Cost of essential work on Whanganui River bank will be ‘significantly higher’ than planned

The first port of call at the site is protecting a gas main. Photo / Bevan Conley

Repairing dropouts on the Whanganui River at Somme Parade is likely to cost far more than the $2.5 million originally planned for.

Whatever the final tally, Whanganui District Council will need to fork out 40 per cent of it.

Immediate works were necessary to support the gas main to prevent the risk of damage as a result of any further subsidence.

A report from council transport manager Damien Wood says a gas main, which supplies all properties upriver of the slips and across the river via the rail bridge to Whanganui East, has been put at risk as a result of the dropouts.

Wood told a council operations and performance committee meeting that work began on June 26.

“We will be advancing the construction of the full repair once we have the necessary design signoff and once we’ve completed the temporary works, which we estimate will take four to five weeks.”

Subsidence opposite 131 Somme Parade began during Cyclone Dovi in February last year, with further dropouts following.

The report said the latest occurred in May this year, downstream of the initial site, resulting in a significant increase in the amount of work required to repair the entire site.

Whanganui District Council contractors bored holes near the slips last November to work out what to do with the site.

At present, subsidence was affecting the Somme Parade footpath and berm but not the road itself, Wood said.“That’s not to say further erosion could not impact that, hence why we are proceeding with a degree of haste in the repairs.”

Wood said a final price was yet to be determined but it would be “significantly higher” than first thought.

Waka Kotahi, which will provide 60 per cent of the funding, had indicated its continued financial support.

If it comes in at $5 million the council will need to contribute $2m. That will move to $2.8m if the final cost is $7m.

“Once we’ve got the detailed design signed off we’ll have a quantity surveyor do an independent cost assessment to inform the likely final construction costs,” Wood said.

“We will bring this information back to council. This is essential emergency work. This is protecting our primary access to the suburb of Aramoho.”

A plan for the site has been designed by geotechnical engineering firm O’Sullivan & Associates and involves installing two layers of SP4W sheet piling along the site.

Wood said iwi and hapū believed the design had the least amount of visual impact on the awa (river).

Councillor Jenny Duncan said the numbers around cost had the “potential to get extremely big” and asked whether the work would future-proof the entire stretch of riverbank.

Wood said it wouldn’t but an extension of the work - towards the Aramoho rail bridge - would need to be considered eventually.

Mayor Andrew Tripe said it looked like a site that was very vulnerable to erosion.

“Is there any thought while you’re doing the emergency works to do some preventative works at the same time?,” Tripe said.

Wood said that may not be funded by Waka Kotahi and preventative work would be impacted by budgetary constraints.

“Subject to availability of funds, it is, as always, something that can be looked at,” Wood said.

Councillor Kate Joblin said she was sure that council staff did their very best to get works that “might be described as presentative” into the emergency works category.

Mike Tweed is an assistant news director and 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 the Whanganui District Council.