How a sinking South Dunedin is dealing with rising sea levels

St Clair beach, Dunedin, around 1905. The sea level relative to the land has risen 14cm since then. Postcard from the National Library of New Zealand collection.

So what would a managed retreat from rising sea levels look like? Lynn Grieveson takes a look at how the Dunedin City Council is trying to avoid the mistakes of Kapiti Coast and Christchurch by educating residents in the former marshland of South Dunedin about the risks of rising sea levels, and by proposing to make new homes relocatable.

South Dunedin is one of the most densely populated places in New Zealand. Many of the small sections were originally tent sites of early settlers and gold miners. The location was flat and close to the rapidly expanding town centre - but it was also a marshy wetland previously occupied by wading birds and eels and bordered by mudflats and lagoons.

Undeterred, those early residents filled in the swamp with dirt and rocks from surrounding hills and sand from the nearby dunes and beach, and built more permanent homes.

Now Dunedin City Council is telling current residents that any future homes on the land may have to be another sort of temporary structure – not tents, but relocatable buildings – because the land is sinking and sea levels are rising.

Letting it sink in

Around the world, local governments are wrestling with how to plan for the day that rising sea levels and increased rainfall mean it is no longer viable for a town or development to remain in its historical location. The name for the process of abandoning the homes, buildings and infrastructure too susceptible to the impact of climate change is “planned”, or “managed”, retreat.

Not surprisingly, it is always a challenging discussion to have with residents and land owners.

Attempts by Kapiti Coast District Council and Christchurch Council to include coastal hazard notifications on LIMs ended in failure, following ratepayers' revolts and independent reviews.

Dunedin City Council and Otago Regional Council are hoping that an approach of slowly educating and informing residents about the geological realities they face will allow a plan for managed retreat to gradually develop.

In this they are following an approach that meets with the approval of the Parliamentary Commissioner for the Environment, Jan Wright, who says when it comes to talking about the worst-case scenarios of climate change, “if it's a more of a slowly unfolding thing it's a bit easier”. She has previously described the effects of climate change as a "slowly unfolding red zone".

“What went badly wrong in Kapiti and Christchurch, apart from far too much zeal connected with the problematic word 'precautionary' … is that people were just getting a letter in the letterbox, where it has gone from go to whoa in one step,” she says.

“In east Christchurch they had just got through the earthquakes then they got a letter saying you have got to worry about things in 100 years’ time – ‘really? The end of the century is not our concern at the moment’.”

So Otago Regional Council now has a series of videos on its website, where natural hazards analyst Sharon Hornblow talks about how that makeshift reclamation of the South Dunedin marshes has been settling down, resulting in subsidence. And how the land is so low-lying that a lot of South Dunedin is actually below the level of the high tide.

Add to that data showing that the sea level at Dunedin wharf in relation to the land has gone up by about 14 cm in the last 100 years, and any residents watching the videos may well get a sinking feeling.

"We've made it a lot easier to live on, but beneath the surface there is still a very dynamic, marshy sort of environment which responds to different groundwater conditions and the rise and fall of the sea," Hornblow says in one of the videos.

Mitigating the loss

But despite the educative approach, the response to the proposal by the Dunedin City Council that new buildings in the so-called "coastal hazard zone" (including South Dunedin) must be relocatable was met with opposition not just from land owners and developers but also from the District Health Board and even the Ministry of Education.

The original proposal in the Council's proposed new district plan (its 'Second Generation Plan' or 2GB for short) called for not just new homes in the zone to be relocatable but also "sensitive" buildings where people were regularly present and often asleep or vulnerable - such as schools, hospitals and motels.

This is intended to mitigate losses in the event of a managed retreat due to future worsening climate change effects.

Both the Southern District Health Board and the MOE, who have a hospital and schools in the affected zones, described the requirement as impractical and too prescriptive.

The Council has now pulled back, with planners instead recommending that all new homes under nine metres in height must be relocatable.

"I think probably the focus of that provision was always on houses rather than bigger, commercial buildings," says Dunedin's City Development Manager Anna Johnson. "I guess over a certain size and construction you can't really design buildings to be relocatable. It's more of a pragmatic change than a policy change."

She says the requirement for new houses to be able to be moved elsewhere once sea level rise becomes more extreme remains in the proposed district plan.

Adding that the hearings into the 2GP's natural hazards provisions, including the relocatable requirement, start next week (April 19th), Johnson says: "That is what the Planner is recommending; that that be retained. The Planner does have the right to revise the recommendations at the end of the hearing, but they obviously haven’t had any evidence presented to date that's caused them to rethink."

The hearings will run until the beginning of June, with a decision on the 2GP expected at the end of September at the earliest.

Do we stay or do we go?

Despite the recent extreme weather events, it is probably still a long time before South Dunedin becomes one of our first large-scale managed retreats.

But the reaction to this attempt to mitigate the future losses to home owners shows how challenging planning for it can be for local governments.

Limiting or placing restrictions on new development, as Dunedin City Council is attempting, is the first step.

The next step is engaging local communities in deciding on the triggers that should set the various phases of a managed retreat in motion.

A 2014 study by the University of Melbourne found that residents surveyed in the Lakes Entrance region of Victoria said that "five or more days of flooding of the main street or the main road into town" would be a first "trigger for adaptation", resulting in more stringent controls over new developments and starting to plan for the relocation of critical infrastructure. The next trigger, leading to relocation of infrastructure and homes, would be "two or more 1:100 year flood events in a year."