Whangārei's town basin and central city face twin threats from sea level rise and flooding as this photo from January this year shows Photo / Tania Whyte
Whangārei's town basin and central city face twin threats from sea level rise and flooding as this photo from January this year shows Photo / Tania Whyte
A harbour entrance causeway to control flooding and sea level rise impacts for Whangārei city and the proposed Marsden Point port rail spur should be considered, a Bream Bay councillor says.
Whangārei District Council (WDC) Cr Ken Couper’s call came at a council briefing meeting last week where the council’s acting stormwater manager Andew Carvell addressed politicians about managing Whangārei flooding and the impacts of sea level rise.
The latest developments in addressing Morningside’s flooding issues were also looked at.
Cr Paul Yovich said dealing with flooding affecting Whangārei city should be about the big picture.
A causeway across the harbour from Morningside’s Rawhiti St to Onerahi or from Rawhiti St to Port Nikau would help address the suburb’s flooding impacts on a larger scale.
But Couper said that concept should be expanded further to the whole of Whangārei Harbour. A harbour-mouth causeway would also protect the former Marsden Point oil refinery.
The 100 square kilometre Whangārei Harbour drains a 300sq km catchment.
Deputy mayor Phil Halse said the council should stop granting resource consents for development, which came with hard surfaces, in inappropriate areas.
The council has been looking at putting in gates on Kaka St to shut it off during flooding to stop cars driving through and creating waves that affect local businesses. Photo / Tania Whyte
Morningside had not flooded in the same way before Mitre 10′s store had been built in the suburb. Pacific Motors’ Porowini Ave site also had a large area of hard surfaces.
Mayor Vince Cocurullo said the council was looking at putting in gates on Kaka St to shut it off during flooding to stop cars driving through and creating waves that affect local businesses.
Efforts to achieve this to date, with cones, tape and staff, had been ignored by drivers who continued travelling through floodwaters.
Carvell said a combined storage tank and pump system would be the likely best solution arrived at for Morningside flooding.
This would be a combination of a 60,000 cubic metre storage tank and 2000-litre-per-second pumping station, its principles much like the Hikurangi Swamp’s flood management scheme.
He said it would be cheapest if the tank could be above ground. A location under the ground at Morningside playground near Morningside School would be the likely best option for an underground tank.
Morningside’s 110-hectare catchment is made up of mostly impervious land, half of this steeper land along with 22.5ha of lower flats including Morningside School, Kaka St and Porowini Ave.
Carvell said short, intense storms of at least 30 minutes caused Morningside’s worst flooding, because of its small steep catchment.
He said the suburb’s stormwater systems could deal with one-in-two-year storms, as long as its channels were clean and the tide was not high at peak rainfall times.
“The impact of flooding is exacerbated by climate change,” Carvell said.
He said a feasibility study for the catchment’s flood management would likely be finished by December.
Work was proceeding towards the best option, which could be in place by July 2024, depending on available council budget.
Cr Yovich’s causeway call would involve water behind the structure being drained to a lower level ahead of heavy pending rain, so it could accommodate expected runoff from up the catchment.
This rainfall would then be released down the harbour on appropriate outgoing tides.
Carvell said any causeway development would need to be considered in conjunction with the community. Ecological and environmental considerations were also important.
Carvell said the Whau Valley dam could, to a degree, potentially also be used in the same way to reduce Whangārei city flooding from further back up the harbour catchment. Careful thought would be needed to the dam’s provision of city water supply as part of that.
A sign of the times during Morningside Whangārei flooding in March last year. Photo / Michael Cunningham
Cr Yovich said consideration of Whangārei flood modelling and management needed to take the city’s northern area into account.
Further major development was planned for Vinegar Hill with a major roundabout at Springs Flat. The stormwater from this area would flow down to the city via Whangārei Falls.
WDC acting stormwater manager Andew Carvell said upper catchment areas of the Whangārei city’s flood modelling area - from Springs Flat and Vinegar Hill in the north, west to Whau Valley dam and Kara Rd and south to the southern end of Port Nikau – were affected by different flooding issues.
Low-lying areas such as the Town Basin were threatened by coastal inundation. They were heavily affected by tides whose impacts reached the confluence of the Waiarohia and Raumanga rivers and stretched back up the Hātea River as far as Mairtown’s Rurumoki St.
Residential areas such as Mairtown, that were above flood plains, were threatened by stormwater network constraints.
Properties next to waterways in places like Spring Flat and near Waitaua Stream were threatened by localised flooding and constraints in the natural course of their flow.
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