This Mangawhai home was destroyed in a landslide caused by torrential rain that fell north of Auckland on February 24. Wreaking an estimated $300m in damage each year, landslides are one of the country’s biggest natural hazards. Photo / Supplied
This Mangawhai home was destroyed in a landslide caused by torrential rain that fell north of Auckland on February 24. Wreaking an estimated $300m in damage each year, landslides are one of the country’s biggest natural hazards. Photo / Supplied
New Zealand’s landslide risk could jump significantly under a worst-case climate change scenario, a first-of-its-kind analysis shows.
Wreaking an estimated $300 million in damage each year, landslides are one of the country’s biggest natural hazards – as shown by the thousands of slips that came down in Auckland alone over our extreme summer, leaving hundreds of properties condemned.
In some of the most serious recent incidents, Remuera resident Dave Lennard was killed when a landslide crashed into his Shore Rd home on January 27, while several people were injured inside a collapsed seaside bach at Manukau Heads on February 1.
Further south, February’s Cyclone Gabrielle left hill country dramatically scarred by slips across the North Island’s East Coast, where big deluges have grown up to four times more common over recent decades, and now drop up to 30 per cent more rain.
Such was the vast number of landslides that, months later, scientists were still working to map them in detail.
Now, a just-published study has shed more light on the potential for increasing extreme downpours to trigger more slips in our rural landscapes.
Its lead author, Hugh Smith of Manaaki-Whenua Landcare Research, said our previous understanding of this rainfall-driven risk had been limited.
Masses of slips tore their way across hill country in Hawke's Bay, in the days immediately after Cyclone Gabrielle. Photo / RNZ / Sally Murphy
“We knew that landslides tend to occur when storm rainfall exceeds 125mm to 200 mm in 48 hours, but the number of landslides can vary greatly depending on rainfall and landscape factors,” he said.
“We also had little information on the relative importance of rainfall versus landscape factors for triggering landslides within storm-affected areas.”
By analysing freshly-collected data, he said, we could pinpoint those factors that influenced susceptibility – and also quantify just how many landslides might be expected under different rainfall scenarios.
In their study, Smith and his colleagues gathered high-resolution satellite imagery of large areas of the upper North Island that were hit by storm-triggered landslides over 2017 and 2018.
They then compared the imagery with a trove of weather radar data captured by MetService.
After calibrating that data with measurements from various gauges across the study area, the team built a sophisticated new statistical model, enabling them to run different scenarios and tease out spatial patterns.
While it was well known that landslide risk was higher on steep slopes in areas of pasture-covered, weak sedimentary rock – much like in the East Coast – the analysis went further by over-laying a range of rainfall factors.
The most important of those were found to be the maximum intensity of 12-hour bursts of rainfall – and also how much had dropped over the 10 days before the storm.
“Prior rainfall increases soil moisture, which, combined with sustained high rainfall during the storm, leads to saturation and the increased likelihood of slope failure,” Smith said.
“We also observed a distinct step-change in the spatial density of landslides - or the number of landslides per square kilometre - when rainfall exceeded a critical threshold for pastoral areas on weak sedimentary rocks.
“Once the 12-hour rainfall intensity exceeded the intensity corresponding to the 10-year recurrence interval by 25 per cent or more, the mean landslide density increased more than three-fold, compared to lower rainfall intensities.”
This major slip came down last week on the rural State Highway 75, the Christchurch-Akaroa Rd. Photo / George Heard
Those levels of extreme rainfall happened to be consistent with what Niwa had projected, under the highest levels of warming in New Zealand, by late century.
“This suggests the potential for a significant increase in the number of landslides per storm, under this worst-case scenario.”
Meanwhile, the modelling also showed how forests could help lower the risk, regardless of the types of landscape or rainfall levels involved.
“This finding provides further evidence for the use of targeted tree planting to effectively reduce landslide susceptibility.”
The study, published in the journal Geomorphology, comes as crews were working to clear more than two dozen slips that came down along the East Coast during the region’s latest big storm.
Gisborne District Council principal scientist Dr Murry Cave said the rain left the area highly saturated and the land like “porridge” - and urged people to let the council know if their properties had been hit by slips.
