Washed out

What happens when the rain falls on a city? (15 minute read)

I’m less than a kilometre from State Highway One where it cuts through the Ngauranga Gorge with six lanes of high-speed traffic. The road has a grade of 8%, making in one of the steepest motorways in New Zealand. It’s also one of Wellington’s busiest roads, with 74,000 vehicles per day using it in 2018, and numbers haven’t dropped since then. Many cars manage to make the uphill journey at something close to the 80 kilometre per hour speed limit, engines whining with the strain, but trucks grind their way up at an agonisingly slow pace. Going the other way, most drivers keep their foot on the brake the whole way down.

Although the name looks like a Māori word, Ngauranga is not its original name. There are a couple of versions of what it once was – older sources mention a pā named Ngā Hauranga, but government agencies working with local Māori are now using Ngā Ūranga. When I try and understand the history of the area, I come across a number of images showing waka¹ either approaching the shore or pulled up out of the water. Since ūranga means a landing place, that makes sense.

One feature of many of the early images is a peaceful stream named Waitohi. It’s hard to imagine driving through Ngā Ūranga today, but streams don’t simply disappear because we build six-lane highways over them. Deep underground, the Waitohi still flows down the gorge before washing into the harbour.

Waitohi, however, isn’t entirely underground. One branch of the stream flows through regenerating forest in the Tyers Stream Reserve, and that’s where I am, listening to the sound of water trickling over rocks and the squeaking calls of piwakawaka or fantails. I can hear nothing of the motorway, and although I can see that the forest is not original I’m delighted to be discovering another hidden corner of Wellington where there is space for nature.

Waitohi looks good here (image credit: Melanie Newfield)

But all is not well with Waitohi. At various points beside the stream it’s obvious that there’s a huge pipe running alongside it, and that pipe is part of Wellington’s aging wastewater network, that is, it’s a sewage pipe. At one point, I see a manhole cover over an access hole, and beside it I see the very obvious sign that at some point recently raw sewage has been flowing out of it, along the ground for a couple of metres and then straight into the Waitohi Stream. There’s even the remnants of toilet paper visible on the ground. I feel dirty just standing near it, and I’m also conscious that this overflow is upstream from where I’ve been walking. I’ve been wading through the water and my boots are now full of stream water which is contaminated with who-knows-what.

The raw sewage overflow is not a problem which results from too much sewage – too many people flushing their toilets at once or even too many houses attached to the network, although Wellington’s growing population doesn’t help. The problem is that when there is heavy rain, and this is Wellington so it happens quite often, some of that rainwater gets into the sewage system and overloads it. The pipes can’t cope with the increased volume, and so they overflow either in planned locations or directly to the stream.

Waitohi doesn’t look so good here. The pipe in the background is one of the sewage pipes flowing into the main pipe, and the dark smear on the ground is where the manhole overflowered (image credit: Melanie Newfield)

I’ve come across this issue before, when I looked at wastewater, but I realise that I haven’t looked at the other side of the issue, which is stormwater. To learn more, I speak to ecological engineer Stu Farrant from Morphum Environmental. As well as working on water issues all around New Zealand, and a few cool places overseas such as Andra Pradesh in India, Stu grew up in Khandallah (the suburb where I now live), mucking around in the headwaters of the Waitohi Stream at the top of Kim Street and Khandallah Park. He tells me that his love of waterways, and his awareness of the threats, developed early and was heavily influenced by his connection with the Waitohi.

“I was probably less than 10 years old, and at the top of that stream there used to be a beautiful little fresh water spring where the Waitohi began. I had a little fort there, and used to collect koura and things. Then one day I went up there, and it was all just completely destroyed. That was when they developed Kanpur Road in Broadmeadows, they just bulldozed everything straight over the bank and completely filled in my little paradise.”

Much of the attention on New Zealand’s water problems goes on agriculture, particularly livestock and their immense quantities of manure (I calculated it at one stage and it is truly staggering). There’s no doubt that this is a problem, but on the whole the streams and rivers in urban areas are in a worse state than those running through farmland. When I speak with Stu, I realise that we have many more impacts on the water in cities than I had imagined.

He explains: “people who work in the industry consider stormwater to be water that's generated from unnatural surfaces or processes. So stormwater becomes stormwater when it's fallen on your roof, or the road, or a sports field or another artificial surface. But water that comes out of the forest or seeps out of the hills in a natural catchment is just stream flow. So stormwater is water that’s been impacted in some way by the activities of humans and development. It isn't just water that comes from storm events either. It's from all rainfall events. And ecologically, the biggest impacts are from those small rainfall events rather than the big floods.”

