One of the most disrupting things we do when we build cities, is also one of the least obvious to those who walk along the streets. Our cities upset the water cycle. As a basic rule, a city is developed to prevent the natural flow of water from taking place. When people develop a place to live, plants are the first things to be removed, usually to be replaced with hard surfaces such as roads, sidewalks and roofs. While the original cover was sponge-like, letting water in, the latter is mostly impermeable, forcing water to accumulate and run down it. This is how we create stormwater, and the numerous serious problems that go hand-in-hand with it.
How we make stormwater from rain
In simple terms, much less rain can inﬁltrate the soil; a docile and almost unnoticeable process. Infiltration maintains soil moisture and allows water to migrate to surface water or groundwater systems or evaporate back into the atmosphere.
Instead, when rainwater hits a hard surface, it quickly gathers to run down the most suitable channel, like a street. In cities, rainwater thus becomes stormwater. It’s the water that runs off areas such as roads, roofs, footpaths and parking lots when it rains. Runoﬀ from spongy areas like gardens, lawns and parks also contributes when rainfall is intense.
Building cities can reportedly increase the runoﬀ rate by up to 50% compared with natural conditions. In extreme cases, the peak ﬂow can be as much as 6.8 times that prior to development.
How rain becomes dangerous
As a result, in cities, unmanaged stormwater is dangerous. So, people had to step in again, to manage the danger they created. The ﬁrst purpose of stormwater management and infrastructure is to keep people safe from ﬂooding. Usually, this is done by quickly directing water into streams or other natural water bodies via roads and storm drainage systems.
This arrangement has resulted in a number of problems. Most concerning is what it is doing to those waterbodies to which the stormwater is directed. Not only is stormwater dangerous because it can create floods, but also because of what is in it.
How rain becomes dirty
Stormwater is often very polluted. Though rainfall is usually rarely contaminated, this changes drastically as soon as it hits the surface of a city.
Here, the water picks up microbial pathogens and other hazardous substances that can easily spread via the rivers, lakes and wetlands that the stormwater is destined for. In fact, urban stormwater has become one of the largest sources of contaminants to surface waters throughout the world.
More problems with stormwater
Except for ﬂooding and pollution, there is an array of knock-on eﬀects from replacing green areas with grey surfaces.
For one, it gets hotter. Heat from the sun is dissipated by water that is transferred from the land to the atmosphere by evaporation from the soil, and transpiration from plants. Plants also intercept heat from the sun. Because the planted and earthy surfaces are less when we build over it, cities are hotter. In order to keep living conditions comfortable, more energy is necessary to regulate temperatures in buildings, requiring vast amounts of water to generate.
The natural ebb and ﬂow of rivers and other waterbodies is also disrupted – now there can be sudden floods and water that gushes there from the stormwater drains. This causes big changes to the natural hydrology and ecosystems. Altered river channels, erosion, sand build-up and reduced biodiversity are all quite common in these systems nowadays.
It’s time for change
The priority for stormwater managements is still mostly to prevent ﬂooding (the quantity of stormwater). Yet, it’s become clear that the quality of stormwater urgently needs attention, as well as the environment into which the stormwater is released.
An alternative approach is to consider stormwater as a resource. Instead of sending it down the drain, we can use it! It can be seen, again, as a valuable element of the urban water cycle.
This is referred to as Sustainable Drainage Systems. Lots of people simply call it SuDS. It’s where the whole concept of a water resilient city started. This approach often makes use of nature-based (green) infrastructure (like wetlands) to manage stormwater in line with the ideals of sustainable development.
SuDS flip our normal idea of development on its head. Ironically, the key is to bring back those characteristics of the water cycle that urban development tried hardest to annihilate. In contrast to the traditional approaches of stormwater management, sustainable stormwater management embraces the natural water cycle. And, with careful management, stormwater can be used as a valuable resource. Once again, we can look at the rain as a blessing to the community that it falls on.
Ins and outs of sustainable stormwater
“SuDS put water back into the ground,” says Prof Neil Armitage, Deputy Director for the Future Water Institute at the University of Cape Town.
Simply put, you get the ground to return to the permeability that you had before development, he says. Prof Armitage explains that the functioning of the urban hydrological environment can be restored through various interventions. “It behaves like a natural system even though it does not look like a natural system.”
