Landslides are New Zealand’s most costly natural hazard, but can we mitigate our way out of their impacts?​

It’s been a wet winter.

Record-breaking rainfall in Wellington sent sodden earth tumbling down slopes all over the city in July – more than 600 slips in all. In Nelson, damage to roads and buildings from landslides and flooding in August is projected to cost more than $100 million to repair.

Slips have the power to take out our major infrastructure in a matter of seconds, and in a hilly country like New Zealand, that risk is never far away. Motorists on slip-stricken major highways, like the road into Milford or the Mangamuka Gorge in Northland, face frequent closures and disruptions. The Manawatū Gorge in the lower North Island was permanently closed after severe slips in 2017 left the road impassable.

“Landslides are the most costly natural hazard in New Zealand,” says Andrea Wolter, an engineering geologist at GNS Science.

“On average they cost about $250 to $300 million a year.”

While slips are most commonly seen in New Zealand’s more remote mountainous regions like the Alps and around Ruapehu, Wolter says in the past two years GeoNet has responded to slips across every corner of the country.

“As long as there’s a slope, there’s a possibility that you will watch a landslide happen.”

Landslides are reported in New Zealand year-round. What made this past winter so disastrous, Wolter says, was the record rainfall in July.

“That saturates the ground, makes it really wet and prone to landsliding.”

In Nelson, close to 40 homes are still red stickered after August’s flooding, with some hillside dwellings completely inundated with dirt. Slips in Wellington collapsed onto residential streets, leaving people’s cars buried.

The number of landslides reported annually has been increasing over the years.

“Twenty years ago, in Wellington itself, there were anywhere from 100 to 200 landslides reported a year, and now we’re up to 800 to 1000 landslides a year on average,” says Wolter.

Some of it can be attributed to an increase in reporting, but climatic changes are also to blame.

“What we’re seeing is an increase in extremes, so both an increase in dry days and extreme storms. And these extremes affect how stable the land is around us.”

Liam Wotherspoon, an associate professor in the department of civil and environmental engineering at the University of Auckland, says the damage caused by slips all comes back to the nature of our landscapes.

“As infrastructure networks have developed, the road then becomes quite a nice corridor for us to put other infrastructure along … power, telecommunications,” says Wotherspoon.

“And that’s for efficiency, but the downside can be if we have these sorts of land instabilities occurring, that it can take out quite a few networks at the same time.”

Being forced to cluster our critical infrastructure networks together can’t be helped when faced with New Zealand’s challenging topography. But it means there is often obstructive, expensive, time-consuming repair work going on in some of our most important roads.

“New Zealand is a beautiful country, and it’s a beautiful country because of how chaotic the landscapes are.

“The Manawatū Gorge is a good example of we can only go so far with what engineering approaches can do before we have to take a step back and think – is this actually the right approach? Is this someting we can manage moving forward?

“[With the Manawatū Gorge] Nature is telling us that we’re really not going to be able to create a road corridor going through here that’s going to be safe for the users and is going to be stable based on the geology that we’re dealing with.”

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