LaharsLahars fromfrom MountMount RuapehuRuapehu mitigation mitigation andand managementmanagement Harry J.R. Keys Dept of Conservation (DOC) Mt Ruapehu (2797 m) is a composite andesite strato- Private Bag, Turangi volcano in the central North Island of New Zealand. New Zealand It is the largest and most active of the three volcanoes [email protected] of Tongariro National Park World Heritage Area, and has a crater lake (107 m3) and glaciers. North Lahars from Crater Lake are a significant hazard on the Island Mount volcano and in its draining valleys and surrounding Ruapehu plains. They threaten New Zealands largest ski area and nationally important infrastructure. Mount Ruapehu and Crater Lake South Island 1945,1959, 1966?,1969, 1968,1971, 1975,1977?, Whangaehu River What is the current lahar hazard? 6 199597 1988 Tongariro River 5 At least 13 lahar episodes have occurred since 1945, most How large might this lahar be? 4 directly associated with an eruption. One (1953) was 3 1945? triggered later by dam-break. Rim collapse, glacier burst Several estimations of lahar size have been made. 19952000 2 Uncertainties include the role of glacier ice, dam- 1953 or rain are other known triggers. There have been no 1 useful precursors for many of these lahars. break dynamics and variation 0 in debris content. Number of lahar episodes Following eruptions in 199596, a dam-break lahar was Extended Blue sky Rain Dam eruption eruption mobilisation break predicted, and a process to assess risk and develop Dam-break and 1-D hydraulic Numbers of different lahar-producing mitigation measures was initiated. models estimate lahar flow- episodes since 1945 rate 40 km downstream of lake to be double that of the 1953 lahar. What has been done? Granular flow model com- A public consultation process with scientific input and bined with paleohydraulic risk assessments was carried out. analysis of the 1953 event using Clarkes glacier burst A sequence of decisions have been made to reduce Auckland Star risks to manageable residual levels. model suggests flow-rate at Tangiwai rail disaster caused 40 km will be similar to 1953. by bridge collapse during Reliable early warning and emergency response systems dam-break lahar in 1953. have been set up (ERLAWS and EDS). Conclusions Automatic lights are installed on Herb Christophers A bund to prevent overflow from the Whangaehu lahar State Highways 1 and 49. The current management solutions reduce lahar risks path into the Tongariro River headwaters has been to very low levels off, and adequate levels on Mt constructed. Ruapehu. However, Crater Lake may be reaching an equilibrium fluctuating about the former overflow Some utility companies have made changes to their level, i.e. the base of the tephra dam. assets so that they can withstand lahars, or are isolated from them. 120 2550 Base of tephra dam 2529.3 m 100 2500 The Minister of Conservation decided that there would Lake level 80 be no intervention at Crater Lake, as engineering at 2450 60 the outlet would provide a short-term solution to only 2400 40 one lahar problem. State Highway 49 bridge being Lake volume as 20 percentage of fullness 2350 Lake level (m.a.s.l.) Crater rim deformation is regularly surveyed. raised and strengthened. Fullness (%) above 2440 m 0 2300 199697 98 99 2000 01 02 03 04 05 06 Crater Lake DOC sensors Genesis Energy This means that dam collapse may be some way off sensors Whakapapa ski area Whangaehu Valley and raises the question of what are appropriate levels NZ Alpine Club Hut Eruption Detection System Raised footbridge Tukino Skifield of readiness for the longer term and how they should Improved staff training Signage be sustained. Long-term solutions are needed for Public awareness campaign Tukino repeater Whangaehu footbridge managing the risks associated with this very active Bund obstructing potential volcano. lahar spillover into Tongariro R. headwaters Crater monitoring Chute overflow from Tongariro Eastern Ruapehu Lahar Alarm and Warning main channel Lake, dam and rim Mt Ruapehu River System (ERLAWS) o and microwave Volcano (GNS science) Radi Tokaanu Crater Lake Power Station State Highway One Lake Taupo Tangiwai Bund Mt Ruapehu Close visitor area ERLAWS lights, signs Eastern side of RANGIPO Automatic warnings Highway bridge raised Ruapehu with lahar when set thresholds DESERT Traffic control DOC/GNS are exceeded sensor sites and bund DOC and Emergency ERLAWS gates and lights Genesis sensors services Police Traffic control Stop trains Rail Whangaehu 1 Train alarm and control Aqueduct site Automated gates and Agency integration River Extra survey work lights on SH1 and SH49 Pine harvest upstream Checks on the day Sensors linked to acoustic flow monitors installed at 49 three sites in upper Whangaehu lahar path. Waiouru 0 5 km Data from 8 sites telemetered to Tokaanu Power Station and displayed via the internet in near real-time. Others roads and bridges Flood tower Alarms automatically sent to police, rail and road Individual notification Rail warning system authorities and other agencies; plus automatic barrier ERLAWS lights and signs Horizons web cam arms at highway bridge, and flashing lights and signs. Agencies have developed an integrated response plan. Rail and power companies have additional sensors and The 300-m-long bund alarm systems. Department of Conservation under construction Te Papa Atawhai More at www.doc.govt.nz/whats new/issues DOC Science Poster no. 87 (edition 1 June 2006) prepared by Chris Edkins. Other titles available from DOC Science & Technical Publishing, Dept. of Conservation, Box 10420, Wellington..