SuDS is about reengineering our urban drainage system to achieve four things, he says. The ﬁrst is managing stormwater quantity, with the aim of controlling the amount of runoﬀ as close to the pre-development system as possible.
The second relates to water quality, and using the system to treat water to an acceptable standard.
The third aim is to change the system into something beautiful, and creating a place where people want to be.
The fourth is to use it as method to improve biodiversity and the associated ecosystem services.
In a water sensitive city, SuDS commonly convey stormwater through a network of plants and waterways to create blue-green corridors of open spaces and productive landscapes. At the same time, these systems detain water to protect communities from ﬂoods.
What sustainable stormwater looks like
In comparisons to traditional stormwater systems, SuDS can be all over the place. It can even be placed right at the source of the stormwater. One example is plants on rooftops (green roofs). Or, runoﬀ from rooftops can be captured and stored in tanks for reuse (rainwater harvesting). Another example is a soak-away or rain garden. These are pits packed with coarse material that help to detain runoﬀ and let it inﬁltrate into the soil. Permeable pavements do the same job.
It can also take on a larger size. In roads and parks, filter strips can be used. These are planted areas to manage shallow overland stormwater by letting it soak into the ground. Trenches ﬁlled with rocks can be built to receive stormwater runoﬀ from neighbouring residential properties. Instead of transporting the water away via a culvert and underground pipe network, the water is allowed to inﬁltrate into the ground instead.
There can be even bigger systems. Detention ponds are large depressions that temporarily store stormwater to reduce floods downstream. Constructed wetlands attempt to mimic the characteristics of natural wetlands through the use of marshy areas and plants. In the process they can create beautiful, open spaces for people to enjoy and for wildlife to make a home.
Clearly, the design and nature of sustainable stormwater management systems are in stark contrast to the characteristics of conventional, grey stormwater infrastructure. It entails veering far away from the familiar and trusted engineering solutions.
It’s perhaps not that surprising that it has not seen a lot of uptake in many countries (including South Africa). Yet, should we embrace it, the benefits are many.
The wins of sustainable stormwater management
Two of the main beneﬁts of SuDS have to do with water quality and quantity. If done correctly, SuDS reduce the amount of stormwater, because water is kept on site and less water turns into runoﬀ. Additionally, more water inﬁltrates the soil to recharge groundwater. This leads to less ﬂooding.
It is also widely accepted that sustainable stormwater management systems have the ability to improve the quality of stormwater. Depending on the type of application used, pollutants can be captured and treated through physical, chemical, and biological processes.
These functions are performed basically for free and if not, mostly at a smaller cost than traditional treatment processes – as much as 5% to 25% more economical than conventional systems. A green roof, for example, can lead to further savings because it regulates temperature too.
Healthy waterways also help to maintain healthy biodiversity, creating beautiful areas to thrive in for people, animals and birds. The greening of cities and the conservation of natural features can add to the aesthetics of an area, creating healthier and more resilient urban environments. This can have a positive knock-on eﬀect on property values. The beneﬁts of stable riverbanks and decreased erosion are many. Less infrastructure and land for farming are in danger, for example.
Yet, there’s more. Sustainable stormwater management also calls for the use of stormwater. More and more, stormwater is seriously being considered as an alternative, major water source for use by cities.
A new water tap
The idea of using stormwater in cities is slowly starting to become more popular. In many Australian cities and towns, for example, stormwater is now seen as a valuable alternative source of water. It is popular for watering public green spaces. In South Africa, it remains a resource almost entirely untapped, though there are some examples from across the country where it’s harvested from parking lots and commercial properties.
Still, there are many challenges. These include people’s resistance to new approaches, underfunded water management institutions, a lack of political will, and a shortage of the necessary skills and capacity to operate and maintain the harvesting process.
In South Africa, at least, this is slowly changing as the magnitude of the country’s water crisis is becoming clearer. During the 2016 drought the concept was thrust in the spotlight as part of a suite of alternative water resources under investigation to supplement traditional water supplies.
Mostly, the view is that the most realistic option would be to store stormwater runoﬀ for non-drinking uses like watering gardens and ﬂushing toilets. Though it can be treated to drinking water standards, this option can be expensive. Still, there is one example of successful, large scale harvesting of stormwater that has been ongoing in South Africa for decades. The award-winning Atlantis Water Resource Management Scheme has proven that reuse of stormwater is possible, though it is a complex system dependent on expert management.
