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 Emergency Management Act 2002). 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).
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.
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 unstable conditions. However, landslides cause few deaths in New Zealand because there are few settlements in mountainous terrain and the population density is low.