Debris flows are caused when large amounts of debris and sediments enter flood waters. As rain water washes into a valley it can pick up material such as mud particles, rocks and vegetation. This picks up speed and debris as it travels down the valley and gradually takes on the characteristics of a river. The faster the water flows, the more the water can pick up, including rocks, trees, cars and parts of buildings. This creates a fluid mix of water and debris which may be as thick as wet concrete.
The speed and the enormity of the carried particles make debris flows very dangerous. Debris flows can threaten people’s lives and property and damage our environment. Debris flows regularly damage bridges, resulting in expensive road maintenance through coastal and mountain regions.
Debris flows are more likely to occur when:
- land is steeply sloped
- soils are highly altered
- exceptionally heavy rain occurs
- soils are already saturated
- remains of the previous debris flows are present.
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 due to volcanic activity.
- 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 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 multi-million dollar Eastern Ruapehu Lahar 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, the Whakatane District Council looked at constructing a debris detention structure. In December 2012 it was agreed there was no affordable engineering solution and the council is now looking at planning and regulatory options to control future debris flows.
- Planning for an emergency response.
- Hipaua: We work with the Taupō District Council to monitor the area, identify risk and develop community-based contingency plans. With the Taupo District Council we have set up a Waihi Risk Project Team and commissioned consultants’ reports to outline the hazards and undertake ongoing monitoring. We will continue to work with local authorities and other agencies to develop site-specific contingency plans and provide community education on the site risks and agency responsibilities.
- Thames: We have worked with GNS to study the potential for debris flows from Karaka Stream at Thames, Coromandel. The report can be viewed here.
For policy information on how Waikato Regional Council manages debris flow risk, check out the Regional Policy Statement.