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Image: Archi Banal
Image: Archi Banal

OPINIONScienceFebruary 27, 2023

For safe places to live, look to the land

Image: Archi Banal
Image: Archi Banal

Intelligent land use starts with the make-up of the land, and considers timeframes longer than living memory, writes geologist Ursula Cochran.

I’ve seen this destruction before. Napier, Wairoa, Gisborne completely cut off. Pōrangahau, Tutira, Ruatoria – no word from small towns across the North Island’s east coast. No water, no power, no comms, no road access. Other major cities severely compromised at the same time. People dead and missing. Not in real life. Not in a nightmare. But in my best attempt to imagine the impacts of a magnitude 9 earthquake on the Hikurangi subduction zone. 

In a subduction earthquake, flooding will come as a tsunami instead of heavy rain, damage will come from ground shaking rather than water-borne debris – there are differences of course – but Cyclone Gabrielle has painted a strikingly accurate picture of the magnitude of chaos. Devastatingly, it’s not a picture, it’s real life. 

Now is not the time to be anticipating a megathrust earthquake in New Zealand – we’re in emergency response mode. But now is definitely the time to be stacking up reasons for taking a radically different approach to rebuilding our towns. 

Earthquakes may have been the biggest, scariest natural hazard prospect for driving improved resilience across the country, but extreme weather events are now a bigger challenge because of their climate-change-induced increasing frequency and intensity. The good news is that by proactively managing the likely impacts of climate change, we also improve resilience to earthquakes and other hazards. In fact, if we want to reduce risk rather than react to it, we must take a multi-hazard approach to solutions.

Let this be our opportunity. Our chance to step back and think about where it’s sensible to live in this country. I believe in engineering and its ability to keep us safe, but as a geologist, I know there’s a simpler, more fundamental consideration before we turn to human intervention. 

Look to the land itself – it has all the answers. Flat land on a valley floor is a flood plain. No matter how small the river is most of the time, it will be capable of filling the valley wall-to-wall occasionally because that is how the valley formed. Steep cliffs or banks are hills that have been cut into from below by the sea or rivers or faults (or by people to make roads). They will erode slowly and occasionally collapse in landslides because that is how they were formed. Wetlands or estuaries that have been drained will liquefy in earthquakes because they were made from wet mud. Storm ridges above a beach will be inundated by the sea occasionally because that is how they were formed. Rock type and shape of the land should be our primary guide for where and how to live.

There’s a long legacy of poor land use to overcome, but the early history had innocent beginnings. Mana whenua and early settlers needed to live close to sheltered harbours and sources of water and kai. As settlements grew, it made sense to expand out from the original positions. There is no one to blame for the sometimes-treacherous locations of our towns and cities. 

Gisborne
Flood waters rose more than 1m past Cyclone Bola inside this Gisborne home. (Photo: Amy Spence)

What is unforgivable in recent times is the approval for new buildings to go up on land known to be prone to floods, sea level rise, landslides, liquefaction, tsunamis and fault rupture. How is this happening?! Yes, we have a shortage of housing, but we also have ample land that hasn’t been formed by geological calamities. 

Intelligent land use starts with the make-up of the land, and considers timeframes longer than living memory. River valleys are perfect places for farming and horticulture. Clifftops and beach ridges are great for walkways and lookouts. But do we want permanent dwellings in harm’s way? Do we really want people subjected to the danger and heartache we’ve witnessed in recent weeks? 

Ground can be improved. Structures can be strengthened. Stopbanks can be built higher. We can stay in the same places if we invest in appropriate engineering solutions. But the destruction wrought by Cyclone Gabrielle means that some settlements will be almost starting from scratch. Isn’t that the best time to rethink? Let’s give our rivers more space, keep away from landslide-prone hills and rebuild around the safer edges of existing settlements. Visionary leadership will be required, as will hard work to figure out the financially-viable, legal, community-enhancing and equitable way to do this, but people are worth it. 

As we did geological fieldwork in peaceful Hawke’s Bay valleys full of orange orchards and asparagus fields, we tried to imagine the chaos we were finding evidence for. Sediments beneath Ahuriri (Napier), Pakuratahi (next to Esk Valley), and Wairoa Lagoons show a history of uplift, subsidence, tsunamis and floods. When sites further south were also investigated, it became clear that a great earthquake was a rare but realistic possibility for the North Island’s east coast. 

East Coast Lab was vital for translating this science into action. Hikurangi Response Planning was born. Deaths and injuries are anticipated to be in the thousands for a magnitude 8.9 earthquake, but otherwise the plan outlines a similar situation to what is before us – extensive damage to buildings, water supply, transport networks, power, communications, medical facilities, marae, emergency coordination centres, farmland and businesses. This plan demonstrates how much overlap there is in the devastation inflicted by different hazards, strengthening the case for an all-hazards approach to future mitigation measures. 

Hawke’s Bay and Tairāwhiti civil defence emergency management groups, hapū of Ngāti Kahungunu, Ngāti Porou, essential service providers, many businesses, community groups and individuals will have planned for an event as extreme as Cyclone Gabrielle. They are doing an exceptional job responding in incredibly challenging circumstances. 

Rooftop rescues by the Air Force in Esk Valley, Napier (Photo: Royal New Zealand Air Force)

Remember, though, we are all part of the emergency response. In any disaster zone, the best first responders are those already on the ground – you and your neighbours. Emergency management teams within councils are tiny. They rely on trained volunteers and councillors to help in an emergency, and they call in support from other agencies. These responders need time to mobilise and gain access in difficult circumstances. There is no magical fleet to arrive at your door immediately. There is no silver bullet technology. Locals helping locals is usually the only way – just as we have seen in such brave and innovative ways these last few weeks.

Hence the need to know your hazards and be prepared. Then you can self-evacuate in time, have basic needs met in the first weeks after an emergency, help others or be helped before officials arrive. Although nearly 90% of New Zealanders know it’s their responsibility to look after themselves and their family in a disaster, only 13% of people have taken all the recommended preparedness actions. If you find it daunting preparing for the worst, do it with others, imagine you’re packing for a remote camping trip, and you’ll end up with the right kit.

While personal and community preparedness is essential, there are weak links higher up the chain. At the top of the chain, of course, is the need to halt climate change. Further down the chain is the need to avoid the impacts of climate change and other natural hazards – there are limited options here but building on safe land is a crucial starting point. Then comes minimising or controlling the impacts – this is where engineering (of buildings, infrastructure, protection measures) has a big role to play. We also need more time and resources put into preparedness at central and local government level. But let’s not go too much further down the chain or we get to accepting the impacts and finding ourselves stuck in the cycle of emergency response we’re tired of.

To save ourselves from the trials of the bottom of the chain, New Zealand needs an urgent switch to prioritising work at the top of the chain. The current restructure of the Resource Management Act has the power to make this transformation if natural hazard risk is considered collectively and prioritised appropriately. The prime minister is sounding determined to “build back better”. So right now, in the immediate rebuilding from recent events, let’s look to the land for safe places to live.


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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.