The Quake Centre was proud to be a part of the 2017 NZ SOLD (Society of large Dams) Workshop, with speakers from New Zealand and around the world gathered to talk about risk informed decision making (RIDM).
CASE STUDY IN RIDM: MOUNT RUAPEHU AND THE TANGIWAI DISASTER
Dam Safety Intelligence Engineer, Chair of the NZSOLD Young Professionals Group and workshop presenter Dan Forester says, “RIDM is not a new thing. Humans have been making risk averse judgements and responses since the dawn of time. Perhaps what has changed is the level of information available and an increasing intolerance for injury or loss of life.”
Dan takes us back to one of New Zealand’s darkest days, the tragic Tangiwai Rail Disaster on December 24th 1953, an indirect result of a major eruption of New Zealand’s most active volcano Mount Ruapehu, eight years earlier. The loss of 151 passengers devastated our young nation, and forever altered scientific procedures in dam safety risk management.
The 1945 Mount Ruapehu volcanic eruption had caused the formation of a natural volcanic rock and ash barrier at it’s crater rim. A warning from geologists that, “the state of the crater wall was a reason for concern,” unfortunately went largely ignored by authorities.
On Christmas Eve, 1953, the crater lake dam finally gave way. An immensely destructive lahar of water, ice, mud and rocky debris surged down the steep gradient of Mount Ruapehu and into Whangaehu River, slamming into the concrete pylons of the Tangiwai railway bridge and severely weakening the structure.
Minutes later, without warning, the Wellington–Auckland night express train, with 285 souls on board (arms and suitcases laden with Christmas gifts) plunged from the collapsed bridge into the Whangaehu River. The catastrophe made headlines the world over and was, at the time, the world’s eighth-deadliest rail disaster.
Tangiwai had lived up to its name meaning ‘weeping water’ in Māori.
Ever since, Mount Ruapehu, as well as other volcanoes, crater lakes and dams throughout the country, have been strictly scientifically monitored.
“What was immediately recognized with the Tangiwai Disaster was the need for an accurate understanding of what Mount Ruapehu’s crater lake risk looked like,” says Dan. “A number of scientific and engineering assessments were undertaken, and work carried out with dam and internal erosion experts to determine the potential failure mode.”
Part of the emergency planning was installing warning systems that included lake level monitoring sensors, an embedded trip wire, cameras and geophones to detect vibrations. As well, warning systems along surrounding railways, bridges, houses and roading.
As was the case in 1945, further major eruptions of Mount Ruapehu in 1995 and 1996 also formed a tephra barrier on the rock rim of the crater. (Tephra is a greek word meaning material deposited by air in a volcanic eruption). However, this time around, there were the necessary precautions in place.
“Scientific analysis predicted failure of the 1995/96 tephra formation to occur at, or approaching, a crater lake level of 2536 metres,” says Dan. “Some twelve years later, in 2007, as lake levels began approaching this predetermined mark, the department of conservation, civil defense and other agencies went on high alert.”
Testament to the accuracy of crater lake level monitoring the tephra barrier gave way at very near to the level predicted. The alarm and warning system was successful, as was the emergency response. No injuries or significant damage was recorded. Accurate scientific risk assessment had provided valuable information for decision makers to help find long term solutions.
“In regards to dams and public safety the Mount Ruapehu lahar case study is an example of risk informed decision making at its best,” says Dan. “Understand the risk, decide what to do about it, nobody dies.”
“Continuing to develop our understanding of the impact of natural hazards such as volcanic and seismic activity in regards to dams is a very important and ongoing growth area, with each new event bringing new learnings.”
The UC Quake Centre is proud to partner with NZSOLD and help take the necessary measures for better community resilience in New Zealand, and around the world.