A resident wades through a large landslide on Domain Crescent in Muriwai following Cyclone Gabrielle, February 14, 2023. (Photo: Phil Walter/Getty Images)
A resident wades through a large landslide on Domain Crescent in Muriwai following Cyclone Gabrielle, February 14, 2023. (Photo: Phil Walter/Getty Images)

ScienceFebruary 22, 2023

Landslides and law: The questions Cyclone Gabrielle raises about where we build

A resident wades through a large landslide on Domain Crescent in Muriwai following Cyclone Gabrielle, February 14, 2023. (Photo: Phil Walter/Getty Images)
A resident wades through a large landslide on Domain Crescent in Muriwai following Cyclone Gabrielle, February 14, 2023. (Photo: Phil Walter/Getty Images)

After Muriwai suffered a devastating landslide in 1965, it was judged too dangerous to build again on affected land. So what changed? Geologist Martin Brook tells the story.

Given the death toll, it’s important we consider the impacts of Cyclone Gabrielle sensitively. But we must also begin looking into the history of land-use and planning decisions in areas worst hit by landslides.

One such area is the beach community at Muriwai in West Auckland, where two volunteer firefighters were tragically killed in a landslide.

Several homes were cut off by slips. Residents on the steep terrain of Domain Crescent were told to evacuate on foot, rather than drive, because the land was so unstable.

Landslides can be a deadly hazard, but only when people are exposed to them. A landslide high in the Tararua mountain ranges is unlikely to pose a risk to anyone. But living near or within a landslide zone poses a clear risk.

This can be summarised as: risk = hazard x exposure x vulnerability.

Muriwai offers a case study of that equation. We already have a good understanding of the soils, landscape, geomorphology and exposure to landslide hazards – as well as the history of planning decisions that allowed houses to be built on land prone to slips.

An unstable history

Much of Muriwai, like other parts of Auckland’s west coast, is underlain by Kaihu Group sands. These are geologically young (Pleistocene age, less than 2.6 million years old) and form the high country around Muriwai.

The sands are weak and are poorly cemented, or completely uncemented, meaning there are “pore” spaces between the grains that are filled with air. During rainfall, water starts to fill these pore spaces.

Initially, this has a suction effect (negative pore pressure), whereby the water pulls the sand grains together, increasing strength. As water content increases, however, this negative pressure drops, and the sands fail and flow.

A good analogy is sand on a beach. If a little water is added, a steep-sided sand castle can be built. But if too much water is added, the castle collapses rapidly as a “flow-slide”.

A prominent geomorphological feature of Muriwai is an escarpment of soft Pleistocene Kaihu Group dune sands that forms the crenulated ridgeline immediately west of Oaia Road. These crenulations, or “embayments”, represent the headscarps (or source areas) of landslides.

The figure below is a digital elevation model (DEM) based on 2016 data gathered by the remote-sensing method LiDAR. This uses airborne laser scanning of the land surface, which removes vegetation and exposes the land surface “geomorphology” underneath.

This digital elevation model shows former landslides, roads and properties in Muriwai. (Image: Supplied)

Landslides are denoted as “L”. Houses on Domain Crescent and Motutara Road are at the foot of the escarpment, below landslide source areas. They are constructed on Kaihu sands, with some of the houses built on debris from former landslides.

Landslides and the law

In August 1965, following heavy rainfall, fatal landslides over 200 metres long occurred on consecutive days at the south-east end of Domain Crescent, destroying houses and killing two people. The landslide extent is denoted in red hash in the figure above.

A 1966 New Zealand Geographer article recorded that witnesses said the landslide moved at 90 kilometres per hour. Soon after, it was reported a Rodney District Council engineer had stated no new houses would be built on the 1965 landslide footprint. This held until the early 1980s, when gradual house construction began again.

The timing of this new construction (denoted by the yellow arrows in the figure below) is intriguing. In 1981, the Local Government Amendment Act (section 641A) allowed councils to issue building permits for houses on unstable land prone to erosion, subsidence, slippage or inundation. Councils were also absolved of any civil liability.

