Hawke's Bay's Thomsen family rise from the mud after Cyclone Gabrielle.
Cyclone Gabrielle caused extensive shallow landslides in Hawke’s Bay, leaving significant scars on the landscape.
Dr Ashton Eaves attributes the landslides to slope and land cover, with heavy rainfall and weather accelerating the process.
Hawke’s Bay Regional Council says approximately 252,000ha of Hawke’s Bay hill country has been identified as having a high risk of erosion.
Hawke’s Bay’s scenic rolling hills are one of the most visually charming landscapes in New Zealand.
Painted by artists like Rita Angus and Freeman White, the fruit bowl of New Zealand consists of a central belt of flat land flanked by hills and ranges on either side slowly sloping offinto the Pacific Ocean.
However, where immaculate green and brown rolling hillsides once stood, large pockmarks now blight the once-spotless landscape.
It’s a legacy of what the region went through when Cyclone Gabrielle hit two years ago.
Hawke’s Bay Regional Council senior scientist Dr Ashton Eaves says the scars are actually shallow landslides.
Hills on farmland near Ōtāne are still pockmarked by landslips caused by Cyclone Gabrielle two years on. Photo / Julian Aitken
“‘Scars’ I guess is a bit more of a colloquial term, because people don’t like to see them and so they associate it with a scar,” he said.
A similar sight was seen on the hills northwest of Napier after Cyclone Bola hit the region in 1988.
Much of the land affected at that time was steep hills cleared for farmland. As the soil became sodden, upper layers separated from lower layers and slid down the slopes.
Much of the soil affected during Bola was derived from soft mudstone rocks, which are particularly vulnerable to slips following heavy rain.
In September 2023, GeoNet said it had mapped over 140,000 landslides around the North Island caused by Gabrielle, calling it “potentially the largest landslide-triggering event on record in Aotearoa”.
Eaves says while it was to some degree a natural process, the usage of the land also plays a large factor.
“Particularly in sheep and beef farmland,” Eaves said.
“That particular land use, they’ll have pasture rather than indigenous forest, for example.
“But that natural process is accelerated because you have a massive event like this – even native forest goes.”
Eaves says the cause of the landslides comes down to two main factors.
“Fundamentally, slope is going to be your main driver, as well as land cover,” he said.
“But there’s a certain percentage that just goes because it can’t tolerate a certain amount of rainfall at that slope.”
Eaves says as soon as the pool of water from the rainfall exceeds the shear stress of the soil, it will then slide back.
What Eaves remains uncertain about is just how long these scars will take to heal, calling it the “million-dollar question” because of how steep some of the slopes are.
“On these affected bits of land, you’re talking [a] 20, 30 to 50, 60, 70-degree slope ... chances are that it’s going to take decades to 100 years, if at all, to patch up.”
Eaves says if a cyclone hits the region every 30 to 40 years, the slips will continue activating because the development of the soil can be slower than the cycle of the cyclones or weather events.
“That’s where you end up with this issue of: How do we mitigate it in the first place?” he said.
Hawke’s Bay Regional Council says approximately 252,000ha of Hawke’s Bay hill country has been identified as having a high risk of erosion. It estimates this land produces on average 3,272,686 tonnes of sediment that flows into the region’s waterways every year.
The council recommends identifying areas of erosion-prone land, planting trees, as well as fencing and planting on retired marginal land. The council offers poplar and willow poles for sale.
Jack Riddell is a multimedia journalist with Hawke’s Bay Today and spent the last 15 years working in radio and media in Auckland, London, Berlin, and Napier. He reports on all stories relevant to residents of the region.
