2007 Volume 60 Issue 2 Pages 59-65_1
On 18 March 2007 the summit Crater Lake of Mt. Ruapehu, New Zealand, breached a barrier of tephra emplaced by eruptions in 1995-96, resulting in the rapid release of 1.3 million m3 of water. The flood rapidly bulked by entraining snow, ice, rock debris and alluvium along the steep gorge of the upper Whangaehu River to form a debris flow that then transformed downstream into a hyper-concentrated and then sediment-laden stream flow during its passage to the Tasman Sea 155 km away. No lives were lost and infrastructural damage was minimal due to a comprehensive warning system developed in the decade before the lahar. A previous break-out lahar in 1953 caused a railway disaster at Tangiwai with the loss of 151 lives. The lahar flowed as a multi-peaked debris flow in the Whangaehu gorge, 7 km downstream of Crater Lake. Automatic lahar measuring equipment, including water level gauges, flow velocity sensors, conductivity samplers and seismographs were installed at key sites along the flow path, and were supplemented by sequential photographs captured by automatic monitoring cameras and observer teams. Flow velocity averaged 30 km/hr and water level rose by 8.2 m in 4 minutes in the gorge. The lahar caused riverbed aggradation in many places and environmental disturbance to the channel regime.