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Hazard

Landslides and debris flow

Hazard Knowledge

Deforestation, heavy rainfall, undercutting of slopes, inappropriate construction and buildings with weak foundations can lead to the sudden failing of the integrity of slopes causing a landslide or debris flow.

Steeper slopes forming a V shaped valley with large boulders, 1 to 6 metres in diameter, or those exposed to excessive water flows are more prone to landslides. Areas with a history of landslides are also more prone; many at-risk sites have been identified and precautions put in place.

Landslides can be categorised into three types:

  • Falls: large pieces of debris falling through the air or bouncing down steep slopes
  • Slides: materials moving in contact with a slope. Slow moving slides on shallow slopes are possible.
  • Flows: liquid or semi fluid movement of materials down a slope, typically fast moving

Debris flows and landslides are powerful enough to move cars, large trees and buildings within the flow. They can be over 20 metres wide and can move at 35mph, making them difficult to outrun or outmanoeuvre.

Landslides have a high casualty rate and also damage property, infrastructure and utilities. Temporary mortuary arrangements may be required.

Damage to utilities can cause secondary hazards such as fire and gas leaks. The impacts on road infrastructure may affect the response to an incident. Debris flows and landslides can result in building collapse, trappings or cause blockages that result in flooding.

The disturbance caused by a landslide may result in secondary movements and collapses.

See Hazard Unstable surfaces