Disagreement over Dunedin sea level predictions

Professor Jim Flynn has provoked some discussion after making some radical predictions about potential rapid sea level rises in South Dunedin and the rest of the low lying parts of the city.

These predictions have been disputed by other academics.

ODT: Flynn’s sea level figures disputed

Associate Prof Nicolas Cullen and senior lecturer Daniel Kingston have described the predictions by Jim Flynn as “very unlikely”.

On Monday, Prof Flynn told a Dunedin City Council public forum huge erosion of polar ice that had begun in 2014 meant predictions on the rate of climate change had changed.

He said there were 1932 homes in Dunedin, mainly in South Dunedin, built below 25cm, and a 25cm sea-level rise could occur before 2034.

He said by 2090, South Dunedin would be under 5m of water, along with Forsyth Barr Stadium, the Otago Polytechnic and other low-lying areas.

That’s fairly extreme. Many would claim bonkers, with some justification.

…approached for comment on the predictions, Prof Cullen and Dr Kingston disagreed.

Prof Cullen, whose expertise includes glaciology and climate change, said Prof Flynn’s figures were “questionable”.

He said the observed sea-level rise was 3.4mm a year, which was “quite high compared to historical estimates”.

If that rate continued for the next 20 years, there would be a total 68mm sea-level rise – “quite different”from Prof Flynn’s estimate.

“I agree that there is evidence that the two large ice sheets [Greenland and the Antarctic] may contribute more to sea-level rise in the future than previously thought, but I would think it is very unlikely that South Dunedin will be under 5m of water in 2090.”

Dr Kingston, whose expertise includes atmospheric circulation patterns and climate change, said he agreed with Prof Cullen.

“The numbers in that article, like 5m by 2090, are at the very, very extreme end of what’s likely.”

To Prof Flynn’s suggestion the more recent erosion of polar ice had changed what could be expected in terms of sea-level rise, Dr Kingston said, “I don’t think the matter is settled by any means.”

A rise of 5m would mean the entire West Antarctic ice sheet disappearing, or the entire Greenland ice sheet disappearing.

“The likelihood of that happening by the end of the century is low – not impossible – but the real extreme end of the situation.”

Flynn’s claims may cause some concern. Things like insurance cover and resale values of low lying properties won’t be helped by worst case scenarios being portrayed as likely.

I’m at 100, so way out of the danger zone but even a half metre rise would cause significant problems in the drained wetlands of South Dunedin where flooding cause major problems in 2015.

Jim Flynn can be a very interesting dude but he isn’t helping anyone with worst case claims that amount to scaremongering, but perhaps his aim was to attract attention to a real potential problem with a bit of exaggeration.

The problem is that claims like this are easy to dismiss as nuts.

Dunedin council admits flooding fault

The Dunedin City Council has admitted that a blocked pumping station meant that the flooding in South Dunedin last year was about 20 cm deeper than it would otherwise have been if the pumping station was fully effective.

This was revealed at a public meeting where anger at the council was expressed.

RNZ: Dunedin council concedes flood worsened by faulty pumping station

South Dunedin residents have been waiting for a year for its council to front up for the flooding – and last night it did so en masse. At least eight city councillors, the chief executive and her two deputies were quizzed by 200 locals about what happened last June, and what will stop it happening again.

Chief executive Sue Bidrose told the crowd of 200 people the council had reports showing the flood was caused by more rain falling than the stormwater system was designed to cope with.

But Dr Bidrose made a major concession, saying the council now accepted a key pumping station was blocked, adding an extra 20cm of water to the area.

20 cm makes a big difference as to the severity of flooding or whether houses got flooded at all.

Surrey Street, South Dunedin.

Surrey Street, South Dunedin. Photo: RNZ / Ian Telfer

She said the council was fixing the pumping station, had all the drains and mud tanks in South Dunedin fully cleared and had new procedures when heavy rain was forecast.

Checking that flood protection was in good working order prior to forecast heavy rain seems a fairly basic thing. That new procedures are required to remedy fundamental flaws does not give one confidence in their council.

I think Sue Bidrose is generally doing a good job trying to sort out problems in the council. There seems to be less public confidence in the mayor.

But it will take more than her words to sort out the ill feeling with residents, who said they felt neglected and betrayed by the council, and especially by Dunedin Mayor Dave Cull.

Shortly after the flooding, Mr Cull linked the event to climate change and warned South Dunedin may have to beat a managed retreat.

Local woman Kathinka Nordal Stene said she was shocked Mr Cull undermined the community at the time when it most needed his support.

