The NZ Transport Agency says there is no evidence to support suggestions that a lack of maintenance or earlier work upstream were responsible for the failure of South Westland’s Waiho Bridge on Tuesday this week.
Transport Agency System Manager Pete Connors says given the massive force of the swollen river towards the north bank and the huge amount of rock pounding the bridge’s piers, on top of debris build-up over the course of the day, it is unlikely anything could have been done to save the bridge.
However, the bridge’s river protection system had undergone maintenance and additional rock work just two weeks earlier.
Mr Connors says claims that a temporary stopbank had directed the river channel towards the north bank are not correct. The temporary stopbank used by crews and machines to access the river’s groynes for rock wall repairs had eroded, as it was designed to do, before the flood peaked on Tuesday.
The river had been building up in the centre in recent months, pushing its main channel towards the north bank, he says. That was beyond anyone’s ability to alter.
The massive rainfall on the West Coast on Tuesday 26 March, resulted in extremely high flows in many rivers, including the Waiho which flows out of Franz Josef Glacier.
The Waiho is one of the South Island’s most dynamic river systems, carrying massive amounts of boulders and rock in its water.
In recent months the river had built up material in the centre of the channel, pushing its main channel across to the right/ north side bank.
When combined with the significant flood event, this resulted in extremely high river velocities in the main channel which aggressively attacked the rock groynes that protect the bridge and the rock embankment protection at the northern abutment (where the bridge joins the bank).
The scour and displacement of rock resulted in the shallow pad foundation at the northern abutment being undermined. The northernmost pier (which stands in the river) appears to have been hit by a large boulder, causing the pier to buckle and fail, causing the collapse of the main northern span. Once that entered the river floodwater, subsequent bridge spans were pulled downstream off their pier foundations.
Two weeks ago, after short periods of high water flows in the river, (called freshes), resulting in some rock displacement, Transport Agency crews repaired the Waiho River Bridge’s existing bunds (protective rock walls) on the north side, patching and filling in gaps in the rock work and reinforcing the ends of the groynes. This is part of the regular maintenance required around this river.
Once completed, the crew removed the top from the temporary rock bund which was built to access the groynes, so it would not influence where the river went later and would erode away quickly. That temporary stopbank was washed away by the time the river started to rise and push hard at the bank.
The river pounded the north side all day and eventually undermined the abutment, however the reason for the bridge collapsing is believed to be due to a large rock at high velocity hitting the first bridge pier. Once the pier was hit, that side of the bridge could no longer hold.
The river protection works upstream of the bridge largely all remained in place.
The pier failure resulted from the forces created by debris and rocks in the river.
The Transport Agency is fully committed to replacing the bailey bridge as soon as possible, aiming for 7–10 days, to reconnect the South Westland community and the tourism loop around the South Island.