Debris flows

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What are debris flows?

A debris flow is a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope under the influence of gravity. Debris flows can travel extremely fast, and the fluid mix of water and debris may be as thick as wet concrete. The faster the water flows, the more the water can pick up, including rocks, trees, cars and parts of buildings. The speed and the size of the carried particles mean debris flows can be very dangerous. Debris flows can threaten people’s lives and property and damage our environment. They regularly damage bridges, resulting in expensive road maintenance through coastal and mountain regions.

What causes debris flows?

Debris flows can be caused when large amounts of debris and sediment come into contact with water, for example a landslide that occurs during a large rainfall, or floodwaters eroding a stream channel. 

Lahars are also a type of debris flow, where the ‘debris’ is volcanic material. The source of a lahar may be a crater lake, a dam collapse or heavy rainfall washing ash from the slope of a volcano.

Debris flows are more likely to occur when:

  • land is steeply sloped
  • soils are highly altered
  • heavy rain occurs
  • soils are already saturated
  • remains of previous landslides are present.

Past debris flows

In the past, debris flows in the Waikato region and around the country have caused the loss of lives, extensive damage and changes to our landscape.

  • 18 May 2005: A debris flow in the catchment behind Matata caused extensive damage and closed SH2 and the railway line for many days.
  • 17 February 1985: A debris flow at Te Aroha killed 3 people.
  • December 1953: A lahar flowed down Mount Ruapehu, washing away a rail bridge and killing 151 passengers. Mount Ruapehu is particularly prone to lahars, a special form of debris flow that occurs in volcanic areas.
  • 1846 and 1910: Debris flows originating from the Hipaua Steaming Cliffs killed 65 people at Little Waihi, Taupō.
  • Prehistoric debris flows built the land beneath Thames over the last 7000 years. 

Debris flow management

Debris flows can be managed in a number of ways. However, due to the potential size and destructive capabilities of debris flows, management is not always a successful or viable option.  

After the 1953 Lahar on Mount Ruapehu, the Eastern Ruapehu Lahar Alarm and Warning System was set up to provide warning on the Desert Road and to Tangiwai. This system was tested by a lahar event in 2007. The lahar was successfully detected by the warning system resulting in the closure of a nearby road.

After the 2005 Matata debris flow, Whakatane District Council looked at constructing a debris detention structure. In late 2012 after extensive investigations, the Council accepted expert advice that there was no feasible engineering solution to mitigate against debris flows from the Awatarariki Stream Catchment. The council is now working on planning and regulatory options to control land use in the area.