Cyclone Gabrielle: understanding the dynamics behind landslides

Damage from landslides typically exceeds $300 million  a year, but that cost will be an order of magnitude higher in 2023, thanks to the impact of Cyclone Gabrielle.

While they may have come as a shock to those who lost homes and livelihoods, many of the earth movements in the Gisborne region came as no surprise to School of Environment Associate Professor Martin Brook.

“They were in predictable areas, and they sped up as the slopes started moving more quickly during Cyclone Gabrielle.”

What’s more, Professor Brook says the resulting damage to some properties was also preventable because landslides were exacerbated by human factors such as clearing trees and vegetation off slopes and cutting into slopes for roads and residential subdivisions.

“The soils are weak and very sensitive to changes in moisture content, very much the last place you’d build houses – but that’s where people were building.”

The unstable land around Gisborne was very much on Brook's radar before Gabrielle struck, thanks to a four-year research project funded by the Earthquake Commission with ‘in-kind’ support from the Gisborne District Council.

“From a scientific standpoint, it actually gives you a really good opportunity to understand what is generating these landslides,” says Brook. “Then you can start unpacking; where have they occurred? What size are they? Are they all on north-facing slopes, for example?”

According to standard classifications, there are 32 different types of landslide which are categorised by factors such as their failure mechanism, the speed at which they move and whether they’re constrained by topography, which could ‘super elevate’ material out of a channel.

“Understanding the type of landslide is really important,” says Brook, which is why his research involves a range of techniques – from site visits to soil samples and remote sensing. Another key tool is LIDAR (Light Detection and Ranging) imaging, which produces high-resolution digital elevation models of land surfaces that are fed into predictive models.

Landslides are typically triggered by either earthquakes, which produced an estimated 40,000 landslide events when Kaikoura was struck in 2016, or heavy rainfall which can typically generate more than 10,000 landslides in a single major weather event.

Brook's research also draws on data from the European Space Agency’s Copernicus Sentinel-1 satellite which has a 12-day revisit time and provides raw data that needs further analysis with algorithms to make sense of it.

Slope movement can be calculated in millimetres per year, and across Europe, the so-called InSAR monitoring has revealed how far and fast slopes have moved over the past five years – something Brook says would be great for New Zealand.

“If we had something like that in New Zealand it would be fantastic. That’s something that GNS, the other Crown Research Institutes or MBIE or EQC need to do.”

In the case of Gisborne, 2023 has produced what’s known as MORLEs – Multiple Occurrence Regional Landslide Events – which Brook believes were related more to land use change than climate change.

“Land use change practices were causing a big problem in Gisborne, and it just makes the landscape less resilient when a big dump of rainfall comes along.”

Nevertheless, climate change is expected to have an increased influence on the frequency of landslides because of extreme weather conditions, which include rainfall intensity and rising temperatures.

In very dry summers, Brook says the clay-rich soils which are common around Gisborne and parts of Auckland are likely to shrink and crack with deep fissures which destabilise and weaken the soil – and allow rainfall to penetrate more deeply.

“It’s not just rainfall, but temperature extremes are probably an issue as well.”

Which is why events like Cyclone Gabrielle are so important in terms of understanding the dynamics behind landslides.

“It’s a range of factors; you’ve got to understand the soil and the rock properties. You’ve got to understand the shape of the landscape known as the geomorphology.”

Looking ahead, Brook is concerned about ‘legacy issues’ created by the Local Government Amendment Act 1981 which absolved local authorities for any civil liability arising from building on unstable land.

“That 1981 Act has left a legacy in New Zealand. There’s some really bad building and it’s a problem.”

In particular, he sees a need for realistic setback distances in building codes– like the rigid distances applied in the Californian city of Santa Cruz – which would provide greater protection for buildings at the top and toe of slopes when material is brought down in a landslide.

To that end, he says that the recent slope stability hazard maps and planning controls included in the Hutt City Council’s District Plan “sounds like a great idea”, although something “more nuanced” might be required in Auckland because of its varied geology.

However, Brook is disappointed that Auckland City Council’s ‘Making Space for Water’ initiative, announced in early 2023 as part of the city’s flood recovery program, doesn’t reference landslides in its flood management plan.

“Floods and landslides are two sides of the same coin in terms of a storm event,” he says. “It needs to be a bit more holistic, I would have thought.”

He also questions whether there’s a level playing field when it comes to red stickering properties after major weather events – especially given that it’s such an emotive issue and “potentially condemns somebody to a financial disaster.”

Decisions are currently being made by structural engineers, but Martin says that a team of people – including geotechnical engineers or geologists – should look beyond the structural integrity of a building and include the land around it.

“The accuracy and the efficacy of this whole stickering needs to get looked at, in my view.”

One initiative that he’d like to see is the establishment of a Central Office of Geotechnical Control along the same lines as that established in Hong Kong after the 1972 landsides where at least 156 people died when several apartment complexes and houses were wiped out.

“The events this year have already cost the public and private sector 12 billion dollars, so I think that spending a few million setting up a satellite monitoring service would not be a big deal.”