Journal of the Japan Society of Erosion Control Engineering, Vol.60,No.2,2007
Ruapehu Crater Lake break-out lahar,North Island,New Zealand
Tomomi MARUTANI, Takashi YAMADA, Masanobu KIMURA, Hideji MAITA,
Vern Manville, Graham Leonard and Noel Trustrum
Abstract
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 mR 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.
Key wordsFlahar (volcanic mudflow), Crater Lake break]out, hyper]concentrated
flow, automatic lahar measuring equipment, automatic monitoring camera
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