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:
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.
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(external link) 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.
For policy information on how Waikato Regional Council manages debris flow risk, check out the Regional Policy Statement(external link).