Work to protect Palmerston North houses from a "Noah's Ark" flood is underway, but the flow-on effects are threatening properties outside the city.
Horizons Regional Council will soon begin talks with Aokautere and Longburn property owners who will bear the brunt of flood waters once stopbank upgrades are completed to protect the city.
Work to upgrade the the city's stopbanks to withstand a 4500 cumec flood is in its third year.
The council is now preparing to lodge resource consents for the next two stages, from Albert St to Roxburgh Cres and at Napier Rd.
But modelling work has revealed that water kept out of the city will increase the level of flooding on the other side of the river, and effects previously thought to be minor may be worse than planned.
In particular, houses in the Staces Rd area and Longburn and the Turitea Science Centre by Massey University will be affected.
These areas would have flooded regardless, but the Palmerston North protection works will add up to 150mm of water.
Until mitigation had been worked out, the resource consent application couldn't go ahead, Horizons group manager operations Allan Cook told the council this week.
But the two sections of stopbank were better being dealt with together, he said.
"My preference is to deal with it as one and take the risk of a delay in that work."
It was necessary work though.
While a flood of that size was rare, for some houses it was the difference between having water inside and having no floodwater cross the threshold.
"They are very rare effects, but nevertheless, where we can, we would like to be able to mitigate [them]."
It was important to get that message across to the public, and find the best way forward, he said.
"We suspect the situation, certainly over on the left bank, is probably worse than people understand it to be."
Senior design engineer Graham Doull said it was necessary to work out which properties would get water inside, and the best way to deal with the situation.
In Staces Rd there were four houses that needed particular attention, he said.
But raising the stopbanks on the other side of the river was not an option.
"The water has to go somewhere ... We'd probably double the risk of the city being flooded."
Depending on the building types, options included raising houses so the floor levels were higher, or landscaping so the water would flow around the house. Consent could be sought by September.
AT A GLANCE
Without upgraded stopbanks, 3600 houses in Palmerston North city would have water through them in a 500-year flood or 4500 cumec event.
There has never been a flood of that size on record in Palmerston North the 2004 flood was about 3500 cumecs.
Horizons group manager operations Allan Cook emphasised a flood of that size was of "Noah's Ark" proportions, and incredibly rare.
But the City Reach project was protecting the city against the possibility because of the level of potential damage.
The rural protection standard is 3700 cumecs.
All ratepayers in the Lower Manawatu Scheme pay $17.30 per $100,000 of capital value toward flood protection.
WHAT WOULD HAPPEN?
How different areas will be affected in a 4500 cumec flood after the Palmerston North works:
Staces Rd Aokautere floodplain area: There will be increased flood waters, offset by a rural stopbank upstream. There are 19 houses on the floodplain.
Twelve will flood to a depth of 400mm regardless of whether the Palmerston North works proceed; three will not flood in either case. Four are marginal, and the works will make the difference between flooding and not flooding.
Turitea Science Centre: It is quite floodable in a 4500 cumec event, and the Palmerston North works will make it worse by about 100mm.
Full protection is available, at a cost of about $1.24 million. Talks will be held with affected parties to discuss the best way to fund the works.
Longburn: In a 4500 cumec flood Longburn would experience serious flooding, and this would be made worse by the Palmerston North works.
A lot more investigation is required to determine where the water would go, and the best way to mitigate the impacts.
- KATIE CHAPMAN/Manawatu Standard
Is New Zealand's airport security stringent enough?Related story: Risky objects bypass Wellington Airport security