The Clues Hidden Within NZ’s Largest Urban Landslide

Wednesday, 12 June 2024, 5:34 pm
Article: Max Frethey - Local Democracy Reporter

New data from an active Nelson landslide is hoped to provide valuable insights into how to manage the risk of the hazard for councils and landowners across the country.

The Tāhunanui Slump is the country’s largest active urban landslide, with 120 homes built on it, and has shifted several times in recent history – most recently during the August 2022 storms.

“Ideally, it would be nice not to build homes on this,” said Chris Massey, an engineering geologist at GNS Science.

“But if you don't know where they are, then it's really difficult to avoid them.

“That's the point of the research that we're doing, to try to identify where these are, to hunt for them. Then if we at least know where they are, then we can try to try to avoid them.”

Groundwater, rainfall, and movement, both on the surface and at depth, are being monitored and analysed for patterns, and compared to years of historic data.

“We can use those patterns to look for those types of similar patterns elsewhere in New Zealand, and then help us identify where we might have similar types of landslides.”

The current project is funded by $240,000 from the Earthquake Commission (EQC) but leverages hundreds of thousands of dollars from other projects to amalgamate several different data sets into an expansive model.

The study is on top of the $300,000 of Government support Nelson City Council secured last year for monitoring of the slump over the next decade.

Draft reports are hoped to be complete by the end of the year, which will then be reviewed ahead of being finalised.

Nelson mayor Nick Smith said he was “hugely excited” about the Tāhunanui Slump study.

“There are 120 homes here, there are millions of dollars of infrastructure, and we've got to manage that on the site as best we can. We've got confidence that we can do so safely here, but the more science and information we have, the better we can manage those risks.”

The study could provide the council with better information on how to keep people safe, what sort of infrastructure to build, how to zone land, and how best to respond in an emergency.

The council is currently in the process of updating its planning rules, prohibiting development or introducing stronger regulations, for hazard-prone areas of the city.

“I think it is inevitable going into the future that there will be parts of Nelson that we have built on, that we shouldn't have built on, of which we will need to manage retreat,” Smith said.

In some cases, the council has already purchased properties that were too costly to repair following the August 2022 storm.

Wendy Saunders, EQC's champion of land use planning, said that development on the Tāhunanui Slump was one of many “legacy issues” around the country.

“We need to be able to deal with the existing management of the houses and the risks in places like this.”

Understanding landslides will be vital in the future because heavy rainfall, which is expected to increase due to climate change, can cause old landslides to “reactivate”.

“I think we will see more and more reactivation of these old landslides, which we may not even know are a problem at the moment, but may become a problem in the future.”

The information on how to manage risk could also provide insurance companies the confidence they need to continue insuring some properties at risk from landslides, she said.

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