Wind that blew trucks over was sped up by Auckland Harbour Bridge's design, scientists say

Last year's freak accident on the Auckland Harbour Bridge which saw two trucks blown over by wind had two main causes, scientists say.

The first is the design of the bridge, and the second, incredibly bad luck. 

Just after 11am on September 18, a "freak gust" blew a "medium-sized" truck onto a moveable lane barrier, and another "was blown sideways and hit the bridge superstructure", NZTA Auckland operations manager Claire Howard said at the time. 

The wind was consistently already blowing at more than 60km/h that morning, readings showed, but spiked to more than double that. 

Coincidentally, scientists at the National Institute of Water and Atmospheric Research (NIWA) had been developing a computer model of wind patterns in the harbour as part of Team New Zealand's preparation for their defence of yachting trophy the America's Cup. 

They combined this 'Auckland Model' with a three-dimensional model of the bridge, looking at how air moves around it - and found the bridge itself causes the wind to speed up.

The modelling found speeds at road level and between the struts - one of which was damaged and had to be replaced - are 10 to 15 percent faster than the surrounding wind. While the modelling correctly identified the timing and direction of the gusts that September morning, it actually underestimated just how much faster the wind would get as it hit the bridge.

"The effects here are very localised and it is really important to understand these better because of the risk high wind events have to a range of assets such as transport and distribution networks and the potential knock-on to economic impacts," said Richard Turner, a NIWA meteorologist and expert in wind hazards.

The research was partially funded by the Government's Resilience to Nature's Challenges programme, which aims to "accelerate Aotearoa New Zealand's natural hazard resilience through innovative, collaborative science". 

"I imagine that if those trucks hadn't been in the position they were at that exact time, this wouldn't have been such a big news event," said NIWA meteorologist Stuart Moore.

NIWA hopes the research will lead to a warning system that could halt traffic on the aging bridge and prevent a repeat incident. 

"This was an interesting event because it lasted for just a short period of about 10 minutes when the wind really picked up suddenly and equally quickly died down again. It was very dramatic across large parts of the city," said Dr Moore. 

Several lanes on the 62-year-old bridge were closed for a few weeks before a replacement strut could be installed. Commutes to and from the North Shore were lengthened by up to an hour in the meantime.