10,000 Wellington homes could be damaged by landslides in big earthquake
Up to 10,000 Wellington homes could be damaged by landslides if a major earthquake strikes close to the city.
But while landslides may pose the greatest threat to those living in the capital when the big one hits, it could be years before residents know which slopes across the city pose the highest risk.
Brendon Bradley, deputy director at QuakeCoRE, New Zealand's centre for earthquake resilience, estimated 30,000 Wellington homes were on hilly ground, and up to 10,000 could expect damage from landslides triggered by quakes.
GNS Science figures also highlight the threat landslides pose, with 70 recorded across the Wellington region the day after the 7.8 magnitude Kaikoura earthquake in November, following heavy rain.
Bradley said the extent of damage would vary. "Some might be a bit of slumping under the house, causing the floor to tilt by two or three degrees. It won't put any lives in danger, but it will make the house not very habitable," he said.
"We saw after the Christchurch earthquakes that houses in that situation due to liquefaction actually had to be demolished."
In more serious situations, Wellington houses might drop substantially, or be hit by slips from above.
Bradley said the parts of Wellington's hilly terrain that are man-made were much more susceptible to landslides. This includes houses built on flattened hillside sections, and roads where the hillside had been cut into.
Learnings from the damage to Kaikoura's roads suggested Wellington's Ngauranga Gorge and Rimutaka Hill road were also at risk from landslides, with the size and clean up time dependant on the size of the quake.
With the majority of Wellington's hills being between 200 metres and 400m high, the slips would probably resemble the one seen in the suburb of Kingston after the remnants of Cyclone Debbie hit this month.
That slip was about 150m long and 40m wide, and forced the temporary evacuation of three homes.
New Zealand Transport Agency Wellington highways manager Neil Walker said the agency was investigating the stability of the Ngauranga Gorge, and expected to complete stabilisation improvements within two years.
Other roads around Wellington being looked at were State Highway 1 between Plimmerton and Paekakariki, SH2 over the Rimutaka Hill, and SH58 over Haywards Hill, he said.
Wellington City Council chief resilience officer Mike Mendonca said the council was keen to invest in solutions, like retaining walls, but needed to gather more information on at-risk slopes to figure out where best to spend its money.
"We are trying to pull those threads together at the moment so we can make some smart decisions, because we haven't got enough money to go around."
GNS engineering geologist Sally Dellow said earthquake-induced landslides were usually bigger than those caused by rain.
A research project was already under way looking at modified slopes in the Wellington region, she said.
Homeowners could expect slopes around them to be evaluated, but it would take extra funding of about $5 million for the work to be completed in four years.
"GNS Science has been working on this problem for many years, but only recently has the work started to be funded to the level required to deliver the detail needed to give information that can be acted upon."
HOW TO PROTECT YOUR HOME FROM SLIPS
* Drains are critical for avoiding saturation that leads to slips so make sure yours are cleaned out regularly and working properly. Make sure retaining walls have drains at their base and downpipes connected to stormwater systems.
* Check that retaining walls are straight and take photos to compare them over time. If a wall is moving or bulging, it needs attention.
* Take photos of all the cracks around your property – in the soil, in concrete paths and in driveways - before and after quakes or heavy rainfall, then compare them. Changes over time mean something is moving and needs to be checked.
* Stiff windows or doors can also be a sign of ground deformation over a long period.
* If in doubt, call a professional. Changes in drains or retaining walls, and ground or building cracks should be seen by drainlayers, plumbers, geotechnical engineers, or building inspectors.