To understand why this should be, we need to understand the natural water cycle – what happens to rainfall in natural areas. Stu tells me: “in a natural catchment around about 30% of all rainfall that falls gets captured by trees and other plants and gets evapotranspired² back into the atmosphere. About the same amount again will then soak into the soils and slowly come out as springs or base flow (the constant part of a stream or river’s flow). Then only around about 40% or less gets into a stream directly when it rains. So the first thing that happens with rain when it falls on surfaces like roofs or roads is that it dramatically changes the hydrology in terms of the amount and flowrate in the movement of water.

“So the water is hitting these surfaces, and what those surfaces are made from and what they've been used for impacts on the quality of the stormwater that flows off. So you can have contaminants from the surfaces themselves, things like zinc roofs or copper spouting. Or you can have the activities that have gone on – cars on roads is a pretty obvious one or microplastics blowing around in the in the wind. And the other thing that's really important to understand is that negative impacts are not just caused by chemical contaminants. We've also got physical contaminants, like changes in the temperature and the dissolved oxygen level. In Aotearoa New Zealand our native freshwater fish species are very sensitive to temperature in particular.”

Stu explains that there are also changes in the way the water flows. “When we have a big storm it looks like lots of water, and people think that's a huge component of the year's rain. But in fact, it isn't. The vast majority of the volume of rainfall in average years is comprised of frequent small  rainfall events. Even a little 5 millimetre rainfall event generates a significant amount of stormwater that otherwise would have been held in the catchment or, more importantly, evapotranspired. The greatest ecological impacts come from these frequent small events too. If you get a big rainfall event, the fish are quite good at finding somewhere to hide and hanging out in there, as long as these big flows only occur every now and then. But with all these modified surfaces (which engineers call connected impervious surfaces), the rain flows off them and into a stormwater pipe and then to a stream. So the stream levels are going up and down, up and down, every single time it rains. That's the stress that tips our indigenous fish over the edge.”

This is a crucial point. Our wastewater goes to a treatment plant. Stormwater does not. It flows, untreated, into our streams and then the sea. Stormwater from roads is particularly bad, contributing oil and tiny toxic particles from the wear of car tyres and brakes in particular. But we also use chemicals around our homes that are highly damaging to waterways. Washing a car on a driveway pours detergent (and the mix of contaminants you are washing off) into the stormwater system, and therefore our waterways, and detergents are highly damaging in waterways (in fact, there is so much information that I’m thinking about doing a future article on the subject). Another particularly damaging chemical is found in outdoor cleaners, such as those advertised as suitable for spraying on paths and roofs for removing moss and lichen. The active ingredient in these products has the highest hazard rating in terms of its danger to aquatic animals such as fish, but if used as advertised, where is it likely to end up the next time it rains? It’s going straight into a waterway.

So far, most of what Stu and I have talked about are impacts on the environment and the species that live in our streams. But people are affected too, and in ways which might not be so obvious. Stu tells me that the fluctuating water levels in streams caused by all our impervious surfaces increases stream erosion. “You can end up in a situation such as having a stream undermining your driveway and the argument with Council about who's responsible for it and the expensive fix required – these things can be really quite impactful for people.” Stu gives me an example from his own experience: “my neighbours from years ago had subdivided their property and built another house. As part of this they installed a single new stormwater pipe discharging into the stream, and the whole stream just collapsed as a result of that single additional property with erosion undermining private driveways and the public road.”

This is a confronting issue, because we are in desperate need of new houses. There’s a proposal right now to significantly reduce the regulations for building small standalone houses. I haven’t looked at the proposal in detail, but it’s unclear how issues such as impacts on stormwater and wastewater will be managed to avoid a raft of unintended consequences.

It’s crucial though, that as much as possible we get things right when new houses are being built, Stu explains. “Once we've made decisions, retrofitting is always really difficult. It can be technically difficult, or it can be horrendously expensive. Or it’s just not possible, because we've built on all the available land. Retrofitting is generally really hard and opportunities are few and far between to do something meaningful to retrofit stormwater management once we have developed our communities. So new development is the one opportunity that we have in the next 50 years to actually do something to improve the existing situation and avoid any worsening. We need to change our mindset, so that each new development is a regenerative development opportunity to address both the existing situation and the needs of the future.

