographical make-up meant that could change dramatically in strong rainfall, such as the conditions that hit the Bay of Plenty region in the lead-up to Thursday’s disaster.
The research paper said landslides had “significantly altered” the shape of the mountain over time.
Four types of landslides had been identified at Mount Maunganui, including rockfalls and debris avalanches; the latter included a slurry that “flows rapidly downhill” and followed large rainfall like Thursday.
“In the last 20 years, Mount Maunganui has been used as pasture for sheep, along with the addition of a maintained network of walking trails and associated surface water drainage structures,” the paper stated.
“Repetitive fires, including the most recent one on the northern slopes in 2003, are an additional process that influences the vegetation and by association the slope stability conditions at this site.
“The susceptibility of the residual volcanic soils to instability is a widespread problem on Mount Maunganui,” it said.
The paper added “most mass movements” were associated with heavy rainfall.
Talking today, Auckland University professor of applied geology Martin Brook said the 200mm of rainfall recorded in 24 hours would “easily exceed” the rainfall threshold to trigger landslides in many parts of New Zealand.
Brook said given their physical geography, areas such as Bay of Plenty, Coromandel, Northland and the East Coast were “always susceptible” to shallow rainfall-triggered landslides in wet conditions.
“The water weakens the surface soils and underlying highly weathered rock, causing shallow landslides to occur,” Brook said.
“The landscape often provides evidence of prior landslides, such as undulating surface topography, so rural landowners often are aware of landslide hazards on their land.”
Brook added that the deluge had been predicted by various New Zealand weather forecasters.
Lauren Vinnell, a senior lecturer of emergency management at Massey University’s Joint Centre for Disaster Research, said the tragedy highlighted the importance of continued studies on landslides and how to prepare for them.
“Landslides are common across New Zealand but can happen quickly and cause considerable damage,” Vinnell said.
“Sometimes there are warning signs, like small rocks falling, cracks or bulges in the ground, or doors or windows becoming hard to close or open because the frames have moved.
“Keep an eye out for these, especially during rain and after earthquakes, and if you feel unsafe then take action.”
Neil Reid is a Napier-based senior reporter who covers general news, features and sport. He joined the Herald in 2014 and has 34 years of newsroom experience.
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