Rainfall-triggered landslides

Landslides involve the movement of a mass of soil and/or rock down a slope­­.

New Zealand's tectonically active setting and temperate climate is reflected in a highly dynamic landscape where landslides are common. Scales of landslides vary widely, from small weather-related landslides, to large earthquake-related landslides affecting whole hillsides.

While very large failures have the potential to cause major damage in their immediate area, it is the landslides triggered by rainfall which have caused the majority of significant disruption and economic loss in historical times. This impact has mostly been due to their widespread distribution.

Causes of landslides

The majority of landslides are triggered by rainfall, but earthquakes and other phenomena may also trigger landslides.

There are three main factors that contribute to New Zealand's pronounced susceptibility to landslides: geology (including tectonics), climate and landuse change.

Geology - The weak rocks that form many of New Zealand's hill country are often mantled with a shallow soil layer. These materials are highly susceptible to rainfall triggered landslides.

Climate - Heavy rainfall events occur regularly in New Zealand.Summer sub-tropical cyclones are some of the more powerful events that have caused widespread landslides in the past.

Find out more about heavy rainfall events

Landuse - With European settlement, large land areas were cleared of indigenous forest and converted into crop land and pasture, exposing unstable slopes that were previously protected by tree cover.

Major rainfall-triggered landslides around New Zealand

  • A recent example of a serious landslide is the February 2004 storm in the central and lower NorthIsland. More than 60,000 landslides were generated over an area of about 16,000 km2. The total economic impact of this event is estimated to be ~$400 Million.
  • The biggest landslide-generating storm event in historical times, however, was Cyclone Bola in 1988. Affecting the east coast of the North Island, up to 30% grazing area was lost to shallow landslides on some farms, and the total cost of the event exceeded $200 million.
  • In addition, in prehistoric times several events are recognised that are significantly larger.

Read the Water & Atmosphere article 'Drilling Lake Tutira for evidence of climate change'

Current research projects

NIWA scientists work on terrestrial rainfall-triggered landslides, as well as submarine landslides at a variety of scales. Submarine landslides are also a consistent feature of our offshore 'landscape' and are studied as part of our marine geological hazards research.

More information about our marine geological hazards research

Research on rainfall triggered landslides is primarily within the framework of Riskscape. NIWA collaborates with other New Zealand institutes such as the University of Auckland, University of Canterbury and GNS Science to further landslide research in New Zealand.

Visit the Riskscape website 

Probabilistic modelling of landslide distribution - NIWA is working within the Riskscape framework to predict the probability of landslide occurrence for given rainfall events. With knowledge of the stability of the slope, and the pre-rainfall soil moisture content, it is possible to predict where landslides could occur and subsequently the vulnerability of property and infrastructure.

Landslide warning using Numerical Weather Prediction (NWP) and super computing power - NIWA has developed a prototype of a landslide warning model. This physically-based model allows prediction of regional landslide distribution over the forecasting timeframe of an NWP (typically 48hrs).

Contact

Dr Joshu Mountjoy
joshu.mountjoy@niwa.co.nz
Tel 04 386 0336 

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Erosion of hillside affects housing. Kelson, Wellington, August 2006. Photo taken by Alan Blacklock on 8 August, 2006.

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Landslides in very shallow soils overlying marine sedimentary rocks in the Taranaki area.

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Comparison of modelled probability of failure (reds) and observed landslides from the 2004 Manawatu storm (black).

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Widespread shallow instability resulting from intense rainfall in the April 2011 storm in Hawke’s Bay. Photo: Dave Hansford