Part of the hesitance to embrace stormwater is because of the substantial risk that was mentioned earlier. As much as stormwater is a danger to the environment that it is released into, it can be hazardous to people that come in contact with it.
The risks that lurk in stormwater
We really have only a limited understanding of the dangers that lurk in stormwater generated in cities. What is certain is that stormwater contains pollutants, some of which are dangerous to people. Pathogens and micro-pollutant concentrations in stormwater are not well understood, with even less information available about how factors such as rainfall patterns and soil types impact this. It is also often carries sewage, and the abundance of chemical pollutants present in this. This is stuff like artiﬁcial sweeteners, caﬀeine and pain medication (e.g. paracetamol and ibuprofen).
Bad waste disposal and hygiene habits add to the quality of stormwater. Except for raw sewage, household greywater is a common, adding food remains, fats and greases, animal blood and tissue to the list of ingredients. The stormwater systems themselves are thus breeding grounds for disease-causing organisms (pathogens), and attract fies, rats, mice and cockroaches.
Moreover, one of the biggest headaches of stormwater management is solid waste. Vast amounts of litter commonly block the systems. As a result, polluted stormwater can stagnate, becoming a toxic substance.
A blessing and a curse
So, though often touted as a wasted source of water, the way that stormwater runoﬀ is produced by our cities and settlements does not allow us to approach it without being careful. Additionally, the opportunities to do so vary greatly from city to city – as much as the rainfall patterns, topography, geography and land-use vary from one to the next.
As with most other aspects where water is involved, each city has to make its own way forward when transforming to sustainable stormwater management. It depends on their location, rainfall patterns and much more (this interview with Dr Debra Roberts from Durban, South Africa is a great explanation of the unique potential of different cities). Yet, many cities and towns featured in this blog have made vast progress, and are reaping the benefits. Singapore is a great example. Atlantis, mentiones earlier, is another.
As with all other new ideas about how we can solve out water crises, the questions should really not be if we can do it anymore. Especially if it has been proven to work elsewhere, rather, we should only ask how we can benefit too. Until then, we are sending valuable water down the (stormwater) drain.
- The blog is part of an ongoing series tackling the problem of how we will live with less water (by the way, this is why we will have to). The first article looked at how cities developed, and what went wrong. The second, looked at how a new direction in how we develop the places we live can help solve our water problems (it’s an overview of Water Sensitive Urban Design and the concept of a ‘livable’ place). The third dug into the details of South Africa’s water crisis (it’s not a pretty picture). Then, we looked at the one thing that cities that need water MUST do.
- None of this would have been possible without the support of the Water Research Commission (WRC) in South Africa. The series of blogs are excerpts from the book I wrote for them on Water Resilient Cities.
- Alternative Technology for Stormwater Management, the South African Guidelines for Sustainable Drainage Systems, by Neil Armitage, Michael Vice, Lloyd Fisher-Jeffes, Kevin Winter, Andrew Spiegel and Jessica Dunstan
- Alternative Technology for Stormwater Management, Sustainable Drainage Systems – Report and South African Case Studies, by Neil Armitage, Michael Vice, Lloyd Fisher-Jeffes, Kevin Winter, Andrew Spiegel and Jessica Dunstan, report to the WRC (no. 1826/1/13)
- Fieldnote, Managing floods and droughts, lessons for municipalities, published by WIN-SA and the WRC
- Stormwater Management in a Water Sensitive City. Melbourne, Australia: Cooperative Research Centre for Water Sensitive Cities, by Wong T.H.F., Allen R., Brown R.R., Deletić A., Gangadharan L., Gernjak W., Jakob C., Johnstone P., Reeder M., Tapper N., Vietz G. And Walsh C.J. (2013)
- Water Sensitive Urban Design (WSUD) for South Africa: Framework and Guidelines by Neil Armitage, Lloyd Fisher-Jeffes, Kirsty Carden, Kevin Winter, Vinothan Naidoo, Andrew Spiegel, Ben Mauck and Daniel Coulson, Report to the WRC (no. TT 588/14), by the Urban Water Management Research Unit