14 February 2023
Landslides in North Island of New Zealand from Cyclone Gabrielle
Posted by Dave Petley
Landslides in North Island of New Zealand from Cyclone Gabrielle
As forecast, Cyclone Gabrielle has struck New Zealand, bringing very high rainfall totals to North Island. This comes on top of the recent heavy rainfall events in the area, which have already triggered many landslides. The Prime Minister has declared a National State of Emergency for only the third time, but the true picture remains a little unclear as areas are cut off with limited communications.
The northern part of New Zealand is having an extraordinary start to the year – remember, this is summer there. Met Service tweeted this graph of cumulative rainfall for Auckland Airport, including data for the years since 2019 and the long term average:
To help put 2023 in context for Aucklanders, here is the Airport rain accumulation plot (red line is 2023, showing a total of 540mm).
The Airport has seen about half (48%) of its annual rainfall in just 45 days. Drier weather for the coming days, https://t.co/tks2Q488UD ^GG pic.twitter.com/vyA1UOmNE7
— MetService (@MetService) February 14, 2023
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News of landslides is starting to appear – we can expect that there will have been many, many slips once again. There is information about a tragic landslide in Muriwai in Auckland, which has left one firefighter critically injured and another missing. As I write the search is ongoing, but the situation seems to be grim. It appears that there have been at least two landslides in Muriwai – this image shows the aftermath of another event:-
The aftermath of a landslide in Muriwai, Auckland following Cyclone Gabrielle. Image from NZ Herald.
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Meanwhile, another significant landslide has occurred in Karekare. There are reports that five houses have been destroyed, whilst Newsroom has this image of the aftermath:-
The aftermath of a landslide in Karekare in New Zealand, which destroyed five houses. Image from Newsroom.
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In the rural areas, it is likely that there will have been some damaging landslides. NZ Herald has tweeted this image of a large failure on the Awhitu Peninsula:-
A huge slip has emerged on the Awhitu Peninsula! pic.twitter.com/iK9DuSQu1b
— nzherald (@nzherald) February 14, 2023
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It is interesting that the crown of the landslide is at the road. Stuff also has a report of a landslide that partially buried a truck on the Matatā straights in the Bay of Plenty:-
The aftermath of a landslide in the Bay of Plenty in New Zealand, triggered by Cyclone Gabrielle. Image posted by Stuff.
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Stuff has a gallery of images of the aftermath of the storm, many of which show landslides.
Cyclone Gabrielle is now moving away from New Zealand, so the situation should start to improve overnight, but the damage is clearly very extensive.
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Readers may wish to note that the Muriwai landslides are tragic events, but not unprecedented. Heavy rain in August 1965 caused two highly mobile small (“debris avalanches”; Wright, 1966), destroying 2 houses at Domain Crescent, killing two people, with a 250m runout and speeds estimated as 90 kmph. Lawrie Wright (1966) wrote a short paper on this. Additionally, a news article a decade ago stated that the local authority said that no new houses would be built in the 1965 landslide runout track (https://www.stuff.co.nz/auckland/local-news/9041058/Old-memories-revisited). However, while this planning decision held for a while (historic imagery is available here: https://retrolens.co.nz/), several newer houses have since been built within the 1965 landslide footprint on Domain Crescent (e.g. numbers 57,59,36). Domain Crescent has been affected again in the Cyclone Gabrielle event, as well as other streets in Muriwai.
Ref:
Wright, LW (1966). The Muriwai debris-avalanche: some aspects of its form and genesis. New Zealand Geographer 22: 90–93.
The scarp of the Awhitu landslide being at the road isn’t terribly surprising – the crest of the hill is in front of the road and the ground falls away behind the road, and the roadbed has probably been compacted to be stronger than the surrounding soils, so the failure would begin in the soils weakened by the drainage soaking in.