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.
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.
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.
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.
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).
Dr Joshu Mountjoy
Tel 04 386 0336