Three leading options for the protection of flood-prone Anzac Pde properties have emerged - but each comes with the question of who will pay.
The solutions have been floated by Massey University researchers Professor Bruce Glavovic and Dr Martin Garcia Cartagena and include strengthening existing stopbanks, raising houses above flood level or buying out property owners.
Garcia Cartagena outlined a total of seven options during public meetings with residents this month but only the three mentioned were feasible and would make a difference.
The first was raising and strengthening stopbanks, which are currently built for a one-in- 30-year flood.
Getting them to a one-in-50-year standard would cost an estimated $6 to $7 million.
But there would still be the potential for water to creep underground behind them, as it did in 2015, and a Horizons survey found Whanganui residents didn't want to pay for the work.
With climate change, a one-in-50-year flood level may not be enough, the researchers said.
The 2015 flood was estimated as a one-in-130-year or a one-in-150-year flood.
Raising the stopbank would also reduce the amenity value of Kowhai Park.
Buying out property owners was definitely feasible - but it depended on "political will", the researchers said.
Buying out the owners of the 33 properties that would flood by more than 1m in a one-in-50-year flood could cost $9.5 million, at the properties' rateable value.
For the 50 properties flooded in a one-in-200-year flood, it would cost $15.6 million.
It's not certain that Government would pay for this, and insurance probably wouldn't either, Garcia Cartagena said.
Owners elsewhere in New Zealand are in the same situation, and getting Government to pay could rely on major damage and a legal fight.
Raising houses above flood level is possible for some, the researchers said. The cost of this is being calculated and will be available next year.
It depended on the individual house construction, and the raised houses could obstruct neighbours' views.
Also, the soil in Anzac Pde is prone to liquefaction and very deep piles would be needed.
Anzac Pde resident Des Thiele and his wife bought their house 10 years ago and were flooded in 2015.
"Nothing has happened since, and nothing is being offered as a solution," he told the Chronicle.
"The only real thing is an early warning notification. Last time we had half an hour's notice. If we had known at lunchtime it would have been better," he said.
David Cotton, chairman of Horizons' catchment operations committee and a councillor didn't want to voice an opinion about the flood resilience options until he has heard from all the stakeholders.
"I want it to come to me with fresh eyes," he said.
He's delighted with the information Massey has gathered in its $130,000 contract. One thing that stood out was how few Anzac Pde residents knew about his council's early warning system. Another was how relaxed residents who hadn't been through a flood felt about their prospects, compared to those who had been through a flood.
"The take-home message for me is it's time to make a decision - a long-term strategic view for the next 10- to 15 years," Cotton said.
Fellow councillor Nicola Patrick has been frustrated by the delay in a solution for Anzac Pde, but also thinks responding straight after a big flood might not have been the best.
"That said, we are constantly just missing another major flood."
The current Horizons councillors will make the right decision, she believes, when they have all the information.
Flood resilience for Anzac Parade is a complex and expensive issue, she said, with considerations about affordability and precedent-setting.
"I'm waiting to see how it all comes together and whether there might be a multi-faceted, multi-year solution. That's where I think it's going to end up."
Massey University has been contracted to amass the information about flood protection that Horizons Regional Council will need before making a decision on action.
Its report is due with the regional council in July or August next year.
As part of it, researcher Kath McDowell interviewed 87 residents during October.
She found 32 of them had bought their property since 2015, and there were also 14 new tenants. Another 35 of those she interviewed were living in Anzac Pde during the flood and 30 per cent knew about Horizons' early warning system and used it.
Low-lying houses in Anzac Pde had their worst recorded flood in June 2015, when the Whanganui River rose to 4775-5150 cubic metres per second (cumecs).
There were 16 properties where water levels exceeded 1m, 13 of them downstream from the Dublin St Bridge.
Glavovic said rainfall and flood danger was increasing with climate change.
He said a "hundred-year flood" had a one per cent chance of happening in any year - but that's just probability and it could happen more than once in a year.
"Is once every 10 years comfortable? What about once a year? Would that change your view?" Glavovic asked.
Other possible solutions included work on the Matarawa Stream, beefed up early warning systems, changes to land use in the catchment and dredging.
The Matarawa Stream backed up in 2015 because the Whanganui River was so high. All of its water could be diverted through the Mateongaonga Stream, but that would make no difference to flood levels, Horizons' northern region engineer Wayne Spencer said.
Dredging the riverbed has been talked of as a solution but Garcia Cartagena said the 2015 flood wasn't caused by a river channel clogged with sediment.
In fact, the channel had more capacity in 2015 than in 1995.
Dredging will be done as part of Te Pūwaha, the port project, but only in the port area and to aid navigation.
For more on the Anzac Pde Resilience-Building Project, see www.horizons. govt.nz/anzac-parade.