Mass Flow Mitigation Options
Dr Jérôme Lecointre
Lahars (volcanic debris flows and hyperconcentrated flows) and debris avalanches initiated by sector collapse represent two critical hazards associated with the evolution of andesitic stratovolcanoes. Structural mitigation options aim at controlling and reducing the impact of mass flows. A range of suitable engineering designs has been tested overseas in unstable, erosion-prone mountain environments (e.g. against debris flows in British Columbia; see VanDine and Bovis, 2002).
Sabo construction adapted to lahars generated on steep-flanked volcanoes include: concrete slit dams and steel grids/nets to trap coarse material; lateral walls and diversion dykes ( bunds ) to contain the flow; and terminal berms associated with sediment retention basins to stop the moving debris mass in the unconfined lower reaches of the river catchment (ring plain). Comprehensive, well integrated protection systems associating these different technologies are successfully used in Japan, Indonesia and The Philippines (Anon., 2001).
In order to protect inhabited areas and key infrastructure exposed to these recurrent hazards, it is necessary to carefully consider (1) the topography of the affected river catchment and (2) the physical properties of the mass flow (rheology; flow behaviour; hydrology) along its course. This can be achieved through the acquisition of detailed, high-resolution surveys/profiles of the stream channel, the sampling of the fluid-sediment mixture, the record of dynamically controlled parameters, and the mapping of inundation limits coupled with flow modelling (simulation codes).
Engineering solutions are realistic options only for small to medium size mass flows (up to 10 5 to 10 6 m 3 or sizes 4-5 for debris flows; Jakob, 2005). Other limiting factors are the building and maintenance costs of these structures, and their environmental impact in protected areas.
We are currently looking at practical and cost-effective solutions for a number of critical sites located on both Taranaki and Ruapehu volcanoes (road, industrial and tourist infrastructures), based on the application of recently developed volcanic emergency scenarios and for an accepted level of risk (Lecointre and Neall, 2005).
- Anon., 2001. Sabo in Japan - Creating safe and rich green communities - Ministry of Land, Infrastructure and Transport/Japan Sabo Association, Tokyo, Japan: 36p.
- Jakob M., 2005. A size classification for debris flows. Engineering Geology 79: 151-161.
- Lecointre J.A., 2004. Structural mitigation options for volcanic mass flows. In: Manville, V. & Tilyard D. (eds.) Programme and Abstracts, Geological Society of New Zealand/New Zealand Geophysical Society/26th Annual Geothermal Workshop combined Conference "GEO3", Taupo, New Zealand. GSNZ Misc. Publ. 117A: 60-61.
- Lecointre J.A. and Neall V.E., 2005. Taranaki awakens! An emergency management eruption scenario for Taranaki Civil Defence and Emergency management based on eruptions at Egmont Volcano in the last 1,000 years. Report for the Taranaki Regional Council: 11p.
- VanDine D.F. and Bovis M., 2002. History and goals of Canadian debris flow research, a review. Natural Hazards 26: 69-82.