Tide turns on ommunity
Decades before 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. National correspondent Charlie Mitchell reports in the first of a two-part series on the community’s challenge.
The rain started just before the sun rose, and didn’t stop for 24 hours. It was one of those floods where, if you were exposed to it, you remember the year, the month, the day, a long time afterwards; one of those events that marks a change in the social fabric of a community, that becomes apparent only after the roads have reopened, after the sodden carpets have dried.
They keep you on edge any time it rains, when the gutters start to pool and it threatens to start all over again.
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 about 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.
The army was called in to help sandbag coastal homes, and one of its Unimog trucks was sent to evacuate children from a primary school. A retirement home was ruined, and its residents, some of whom had dementia, were unable to return for months.
When all was said and done, about 1200 homes and businesses were damaged by water. Insurance payouts topped $28 million, but the cost of the event was much higher.
The flooding was not just severe, but unfairly so. It was as though circumstances had conspired to direct the damage where it would do the most harm, a consequence of incremental decisions made over the course of a century.
Most of Dunedin is hilly. Houses were built on the low, jagged slopes of an extinct volcano, from where they looked over the harbour and the freshwater marsh, the swamps and the dunes on the shore, an area that came to be known as ‘‘The Flat’’.
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 Flat became one of the most densely populated areas in New Zealand, and remains so; some of the old miners’ cottages, shoulder to shoulder on narrow streets, are still there on tiny sections.
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.
And so on June 3, 2015, from about 5am, the water rushed downwards. It pooled in the open parks, the sports fields, and the school grounds. The network of pumping stations and drainage pipes, many decades old, were not up to the task. Thousands of people woke to a flooded city, and a grim omen.
Knowing the earth
On a bright winter’s day in June this year, four years after the floods, a giant drill bore down into the Earth.
The threat of climate change, and how it is likely to affect South Dunedin, has exposed how little was known about a place that became home to more than 10,000 people and billions of dollars worth of infrastructure.
What happens on the surface during heavy rainfall is no secret – it floods gratuitously. But how is that shaped by forces deep below the surface, which are far less obvious, but no less significant, to the fate of South Dunedin?
Hence the drill. Its purpose was to drill down as far as it could go. It collected data about the different layers of sediment, and how the groundwater moves through them; all crucial evidence that, until now, has been a mystery. ‘‘Until fairly recently, if you were walking around South Dunedin, you wouldn’t know, for example, what the depth of sediments was between you and the bedrock below,’’ says Ben Mackey, a hazards analyst at Otago Regional Council.
‘‘We’re really trying to fill in some of those basic, first order questions.’’
If adapting to climate change is a maunga New Zealand must climb, the greater South Dunedin area is its Aoraki. It is a tangle of physical, social, and economic components, all of which bounce off each other in complex ways.
This collision of different spheres – scientists and engineers and social researchers and politicians and community advocates – is a case in point for the complexity of learning to live
with the effects of climate change.
The people who live in South Dunedin already recognise this complexity in their lived experience. Its most obvious form is a long-known geological quirk in the South Dunedin suburbs – if you dig into the ground, you don’t have to go far to find water.
The water table is not just high, but unusually so, to the point where modest amounts of rain can cause flooding. And unlike most aquifers, the water table is, at least in part, connected to the sea – when the tide rises, so too does the groundwater.
It’s as though the community was not built by the sea, but on top of it. There are stories of builders waiting for low tide to dig foundation holes, and of gardens dying due to the salty water pooling after modest rain.
‘‘It’s very obvious to me there’s a problem, as it is for a lot of people,’’ says Eleanor Doig, who lives in low-lying Musselburgh.
‘‘You say the word ‘water’ around here and everyone gets anxious.’’
The water is inescapable, she says. Two years ago, it became so bad she put in drains and a pump at her home.
‘‘Up until then, I had to walk out to the washing line with gummies on,’’ she says.
‘‘Even if it was just rain, the water would be halfway up my gummie . . . We’ve had to raise our flowerbeds because the soil is saline.’’
It’s not just the high water table. South Dunedin, like a lot of dense urban environments, is impervious – all the concrete and other hard infrastructure stops water from seeping into the ground. As a whole, South Dunedin is 60 per cent impervious, but some pockets reach 100 per cent. When it rains, the water has nowhere to go, and must be manually removed.
Because it is a basin, this water is topped up by rain bouncing off the hills. Even if the water can be absorbed, when the water table is high, the water has nowhere to go. It rises upon itself, on to the surface and into the lives of thousands of people living there.
Climate change factors into this in a few ways. First is the apparent connection between the groundwater and the sea. Sealevels are certain to rise – at least modestly, and potentially catastrophically – which, if reflected in the groundwater beneath South Dunedin, presents an obvious problem.
At its lowest points, some parts of South Dunedin are within 30cm of the water table, and much of it is within one metre. On top of this, it appears the land is sinking by about 1mm a year, adding to the rate of relative sea-level rise.
How and where this causes flooding to happen is not as simple as one might expect, says Dr Simon Cox, a principal scientist at GNS Science.
He has been studying the water table beneath South Dunedin, and says it would be wrong to think of it like a bath, slowly filling with water until it floods over the top. It has a rigid shape, which accentuates when it rains and in response to the tides. It sloshes around, higher in some areas and lower in others, being pushed around by the sea.
It means the way the aquifer looks now may not be how it remains, once the sea more forcefully asserts itself.
‘‘We’ve learned it’s a bit harder than we thought it would be,’’ Cox says. ‘‘It’s very different from looking at a normal aquifer. A normal aquifer would perhaps be made up of gravels and have a lot more variability and quicker responses to changes from rainfall.
‘‘The challenge for us is to figure out what the shape of that is going to look like as it gets pushed more and more from the coast, and where it starts to emerge through the ground and cause springs and get closer and closer to the surface.’’
For people on the surface, this means it will be difficult to predict where groundwater flooding is most likely to happen.
Scientists are playing catchup. 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.
‘‘It’s fair to say we know an awful lot more than we did two years ago,’’ Cox says. ‘‘There’s something like $2b of assets in South Dunedin. If you were going to spend $2b in assets in this day and age, new, you would do a lot more investigation into the ground than we have to date, to make sure you understand it before you put that investment in place.’’
The natural environment of South Dunedin is complicated in itself, but there’s another layer to add on top.
Like most urban environments, South Dunedin is underlain by pipes. Because the water table is so high, the pipes are, in some places, ensconced in water.
They 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.
Under most climate projections, not only will the sea level rise, but rainfall volumes are expected to become more intense, increasing the probability of events such as the heavy rain that caused the 2015 floods.
Part two, tomorrow
The South Dunedin community is fighting for its future.