Manville, V.R. 2007 Modelling the predicted break-out lahar from the Crater Lake of Mt Ruapehu, North Island, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2007/03 30 p.
Abstract: During 1995-96 eruptions of Mt. Ruapehu volcanic activity expelled the summit Crater Lake and deposited c. 7 m of tephra on the stable rock sill that normally forms the outlet to the 9 million m3 basin at an elevation of 2530 m. Subsequent refilling of the crater has raised the possibility of a large lahar triggered by a break-out of the lake with the potential to release up to 1.65 million m3 of water. The level of Crater Lake is currently c. 1.4 m above the hard rock rim (May 2006), but forecasting of future levels is complicated because of climatic influences and heating cycles within the lake. The last time such a situation occurred was in December 1953, when breaching of a barrier of similar volcanic material, deposited during the 1945 eruption episode and buttressed by the Crater Basin Glacier, released c. 1.8 million m3 of water into the headwaters of the Whangaehu River. The outflow, peaking at an estimated 300-400 m3/s, rapidly entrained snow, ice, and volcanic debris to form a lahar (volcanic mudflow) with a maximum discharge 5-6 times larger by the time it had reached the terminus of the Whangaehu Gorge 10 km downstream. The lahar reached the Tangiwai railway bridge approximately 39 km downstream in a little over 2 hours, causing damage that resulted in the loss of 151 lives when the Wellington-Auckland express train fell into the lahar-swollen river. With this impetus, the current situation has been the subject of intense scientific scrutiny lead by the Department of Conservation, the agency responsible for managing the Tongariro National Park in which the Crater Lake is located. Numerical modelling of the size of the predicted break-out lahar, commissioned by the Scientific and Technical Advisory Panel (STAP), during 1998-2002 yielded a ‘standard model’ (based on a fixed breach width) involving a peak outflow from the Crater Lake of 480-850 m3/s, and a median peak discharge (based on an attenuation curve calibrated by a single data point) at Tangiwai of 910 ± 105 m3/s (i.e. 1.4-1.7 times the size of the 1953 lahar). Overall, the likelihood of the lahar being larger than the 1953 flow was assessed at >85 %. Further work between October 2002 and June 2003 revised the official estimates of lahar size upwards, to a most likely discharge (65% probability) at Tangiwai of 1200 m3/s (1.9 times bigger than the 1953 lahar), and a maximum discharge of 1700 m3/s (2.7 times bigger than the 1953 lahar). Independent analysis of the Crater Lake situation predicts a different median-sized lahar and worst-case lahar due to differences in the breach outflow model and the Whangaehu River lahar attenuation curve used. Probabilistic analysis of the predicted Crater Lake break-out lahar using a parametric model based on the most likely range of three independent input parameters suggests that the median peak outflow will be c. 220 m3/s. Development of a l ahar attenuation curve calibrated against 10 data points from historical lahars converts this outflow value to a lahar of c. 325 m3/s at Tangiwai (i.e. 0.55 times the size of the 1953 lahar). The probability of the lahar being larger than in 1953 is assessed at 3/s yielding a lahar peaking at Tangiwai at c. 1000 m3/s (i.e. 1.6 x bigger than in 1953), but the independent analysis places the probability of a flow of this magnitude at 1%. (auth)