South Dunedin map; overview of S.Dn and sea level rise

Charlie Mitchell's article on stuff.co.nz is a look from several angles at South Dundin. The history of growth, 2015 flood, ongoing studies into drainage and water table, factors that add risk to the suburb, and social issues. I've picked out a few passages to tempt readers to go to the whole article, which includes photographs and maps.

DOWN UNDER

The community most-exposed to sea level rise is also one of the poorest

Decades before we knew climate change was anything to worry about, a community was built over water in South Dunedin. Now, as it rises around them, ‘water’ is a fearful word for the people who live there.

CHARLIE MITCHELL
National Correspondent

https://interactives.stuff.co.nz/2019/11/south-dunedin-climate-change-sea-level-rise/

.......For the southern suburbs of Dunedin, one of those floods began on the morning of June 3, 2015. A low pressure system from the south had been expected to cause “persistent rain, heavy at times” across coastal Otago, MetService had said that morning, but the rain proved to be more intense than forecast. 

By around midday, a month’s worth of rain had already fallen on coastal Otago. The same amount had fallen again by 6pm, and once more by 6am the following day, when it finally stopped. In South Dunedin, over the course of the day, flooding had caused ponding up to half a metre deep in some streets, floodwater likely contaminated with sewage spilling into houses and businesses.....

......As the city grew, primarily on the back of the gold rush, it repurposed those flat but swampy areas south of the city centre for housing. The housing was of low quality, with dubious foundation. The swamp was filled with whatever filling material was on hand, usually sand, and filled to only slightly above the water table....   

....When The Flat was developed, it probably wasn’t understood that a flat area by the sea, beneath a large, hilly catchment area, functions as a basin. It might not have been a problem until the population grew and the ground was paved with asphalt, and modern life necessitated the use of water that needed to be disposed of within the restrictive laws of gravity, meaning this water, with or without human intervention, would inevitably travel downwards....

 ...Until recently, there were only four groundwater bores in South Dunedin, all installed within the last decade. There are now 17, each of which records groundwater levels every 10 minutes. 

In the near future, this data could be used to model the probability of groundwater flooding in particular areas, an exercise similar in nature to weather forecasting....

...The pipes are old, which means they leak. This may, strangely enough, be a good thing, at least in one way – it is likely the high groundwater is being drained by these leaky pipes. It raises the possibility that upgrading the network could be a hindrance, rather than a help.

It also means that combined with the pumping at the surface, South Dunedin has been kept artificially dry for a long time, which may have concealed the severity of the problem....

 ...A few months after the 2015 floods, then Parliamentary Commissioner for the Environment Dr Jan Wright released a report into sea-level rise. ... It made it clear that South Dunedin was the single biggest community in New Zealand exposed to sea-level rise, at least in the medium term. 

The report found around 9000 homes nationally were within 50cm of the mean high tide mark, 2700 of which were in Dunedin, more than Auckland, Wellington and Christchurch combined....

 

...“There’s an archetype of South Dunedin, as people tend to see it – a low socio-economic area – but just like any other place, people love it for a range of reasons,” says Dr Janet Stephenson, head of the Centre of Sustainability at the University of Otago, who researches community adaptation to climate change. 

“That sense of attachment and passion is something we have to take into account when we’re looking at climate change impacts.”

The researchers put together an interactive map in which they could overlay social and economic data with a height above sea-level map, to see if there was a correlation.

There is. The areas closest to sea-level tend to score highly on the social deprivation index, meaning its residents are likely to have low incomes and to be renting their homes....

 ...After the 2015 floods, there was a breakdown in trust between the community and the council. The dispute was somewhat complicated, but it centred around whether the extent of the flooding was inevitable, or had been exacerbated by poor management of South Dunedin’s infrastructure.

The council had been quick to attribute the flooding to the sheer amount of rain; the day after the rain stopped, then mayor Dave Cull cited climate change as a potential contributor to the flooding, and floated the prospect of managed retreat at some point in the future....