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Landslides

by  Michael J. Crozier

Sweeping boulders, trees, houses and people before it, a landslide can thunder down with terrifying speed. Mountainous New Zealand, already prone to the triggering factors of earthquakes and rainstorms, is vulnerable to these dramatic and destructive events.


Gravity always wins

Landslides are a natural process that removes material from hills, mountains, and coastlines, gradually lowering and flattening the topography.

Unstable land

Compared to many other countries, New Zealand has a high number of landslides. As mountain ranges are still being uplifted, they have high relief and steep slopes. Rocks are often weakened by folding and faulting, and there is continual earthquake activity. Away from the mountains, much of the land is hill country, formed by rivers cutting into soft clay rocks. These unstable slopes are often covered by weak soils derived from volcanic ash or loess. New Zealand can also experience high rainfall.

Slips

New Zealanders often use the term slip, meaning landslide. Weather forecasters warning of heavy rain and radio announcers giving road reports, often make reference to ‘slips being likely’, ‘slips closing roads’ and ‘roads re-opening after slips were cleared away’.

River sediment

Much of the sediment entering the headwaters of New Zealand’s rivers comes from landslides. Even in the downstream reaches of large rivers, landslide material forms much of the sediment. During Cyclone Bola, which hit the East Coast in March 1988, landslides produced over 20 million tonnes or 64% of the sediment of the Waipāoa River. Over the long term however, landslides contribute about 10–20% of the sediment load.

Landslide types

Some landslides displace whole mountainsides, taking millions of cubic metres of material, at up to 200 kilometres per hour. Others are shallow or slow, moving only a few centimetres a year.

Landslides are classified by the material (rock, debris, or earth) and the movement (topple, fall, planar slide, rotational slide or slump, flow, lateral spread). Some move other materials, in a variety of ways (complex landslides). All types occur in New Zealand.

Causes

On unstable slopes, three things are important in producing significant landslides around the country:

  • Pre-conditions, such as weak rock and steep slopes.
  • Preparatory factors – processes that further weaken the slope, such as deforestation or undercutting by streams.
  • Triggering factors. In New Zealand the most common are intense rainstorms (100 millimetres or more in 24 hours) and large earthquakes (magnitude 7 or greater).

Human impact

Deforestation has had the most dramatic impact on the stability of hillsides. Initiated by Māori, it was greatly accelerated by Europeans burning the bush for farming, particularly between 1880 and 1920.

European farming practices, which converted bush to pasture, have increased landslide activity by about seven times its natural rate in hill country with soft rocks. Road construction and subdivision earthworks can also destabilise slopes and increase landslide rates.


Alpine, low mountain and layered rock landslides

Alpine landslides

In alpine terrain made up of hard rock and steep, high slopes, landslides range from huge rock avalanches that can shape mountain peaks and travel several kilometres in minutes, to massive rock slides, rock falls and debris flows.

It can be difficult to distinguish between rock avalanches, and ice or snow avalanches. Most avalanches originate as falling snow or ice, which can include rock and other debris. Avalanches of rock can also pick up ice and snow. The avalanches mentioned here originate as falling rock and are characterised by rapid, turbulent movement.

How are the mighty fallen

The Aoraki/Mt Cook rock avalanche of 14 December 1991 was a classic example of an alpine landslide. Twelve million cubic metres of the east face of the mountain dropped away, lowering the summit by 10 metres.

In volcanic mountains, a debris flow known as a lahar can be generated from rapidly melting snow or a crater lake bursting. This highly charged flow of sediment and water can carry large blocks at high speed for many kilometres down a river bed.

Many alpine landslides are considered catastrophic because of their volume, speed and distance. In populated mountain regions of the world, they are responsible for widespread death and destruction. New Zealand’s mountain lands are largely uninhabited except for a few tourist locations, and so represent more of a potential hazard than a current threat.

Low mountain landslides

In lower mountain ranges and hard-rock hill country, a range of landslides from small wedge failures to large, deep-seated rotational and planar slides occur. Some are instantaneous, while others move gradually over long periods.

In Central Otago the Cromwell Gorge landslides, investigated during the construction of the Clyde dam in the 1980s, were slow moving, causing the hill slope to sag. Such processes are often controlled by structures within the rock, and by the rate at which rivers are eroding the base of the slope. While these landslides are not an immediate threat to life, they are long-lived and can gradually destroy roads and structures, and block rivers.

Layered rock landslides

Layered rock landslides are controlled by the way different rock types rest on each other and their direction and degree of tilting. The geological conditions that lead to this sort of landslide are widespread throughout New Zealand.

Commonly, a hard, permeable rock (such as sandstone, limestone or volcanic rock) rests on a softer rock such as mudstone. In these circumstances the weight of the overlying rock cap, together with a build-up of groundwater under it, causes the softer rock to lose its strength and deform. (Think of a custard square with the pastry crust being the upper rock and the custard as the deformed mudstone beneath.)