Images showing the extent of the 1965 landslide in Muriwai and where new homes were built two decades later. (Images: Supplied)

Concern about the effects of section 641A was highlighted in 1986 by highly respected engineers Nick Rogers and Don Taylor in a paper published in New Zealand Engineering magazine, titled “Safe as houses”. While the Building Act 1991 and 2004 have improved matters, we are still dealing with section 641A’s legacy.

The Earthquake Commission (EQC) Act in 1993 was an important step forward for natural disaster insurance. But it stipulated that compensation can be refused if a house was constructed on unstable land.

In the 1990s and early 2000s, the Rodney District (which includes Muriwai) was ranked first nationally in having EQC claims rejected on the basis that houses had been built on existing unstable ground. The then EQC chief executive, David Middleton ONZM, appeared on the TV show Fair Go explaining this.

Real and moral hazards

No amount of geotechnical expertise or planning control can produce absolutely zero risk. But communities should be able to assume potential hazards are identified and they are not exposed to them.

Geomorphological mapping of landforms using high-resolution LiDAR DEMs can prove useful in planning and decision-making, as well as landslide susceptibility mapping. This is where a range of parameters – slope angle, soil type, thickness, rock type, vegetation cover and land use – are layered on top of the DEM, within a geographical information system.

The parameters are statistically modelled and a landslide susceptibility map is produced. In many parts of New Zealand, this map will probably not bring news some homeowners and land developers want to hear.

But such a map can be useful for hazard zoning. As the tragic events in Muriwai have shown over the years, the set-back of buildings below slopes is sometimes just as important as set-back from cliff edges at the top of slopes.

Other mitigation strategies include real-time monitoring of risk either in-situ or by satellite. Ultimately, costly slope engineering can be a solution.

However, as Rogers and Taylor wrote in 1986, property owners are often willing to accept risk until the hazard eventuates. In other cases, a “moral hazard” exists where there aren’t incentives to guard against risk because of protection from its consequences by insurance or EQC coverage.

Unfortunately, this risk can also tragically extend to third parties. Whether such risk-taking behaviour continues after the Auckland floods and Cyclone Gabrielle remains to be seen.

But understanding landscape geomorphology and using it as the basis for more resilient planning so we can truly build back better, or undertake managed retreat, is now imperative

Martin Brook is associate professor of applied geology at the University of Auckland.

This article is republished from The Conversation under a Creative Commons licence. Read the original article.

Keep going!
A map of rainfall in NZ over two summers (Maps: Chris McDowall)
A map of rainfall in NZ over two summers (Maps: Chris McDowall)

ScienceFebruary 20, 2023

A climate tale of two summers

A map of rainfall in NZ over two summers (Maps: Chris McDowall)
A map of rainfall in NZ over two summers (Maps: Chris McDowall)

Three years ago, the North Island experienced droughts while the South Island flooded. This year, it’s the opposite. 

In January 2023, Auckland experienced its wettest month on record. The heaviest rains occurred on the evening of January 27 when the city received a deluge that overwhelmed stormwater systems and resulted in widespread flash flooding. Over 24 hours, the Albert Park weather station recorded 280 mm — more rain than it typically receives over an entire summer.

Cars floated down suburban streets. Already sodden hills slipped and collapsed into buildings. Homes and businesses filled with water. Rivers changed shape. Four people died. Countless pets and other animals were lost. Emergency responders were superhuman.

Flooding in Wynyard Quarter, Auckland. (Photo: Lynn Grieveson/Getty Images)

Cyclone Gabrielle arrived two weeks later. Once again Northland, Auckland, Coromandel and Bay of Plenty experienced flooding, this time accompanied by gale-force winds. Tairāwhiti and Hawke’s Bay suffered its worst effects and the true extent remains unclear. Half a million people without power. Ten thousand people displaced. The government declared the third national state of emergency in New Zealand’s history. Burst floodbanks, landslides, flooded homes, rooftop evacuations, communication outages and death upon death

One fucking disaster after another.

Throughout these events my mind keeps returning to a climate change documentary series I worked on with the Spinoff in 2020 called 100 Year Forecast. Specifically, I think about the opening lines of episode two, “Where New Zealand will get wetter and drier”.

