Sponge city: Our urban landscapes have untapped potential to absorb stormwater
January 27, 2020
How can cities harness rainwater? It starts with making the decision to forget about paving—and choosing to lead with landscape.
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Urban floods happen when the stormwater hits the roads and the capacity of the storm-drain system is exceeded. Flash floods are increasingly likely in cities, because climate change increases the frequency of large storm events, and cities pave over precious absorptive green spaces. What if cities circulated more stormwater through buildings, allowing that water to get soaked up by our urban landscapes?
The idea of moving water is as old as the aqueduct. These days, people recognize that there is not a single macro move that does not have macro repercussions. This has caused our Anthropocene—aka human impact—state of natural imbalance. Now that we have moved into the era of the Internet of Things, we have the predictive micro technologies to work with nature.
The most damaging floods are those that arrive without warning—flash floods. They are challenging because of our inability to predict when they will come. But we know they will come, and we can model their damage. And we know that the increase in flash floods is because we do not have enough green infrastructure—a strategy of water management that involves creating “green landscape as sponges”—to absorb them.
Flash floods are increasingly prevalent in our cities as climate change increases the frequency of large storm events and cities pave over precious absorptive green spaces.
So, what’s the solution?
We must consciously and collectively decide to harness our water and “lead with landscape.” We must design and develop parks and open spaces to which our buildings respond, rather than the other way around.
Cities are made up of people and people are made up of water.
The development of a stormwater strategy begins by understanding the slopes and the topography, calibrating the permeability of the surface, and creating permeable parks with trees for erosion protection. We also must understand the hydrology. This approach can reduce the risk of flooding and increase the availability of water for all. The best parks are designed to be both infrastructure and an open-air space of joy. They receive hold flash floods, while offering a place of gathering.
Forget about paving—it’s about the “sponge” method
We must take a “sponge” approach to cities that we have been paving over for decades. Here in Toronto, as we build our city, we continue to add hard surfaces. We have a flooding issue that can be cured by taking a more interdisciplinary approach to the waters that we receive from the sky. If we get our water-absorption strategy right, we get our parkland dedication right. If you have the space for managing water, then you have space for gathering.
This requires an infrastructure shift in mindset. As I hinted above, let’s move away from paving and piping water, which restricts its power. Let’s help water to slow down and spread its restorative purpose across our city. Let’s store water on the roof, water the gardens and parks and tree stands, and use it as an alternative water supply within building systems. By putting enough parkland on our sites to deal with stormwater, we contribute to the health, wealth, and community-building structures of our city.
Urban flood acupuncture can relieve some of the pressures
Now, with the increased accuracy of predictive flood modelling, we have the tools to fix our problem with urban flood “acupuncture.” By choosing how and where to let the water run—as if selecting spots during an acupuncture treatment—we can relieve roads, streets, and basements of their current flooding pressures.
The prospect of an attractive, parks-rich, climate-resilient approach to urban development is within our grasp. It’s evident in many of the new developments that are happening in Toronto and other cities around the world.
Support from both public and private agencies is critical to moving in the right direction. I acknowledge that without public and corporate buy-in, it will be harder to secure political support for transformative policy planning that will stretch across several electoral cycles.
Bioretention, shown in this urban rain garden, is a process that removes sediment and pollutants from stormwater by filtering it using soils and plants.
Small changes, big improvements
Small policies can also result in huge changes. For example, when the City of Toronto mandated that all residents disconnect their downspouts from the sewer system in 2007, it changed the game. Now, instead of rooftops full of water flowing into our sewers, the water gets released where we want it—into our private gardens and public parks. Tree planting is another wonderful, simple, and transformative act. Micro changes are critical and reap the largest and most long-lasting benefits for our cities.
Lately, I’ve been inspired by the use of scalable wetlands by colleagues of mine in England, where three floating island ecosystems have been installed in Killingworth Lake, North Tyneside, to improve biodiversity and water quality. The wetlands are part of a scheme that provides flood protection to over 3,500 homes, improves water quality, and provides amenities and biodiversity. Instead of flowing back to the sewerage system, overflows from the lake will spill into natural grassed areas alongside and drain back to a local watercourse instead.
Cities are made up of people and people are made up of water. Protecting that resource and spreading it effectively across our urban landscapes impacts our daily rituals, our health, and our happiness. As you move through your daily urban routine, consider your moment-to-moment relationship with our most valuable of resources. Visit an online water calculator to measure your true relationship with water, or contact me to keep this important conversation going.