New Zealand's largest urban landslide studied to 'hunt out' others

Researchers are building an advanced 3D model of the country's largest urban landslide to hunt out similar areas of instability across New Zealand.

The Tāhunanui Slump is located between Nelson Airport and the city and is prime real estate because of its extensive views of Tasman Bay.

It's a slow moving landslide of 30 hectares, or the equivalent of 30 rugby fields, and 40m deep. There are about 120 homes within the slump area.

Nelson Mayor Nick Smith said there had been five instances he was aware of when the landslide has been activated. "If we knew the risks that were here, 80 to 100 years ago, we wouldn't have built homes," he said.

GNS Science's Chris Massey says the slump has moved "tens of centimetres over a number of decades". He agreed while it didn't sound like a lot, it had a big impact on infrastructure.

"Even 10 centimetres or a few centimetres could cause cracks to develop and then, in a house, it causes quite a few problems."

In 2011, the landslide was activated and a resident told 1News of seeing it move: "We were watching our neighbour's property just slide down the hill, into their backyard."

Heavy rain in August 2022 activated it again, damaging a number of homes with some parts of the area still not repaired almost two years on.

GNS Science, the Earthquake Commission and others are working to build a 3D model of the Tāhunanui Slump to identify similar areas.

"We can then use that information and the patterns of movement to then hunt for similar landslides elsewhere in New Zealand," Massey said.

They would collate information such as borehole data, rainfall data and others to eventually input it all into software that could help predict what would happen to the area during the likes of storms and earthquakes.

The researchers were confident there would be other sites around the country they were not yet aware of.

"With all the recent weather events and with more intensive rainfall expected with climate change I think we'll see more and more reactivation of these old landslides."

Smith said the information would also be helpful for the council's planning and in future weather events.

"We're hoping to get from this GNS report a clearer idea of the scale of rainfall events that will trigger movements, that in the worst case scenarios enables us to be able to manage Civil Defence and make good decisions about when to evacuate."