Properties on Anzac Parade were inundated during the 2015 flood in Whanganui. Photo / NZME
A one-in-200-year flood in Whanganui could kill eight people and damage more than 4000 buildings.
That is according to Horizons Regional Council’s regional flood vulnerability assessment (FVA), completed after 18 months of work.
A report from the Horizons catchment operations group manager, Dr Jon Roygard, said Whanganui was the second-ranked town or settlement for flood vulnerability in the Horizons region, behind Palmerston North.
“Whanganui has 34,005 buildings, of which 8017 are estimated to be exposed to flooding, with 4319 damaged by flooding,” it said.
“The reinstatement cost of this damage is estimated at $394.4 million.”
The report said 23% of the city’s resident population was exposed to flooding, with an estimated eight fatalities from swiftly rising water in some places.
Roygard told the Chronicle the first part of the assessment involved looking at Horizons’ flood modelling from 1990 onwards.
“We did a stock-take of what we had and where, and pulled it into one location.”
The models did not cover the whole region, and existing ones had to be reviewed for their suitability and where they were useful, he said.
They range from level A to D, with A being a basic assessment and D being site-specific, including all assets and structures.
“We collect a lot of flow information, as do other agencies like Niwa [National Institute of Water and Atmospheric Research, now part of Earth Sciences NZ], so we also updated that to try and get records as long as we could, then did some flow statistics,” Roygard said.
“What we would normally do is target a particular area, but we looked to produce a full regional model.
“Twelve models were produced for big areas across the region, then we pulled them together into a single flood map.”
Flow information refers to the measurement, monitoring and forecasting of movements in rivers, lakes and groundwater.
Roygard said the FVA was “very high level” and did not include stormwater networks.
“We have 52 towns and settlements, and we want to work through the relative flood risk for each of them.”
At a Horizons meeting on February 24, Roygard said it was no surprise Whanganui and Palmerston North ranked first and second for flood vulnerability because they were the region’s two biggest urban centres.
Woodville was third, followed by Levin and Foxton.
Marton in Rangitīkei District was eighth.
He said Taihape (Rangitīkei) and Taumarunui (Ruapehu) ranked comparatively high for estimated fatalities in a one-in-200-year event, at three and nine, respectively.
Horizons worked with Earth Sciences NZ and other partners to calculate the number of potential fatalities involved in flood events, Roygard told the Chronicle.
“What we would caution is that those numbers are broad estimates based on a technique.
“They won’t be absolutes, and we work really hard to prevent fatalities with our early warning systems and emergency response plans.”
Speaking to the Chronicle last year, Horizons’ emergency management officer, Chay Hook, said the public could sign up to the council’s river alert system, which automatically calls people if rivers and streams reach a certain height.
“For us, it’s about preparing for the next event. We take the lessons learned and lessons identified previously, and put it into practice with the resources we’ve got,” Hook said.
Roygard’s report said the 12 maps produced for the region had flood predictions for 20-, 50-, 100- and 200-year rainfall events.
Three climate change scenarios, for warming of 1C, 2C and 3C, for the 200-year event were also run.
According to the FVA, a one-in-200-year flood with 3C warming in Whanganui could result in 126 deaths and damage to 9522 buildings, with damage of $976m.
The major flood in Whanganui in 2015 was deemed a one-in-130-year event, with a 0.77% chance of it happening in any given year.
Roygard said a one-in-200-year event was often talked about as terminology for a flood, but Horizons used annual return intervals (ARIs) and annual exceedance potentials (APIs).
“With an ARI, you can expect one that size within a 200-year period,” he said.
“[An AEP is] more like a probability. There is a 0.5% chance that-sized event will happen this year.”
His report said the FVA highlighted the need for regional councils and district and city councils to work together on flood resilience to produce more accurate models and to align policy responses and works on the ground.
At the meeting, Roygard said a “textbook example” of a localised flood mitigation initiative was at Pūtiki in Whanganui.
“We’ve gone about updating flood information, produced a report and had that agreed with the district council, NZTA [NZ Transport Agency] and the community,” he said.
“Engineers have come back with a report saying, ‘here are 10, 15, 20 different options to look at, ranging from this price to that price’.
“That’s still quite high level because we haven’t spoken to any landowners or worked out if it’s feasible.”
He told the Chronicle the FVA was a starting point.
“Like many things in the technical space, you’ve got to do it the first time to get to more refined versions.
“We know there is more work to do.”
Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is on local government, primarily the Whanganui District Council.