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Landslides and law: Cyclone Gabrielle raises serious questions about where we’ve been allowed to build
Opinion – 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.
Firefighters lower flag in remembrance of their colleagues who were killed. Photo: RNZ / Felix Walton
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 home hit by a landslip in Muriwai. Photo: RNZ / Tom Taylor
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.
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.
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 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.
Homes in Muriwai were destroyed. Photo: Supplied / Jacco Foote
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 geo-technical 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 an Associate Professor of Applied Geology at the University of Auckland
SOURCE RNZ
The chief executive of the Parliamentary Service is confident there will be enough room to accommodate the increased number of MPs and their staff coming into Parliament.
On current numbers, up to 131 MPs could be sitting in the Chamber following the election, brought on by a record overhang result.
Clerk of the House of Representatives Dr David Wilson told the Spinoff that currently the debating chamber had enough chairs to seat 124 MPs – including the Speaker of the House – and work was being done to accommodate more by altering the seating arrangement.
In a statement, Parliamentary Service chief executive Rafael Gonzalez-Montero said planning for a larger Parliament had been going for “some time”.
“Now that the election results have confirmed an increase to the number of members we are looking at several options.
“We are confident we will be able to accommodate all members within the Chamber.”
VNP / Phil Smith
Gonzalez-Montero said office accommodation for members and their staff would be “stretched” for a short time, but the new Members’ Building was only a few months away from “practical completion”.
That building – located behind Parliament – was due to be ready to occupy in April next year and had capacity to accommodate around 30 backbench MPs.
“All members should be in fit-for-purpose accommodation from that point,” Gonzalez-Montero said.
Following the 2023 election, 123 MPs sat in the Chamber – a record number at the time. That decreased to 122 following the resignation of National MP Judith Collins.
Collins held the seat of Papakura and stepped down in May and Parliament voted against holding a by-election because it was too close to the election.
The election result also meant high-ranking National Party MPs like Chris Bishop, Paul Goldsmith, Nicola Willis and Gerry Brownlee – Speaker of the House and an MP for 30 years – would likely not make it back into Parliament.
Only resigning or retiring MPs were permitted to give a valedictory speech meaning none of them would get the chance to give a valedictory statement.
Asked if their would be special allowances for valedictories, Wilson said in a statement that normally only MPs were allowed to speak in the House but the House could suspend its own rules or temporarily alter them.
“It could do so to allow former members to address the House.”
Wilson said no such arrangements could be made until the next Parliament opened and members sworn in.
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