Consequently, the hard rock cap slides over the soft rock, or the underlying rock fails, leading to a collapse of the hard rock cap. An example of this is the Abbotsford landslide disaster, which destroyed 69 houses in a suburb south-west of Dunedin on 8 August 1979. Many such landslides were first recognised and described in detail from the Dunedin district in 1940 by the pioneering work of the geologist Noel Benson.


Hill country, regolith and submarine landslides

Soft-rock hill country

Two types of landslide are common in New Zealand’s hill country, particularly in the North Island – rotational landslides and creeping earthflows.

Rotational landslides

Rotational landslides are dominant in the soft, Tertiary (less than 65 million years old) marine sedimentary rock that makes up much of New Zealand’s hill country. They generally consist of a series of rigid blocks that have slumped downward, often rotating on a curved surface. Small ponds often form on the surface of the landslide, and extensive flow lobes (tongues of material) spread down the slope.

Such slides can deform whole hillsides, and many block, at least temporarily, the local drainage systems. If the rock layers are not strongly deformed (such as those in the Taranaki–Whanganui region), hillsides tend to collapse with little sideways movement of material. However, if the layers are dipping out of the hill slope, landslides can travel for considerable distances.

Slip-sliding away

Herbert Guthrie-Smith wrote of his problems with slips in the soft mudstone at his Hawke’s Bay farm:

‘Sometimes a whole hillside will wrinkle and slide like snow melting off a roof, its huge corrugations smothering and smashing the wretched sheep, half or wholly burying them in every posture.’ 1

Creeping earth flows

Also common in soft-rock hill country are deep-seated creeping earth flows. They move a few metres each year, normally during the winter months when the ground is saturated. Like glaciers, they flow and slide over their base, producing crevasse-like tension cracks and hummocky terrain. Earth flows tend to occur on gentle slopes and in soft, clay-rich rocks containing minerals that swell and shrink as they absorb and release water.

Regolith landslides

Regolith landslides are New Zealand’s most widespread and noticeable type of landslide. Regolith (from the Greek meaning blanket of stone) is the covering of soil and loose rock fragments that overlies bedrock. Regolith landslides occur mainly in this surface material and involve very little of the underlying bedrock. Although they can occur individually, it is more usual for large numbers to occur simultaneously over wide areas during intense rainstorms, such as Cyclone Bola on the East Coast in 1988, and the Manawatū–Whanganui rainstorm of 2004.

Regional extent

The typical landslide in these events is shallow and rapid, immediately converting into fast earth flow (with the consistency of wet cement). On average these run out to distances equivalent to three times the length of the initial slide, and occasionally up to 10 times. Regional events, numbering hundreds of individual landslides, spread over thousands of square kilometres of land, are often yearly occurrences. They also take place whenever large earthquakes or rainstorms occur.

Damage

The amount of soil erosion they cause was first documented in 1921 by naturalist Herbert Guthrie-Smith in his book Tutira. Later, geographer Kenneth Cumberland systematically examined regolith landslides in Soil erosion in New Zealand (1944). While landslides of this type are generally not life-threatening, they exact a huge toll each year on roads, railways, farm infrastructure and pasture production.

Deforestation has greatly increased susceptibility to these events. Within a short time they have extensively depleted productive soils formed over thousands of years. They represent New Zealand’s most costly landslide hazard.

Submarine landslides

Submarine landslides occur most commonly offshore on the continental slope. Bathymetric surveys, carried out by boats bouncing sound waves off the sea floor, indicate that some are much bigger than any seen on land. As they are an important trigger of tsunamis, recent attention is being given to understanding them.

Footnotes
  1. W. H. Guthrie-Smith, Tutira: the story of a New Zealand sheep station. 4th ed. Wellington: Reed, 1969, p. 37. › Back

People and landslides

Protective measures

As New Zealand’s population grows and spreads, producing more buildings and road networks, the risk from landslides increases. This is addressed by legislation that controls resource development (Resource Management Act 1991), building activity (Building Act 1991), and emergency management (Civil Defence and Emergency Management Act 2000). The insurance scheme administered by the Earthquake Commission also helps to spread and mitigate the risk. Soil conservation and forestry techniques are employed to reduce landslides on productive land.

Threats to property

In situations where buildings and transport networks are threatened, engineers, geologists and other earth scientists investigate and then set up methods for controlling landslide activity and reducing the risk to property. Developers of new subdivisions on hillsides are often required to hire experts to assess the stability of slopes and the likelihood of landslides.

