What is a lahar?
A lahar is a rapidly flowing, flash flood of rock debris and water from a volcano. Large lahars can be dangerous because they can destroy, erode or bury obstacles in their path1. A lahar from the Mt Ruapehu Crater Lake in 1953 destroyed the railbridge at Tangiwai, causing the deaths of 151 people.
At least 13 lahar episodes have occurred since 1945, most directly associated with an eruption. One (1953) was triggered by dam-break. Rim collapse, glacier burst or rain are other known triggers.2
How would a lahar affect kayakers?
A lahar will primarily affect the Whangaehu River (sections above and below Collier's Bridge). It is estimated that a lahar would take 3.5-4 hours to reach Collier's Bridge on the Whangaehu. If it overtops the protective bund, a lahar could enter the Tongariro River above the Rangipo dam, affecting Access 14, Access 13 and Access 10 sections in 2-3 hours [a]. It is also possible for a lahar to affect the Whanganui River.
The steep walls of these rivers, particularly the Whangaehu mean that escaping from the lahar would be difficult.
Kayakers from all over New Zealand kayak these rivers. Clubs from throughout the North Island including Wellington, Hutt Valley, Wanganui, Wairarapa and Ruahine are regular users of the Whangaheu, usually kayaking down to Collier's Bridge.
The Ruapehu Lahar: Emergency Management Plan (Southern) (RLEMP) was developed by the Southern Ruapehu Lahar Planning Group (SRLPG) consisting of the Ruapehu District Council, Horizons Regional Council, Ministry of Civil Defence and Emergency Management (MCDEM) and Police along with key support agencies the Department of Conservation and the Institute of Geological and Nuclear Science amongst others.
A warning system has been established to detect a dam break and trigger an emergency response. A monitoring programme is in place and regular lake inspections are conducted. The Crater Lake Warning Level summarises the situation3.
The degree to which emergency planning accounts for kayakers is unknown, so kayakers are advised to take all necessary precautions before running sections at risk from lahars, according to good kayaking practice:
- Find out about known hazards (Recommendation 1.1)
- Ensure a late return will be escalated (Recommendation 1.4)
- Carry emergency equipment (Recommendation 1.9)
- Know emergency exit points (Recommendation 3.6)
How to check the situation
- Monitor information at this website: http://rivers.org.nz/article/lahars_and_kayakers.
- Monitor the DOC Crater Lake Status Report
- Sign up to the Lahar SMS/Txt alert service
1. Mount Ruapehu Crater Lake Lahar, brochure, Civil Defense
2. Lahars from Mount Ruapehu - mitigation and management (poster). Keys, H. Department of Conservation. http://www.doc.govt.nz/upload/documents/about-doc/news/issues/lahars%20poster.pdf
3. Refer diagram at http://www.ew.govt.nz/enviroinfo/hazards/naturalhazards/volcano/craterlake.htm