South Dunedin’s future: ‘Look after the dunes’

Dunedin’s St Kilda Beach. PHOTO: GREGOR RICHARDSON
Dunedin’s St Kilda Beach. PHOTO: GREGOR RICHARDSON
If the sea breaks through St Kilda’s sand dunes "all bets are off" for South Dunedin, a top scientist says.

GNS Science principal scientist Simon Cox yesterday took questions from Otago regional councillors after he presented what he called an "unapologetically long and difficult" report into South Dunedin and Harbourside groundwater.

The 103-page report, produced by GNS Science in partnership with the Otago Regional Council, has been touted as an important tool for determining ways South Dunedin can respond to the challenges of climate change.

The report struck a balance between providing confidence, understanding the problems "and making it clear that you have turned on the light and you are not walking around in the dark", Dr Cox said.

"I would like to think that if I was a property owner in South Dunedin, I would like the best possible science on my property.

"If I’m scared, I want that done.

"I don’t want a broad brush from central government that just sort of says ‘All of South Dunedin is doomed’.

"I want to know that the best possible science is being done on my property."

Simon Cox
Simon Cox
The just-published report said the ground was a lot less permeable than first thought and the land was "a lot less prone than we feared".

It provided rough timeframes for when hazards would emerge.

For the first 30 to 40 years, the primary concern would be rainfall on to ground that was losing its capacity to absorb it as the water table rose.

Then, as sea level rise reached 40cm, the shallow groundwater below the low-lying suburb would begin to surface.

At 60cm to 70cm of sea level rise, coastal inundation suddenly became a problem at Portsmouth Dr, he said.

Providing rough timeframes meant authorities would not get too caught up in one investment, mitigation, or adaptation method if another hazard was going to take over later on, Dr Cox said.

The report, though, was "totally predicated" on protection from the sand dunes, he said.

"If we get a tsunami and then a storm surge event and erosion cutting through those dunes — and the sea breaks through the dunes as it once did in the past ... then all bets are off.

"So look after the dunes."

There could also be further flow-on effects, not yet understood, if the stormwater network became "drowned".

Still, the report — based on four years of study — was leading edge.

It was based on a groundwater monitoring network of 35 bore holes, which although not as widespread, was as good as the one established in Christchurch after the earthquake, Dr Cox said.

Typically, both in New Zealand and overseas, monitoring focused on groundwater as a resource — rather than as a hazard, as it did here.

And even in places that were catching up, people tended to think about the "chronic condition at the end" rather than the transition to that end result, he said.

The South Dunedin groundwater study addressed the years ahead that people had to live through, respond to, get insurance for, or work with banks or buy houses during, and make decisions through, he said.

"That’s where I think it’s quite unique.

"Our ambition has always been to understand the effect of building a city on top of a natural system," Dr Cox said.

"We’ve studied a natural system with natural fluctuations as best we can, but clearly there’s a whole built infrastructure that affects it.

"What we need to do is to go back to looking at the relationships between the two ...

"Rain will make groundwater rise, the next rainfall and the runoff will be affected by whether that’s had time to go down.

"It’s the relationships between the built system and the natural system that for me still have many questions."

He said he wanted to understand more about the wastewater network, the stormwater network and the ground and groundwater around it "and what the vulnerability of the system is".

"That’s an area, I think, where a focus should be."