Deluge put Whangarei detention dam to its first test

Now you see it, now you don't; the $11 million dam during and after the recent test.

The $11 million, slow-release flood detention dam in the gully between Whangārei suburbs Maunu and Raumanga has finally been put to the test, and it worked.

The deluge on June 20, which dropped about 120mm rain – roughly a month's worth – on urban Whangārei in 24 hours, was the first significant volume of water to collect in the dam.

Called Hopua te Nihotetea, it is designed to lessen flooding in the CBD, parts of Morningside and Raumanga.

But with it capable of holding up to 1.8 billion litres, what was a record rainfall event this month hardly touched its sides.

Water reached about 5 metres up the 18 metre high banks - equating to an estimated 40 million litres that did not gush down Raumanga Stream.

Northland Regional Council deputy chairman and chairman of the Whangārei Urban Rivers Flood Working Group, David Sinclair said the rain in that torrential 24 hours did not fall fast enough or in large enough amounts to have posed much risk downtown.

However, it was reassuring to see the dam acting as it should, temporarily storing floodwater that, in the past, would have fed straight into the Raumanga Stream and merged with other swollen streams, Sinclair said.

"It's important to bear in mind that even in a big storm, Hopua te Nihotetea isn't designed to completely stop the CBD from flooding as the Waiarohia Stream and tidal Hatea River also flow into the downtown area.

The dam's critical role in a large storm would be in slowing floodwaters and cutting flood depths in the CBD by up to half a metre."

Ultimately, the dam is expected to save about $1.4 million in damage annually over its extremely long operational life, which could be more than 100 years, Sinclair said.

Because it's a detention dam, not a storage dam, for the vast majority of the time it holds no water at all.

It was opened two years ago and is being paid for via a targeted rate on more than 17,000 properties.

When it was built it was the largest project of its kind undertaken by the regional council.

Webcams capturing images at the dam can be viewed on www.nrc.govt.nz/Environment/Webcams