A big thank-you to all those who assisted during the Dunedin October floods including emergency response, contractors, neighbours and the army.
I am very sorry for homeowners on the Dunedin flat and elsewhere who were flooded again, just nine years after the last time, plus the hill-side homeowners who experienced devastating slips.
This has been mistakenly described by some as a 1-in-100-year rain event: it was not even close.
Stormwater flooding is primarily caused by high-rain intensity, not total rainfall. The Dunedin City Council subdivision and development design manual (2010) requires stormwater systems be designed for (i) 1-in-10-year rain events, with all stormwater retained in the pipe system, and (ii) 1-in-100-year rain events, allowing for some surface flooding, but remaining below building floors, i.e. no water in the house.
Niwa data states that a 1-in-10-year Dunedin rain event is 18mm per hour. According to the 2015 DCC report about stormwater infrastructure performance during the 2015 flood, the South Dunedin stormwater system was originally designed for (i) 14mm per hour, with all stormwater retained in the pipe network, and (ii) up to 19mm per hour, with some water backing up into street channels.
The peak rainfall intensity in the 2015 flood was only 12mm per hour and the 2024 flood only 8mm per hour.
We can easily see that the stormwater system’s original design and current capacity falls well short of the DCC design criteria of 18mm per hour. Quite simply, the South Dunedin 2015 and 2024 flooding was due to an undersized stormwater system.
Following the 2015 floods, the DCC eventually addressed stormwater infrastructure failings. The joint DCC/Otago Regional Council South Dunedin Future programme started investigating climate-change adaptation options to be implemented sometime in the future.
This work is ongoing, with final options expected to be selected by 2025/26. However, having a proposed plan by then, won’t guarantee that any activities will be budgeted or actually occur.
The down side of this all-encompassing climate-change analysis has been climate-change paralysis, with no substantial stormwater improvements made in the nine years since 2015.
This lack of any real action was confirmed during the DCC infrastructure services committee (October 15, a few days after the recent flood. Staff reported that no action had been taken to divert the Kaikorai Valley wastewater pipeline away from South Dunedin, as originally proposed after the 2015 floods.
Unsurprisingly, raw sewage under pressure, again, popped the Surrey St manhole lids during the recent flood. In a similar vein, no major action has been taken to divert the hilly suburbs’ stormwater away from South Dunedin.
As expected, stormwater again gushed from Forbury Corner manhole lids into South Dunedin. Both of these inactions resulted in South Dunedin receiving other people’s stormwater and raw sewage into their streets, properties and some homes.
Over the years, inquiries regarding any progress in solving South Dunedin’s stormwater have often been answered with "we’re still doing hydraulic modelling".
How long will South Dunedin have to wait for a fully functioning stormwater system which meets DCC’s own design criteria? This is an unacceptable situation for the 13,000 people in 3000 households who call South Dunedin home, and the numerous shops, businesses and essential services who host thousands of Dunedin citizens each week.
As for those householders who have been flooded once or even twice, the situation is shameful. I wonder how much South Dunedin people’s wellbeing is worth ... surely at least $105 million as recently spent on George St; or even $220m as spent on the new stadium in 2011?
At this stage, South Dunedin is not a victim of climate change, it is a victim of an inadequate stormwater system. Simple engineering with pipes and pumps can sort the stormwater issue now. And, in future, the slowly rising sea-level impact on groundwater can be sorted by dewatering at the sea boundary.
Yes, climate-change justice will be needed sometime in the future. But right now, South Dunedin just needs some stormwater justice.
Let’s replace the "flood-prone" with the "flood-free".
• Julian Doorey is a Dunedin international development consultant.
