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