Is another Bailey bridge the best option for Franz Josef's Waiho River?

Monday, 8 April 2019

Tim Gibb is ferrying up to 30 cars and 100 people a day across the Waiho River in a dump truck after the bridge was washed away in a recent storm. (First published April 2019)

A proposal to let Franz Josef's Waiho River fill up its old flood plain to reduce its energy near the township 'makes no sense', Westland District mayor Bruce Smith says.

The idea has been discussed by West Coast Regional Council after a report two years ago from University of Canterbury natural hazards Professor Tim Davies.

Davies suggested removing the stopbanks would broaden the river and lower the height of the river bed near the State Highway 6 Bailey bridge. However, the new course could inundate hundreds or thousands of hectares of farmland and force the NZ Transport Agency to move the state highway.

Work on the new Bailey bridge to replace the one swept away on March 26, after more than 700mm of rain fell in the ranges nearby, should be finished by Friday.

The State Highway 6 Bailey bridge across the Waiho River at Franz Josef Glacier has always struggled in a precarious, high-energy environment (2003 file pic).

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Flood waters on March 26 took out a section of the Waiho River bridge near Franz Josef Glacier.

Davies, who could not be reached on Monday, has previously told The Press there were many challenges when it came to dealing with such a high-energy environment and that Franz Josef was possibly the most hazard-prone settlement in the country.

Not only did the slumbering Alpine Fault pass directly below the main street - underneath the service station - but the township was positioned in front of an unstable, steep hillside which could fail in an earthquake or storm.

An aerial view of Franz Josef township in South Westland with the Waiho River on the right.

Feeding into the swirling, glacier-fed Waiho River was the flash-flood prone Callery River, which, together or separately, posed a significant flood risk. The aggradation - building up - of the Waiho's river bed also exacerbated the chance of flooding.

Smith said allowing the Waiho to run free would affect farms and 82 people who lived on the south side of the river.

The Bailey bridge over the Waiho River at Franz Josef in quiet weather.

'It'll also wreck the most pristine kahikatea stand left in New Zealand, on DOC estate near the mouth of the Waihao on the southern side.

'It was too precious to be milled and now to think someone would want to run the Waiho through it, it's incredible.

Tim Gibb has set up a ferry service across the Waiho River, which uses a 38-tonne dumper as transport. It provides Franz Josef with a vital link to the south after the Waiho River bridge was washed out during the March floods.

'Unfortunately the district council doesn't have any jurisdiction on the Waiho River other than to protect its own assets. We've already protected those with the floodbank on the northern side, which worked beautifully during the 100-year-flood.'

NZTA Canterbury-West Coast systems manager Pete Connors said the Bailey bridge remained the best option for the dynamic Waiho River.

Work to replace the bridge over the Waiho River last week.

The previous one had been constructed in 1991 as a 'permanent temporary solution' to replace a suspension bridge.

'The beauty of the Bailey is you are able to react and can lengthen and raise it, and also because of the dynamic river bed there, where you don't know exactly where to put the bridge, or how high, or how long.'

The steel-pile piers of the bridge reached down about 15m into the rock.

'Those piers stood up really well to the flood. It was basically just the land-span of the bridge that collapsed, but that bridge lasted 28 years.'

The rising river bed below the bridge was a challenge, Connors said. On average the river bed has risen about 1.7m every 10 years since the late 1950s.

It was possible to alter the length and height of a Bailey bridge to cope with changing river conditions in a couple of months, he said.

Meanwhile, Smith said rubbish that escaped from the Fox Glacier township's old dump after the storm had not travelled as far along the coast as previously said.

Aerial surveys had shown the area of coast affected was about 50km long, from about 10km south of the Cook-Weheka River north to Ōkārito.

Volunteers had done 'an amazing job' but the clean-up of the Fox River bed was 'too hazardous' for them - not because of toxic material but because of the river.

'If there is going to be a problem from the safety point-of-view, then it'll be more [the river]. We're being more focused on getting the right type of volunteer-trained personnel to clear the river.

'We don't want to see lives lost.'