Responding to increased flood risk in South Dunedin and creating an attractive future for the community could cost up to $7.1 billion by 2100, a new report suggests.
Extensive work in such a scenario could include elevating some of the land, pursuing redevelopment there, shifting homes and creating a waterways network.
An alternative option of persisting with a status-quo approach might have its own price tag of $2b over 75 years, but the spend would have few benefits.
Several options are on the table — each one costing billions of dollars — and they have been described as sitting somewhere on the spectrum of "fight and flight".
The "fight" label denotes an approach that would lean heavily on using built infrastructure to boost flood defences.
"Flight" would mean large-scale relocation of homes and infrastructure to lower-risk areas.
Whichever approach ends up being favoured, hundreds of properties will be affected.
Sticking with the status quo could affect more than 5000 properties, while large-scale retreat could affect 3500 to 4000, the report said.
The South Dunedin Future programme was set up by the Dunedin City Council and Otago Regional Council after a major flood in 2015.
South Dunedin Future is developing a comprehensive climate change adaptation plan, and two significant reports were released this morning.
This analysed risks from natural hazards such as surface flooding, coastal inundation and shallow groundwater.
The assessment showed overall flood risk was already high for a large part of the area known as The Flat and this would increase over the next 75 years with climate change, the councils said in a joint statement.
The second report contained seven options for what might be done about the situation.
They had labels such as "Keep the land dry — pipes and pumps", "Space for water — waterways and wetlands" and "Let water in — large-scale retreat".
Councillors from both councils are to consider the material next week and public input is then expected to further shape the eventual approach.
The programme released a list of 16 possible approaches last year and they were combined in different ways to develop seven potential "adaptation futures".
This is expected to be followed by a shortlist of three or four options and then the working out of an agreed approach by the end of next year.
South Dunedin Future programme manager Jonathan Rowe said some options looked very different from how the area was viewed now.
"In all scenarios, you’re looking at several-hundred properties at a minimum that would be changing, and in some, the majority of the suburb," Mr Rowe said.
Projected costings were described as high-level estimates from consultants and not all would fall on ratepayers and taxpayers, he said.
"In some instances, it might be individuals that cover some of these costs, or it might be private developers or insurers or banks, or others."
Benefits for all the options — apart from the status quo — would run into billions of dollars.
Mr Rowe said the report showed the status quo had the worst overall outcomes.
Information in the risk assessment could be viewed within the community as worrying, but data was also helpful, providing more certainty, he said.
"This process might be scary, it might be unwelcome, it might be confronting for some people, but it’s really essential for figuring out how we respond," he said.
"And I don’t want to underplay the opportunity in all this, that while it might involve a lot of change, there’s enormous opportunity for South Dunedin not just to become safer, but also better.
"So in many of those futures, South Dunedin can have better housing, better infrastructure, more green space, more parks, more wetlands, lower flood risk."
Otago Regional Council natural hazards manager Jean-Luc Payan said the risk assessment would be updated as new data came in.
"It is primarily intended to support suburb-level adaptation planning, so we can test how effective various adaptation options are at reducing risk," Dr Payan said.
Dunedin Mayor Jules Radich said the work had reached a stage where the community could have a clear picture of risks and a sense of how
the future might look for
South Dunedin.
Otago Regional Council chairwoman Gretchen Robertson said the latest reports had blended technical science and engineering expertise with wide community engagement.
"This information will empower informed decisions as communities plan for a resilient future."