“February 4th, 2020. Auckland is in the midst of a big, long dry. No rain has fallen for 21 consecutive days. The very same day in Fiordland so much rain is pouring from the sky a state of emergency is declared. […] One country; two hydrological extremes.”

In February 2020, Aotearoa experienced extreme contrasts between Te Ika-a-Māui North Island and Te Waipounamu South Island. The lower South Island had record-breaking rainfall. This led to a state of emergency declared in Fiordland and Southland, as flooding and landslides forced many to evacuate. This was the largest aerial evacuation in New Zealand’s history, with over 700 people evacuated from the affected areas.

While the south grappled with extreme weather conditions, much of the north faced severe drought. A week after the Fiordland and Southland emergency evacuations, the government declared drought in Northland and parts of Auckland. Months of low rainfall had led to water shortages and crop failure. The situation became so severe that the New Zealand Defence Force was sent to Northland towns to assist in drought relief efforts.

This map shows the north in drought and the south experiencing extremely wet conditions.

Extremely dry weather in the north; deluges and flooding in the south.

Three years on the situation has flipped. As Te Ika-a-Māui endures devastating rainfalls, much of Te Waipounamu has experienced record dry weather. During January 2023, 18 South Island locations observed record or near-record low rainfall totals.  Invercargill had its driest January since records began in 1900. Wānaka recorded just 4mm of rain for the whole month.


While climate change cannot be blamed for the existence of Cyclone Gabrielle or a prolonged absence of rain, the scientific consensus is that human warming intensifies these events. We are making everywhere slightly warmer. With every degree of warming, the atmosphere can hold more moisture. A bigger bucket means heavier downpours. But we do not know quite where that water will fall from year to year — just that the future is likely to hold more extreme events as we keep breaking temperature records.

I keep thinking about the contrasting summers of 2020 and 2023. And atmospheric science. And the land. And all those affected. And heartbreaking stories. And the nature of media coverage. And the state of our infrastructure. And the role of policy. And the future of insurance. And action and inaction. And injustice.

Round and round and round.

The complexity makes me realise that I have a cartoon vision of climate change in my head. Dim notions of buckets, greenhouses, thermostats and canaries in coal mines. Metaphors that I mistook for knowledge. They help, but they are not the thing.

Humans have a tendency to avoid uncomfortable truths, whether they’re about the environment, our mortality, or some other difficult aspect of life. The first time I seriously engaged with the prospect of climate change was as a geography undergraduate in the 1990s. Since then I have often worked with environmental scientists and science communicators, getting exposed to the science and implications of climate change.

A car submerged in floodwater near Napier, February 16, 2023. (Photo: STR/AFP via Getty Images)

Despite all those conversations, lectures, graphs and maps, I have not internalised what a changing climate means. I recognise difficult things will come to pass; yet I do not feel the scale and urgency of the issue. Not truly. Not enough.

Staring at maps of February 2020 and January 2023 as gale-force winds bend the oak outside and the radio shares stories of drowning livestock makes the need to reduce carbon emissions feel urgent. But when the sun comes out and the water recedes, urgency risks fading into complacency. It’s a tricky needle to thread. We need to find ways to collectively hold onto our situation's gravity, without becoming overwhelmed and giving up.

‘Like a thousand-piece jigsaw puzzle, each member is vital to the whole picture. Join today.’
Calum Henderson
— Production editor

Seeking guidance, I turned to the final episode of 100 Year Forecast. In the closing moments, environmentalist and iwi leader Mike Smith (Ngāpuhi, Ngāti Kahu) draws a comparison with the early Covid-19 response.

“Just imagine what would have happened if we had two years lead-in time to prepare for Covid-19 – how different things would have been. With climate change we do have that lead-in time. We do have the opportunity to plan for it. And we do have the opportunity to take action.”

These words give me hope. There will be more devastating environmental events. But there is still time to act and limit their severity. The actions needed are straightforward. Reduce your personal emissions. Reduce consumption and waste. Talk about climate change with the people in your life. Demand action from elected representatives.

There is still time to act. We should act.