Whanganui Council uses ‘groundbreaking technology’ to be build case for Dublin St Bridge replacement

Civil engineer Thorbjoerg Saevarsdottir says the bridge's piles extend about 9m below the riverbed. Photo / Supplied

A drone and a state-of-the-art boat have helped build the case to replace Whanganui’s Dublin St Bridge.

Whanganui District Council civil engineer Thorbjoerg Saevarsdottir said the equipment was used to complete a detailed aerial and hydrographic survey of the bridge and the surrounding area of the Whanganui River.

“The ground surface above water level was obtained using a drone with a Lidar scanner that flew over the site,” she said.

“This allowed us to capture highly accurate surface measurements and to see how the terrain changes around the bridge.”

Lidar (light detection and ranging) uses laser sensors on aircraft or drones that fly over a landscape and capture a 3D view of the land.

She said the hydrographic survey – mapping the surface under water – was done with a remote-controlled vessel.

“It used a multibeam echo sounder, which sends sound waves through the water, to map the shape and depth of the riverbed.

“This groundbreaking technology delivers precise readings and measurements.”

The bridge opened in 1914 and spans just over 300m.

Saevarsdottir said the assessments showed that its existing concrete foundations had “performed well”, including during major floods.

“They are still suitable for the current bridge and its restricted weight limits [6 tonnes].

“The piles extend about 9m below the riverbed.”

The assessments would form part of the detailed business case being developed for NZ Transport Agency Waka Kotahi (NZTA), she said.

It would “explore all viable options”.

The Dublin St Bridge. Photo / Bevan Conley

“This detailed picture will support engineering assessments of the current bridge, design, and planning for the next stages of the business case.

“If a case for replacement is successful, there are several steps, including design and consultation, that need to be completed before any physical work on a replacement begins.

“This could take some years.”

The drone and vessel were provided by engineering consultants WSP.

In a statement, the company said it was the first organisation in New Zealand to gain Maritime New Zealand approval to operate remote controlled survey vessels – the Apache 3 and Apache 4.

“This achievement followed months of application and compliance work to ensure they could be used in rivers, lakes, harbours and coastal waters.”

WSP team leader for surveying, Peter Troon, said in a statement that the Apache 4, purchased from Chinese firm CHC Navigation, was used to check bottom contours around piles and pillars on the bridge.

“When it comes to underwater mapping, it doesn’t get much better than this,” he said.

“Putting a staffed hydrographic boat in locations can be risky.

“The new vessel does away with that. Think of it like a water drone – it just needs a land-based operator.”

In March, Saevarsdottir said there were too many variables to put a final cost on the project.

In the council’s Long-Term Plan for 2024-34, it is estimated to cost $69 million.

If NZTA agrees to fund the project, it will provide 62% of the cost.

Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily Whanganui District Council.