The closed section of Day's Track in the Tāhunanui slump zone, Nelson. Photo: RNZ / Samantha Gee
Geologists and engineers are conducting research on Nelson's Tāhunanui slump, the largest urban landslide in the country, to better understand potential failures of other complex slopes across New Zealand.
GNS Science engineering geologist Dr Chris Massey said it was a valuable opportunity to understand how earthquakes and rainfall interact to trigger slope movement.
The 26-hectare slump is a slow moving, active slip on the Tāhunanui hills which was last activated by heavy rain during the August 2022 storm.
Massey said the research data would be collated to create a 3D model of the slump and once complete, it would be calibrated with historical data which could be used to forecast slope movements in future events.
GNS Science engineering geologist Dr Chris Massey at the Tāhunanui slump, Nelson. Photo: RNZ / Samantha Gee
Boreholes have been drilled across the hillside with instruments used to monitor groundwater and movement at depth, gauges on the surface measure rainfall and surface movements are monitored by satellite.
"The idea is that we can take the information from this landslide, because it has been so well-documented over the years, and we can then use that and the patterns of movement to hunt for similar landslides elsewhere."
There was data showing the slump's movement dating back to when it was first identified in the 1890s, Massey said.
Stability problems were encountered on the Tāhunanui Hills during the construction of Rocks Road in the 1890s. Several landslides then occurred in the area after the 1929 Murchison earthquake, then after further earthquakes in 1962 and during heavy rain in 1995 and 2011.
"Now we have got all this instrumentation we can actually link the cause with the movement and that is really rare, because it can be quite difficult to get those data sets together," Massey said.
Grenville Terrace in the Tāhunanui slump zone, Nelson, part of the country's largest urban landslide. Photo: RNZ / Samantha Gee
It was hoped the research could be used to help avoid building new infrastructure and homes in areas prone to landslides, he said.
The data is currently being collated, with hopes the first draft of the model will be complete by the end of the year.
"If you don't know where these landslides are then it is very difficult to avoid them and that is the point of the research we are doing, to try and identify them so we can avoid them because they are costly."
Earthquake Commission (EQC) Toka Tū Ake's champion of land use planning Dr Wendy Saunders said there were likely to be other landslides across New Zealand similar to the Tāhunanui slump.
EQC Toka Tū Ake's champion of land use planning Dr Wendy Saunders. Photo: RNZ / Samantha Gee
"With all the recent weather events and with more intensive rainfall expected with climate change, 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."
Last year, EQC launched a natural hazards portal, which provides details of natural hazard risks in communities as well as settled EQCover claims on residential properties and land the country.
Saunders said it was important landowners living in areas like the Tāhunanui hills had access to good information, like this research, to make informed decisions about risk.
Nelson Mayor Nick Smith said had the risks in the slump zone been known, 100 years ago, it would not have been built on. But the city's hilly terrain and growing population meant it was not feasible to simply prevent building homes on hillsides.
"The Tāhunanui hills are a gorgeous spot and a wonderful place to live, we just have to face the reality it has this risk of landslide movement... there are 120 homes here, there are millions of dollars of infrastructure and we have to manage that as best we can."
The council was reviewing the planning rules in parts of the city which were prone to landslides, with development prohibited in some areas, he said.
Nelson Mayor Nick Smith. Photo: RNZ / Samantha Gee
Smith was confident the risks posed by the slump could be managed safely, but said the more information council had, the better.
"It is going to move again, we just have to make sure we have the best science to manage that movement.
"We're hoping this research report will further help us to make good planning decisions into the future about where we allow people to build and where we don't."
The research is funded by several agencies, with $240,000 from EQC Toka Tū Ake and $300,000 from Nelson City Council (part of the government's $12.3m storm recovery package) along with contributions from Geonet and the Ministry of Business, Innovation and Employment's Hazard and Risk Management Strategic Science Investment Fund.