A landslide has crashed onto Cathedral Cove. Video / Chris Southwick
An Auckland Council engineer says recent storms have left roughly 1000 landslides impacting properties and infrastructure, as a geologist calls for a priority assessment of the city’s current risk.
Nearly 50 Auckland properties have been red-stickered and more than 100 yellow-stickered in the cyclone’s aftermath, on top of the 250 and 800 respectively red and yellow stickered from the January floods.
Access to parts of the West Coast including Muriwai, Piha, Te Henga (Bethells Beach) and Karekare remained restricted due to slips, with 120 teams of council assessors carrying out rapid building and welfare checks around the region.
About 20 homes were evacuated in Piha last night due to land instability, while an exclusion zone was in place in a large part of Muriwai.
“If you have been evacuated, please do not go back to your property until you are advised it is safe to do so,” Auckland Emergency Management deputy controller Rachel Kelleher said.
“Please remember that the ground is saturated and there is a very real risk of further slips.”
Auckland Council’s engineering resilience head Ross Roberts estimated the past few storms had created some 3000 to 5000 landslides around the region, with roughly 1000 impacting properties and infrastructure.
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.
Demolition is carried out on a Shore Rd, Remuera, home hit by landslide in last month's storm. Photo / Alex Burton
Later, amid the worst of the cyclone on Monday night, volunteer firefighter Dave van Zwanenberg died in a slip while helping Muriwai residents evacuate.
Associate Professor Martin Brook, of the University of Auckland’s School of Environment, noted that area had a history of landslides, and was alarmed to see some sections apparently sitting in the old path of a slip that killed two people in 1965.
Brook suspected this was a much wider problem across Auckland, yet, with a lack of data, the extent of the Auckland region’s wider risk to landslide remained unclear.
“Obviously recovery is the immediate priority, but we do need to build more resilience to landslides as soon as possible – and carrying out a lot of susceptibility mapping should be part of that.”
That included building new region-wide maps based from high quality LiDAR digital elevation models, along with Interferometric Synthetic Aperture Radar (InSAR), which used satellite data to measure millimetre-level shifts in surface movement.
“Ideally, this is the sort of thing that needs doing before the next wet season.”
For some slopes that had already come down, Brook said the soil and rock might now be at “residual” rather than peak strength, making them more susceptible to failure in future storms.
While there’d been some engineering work to suggest that residual strength of Auckland’s common Waitemata Group soils wasn’t too far below peak strength, the case was very different for the region’s weaker “quick clays”.
University of Canterbury geomorphologist Professor Tim Davies agreed the recent storms would have changed Auckland’s landslide risk profile.
“The intense rain from Gabrielle, on top of the high antecedent moisture status from record rainfall the previous week, caused many failures that might otherwise not have occurred for many years,” he said.
“Once ground moisture content has reverted to normal levels – which may take weeks – the risks will reduce again.”
A landslide at a property in Piha, which was left cut off after ex-tropical cyclone Gabrielle caused extensive damage. Photo / George Heard
The fact many areas had slipped this week might mean more long-term weathering would be needed to “pre-condition” slopes for further failures, Davies said.
“On the other hand, the slips this week might have destabilised adjacent areas and made them more likely to fail in future storms.”
Roberts said it’d been reassuring to see that many of the landslides that had formed in Auckland’s first big storm hadn’t changed significantly in the second.
“We were quite worried about that.”
He acknowledged Auckland didn’t have a central data repository for landslide risk – namely because it was a challenging area to model – but the council had a work programme underway to better understand the picture.
That included a LiDAR survey of the region’s entire coastline, with the first phase now complete.
Coastal hazards expert Professor Mark Dickson said the latest storm’s impact on the region’s coastal cliffs hadn’t yet been assessed, and in the meantime, he urged people to keep away from cliff faces.
“Any areas where the cliff has been undermined by erosion during the cyclone will have increased risk of future landsliding.”
Dickson and University of Auckland colleagues were working to obtain satellite imagery that would reveal the extent of erosion along stretches of several eastern parts of the North Island.
“Locally in Auckland, we have observed up to 5m coastal erosion at Orewa, and will be ascertaining the extent of cliff erosion at Rothesay Bay with a laser scan over the next few days,” he said.
“More exposed locations are likely to have eroded considerably further than the sites we have been able to observe so far.”
The very end of Auckland's Stanley Point, where major landslides occurred on January 27. Photo / Michael Craig
Fellow coastal hazards expert Dr Emma Ryan said many east coast beaches around northern New Zealand had been in an eroded state even before Gabrielle hit, after a summer of easterly swell events and storms.
“So far, the biggest concerns seem to be around access to beaches, loss of dune barriers and ongoing vulnerability of already compromised dune systems to future storms over the next several months.”
Further afield, in hard-hit Tairāwhiti, experts predicted Gabrielle would leave a long legacy for local erosion problems, just as Cyclone Bola had more than three decades before.
Brook said the cyclone had left large amounts of sediment deposited on slopes, that would likely be reworked in big downpours over coming years and decades.
He felt it ironic that forestry slash from the radiata pines, once planted to provide stability after Bola, were now causing major headaches for the East Coast.
University of Auckland river scientist Dr Jon Tunnicliffe said that the region’s rivers were again revealing the long-term impacts of land use there – namely the clearing of vegetation from terrain sensitive to big rains.
