Kotuku detention dam project may spread into second season

Construction of a massive stormwater detention dam designed to better protect central Whangarei from damaging floods may need to be spread over a second construction season.

Northland Regional Council member Craig Brown, who chairs the Urban Whangarei Rivers Liaison Committee, says contractors believe the $8.6M-plus dam – one of the largest engineering projects the regional council has undertaken – can still be completed early next year as planned.

However, with the project currently more than a month behind schedule, the council may next month call a temporary halt to work until the next construction season in October.

“The delays to date are largely attributable to the complexity of the build and aren’t unusual on a project of this size,” Cr Brown says. “But what we do want to avoid is a situation where we get a partially-completed dam embankment exposed to winter storms; it’s better to simply defer embankment works altogether until October.”

An issue with a small area where the soil was more unstable than initial extensive testing had indicated also needed to be addressed.

Councillor Brown says while delaying the build would likely add several hundred thousand dollars to the overall cost of the project, one bonus is that it would reduce the amount of time the partially-completed dam would be exposed to peak cyclone season in late summer-early autumn.

He says the irony of potentially delaying construction of a dam designed to help prevent the Central Business District from flooding due to the risks posed by bad weather is not lost on councillors.

“It’s in all parties’ interests to try to ensure all works are completed in just the one season as originally planned and everyone’s working hard to achieve this, but given the huge amounts of rain that fell last winter, I think it’s fairly obvious why you wouldn’t want a dam with a partially-completed embankment exposed to that.”

He says ultimately, the 18-metre high dam will hold up to 1.27 million cubic metres of floodwater during heavy rain, about two-thirds the capacity of the existing Whau Valley dam.

The Kotuku dam is designed to trap vast amounts of water and then slowly release it over several days. It’s being paid for via an existing targeted rate on more than 17,000 properties in the CBD and contributing catchment areas.

Councillor Brown says the likely timeline will become clearer next month and a call will probably need to be made then as to whether the project continues – or is delayed until October.

Irrespective, he was confident the main contractor, an experienced Rotorua-based civil contracting business that had successfully completed a number of challenging civil construction projects, would deliver a wonderful asset.

“The most important thing is that we achieve the best outcome for our ratepayers and I have every confidence we will do so. This dam has the potential to reduce flood-related damage worth millions of dollars from just one flood and will be money well-spent.”

Councillor Brown says much of the damage caused by floods depends not only on how deep they are, but importantly, also how fast they are travelling. “When finished, Kotuku dam will help address both of these, slowing floodwaters and also cutting flood depths in the CBD by up to half a metre.”

Councillor Brown says information on the project is available from the regional council’s website via: www.nrc.govt.nz/kotukudam

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