Mount Maunganui ‘misinformation’: Did cutting down trees really cause the landslide?
Amid speculation that the deadly Mount Maunganui landslide was caused by the prior removal of large exotic trees, two scientists told Neil Reid why other factors were the more likely causes.
It was a matter of when – not if – the slope overlooking the Mount Maunganui Beachside Holiday Park failed.
And landslide expert Dr Tom Robinson – senior lecturer in disaster risk and resilience at the University of Canterbury – says that was regardless of whether trees had been removed from the mountain.
In the wake of last Thursday’s devastating landslip – which saw a fast-moving pile of dirt and other debris bury six campers – a succession of theories have been explored and spread on social media claiming the slope failed because numerous large exotic trees had been removed from the slopes of Mauao.
In 2023, Tauranga City Council (TCC) removed eight of the trees – all located on the same side of the mountain as the campground. It said they were “deteriorating, damaging” significant archaeological terraces.
A year earlier, several other exotics were also removed.
Critics – most without geological science backgrounds – have claimed the actions of TCC and local iwi who own Mauao are now partly responsible for the deadly slip that has now occurred.
But speaking to the Herald, Robinson said that argument was too simplistic, pointing out that Mauao has a long history of slips, including in the area overlooking the campground and the neighbouring hot pools.
And Robinson – whose work involves a broad range of techniques including geospatial modelling, scenario modelling, risk analysis and statistical analyses – told the Herald that any slip site should be considered susceptible to more.
“If you’ve had a landslide at a site ... that’s normally a good sign that slope is prone to landslides and that you could get a landslide in the future,” Robinson said.
“The problem we have with landslides is the ‘when’. We can do the ‘where’ relatively well, but the ‘when’ is the hard part.”
The area that came down with such deadly force would have been an “area of weakness” he said, given the large slip recorded in the same spot in the 1970s.
Since then, Mount Maunganui had been battered by many weather systems including Cyclone Wilma in 2011 and the Auckland Anniversary floods and Cyclone Gabrielle in 2023.
Last Thursday’s slip occurred after 274mm of rain drenched the area in a 24-hour period.
“We probably could have known or should have known that slope was unstable,” Robinson said.
“But that doesn’t mean that we could have known that it would go this time and not last year during a rainstorm or the year before, for instance.”
Robinson’s area of specialty – including developing a greater understanding of earthquake hazards and disaster risk – has seen him work throughout New Zealand, the Himalayas, Nepal and Bhutan.
The days following the tragedy have seen a lot of information spread, he said “and a lot of people making assumptions without the technical detail or the technical knowledge” of why the slope came down – especially around claims tree removal could have been a contributing factor.
“We’ve got to be really careful around this,” he said.
Robinson said it’s true that having trees and vegetation on slopes reduces the likelihood of landslides.
But native trees – and not exotic trees like those removed from Mauao – would have performed that task better.
That’s because while native trees often grow more slowly, in most cases they have stronger root systems than exotics, and can hold the earth together more securely.
Exotic trees that grow faster offer short-term stabilisation but their weaker roots mean they are not as suited for long-term erosion control on slip-prone land.
Exotic tree roots “only really work” to reduce slips that are shallow in depth.
“From what I can see of the images of this one, this slip looks pretty deep,” Robinson said.
“This looks like the kind of depth where even if you’d had full trees all over there, this wouldn’t have made a huge amount of difference.
“We’ll need a full geotechnical report to confirm that, but the size of this tends to suggest that [full tree coverage] wouldn’t have made much difference.”
Mauao was also made up of rhyolite – a type of rock known to weaken under heavy rainfall.
“So having trees is better than not, but that doesn’t mean it’s guaranteed to solve the problem,” Robinson explained.
“It’s not like if that slope had been completely forested this would never have happened.”
The 2023 tree removal included three macrocarpa, two chestnut, one pine, one oak and one poplar tree.
At the time, council said their removal aligned with the 2018 Mauao Historic Reserve Management Plan which “highlights the importance of protecting and enhancing the character of Mauao and acknowledges the protection of the historical values including the special cultural and spiritual relationship that tangata whenua have with Mauao”.
The plan was also seeing the gradual planting of more than 12,000 native plants on Mauao.
Aside from a lack of vegetation, Robinson said other factors that contribute to landslides included slope angle and shape, the presence of water, the types of geology below ground, and the direction the slope faces compared to where the rain or the water is coming from.
“When these kinds of events happen, the only way you can really find out what the cause was is with a really detailed on-site geotechnical study,” he said.
“So that means experts to go in, to look at the material, look at the failure zone, look at that slope and really understand what was going on beneath the surface, how water was getting into the slope.”
‘Act like a sail on a boat’: How trees can actually cause slips
Martin Brook – professor of applied geography at the University of Auckland – says there has been a lot of “misinformation online and also disinformation” in the wake of last week’s tragedy.
He told the Herald the benefits of trees on slopes includes their canopies intercepting rainfall, and their roots adding to the strength of the soil.
But in some locations, the presence of trees could actually increase the likelihood of a landslide – and also even its size.
“Trees can actually be problematic for slope stability,” he told the Herald.
“We found that after Cyclone Gabrielle in Auckland and the anniversary storm, some of the landslides there were actually probably caused by trees.
“Trees can act like a sail on a yacht, and so wind loading is a problem, and the tree vibrates backwards and forwards in the wind and eventually gets tilted or uprooted and then all the soil gets exposed and the tree, a mass of soil and tree can then, you know, move down the slope.
“Trees can be heavy, and they add a surcharge to the slope, they add a mass, a weight to the slope, which can increase the driving force.”
After the Auckland Anniversary floods, Brook led a team that investigated the 19 slips that happened on One Tree Hill.
He said Mount Maunganui was clearly a “landslide-prone” location.
“The whole hill is pockmarked by landslides. The council has had various reports done over the years and things like that, so they’re fully, you know, well-versed on the landslide hazards there.”
The day after last Thursday’s tragedy, the Herald reported how a scientific study into the stability of the area – including where the slip collapsed – had revealed long ago that it was prone to “mass movement” slips after heavy rain.
In 2011 Cyclone Wilma had delivered 108mm rain in a 24-hour period that resulted in 80 slips.
“Eight of those ... were debris avalanches, they flowed over 100m down the slopes,” Brook said.
Water could be seen flowing out of saturated soil on the slope near the campground in the hours before Thursday’s slip.
“That was basically a prelude to the landslide,” Brook said.
“It couldn’t drain fast enough, so it was actually exiting the slope.”
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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