One Tree Hill terraces too steep to weather storm

University of Auckland geologist Professor Martin Brook led a team that investigated the shallow landslides that struck Maungakiekie – One Tree Hill – in Auckland Anniversary weekend floods on 27 January 2023.

“These landslides appear to be unprecedented, at least in living memory, so we thought it was worth investigating and documenting for the future,” Brook says.

The geologists found that terraces Māori built for Ngāti Awa chief Tītahi in the 1600s for a pa site that was occupied until 1822 had steepened the terrain, contributing to the landslides in 2023. All 19 landslides occurred on slopes of Maungakiekie with terracing, suggesting human modification was a key factor, Brook says.

“Māori designed the terraces well, because they lasted over 200 years, but the terraces were too steep for the amount of rainfall, which followed a very wet Summer.

“NIWA classified the storm as a one in 200-year event and the fact the terraces lasted over 200 years until that storm indicates they were right,” Brook says.

Given the free-draining nature of volcanic rock beneath and around Maungakiekie, Brook says it’s unlikely further landslides will occur there, unless such extreme rainfall strikes again, after weeks of wet weather.

Keeping grazing animals away from existing landslides and planting shrubs and grasses like vetiver on the maunga could help reduce the risk of future slips.

Larger trees can be blown over in strong winds, increasing the chances of landslides, Brook says.

The January 2023 storm caused $2.2 billion of damage, making it New Zealand’s costliest non-earthquake event ever, until Cyclone Gabrielle wiped out that record two weeks later.

On 27 January 2023, Auckland was inundated with more than 280mm of rain in 24 hours and 211mm in six hours during the storm. Slips also occurred on Mt Hobson and Mt Albert that weekend.

Four people died in the storm, one from a landslide in Remuera.

Brook says more than 600 people have been killed by non-seismic landslides in New Zealand since 1843, more than from any other natural hazard.

Large parts of New Zealand are at risk from landslides, he says.

“Much of the Auckland area is at high or moderate risk from landslides, according to a GNS (Institute of Geological and Nuclear Sciences) report.

“The risk increases when people clear-fell trees or build retaining walls that don’t meet the requirements of the building code, so they don’t have proper drainage behind them.

“If people let spouting on their roofs run down a slope, rather than going into the reticulated drainage system, that’s another risk factor," Brook says.