June/July finish for Kotuku detention dam

Construction of a massive stormwater detention dam designed to better protect central Whangarei from damaging floods is edging ever closer to completion in a single season.

Two workers standing on the earth cut at the top right of this image illustrate the scale of part of the huge dam as it takes shape.

Northland Regional Council member Craig Brown, who chairs the Urban Whangarei Rivers Liaison Committee, says progress on the massive $11M-plus dam – the largest construction project of its kind ever undertaken by the council – is being reviewed weekly.

As the 18-metre tall dam takes shape, those weekly reviews gauge actual progress against detailed targets designed to manage the risks heavy rain could potentially create given the dam's partially-built status.

When complete, the dam is designed to capture up to 1.27 million cubic metres of floodwater during heavy rain in a matter of just hours, equivalent to about two-thirds the capacity of the existing Whau Valley dam.

"Kotuku dam is specifically designed to trap vast amounts of stormwater and then slowly release it over several days," Cr Brown says, "Obviously, we need to make sure that if large amounts of rain are forecast during the build, the dam is able to cope with that safely at whatever stage of construction it is at."

Councillor Brown says construction had earlier fallen behind schedule for a number of reasons, including the complex nature of the build and issues with soil types found in some spots.

"This had prompted serious consideration to calling a halt to work altogether until the new construction season in October, however, after good progress of late, things are now more or less back on track for a single season construction and should hopefully be finished by June or July – subject of course to those ongoing weekly reviews."

When finished the new dam, which sits at the ends of Raumanga Valley Road and Kotuku Street, should slow floodwaters and cut flood depths in the Central Business District by up to half a metre, potentially saving the CBD and parts of Morningside and Raumanga millions of dollars in flood-related damage.

Councillor Brown says much of the damage caused by floods depends not only on how deep they are, but importantly, how fast they're travelling.

The dam is being paid for via a targeted rate on more than 17,000 properties in the CBD and contributing catchment areas.

Those interested in learning more about the project can visit: www.nrc.govt.nz/kotukudam

Progress can also be viewed via webcam at: www.nrc.govt.nz/webcams and clicking on the Kotuku Detention Dam Webcam link.