A small eruption on Mt Ruapehu last week sounded the lahar alarm, but the crater lake wall was not breached. Picture / Alan Gibson
An exercise to test emergency services' response to a lahar was held at Mt Ruapehu yesterday amid real rumblings beneath the active volcano.
Hundreds of people from several agencies were involved in the exercise, which began with the artificial triggering of the lahar warning system.
The Eastern Ruapehu Lahar Warning
System was triggered for real on October 4, when a small eruption sent 6m waves across the crater lake.
Yesterday's exercise was planned before the eruption, and has been held annually for the past six years.
But GNS Science said it followed two small volcanic earthquakes under the mountain on Tuesday.
Duty geohazards officer Ken Gledhill said the earthquakes measured about magnitude 2 on the Richter scale, and indicated that volcanic activity was "happening at depth".
"There's been a quiet period, but it's doing a little bit more at the moment," he said.
The danger of an eruption was low, as earthquakes of at least magnitude 3 were usually required to generate eruptions.
During the exercise, GNS Science acted as scientific advisers for officials co-ordinating the emergency response from two bases in Ohakune.
The Ruapehu District Council, the Ministry of Civil Defence and Emergency Management, the Department of Conservation, police and the Army and were among agencies involved in the exercise.
The officials were given the scenario of a "dam burst" lahar - a wall of water and debris which sweeps down waterways when the crater rim collapses.
This is the kind of lahar that destroyed the Tangiwai rail bridge over the Whangaehu River near Waiouru in 1953, killing 151 people on the overnight Wellington to Auckland express.
Yesterday's exercise was conducted over 90 minutes - the fastest estimated time a lahar would take to get from the crater lake to the rail bridge on State Highway 49.
Traffic on either side of the bridge was stopped for about five minutes when the warning system brought down automatic gates on the road.
Police also pulled gates across State Highway 1 at Waiouru.
Ruapehu council spokesman Paul Wheatcroft said it was the first time traffic had been stopped since the lahar exercises started.
"We add complexity and increasing levels of realism to them each time," he said.
Hundreds of people had been involved, and the system would be tested and refined until a real lahar occurred.
