By Alexandra Cain
The Wellington floods highlight the need for new stormwater standards in urban centres.
Experts say the catastrophic flooding that hit Wellington this year is a wake-up call to urgently rethink the way water is managed in urban landscapes.
When Wellington received 70 millimetres of rain in an hour in a catastrophic flood that hit the city in April, it became crystal clear that more needs to be done to prepare cities for these events, which are only expected to increase as the climate continues to change.
This means urgently rethinking the way water travels through New Zealand’s urban landscapes, especially above ground, to reduce the damage when events like this happen in future.
‘When you are dealing with things that affect the spatial nature of urban places and natural areas, then it becomes a land use planning issue as much as an engineering one, whether we are thinking about establishing new urban areas or working in existing ones,’ says Mark Tamura, Founder of resource consultancy, TamuraHill.
‘We used to assume that, to some extent, you can manage and control the flow of water in urban landscapes below ground, creating much more developable land. Now, we are realising that we also need to make space for it above ground in blue-green networks. So, we are seeing significant parts of some newly urbanised areas providing space in the landscape for the water to move.’
The landscape often determines how easy that is. For instance, cities like Hamilton have an extensive gully network that makes it easier to cater for water flow. Wellington’s topography also lends itself to above-ground water management. But better management and planning is required to avoid a re-run of this year’s floods.
Lack of investment in pipes and other infrastructure is a problem across the country. Many of the pipes under the streets in Wellington are 100 years old and regularly fail. Yet, in a cost of living squeeze, ratepayers are resistant to paying higher rates, arguing that the central government should pay to maintain the pipes, while the government says it is the responsibility of local authorities to maintain stormwater assets.
‘There is a lot of buck passing going on and not a lot of action,’ says Victoria University Professor of Climate Science and Physical Geography James Renwick.
‘We are in the situation where we have old infrastructure that is pretty creaky and the system across the country needs a major revamp. That means putting in new stormwater drains that are at least 50 per cent larger in diameter than the old pipes. But actually doing it is a massive undertaking. When they start digging up the roads, it is just chaos. Unless there is a really good reason to do it, most councils are pretty resistant to taking this on. But we can’t keep going on like this. And we need a stormwater drain system that can handle much more intense rainfalls than the one we have now. We need newer, fit-for-purpose infrastructure.’
What is evident is that the weather patterns of the past are no longer suitable benchmarks when it comes to defining or maintaining design standards for stormwater. Existing design standards are based on a time when it was assumed the climate was stable; but the Wellington floods in April this year and the Auckland floods in 2023 prove that is not the case.
‘Designing for that sort of exceedance comes down to how you manage the water on the surface,’ says WSP Technical Director – Stormwater and Catchment Management James Reddish.
‘We need to protect overland flow pathways to make sure nothing is put in their way, so they have safe passage from mountain to sea, and there is no critical infrastructure or properties in the way. Flooding is not the problem; it’s only when we put infrastructure in the way that it becomes a problem.’
There is now better recognition that overland flow and exceedance need to be accommodated in future urban planning, although retrofitting existing urban forms is one of the biggest challenges. Stormwater standards are another hurdle.
‘The question is how you allow for that overland flow in stormwater standards, because it’s not the way we always thought about infrastructure. How do you have stormwater standards and regulate something that does not relate to asset? Water flows through people’s backyards and down road corridors. It does not have a clear boundary. This is one of the most significant challenges of stormwater compared to water and wastewater,’ says Reddish.
While standards are key, it is important to understand their limitations and the extent to which common standards are useful at all.
‘The size of the pipe you need to convey a certain volume of water may be amenable to common standards,’ says Tamura. ‘But because of the diversity of the environments we are working with, there are limits to how far ridged standards are useful when it comes to managing things like water in the landscape.’
In terms of future planning, experts want to see longer time horizons taken into account, rather than short-term, reactive actions around floods and other natural disasters. This is especially critical as more extreme weather is expected with a changing climate.
Managed retreat may be on the cards in some circumstances, which is challenging to implement. But even worse is locating communities where they should never have been in the first place, causing massive harm.
Another way is creating a vision for what a community looks like over a number of generations, which may mean slowly removing people from areas of significant risk. This will come with consequences and challenges, particularly with current government directions around private property rights. But without change, more properties will get damaged.
‘You either help people reinforce their asset with property-level protections to enable them to live with the risk, or you create space by transitioning the urban environment over generations,’ says Reddish.
‘Every part of New Zealand is going to experience a serious climate event at some stage. So, let’s be prepared for when that event happens with a plan about how you then avoid that happening again and again.’