By Melissa Bray, National Institute of Water and Atmospheric Research
Climate change and AN increase in THE number of extreme weather events means stormwater can’t be ignored.
It would be rare to hear a mayoral candidate or a potential councillor campaign for election on the merits of a good stormwater system. It’s just not sexy enough. Stormwater is one of those issues that stays relatively hidden – until something goes wrong. The 2023 Auckland Anniversary Weekend floods brought the issue to the forefront. Cyclone Gabrielle kept the focus alive.
The Anniversary Weekend floods acted as a wake-up call – a call that has been going on in other parts of the world, sometimes with great urgency. In New Zealand, the Ministry for the Environment has curated a set of flood-related projects around the country. Regional and unitary councils received funding through Envirolink to assess what could be done in their regions and find solutions. Out of this, the National Institute of Water and Atmospheric Research (NIWA) conducted a literature review, titled ‘Nature-based solutions for flood management’.This review looked at both rural and urban flood risk.
Hydrologist Dr James Griffiths has been leading a team of NIWA scientists in the review. Griffiths moved to New Zealand from China, where he looked at the interpretation and application of ‘sponge city’ guidelines in one of 30 national pilot study cities. The pilot cities were used to showcase urban development approaches that promoted rainwater infiltration and storage to reduce urban flood problems. China embraced the sponge cities concept and, by necessity, started to lead the way in urban water landscape development.
Griffiths left China in 2016 and says that, at that time, the term ‘sponge city’ was not commonly used outside of China.
‘As more money was invested into pilot city development, technical guidance and performance stories caught the attention of engineers, scientists, and academics in Western countries,’ he says.
‘The sponge city concept shares many of its underlying ideas with what we are now commonly referring to as nature‑based solutions (NBS). There is also much crossover with the Māori concepts within Te Mana o te and Taiao Te Mana o te Wai, which bodes well for urban spaces in New Zealand.’
NBS are solutions designed to replicate natural drainage processes in cities to address societal challenges like climate change and water security. Typical NBS for urban flood management include soakage, cisterns and large detention basins.
In New Zealand, NBS for stormwater management are familiar sights in our main centres, even if people do not recognise them as part of the stormwater network. The most common are stormwater ponds and wetlands, living roofs or walls, tree pits and rain gardens. There are also examples of stream restoration and daylighting of buried waterways. These solutions are seen as ways to also get the wider benefits of biodiversity, better water quality and the benefits of human wellbeing with outdoor spaces for people. Since the floods, there has been public interest in whether these systems could also be used to reduce flood risk.
Urban Aquatic Scientist Dr Annette Semadeni‑Davies knows that our existing NBS on their own are not the answer in times of extreme weather.
‘The distinction between NBS for flood protection and stormwater management is important to manage expectation of what these systems can (and can’t) do during a flood. The danger is that overselling solutions designed for stormwater management as a means of flood protection could result in public distrust of these valuable assets.
‘If you’ve got a sponge city, you still need somewhere to squeeze the sponge out between storms,’ adds Semadeni‑Davies.
There are a few exceptions where floods have been taken into consideration in New Zealand: for example, the flood detention basins in Halswell, Christchurch and the Greenslade Reserve detention basin in Auckland.
Semadeni-Davies knows about managing a lot of water. She completed her PhD at Lund University in Sweden on managing urban snowmelt. In northern Sweden, when the snow melts in April after a winter on the ground, the stormwater system that has been designed for summer rainstorms often can’t cope with the volume of melt water (especially if the system is blocked by ice), leading to flooding.
‘I got fascinated by snow, and because I have a wheelchair, I can’t go hiking in the wilds, so I decided to focus on stormwater management. I could stay in a warm, dry hotel rather than a tent when doing fieldwork.’
Back in New Zealand, Semadeni-Davies got involved in the Urban Toolbox project led by NIWA, which focused on how to assess the effect of climate change on hazards affecting towns and cities in New Zealand, including on the drainage network.
Semadeni-Davies says the Auckland floods were caused by a combination of different flooding phenomena. The first was that the central city’s stormwater system was not designed to cope with extremely intense rainfalls, thus being overwhelmed by the huge volume of water. This, combined with continuing land use intensification and increasing imperviousness, means the system was not equipped to cope with flooding. There was also the impact of rising streams. These occurred in areas like Henderson Valley and Rānui, where stormwater from surrounding housing and roads caused streams to rise quickly, inundating houses on the floodplains, ‘like a bath being filled’.
In the report, Semadeni-Davies reviewed methods of flood modelling using coding to take data, and add parameters and scenarios – in this case, NBS – to predict what may happen in different situations.
Water Quality Scientist Dr Karine Borne’s background is in soil remediation and water projects. She moved from France to do her PhD at the University of Auckland on stormwater treatment by using wetlands along State Highway 1. Borne’s focus has been on how NBS will work when applied to specific catchments, highlighting the pluses and minuses. This will provide the knowledge for councils on how to implement NBS in their catchment for increased benefits.
After the Auckland floods, there were success stories in dealing with the large quantities of water. Richmond housing development, in Mount Wellington, is a townhouse development on a former scoria quarry. At the time of the floods, 441 homes and apartments had been built. They all made it safely through the flooding. A comprehensive stormwater design was part of the development plan and included a diversified network of more than 50 soak hole devices, rather than one large soak hole.
Greenslade Reserve, on Auckland’s North Shore, worked well with its recently completed stormwater retention basin, and sportsground and park. The playing field became a lake, then, 15 hours later, it had drained and was back to being a playing field. This water would have otherwise gone through houses and Northcote town centre on its way to the sea.
By default, Pukekawa/Auckland Domain saved surrounding properties from flooding by reducing the amount of water. The domain was like a swimming pool directly after the floods, and then the water drained away. This was the case for many other parks around the city.
Auckland Council noted, ‘There was some public concern over the amount of water that collected in the domain; however, parks are often located on floodplains and designed to store floodwater during extreme rainfall.’
The review of NBS for flood mitigation will now go back to councils, which will begin feasibility studies. Griffiths suggests that once these studies are completed, we should have more information about how NBS can be used to improve resilience to climate change and flooding.
‘This information will also allow councils to develop realistic funding strategies for similar projects going forward,’ says Griffiths. ‘The current feasibility studies are an opportunity to fine-tune our approach to more effective use of NBS for future flood mitigation.’