Kathinka was a neighbour of mine in Central Otago when I was very young and she was quite a bit older. I don’t remember much of her, but good to see her in action on this. Her mother was prominent in  protest against the flooding of Lowburn through the Clyde dam last century.

The ODT also reported on the meeting: Anger about South Dunedin’s future

Unanswered questions about the long-term future of South Dunedin and the city’s response to climate change loomed large at a heated public meeting last night.

Attendees heard about the short-term measures the council had taken or was about to take to ensure South Dunedin’s infrastructure would run at full capacity should there be a repeat of last June’s devastating flood.

But the meeting was at its most heated when the long-term future of the area was discussed. Council representatives said it was too soon to say how it might respond to the difficult issues posed by rising sea levels and groundwater caused by climate change.

Ironically…

…SDAG spokesman Ray MacLeod finished the meeting by praising some of the council’s short-term measures, but had harsh words when it came to what he perceived as a “green” agenda on the council.

This agenda had resulted in a council policy of “strategic withdrawal” from South Dunedin “by stealth”, Mr MacLeod said.

Perhaps a fairly Green leaning council that has been seen to have put a priority on cycleways through South Dunedin (that were poorly designed and had to be rebuilt so fire engines could use the streets) could shift focus to pedal powered boats, although mobility scooters are probably more relevant to South Dunedin than bike power.

Seriously, what is particularly concerning is that a Green council that has stated an importance on preparing for climate change was so poorly prepared for a bit of heavy rain.

Council chief executive Sue Bidrose earlier said there was no such plan, but both she and Mayor Dave Cull, whose speech was read out by Acting Mayor Andrew Noone, said such an option could not be counted out, given the serious threat posed by climate change.

There has been more threat from basic council ineptitude.

But it is no wonder some of the many residents of South Dunedin are concerned about the council not counting out a plan for a “strategic withdrawal” from South Dunedin.

Much of the South Dunedin area was swamp in the 1800s before being drained.

Prior to European settlement, much of the area of The Flat was poorly drained and marshy.

Much of the swampy land of The Flat was drained through the efforts of Chinese settlers were notable among early residents in the St Clair area, and largely through their effort the swampy land inland from the beach was drained and converted into market gardens.

https://en.wikipedia.org/wiki/South_Dunedin

There have been potential problems with drainage, low level land and high water tables long before ‘climate change’ and rising sea levels were raised as a potential future problem.

Climate change and South Dunedin flooding

Major flooding in South Dunedin last year is sometimes cited as a sign of future problems with climate change, but it isn’t a simple issue.

Last week the Commissioner for the Environment spoke in Dunedin, reported by the ODT in South Dunedin flood risk ‘an issue’:

‘‘The most troubling example of this in New Zealand, at least in the short term, is South Dunedin,” she said.

‘‘As the sea rises, that groundwater will rise higher and higher over time.”

Last June’s flood was an example of what was to come.

‘‘This flooding from underneath will become more and more of an issue.”

However, the warming of the oceans presented the greatest risk of flooding.

Much of South Dunedin is less than a metre above sea level. It is drained and reclaimed swamp (originally turned into market gardens by Chinese settlers). So obviously a potential rise in sea levels is a concern.

Last year’s floods occurred after the second worst rainfall since records began in 1918. It’s impossible to say if this event was a symptom of climate change or not.

But the causes of flooding were varied and some at least were due to local human effects and poor maintenance of drainage infrastructure. A just released report highlights some of these problems.

ODT: Mud-tank maintenance failure

A long-awaited report into last June’s flood has found 75% of mud tanks in South Dunedin were not properly maintained.

A Fulton Hogan spokeswoman said in a statement last night it was confident it fulfilled the requirements of the contract.‘‘Due to the age and historic nature of the stormwater network, including the mud tanks, effective maintenance is a challenge.”

The failure of drainage infrastructure has been talked about for a while.

And:

The report also raised longer-term issues about the state of South Dunedin’s infrastructure and said further work would be done to find out what investment was needed.

It found changes in South Dunedin meant the stormwater network, much of which was installed in the 1950s and 1960s, did not perform as well as intended when built.

This included the expansion of hard surfaces, which had increased the amount of rainfall that needed to be dealt with by the stormwater system from 45% to 60%.

And:

In a separate issue, it found the screen on the Portobello Rd pump station was unable to be cleared of debris due to the severity of the flood, an issue that would be fixed when modifications were made this year.

So whether the rainfall was a hundred year type weather event or a symptom of climate change there are a number of other factors involved.