Landslides and the Clyde dam

Landslides can be incredibly costly. In the 1980s, when a hydroelectric dam was being built on the Clutha River in Central Otago, there was a budget blow-out. Engineers were very worried about landslides above the Cromwell Gorge, including the particularly troublesome Cairnmuir landslide. When the dam was raised they feared that the groundwater would also rise and that huge, slow-moving landslides might become active. If one of these ploughed into the newly created Lake Dunstan it could produce a wave that overtopped the dam.

To be on the safe side, the slips were stabilised by drilling tunnels to drain the water, and by reinforcing the toe of the slides. The surface of the Cairnmuir landslide was paved and terraced to seal out water, creating a bizarre amphitheatre in the hills above the lake.

Investigating and stabilising the landslides cost $936 million (2005 value).

Road warnings

Large orange and yellow road signs depicting crumbling hillsides will be familiar to many motorists. Before New Zealand adopted symbolic international road signs in 1987, roadside warnings of landslides read ‘Slips’ or ‘Falling debris’, which often confused tourists.

Each year local councils, roading authorities, private landowners and railway operators spend millions of dollars clearing slips from roads and railway lines. It is a never-ending task – there are always more floods or earthquakes to come, and there is plenty of rock and soil waiting to tumble down.

Unlucky hunters

In August 1972 two hunters were returning from a week near the Clyde River in Canterbury’s back country. The weather had been rough with rain and snow, and the Clyde was running high. A rock slide engulfed them as they were traversing a moraine bank 25 metres high and 100 metres long. The sliding rocks killed them and carried their bodies into the river.

Threat to life

Landslides often kill people in combination with other natural hazards such as earthquakes and floods. High rainfall and earthquakes often trigger landslides. The Murchison earthquake of 1929 killed 17 people – 16 as a result of landslides it generated. One estimate attributes 362 deaths to landslide-related causes between the 1840s and the early 2000s.

In general, landslides are more common in New Zealand than many countries because of the terrain and less stable conditions. However, landslides cause few deaths in New Zealand because there are few settlements in mountainous terrain and the population density is relatively low.


Notable New Zealand landslides

While there are countless examples of landslides, some have caught the public’s imagination or are exceptional in some other way.

Abbotsford

On 8 August 1979 a huge area of the Dunedin suburb of Abbotsford slid downhill, taking 17 people and many houses with it. Luckily no one was hurt – but 69 houses were left uninhabitable. The trouble had started more than a decade earlier when residents noticed hairline cracks in concrete paths. The slip moved very slowly until the sudden slide in 1979, which moved at around 3 metres per minute.

This is known as a layered rock landslide – the hill slid on a lubricated layer of clay. The same rock formations (mudstone) cause unstable slopes on the Kilmog Hill, just north of Dunedin. Slope movement is readily visible in the undulating surface of State Highway 1 in this area.

Green Lake

New Zealand’s largest documented landslide occurred some 13,000 years ago in Fiordland. Around 27 cubic kilometres of material collapsed in a hard rock landslide that covered an area of over 45 square kilometres. In effect, a whole chunk of a mountain range collapsed into the valley below. It formed the hummocky terrain typical of many landslides – but on a much larger scale. Some depressions in the surface filled with water, forming sizable holes such as Green Lake, rather than the usual ponds.

Tāhunanui

Part of the Nelson seaside suburb of Tāhunanui sits on an active landslide. The hillside overlooking the beach consists of marine sandstone and overlying glacial gravels. In places the gravels are slumping. Damage to houses and property occurred mainly in the 1890s, and in 1929 and 1962 when earthquakes struck. Because of the damage in 1962, a stormwater system was installed to reduce the amount of water entering the landslide. This seems to have helped, but surveys show that the hillside is still slowly slumping in places, and further movement appears most likely when groundwater levels are very high. Since 1985 further building on the slump has been closely controlled, and in certain areas it is not permitted.

King Country rail crash

One of the worst accidents caused by a landslide occurred in 1923. After days of heavy winter rain, the express train from Auckland to Wellington slammed into a huge slip covering the rails at Ōngarue, near Taumarunui. Seventeen people were killed and 30 seriously injured.

Waihī

New Zealand’s highest death toll from a landslide remains the 1846 Waihī disaster, which claimed 60 lives on the shores of Lake Taupō. After heavy rains a landslide blocked a stream, creating a dam behind it. Three days later the dam collapsed sending a mudflow down the valley that overwhelmed the Māori village of Te Rapa, killing chief Mananui Te Heuheu and many others.

In 1910 another landslide killed one person in a new village near the old site of Te Rapa. After this second event the village was abandoned. The source of landslides is an unstable geothermal area known as the Hipaua Steaming Cliffs. This still causes problems for road engineers working on State Highway 41, which passes between the cliffs and Lake Taupō.


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How to cite this page: Michael J. Crozier, 'Landslides', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/landslides/print (accessed 22 November 2019)

Story by Michael J. Crozier, published 12 Jun 2006