Aotearoa New Zealand is prone to landslides, in part because of the many earthquakes we experience ever year.
Landslides can also be caused by unstable geology, rainfall, environmental change and human landscape modification.
Why do landslides happen
Landslides occur when the strength of a slope is overwhelmed by stresses imposed on that slope. The strength of a slope is determined by:
- The steepness of a slope
- The type of rock and its stability
- The type of soil and how water infiltrates that soil type
The stresses imposed on a slope can be sudden (e.g. an earthquake or heavy rain) or gradual. For example, weathering can cause stress to build up on a slope over time. The accumulation of damage reduces the strength of a slope until a relatively small event may cause the slope to fail.
What creates landslides in New Zealand?
Risks caused by landslides
As a landslide travels down a slope, the mud, debris and rocks can result in damage to land, property and infrastructure such as roads, railways and communication systems. As larger rock and boulders collide with objects in their path, they can start to roll instead of slide. As boulders roll and bounce down a slope, they gain momentum, going faster and farther than other landslide debris and causing damage farther away.
If a landslide blocks a stream or river, this can cause a landslide dam. Some landslide dams become permanent fixtures in the landscape. Others fail after days, months or years due to overtopping of the dam or gradual erosion and pressure imposed on the dam. When a landslide dam fails, the land and infrastructure downstream is at risk of flooding.