Niwa scientists have found the Auckland Harbour Bridge itself helped increase wind speeds to a point at that trucks toppled over.
Part of the bridge had to be closed for repairs for more than two weeks after two trucks fell over in high winds in 2020. The wind gust measured 127km/h at the top of the bridge arch.
At the time, Niwa scientists had been operating a weather forecasting model known as the Auckland Model to predict wind shifts in the Hauraki Gulf, so scientists Drs Richard Turner and Stuart Moore and modeller Amir Pirooz wondered how accurately it could also have predicted the high winds on the bridge.
The results showed that the Auckland Model, which forecasts weather on a much finer scale than other weather forecast models in New Zealand, was accurate on the timing and direction of the high winds but under-forecast the most extreme wind gusts observed on 18 September.
"This is fairly typical model behaviour but what we wanted to know was whether the bridge structure itself had a hand in increasing the wind speeds to the point at which the trucks toppled over," Dr Turner said.
To find out, the team combined the Auckland Model with some Computational Fluid Dynamics - a discipline studying turbulence and fine-scale flow around structures.
What they found was that the bridge itself causes the wind to speed up at road level as well as between the bridge struts. The results showed that on 18 September 2020, modelled gust and mean speeds were increased by 10 to 15 percent close to the road surface with a forecast peak gust of 87km/h.
"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," Dr Turner said.
Dr Moore said he wondered why there had not been more incidents like the September one but concluded what happened may just be down to bad luck.
"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 a news event."
Internationally, scientists are constructing vehicle overturning models to predict which types of trucks are likely to topple in particular weather conditions - something that could also be of benefit in New Zealand, according to Niwa.
The team says gaining a better understanding of the detailed airflow patterns over the Auckland Harbour Bridge could help in developing a warning system to avoid the consequences of last September.
The Niwa scientists now want to look at whether wind breaks erected on the sides of some bridges overseas would be helpful on the Auckland Harbour Bridge.