“We need to understand the natural context in which we exist. The first thing I always try to communicate is how important it is for people to understand that they live within a catchment. When they go and empty their paint pot, or whatever, into the drain, or they wash their car on the street, people often think it's going to the wastewater treatment plan, and don't realise it's going to the local stream which they may have discovered while walking their dog.

“Then with your development, it’s about trying to mimic those natural processes. The first consideration is the natural water balance, managing the volume of water that's coming off. That's trying to replicate that portion that naturally goes into the atmosphere or goes into those shallow soils. There's two ways to do that. One way is to is to have more surfaces the water can get through, using things like green roofs and permeable pavement. The second is through collecting and reusing our rainwater. Having rainwater tanks on all new dwellings is an absolute no brainer. But it has to be connected to permanent reuse rather than just summer garden watering. So you need to be plumbing it into your toilet and your laundry. There’s not much benefit having a rainwater tank that's just used for garden watering, for instance, because we only use that for a few months of the year. Collecting and reusing stormwater also deals with the contaminants that might have accumulated on your roof because they get flushed down the toilet and diverted to the wastewater treatment plant.

“With car parks and roads and things like that, we need to be capturing the water and treating it. There's things like rain gardens and, if you've got the space, wetlands which catch the water. They slow it down and use biological, chemical and physical processes to improve the quality.” I knew about wetlands, because Wellington has a great example of this in Waitangi Park on the waterfront, where a wetland planted in native plants treats the stormwater from Mt Victoria, Newtown, Mt Cook and parts of the central city. Rain gardens were new to me, though, so I looked them up and realised that I’d already seen some in Wellington. They don’t look any different from any other urban garden, but under the surface they have been designed to catch and clean the water running off the street.

In a city which has made such a terrible job of managing its water, it’s good to see something which is improving.

But what about the floods and erosion? Can we manage that better? This is more difficult than simply improving the design of garden beds in the city. Stu explains: “a lot of the existing flooding risk is really to do with where we're living. The low point below Box Hill [in Khandallah], for instance, is where the Khandallah Park and the Kim Street arms of the Waitohi come together. The land's naturally quite flat there. That would have been a beautiful little natural flood plain in the forest. Before humans came here, if there was a large ‘hundred year’ rainfall event, the stream would have spilled into this area of flooded forest. Flooding is a natural thing, and it's a really important natural thing that supports a range of biological processes. It's just that in many places we're living in the way of flooding.”

There are some options, he tells me. “It’s possible to capture the water, for example in a tank, and then slowly release it to match the same rate that it would have been before developing. That can be really effective in reducing the amount of flooding but it can also be really ecologically negative, because it's extending the duration of the increased flow in streams. But there are also parts of catchments where that approach doesn't provide much benefit, because there's no downstream flooding risk. But where there are downstream flooding risks the use of rainwater detention can be a valuable tool.”

Stu points out that there’s no single solution, that we need a combination of approaches. But our urban waterways will still face huge challenges. Thinking back to the sewage overflow I saw in the Tyers Stream Reserve, there’s no easy answer. It results from stormwater getting into the wastewater network in many different ways. Stu says: “the wastewater overflows are a very, very difficult thing to fix once you've got it wrong”.

The solutions are about much more than fixing pipes and building better gardens. There’s something wrong in our relationship with water. I’m reminded of the traditional Māori approach to managing water, which includes a rule that used water must be disposed of on land, not tipped into a waterway. For the last couple of hundred years, we’ve been doing the opposite, and pouring waste of every kind into our streams, rivers, lakes and harbours. We’ve also tried to engineer natural processes like floods away rather than recognising that some areas are always going to flood. Stu sums it up: “with situations like the Auckland floods [in January 2023], many impacted residents were complaining about the council. But the Council can't realistically prevent all flooding when you get that scale of rain. You can't design your way out of all flooding in flood prone areas that have always been subject to flooding which is now worsening due to poor development planning and changing climate.”

If we can find a way for Waitohi to thrive, we will achieve more than protecting a stream. We will protect the fish which depend on it. We will protect the water in our harbour and our fisheries. We will protect our own health and property. But we need to acknowledge that we share this land with water and find ways to live with it instead of simply trying to make it go away.

1. For those outside New Zealand, in this context waka means a traditional Māori canoe.

2. Evapotranspiration is a rather inelegant portmanteau – like brunch only less catchy. It’s made up of evaporation and transpiration. The latter is the process in which plants draw water in through their roots, transport it through the plant and then release it as water vapor through pores in their leaves. So 30% of the water which lands on an area like a forest or grassland either evaporates or is absorbed by plants and then released into the atmosphere.

Image credit: Melanie Newfield