Water inundated South Dunedin once again after the city recorded its wettest day in more than a century earlier this week – a recurring flooding problem partly rooted in the area’s very foundation.
A state of emergency was declared in Dunedin and Clutha on Friday with more than 80 residents forced to evacuate their homes as more than twice the usual monthly October rainfall drenched east Otago in 40 hours.
Flooding and slips effectively cut off Dunedin from the state highway network and locals were urged to conserve water and stay off the roads.
South Dunedin was hit particularly hard by the deluge, with backyards turning into ponds and roads turning into rivers as flooding worsened throughout the day.
But why is the area so susceptible to flooding?
Before European settlement, the neighbourhood known today as South Dunedin was an estuarine wetland connecting Otago Harbour to the Pacific Ocean.
The area colloquially known by Dunedinites as “The Flat” was once marshland, rich in flax, rushes and silver tussock.
Tidal mud flat
Where warehouses and industry stand today, the shore of the Otago Harbour was a tidal mud flat, while coastal lagoons and low sand dunes dotted Dunedin’s south coast.
These wetlands teemed with life, providing local Māori with an abundant supply of food and a place to moor their waka.
However, the discovery of gold to the southwest of Dunedin in the 1860s saw a rapid influx of settlers and miners, and along with it, a huge demand for level, dry land.
Most of Dunedin is hilly, and housing was initially built on the slopes of the long-extinct Dunedin Volcano.
European settlers drained and filled in the wetlands with sand from the coastal dunes at St Kilda and any available fill material.
As a result, much of South Dunedin was built on soft, sandy sediment that was only slightly above the water table.
In those days, groundwater was up to 17cm lower than it is now.
This created an additional 600ha of space on which to build in what was New Zealand’s largest and fastest growing centre.
Densely populated and low lying
Today, South Dunedin is one of the most densely populated and lowest lying areas of the country.
More than 13,000 people reside in the area and groundwater can be located as little as 30cm below the surface.
Reclamation continued for more than a century, culminating in the opening of Portsmouth Drive in 1978, which today marks the boundary of the Otago Harbour.
GNS Science principal scientist Simon Cox said this reclamation provided some protection from coastal inundation but also caused an “unfortunate and compromising” effect of removing pathways for natural drainage.
“The lack of natural drainage outlets, as well as low elevation and fall across the coastal plain, make the low-lying parts of Dunedin prone to surface-water ponding after moderate to heavy rain.”
He said the potential for rain-related flooding has been with Dunedin for a long time and will continue to be the “dominant hazard” in the near future.
“Most rainwater runoff must now pass through engineered stormwater systems, much through a single pump station, en-route to the harbour and ocean.
“Construction of city buildings, roads and pavement surfaces increases the demand on the stormwater system to carry rainwater away.”
When 175mm of rain fell on a single day in June 2015, a key pump station at Portobello failed.
The Dunedin City Council admitted a year later the faulty pumping station had made the flood in South Dunedin 20cm deeper.
Cox said there is concern sea-level rise could drive a permanent and further shallowing in groundwater.
“This would reduce the ability of the land to absorb and store rain, exacerbating present flood issues and can lead to emergence of springs and in the long term eventually flooding from below.”
He said a rising water table presents a “creeping hazard” or a contributor to hazards like rain-related flooding.
“By monitoring and understanding the groundwater system and its behaviour, it enables other organisations to explore the viability of mitigation and/or adaptation options.”
Increasing intensity in future
NIWA climate scientist Gregor Macara said extreme rainfall events like those seen on Thursday and Friday are projected to increase in intensity in the future.
“Greater increases are projected by the end of the century (2081-2100) under high atmospheric greenhouse gas concentration scenarios.”
He added that the positioning of the low pressure system contributed to how the weather event played out.
“In this case, [the system was] drawing in moisture from north of the country, and delivering it to Dunedin with easterly winds, and in this event the low pressure system centred off the West Coast has been relatively stationary, enabling a prolonged feed of moisture into Dunedin.”
Dunedin Mayor Jules Radich told 1News on Friday, following the most recent deluge, that more pipes and pumps were required to deal with heavy rain in the area.
“South Dunedin is a bowl, water accumulates at the bottom.”
He said “something significant” is needed now, adding that it will require financial assistance from central government.
“We’ve done all the fine tuning, now we have to take some significant large steps.
“That is going to happen, I won’t take no for an answer.”
