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Tony Smith 09-04-19 03:43

Bailey Bridge washed away in flood
On NZ's West Coast heavy rainfall has washed away a 30 year old Bailey Bridge. The bridge crossed the Waiho River, the stream flowing from the Franz Josef Glacier.

Due to Climate Change, or not, the Glacier has retreated a couple of kilometres in the past few years and deposited a huge amount of gravel and boulders in the river bed. Since the early 1980's, the riverbed of the 18Km long river has filled with gravel and risen 6.5m, altering the floodpain and where storm runoff can escape. It is now channelled under the bridge.

On 24th-25th March, a storm brought 1086mm of rain in 48hrs (over 40 inches!), flushing the Franz Josef valley and rolling bouders down the river bed.

With 8.2m of water under the bridge over a 200m width of riverbed, the forces on the bridge were massive. Despite this, the bridge withstood the force of water and boulders, but was ultimately destroyed by the north riverbank being scoured away. Instead of just dropping the 1 span that was supported at that end, the strongly connected design of the Bailey Bridge dragged 4 spans into the river.

In view of that excellent performance, the new replacement will be another Bailey Bridge, to be built by the Army!

Hanno Spoelstra 09-04-19 09:26

Impressive, to say the least about the rainfall, and the durability of the bridge.


maple_leaf_eh 10-04-19 06:15

Reliable virgin British steel!

Building long spans across gravel floodplains is just plain risky. Channels form for 10,000 reasons and flow patterns are unpredictable. The engineers ought to have put in more support piers and more expansion joints. Connecting the entire span (IMHO) is what caused so much of this damage. But, Baileys are cheap and very easy to assemble. They require next to zero maintenance and every year of service after the first is a bonus.

dcrfan 24-07-19 11:07

I suspect the lack of additional piers was the increased risk of flood uprooted tree hang ups.

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