Mt Ruapehu Crater Lake lahar hazard

This report documents the results of some additional computational hydraulic modelling of the historic 1953 lahar event down the Whangaehu River (which caused the Tangiwai disaster) and a hypothetical lahar event generated by a future collapse of the Mt Ruapehu Crater Lake outlet barrier. Based on the results of the additional work, the following conclusions were obtained regarding the lahar event generated by a future collapse of the Crater Lake outlet barrier: Peak discharge from the Crater Lake (assuming outlet barrier collapse occurs when the lake is full up to the top level of the barrier) is likely to be in the range 480-850 Bulking of the water flow (due to sediment entrainment) released by the outlet barrier collapse is likely to increase the peak discharge at the end of the Whangaehu Gorge to about 1540-2340 Overflows of the order of 50-290 into the southern "chute' channel and into the Waikato Stream system respectively are likely to occur. Attenuation of the lahar event due to sediment detrain-ment and channel friction effects will occur down the outwash fan and the Whangaehu River parallel to the Desert Road. Peak discharge at Tangiwai Bridge (38 km from the Crater Lake outlet) is estimated to be 910 giving an upper-bound credible discharge of about 1015 Peak discharge at Tangiwai Bridge will be up to 54-72% larger in magnitude than that for the historic 1953 event. The lahar event will also travel faster downstream than the historic 1953 event. Travel time to the Tangiwai Bridge is estimated to be 1.8-2.1 hours compared to about 2.3 hours for the 1953 event.