Tangiwai disaster
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The Tangiwai disaster was the worst rail accident in New Zealand history. It occurred on December 24, 1953 when the overnight express train between Wellington and Auckland passed over the Tangiwai Railway Bridge. The bridge, which had just minutes earlier been weakened by a lahar from Mount Ruapehu, collapsed, sending the train into the Whangaehu River.
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Scale
Of the 285 people on the train that night, 134 survived and 151 died. Of those that died 20 bodies were never recovered; it is believed they were washed 100 kilometres down the river and out to sea.
Enquiry
Evidence given at the commission of enquiry into the disaster revealed that the midstream piers of the railway bridge had been undermined by previous sudden floods, from as early as 1925. While large concrete blocks, weighing several tons, had been placed around the footings of these piers and the space between the blocks and the piers backfilled with gravel, the lahar was strong enough to sweep these away.
Cause
The cause of the lahar that led to the disaster was the collapse of a natural volcanic ash dam that had blocked the outlet of the crater lake on top of Mount Ruapehu. When that dam collapsed, the water from the lake mixed with the material from the ash dam and rushed down the mountainside in a flash flood known as a lahar. Until this disaster, the danger posed by lahars from Mount Ruapehu was appreciated by only a few scientists.
Prevention
A railway line equipped with track circuits has a reasonable but not guaranteed chance of detecting washed-away track.
A lahar warning system was subsequently installed to alert train control to high river flows.
Similar accidents
Similar accidents involving bridge washaways include:
- September 27, 1923 – near Glenrock, Wyoming - a bridge over Coal Creek is washed away and a passenger train derails, killing 30 of the train's 66 passengers.
- Image:Flag of Kenya.svg 1993 - 114 perished in a passenger train which plunged into a river after floods washed away a bridge at Ngai Ndethya.
- See List of rail accidents for more details.