A slump upstream of the Hopua Te Nihotetea Dam face at Maunu could cost $80K to fix.
The rain-sodden slump of a stream bank means 17,000 ratepayers being targeted to pay for the $11 million stormwater holding dam at Maunu will now be charged an extra $80,000.
In the meantime, Northland Regional Council's Environmental Services department has asked for an advance from the Whangarei Urban Rivers reserve to pay to have the slip fixed.
Hopua te Nihotetea, dam at Maunu. Photo/Michael Cunningham
During April, following heavy rainfall and saturated ground conditions, the slump developed on land about 50 metres upstream of the inlet on the dam across the Raumanga Stream.
The collapse - about 450sq m of loose material - is not on the body of the dam.
"The slip is not affecting the dam function or structure," Environmental Services general manager Bruce Howse said.
Slump at Maunu, Whangarei, hopua te Nihotetea. Photo / Michael Cunningham
However, engineers have recommended a repair to prevent material slipping into the inlet channel and interfering with the dam's release of water.
The estimated $80,000 repair cost is unbudgeted expenditure and will be added to the targeted Whangarei Urban Rivers Management Rate on more than 17,000 properties in the catchment area.
"This will extend the reserve balance repayment by three months on the 20-year term, based on no variations to future expenditure and revenue estimates," a report to the council said.
The dam, officially Hopua te Nihotetea but often referred to as Kotuku because of the street it is accessed from, has held and released water at least twice during heavy rain events already this year, although not of significant volume.
The 18 metre high dam, capable of holding up to 1.27 million cubic metres of floodwater, was opened a year ago after nearly two years of construction.
It works by slowly releasing trapped stormwater over a couple of days, reducing downstream river swelling and flooding. It cost about $11 million to construct.
Hopua te Nihotetea will not completely stop the CBD from flooding as the Waiarohia Stream and tidal Hatea River also flow into the area, but will slow the water flow down the Raumanga Stream and other tributaries and cut flood depths by up to half a metre.
The dam took about 16 months to build and is the largest project of its kind undertaken by the regional council. For the majority of the time, Hopua te Nihotetea will hold no water at all but will come into its own in a large storm.
