GNS Science published a new report in January for Councils on landslides that noted they kill more people in New Zealand than earthquakes, tsunamis and volcanoes combined.
Nelson is particularly vulnerable due to our city being nestled amongst steep hills, the frequency of intensive rainfall events and the risk of a major earthquake from any of our eight seismic faults.
Our Council has been grappling over the past 18 months with fixing the damage from the 200 landslides triggered by the August ’22 storm. The damage to dozens of homes is still causing emotional and financial turmoil for many Nelsonians.
Council’s geotechnical work has also identified homes in areas such as Brook St that, although undamaged, are at significant risk from slips on Council land. The worst cases are where the cost of making them safe exceeds their value. We have proposed a pragmatic solution, with Government’s help, to buy out these properties.
The Tāhunanui Slump is NZ’s largest urban landslide and our most complex problem. It covers an area of 26ha overlooking Tāhunanui Beach and has been on the move for centuries. It has 140 homes, of which 45 were damaged in August ’22. We would not allow housing development in such a high-risk area with today’s knowledge but must now manage it as best we can. Sophisticated measuring equipment has been installed and Council has secured Government funding to enable its ongoing monitoring for the next decade.
My PhD in geotechnical engineering at Canterbury University researched the pivotal role water plays in triggering landslides. The big movements of the Tāhunanui Slump occurred in 1929, 1962, 2011 and 2022 in response to intense rainfall. Council is investing close to $10 million in improved stormwater systems on the Tāhunanui hills over the next six years. Work is under way this month on the latest stage in the most at-risk area of Grenville Tce and Moncrieff Ave, which is due for completion in May.
We can never completely remove the risk of landslides but Council’s approach of buying out the riskiest homes, investing in upgraded stormwater infrastructure and tightening the planning rules in unstable areas will make us more resilient.
