A new report shows Cyclone Gabrielle was one of the most extreme landslide-triggering events ever recorded globally.
RNZ can reveal the findings of an Earth Sciences New Zealand analysis of Cyclone Gabrielle landslides, which estimated more than 800,000 landslides were caused by the extreme weather event in 2023.
Cyclone Gabrielle killed 11 people nationwide and destroyed thousands of homes.
Several large scale investigations into the cyclone revealed major failings by local authorities and Civil Defence in Hawke's Bay.
Earth Sciences New Zealand engineering geologist Chris Massey told RNZ an extensive analysis of Cyclone Gabrielle landslides revealed an astonishing picture of its impact to the land, believed to be the worst of its kind in world history.
A gully of pine forestry near Gisborne that slipped in Cyclone Gabrielle. Photo: ALEXA COOK / RNZ
"It was shocking to see the huge areas affected, it went all the way from the Wairarapa to Hawke's Bay, Gisborne - then across to Auckland and Northland."
It took a large team of scientists months to analyse all the data.
"We think it's the largest number of landslides that's ever been mapped from a single storm event in the world."
Researchers identified landslide patterns, finding that because Auckland had been hit with flooding several weeks before Cyclone Gabrielle, larger and deeper landslides were activated there.
While in Hawke's Bay and Gisborne, the land was pockmarked with numerous shallow slips. Massey said in those areas the ground was so saturated by heavy rainfall, over 500mm in just 24 hours in some cases, that the huge number of landslides was inevitable.
The scarred hillsides of Tutira after Cyclone Gabrielle. Photo: ALEXA COOK / RNZ
"The sheer intensity of the rain seems to have overprinted the soil moisure conditions - meaning it didn't matter what the soil moisture levels were before Gabrielle," he said.
However, what did make a difference to if land moved or not, was if had vegetation growing on it.
Land with pasture and scrub was more susceptible to landslides, whereas areas covered in forest and bush were better off.
"The differences were pretty significant with up to 330 landslides per kilometre square in grassland and scrub, but those densities were much lower in indigenous and exotic forests," he said.
A number of landslides caused by Cyclone Gabrielle closed State Highway 2 between Napier and Wairoa for weeks. Photo: ALEXA COOK / RNZ
Although the type of forest growing on the land also made a difference, with native bush performing better than pine trees.
"It supports the concept that indigenous forest is good from a preventing landslide perspective," Massey said.
And with the intensity and frequency of weather events increasing due to climate change, he said this landslide mapping research is a valuable tool for future land use decisions.
"We're going to start losing larger portions of soil, which causes issues not just for agriculture, but for people living downstream of these places. It's not just the landslides, but the debris they generate and the cascading effect," he said.
The data sets would be used for forecasting landslides.
"We're saying 'hey look how many landslides we've got, let's use the models we can develop from these to help us anticipate where they may occur again'," he said.
He hoped the data set would be used by many people for many years to come.
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