A Massey University expert says the cost of raising houses built on flood-prone properties on Whanganui's Anzac Parade would be "eye-watering".
Professor Bruce Glavovic and Dr Martin Garcia Cartagena were engaged by Horizons Regional Council last year to develop a strategy for Anzac Parade flood risk and resilience in a changing climate.
The strategy has now reached the draft stage after interviews with residents and owners of around 100 affected houses, community meetings and consultation with iwi and community stakeholders.
When presenting the findings from the study to the Whanganui District Council's infrastructure, climate change and emergency management committee meeting, Glavovic said quantity surveyors had made roadside assessments.
"It is technically feasible but we are looking at $300,000 to over $500,000 to raise one house by one metre," Glavovic said.
"It is much higher than anticipated, in part due to the liquefaction-prone nature of the soils here."
Glavovic outlined the other options explored and said if nothing was done to mitigate the flood risk to affected properties, insurance costs for property owners were likely to escalate.
Householders paying $50K in 2015 are now paying over $70K on average. There would also be significant ongoing costs to local authorities and the Government for infrastructure maintenance and repairs.
There was also the possibility of loss of life and the question of legal liabilities for councils.
Improvements to the stopbank had been investigated and Glavovic said there were geo-technical challenges and concerns about Kowhai Park as a public amenity.
A buy-out of the affected properties under the Public Works Act was also explored, along with discussions on whether that should be a voluntary or compulsory option, whether it should be pre-emptive or post-flood and how renters might be relocated.
"There is no appetite for compulsory buy-outs at this stage," Glavovic said.
Glavovic described his presentation as a "whistle-stop" summary of possible interventions in the medium to long term.
For a one in 200-year flood, the estimated cost of doing nothing would be $11m, the cost of upgrading the stopbank $27 to $33m and the cost of buying the affected homes at market value would be $28.1m.
Glavovic said given the reality of climate change, the one in 200-year event was "arguably" the realistic option.
Councillor Rob Vinsen asked Glavovic if the option of dredging and widening the awa had been discussed.
Glavovic said it was a good question and one that had come up often during discussions for the strategy.
"It is a common perception that it would make a difference," he said.
"The level of the riverbed is not the issue in terms of flooding."
He said the sheer scale of the river meant dredging was not a realistic option.
While Horizons Regional Council is responsible for river management, which includes the safety of the community, Whanganui District Council is responsible for planning and building regulation, stormwater, road and park infrastructure, and Civil Defence response.
Glavovic said the region is fortunate to have the Climate Action Joint Committee to provide leadership on implementing the strategy.
Glavovic and Garcia Cartagena are collecting final feedback for the strategy until June 10 and expect to have it completed by June 21.