Cliffside slipping was a feature of Auckland's anniversary weekend rains. Photo: Supplied / NZME
Whangārei would cope reasonably well if an Auckland-style rain deluge hit the district, Mayor Vince Cocurullo says.
His comments come as Cyclone Gabrielle, labelled by one New Zealand weather forecaster as one of the "most serious storms of the century" bears down on the top of the North Island.
Auckland-Northland Anniversary Weekend's one-in-200-year rainfall saw four people die, nearly 2000 homes damaged badly enough to have access officially restricted and more than 1500 cars inundated with floodwaters.
So far there have been more than 270 Auckland houses red stickered and more than 1600 yellow stickered.
Auckland airport recorded 245mm of rain on Friday 27 January, its wettest day - part of Auckland's wettest January since records began in 1853. Auckland's Māngere weather station received 859 percent more than its normal rainfall for the month.
Cocurullo, who is also the WDC representative on the Northland Civil Defence Emergency Management Group, said the district would cope reasonably well if such a deluge happened there. There were, however, many variables that would influence exactly how well it coped.
These included the rainfall's geographic coverage, how localised it was, the ground's degree of soil saturation, how much rain had fallen previously and the timing of rainfall in relation to high tides.
Whangārei was not Auckland and the city had a lower population density than Auckland, he said.
Whangārei Mayor Vince Cocurullo. Photo: Northern Advocate / Michael Cunningham
Whangārei was less densely urbanised with more parks per head of population. More green spaces meant the land was more permeable, making it better able to absorb falling rain rather than the rain simply running off over concrete and buildings.
Cocurullo said there had been a lot of infrastructure work done towards building Whangarei's resilience, but that there was naturally however, "always room for improvement".
"There are a lot of things we're still working on," he said.
When asked how well Whangārei's stormwater infrastructure was set up to cope with the demands of a changing climate's rainfall with more intense heavy rain bursts, Cocurullo said the network was up to scratch. It almost fully met existing required national standards, save for a couple of older areas.
The council had recently updated its requirements around stormwater provision through new environmental engineering standards.
He said the time might be coming for New Zealand's stormwater drainage requirements to be reviewed in the face of changing weather. Investigation into tropical Queensland's stormwater infrastructure provision was an option for informing future New Zealand provision of this Three Waters aspect.
Evacuations were part of Auckland-wide Friday 27 January 2023 flood response action amidst huge Auckland-Northland Anniversary Weekend rainfall. Photo: Supplied / NZME
He cited working through different locations, on a catchment by catchment basis, checking the pipes in each and whether they were up to selected specific rainfall intensities, as an option.
That meant asking could a specific catchment's stormwater infrastructure handle a particular amount of rain.
WDC had been working through making sure stormwater and wastewater was not carried together through the same pipes, to reduce the risk of huge amounts of rainfall from getting into the city's wastewater treatment system, and cutting back sewage contamination in any overflowing water, he said.
Whangārei's $11 million Hopua te Nihotetea detention dam in Maunu, had significantly cut Whangārei's city centre flooding since it opened in 2016, he said.
Resilience work also included working on making sure the district's rural roads were up to a standard where they weren't blocked by slips, so could continue to be used in a flooding emergency.
Meanwhile, when asked whether people should be allowed to build in flood and coastal hazard zones, Cocurullo said the council could not stop people building in these locations.
The updated district wide flood and coastal hazard mapping predicts sometimes dramatically increased areas of flooding in a climate change challenged future for Whangārei and Northland.
Cocurullo said the council could only encourage people not to build in flood hazard zones. If they chose to do so, it could only stipulate that all standards including for environmental engineering and building standards were met. Government direction on this would need to be changed through national policy statements which underpinned what council could do.
Whangārei's 489mm January rainfall was the district's highest ever for the month and more than six times its monthly average since records began in 1937.
Local Democracy Reporting is Public Interest Journalism funded through NZ On Air