Are 'Sponge Cities' the Solution to Urban Flooding?

Chicago City Hall's green roof can absorb a lot of water. (Photo: TonyTheTiger/Wikimedia Commons)

The basics behind preventing (or at least mitigating) urban flooding — like we've seen in Houston, Miami and Mumbai — are simple. So simple that new urban building strategies to more proactively deal with it are pretty much exactly what I used to teach my middle-school ecology campers. In my outdoor hydrology lessons I illustrated, via a sprinkler attached to a hose, what happened to water over a (mini) landscape: The water flowed right over the concrete blocks I had stacked up in a stepwise manner, pooling at the lowest spots. In the moss-and-plant-filled diorama next to it, water soaked into the earth and mosses for many minutes until finally the excess started running off the edge of the demo table.

Natural systems have evolved to retain water. Human-made structures tend to waste it. We've recently seen the results of that in areas hit hard by hurricanes — but anyone can see it if they compare an empty, rainy parking lot with a field, garden or woodland of the same size. When we build cities, we favor the short-term solutions: paving over a marshy, muddy wetland to build on top of it, directing water into impermeable plastic tubes, and where green spaces exist, taking down deep-rooted trees in favor of decorative bushes and low-lying plants in containers that sit on perfectly paved courtyards and public areas.

But the solution to flooding is to do the opposite, to mimic nature. A group of public and/or private infrastructure projects aim to do just that. Used together in an urban area, it can then become a "sponge city."

The benefits of intelligent landscaping

The idea isn't just more and larger pipes to move high waters away from inhabited areas, but rather to incorporate ideas like green roofs with swales, urban wetlands, gardens and parks, and permeable pavement.

There's another huge advantage to sponge cities: Most of the flooding mitigation tactics create cooling too. Primarily this is due to using water where it falls. Plants that have absorbed the water caught in place release it into the air as it gets warmer, which naturally cools local air and fights the urban heat island effect, too.

As the video below details, in the Rummelsburg neighborhood of Berlin, Germany, the combinations of swales and green spaces create natural air conditioning: On hot days, locals say you can feel the cooler air. And that's in an area that has no other storm-water infrastructure — it's all down to intelligent landscaping.

If all these initiatives sound expensive, they're not, especially when compared with the costs of installing new sewage treatment plants and the short- and long-term costs of more flooding (including human life and massive loss of housing and infrastructure).

China has been investing in their growing cities in this way, and test projects show that keeping rainwater local works. They have set an ambitious goal that began in 2015. By 2020, the government has decreed that 80 percent of 16 urban areas should absorb and be able to use at least 70 percent of rainwater, and they're even teaching the concept in elementary schools. So far, they've spent about $12 billion.

In Shanghai, for example, almost 100 acres of green roofs and gardens are currently under creation. The government is paying for about 20 percent of this; the rest of the costs are borne by local cities, builders and landlords. In Wuhan, a test in June 2016 showed that sponge-city tactics delayed flooding by hours.

"The rain started at 8 a.m. By about 10 a.m. it had escalated to a rainstorm that lasted until 2 or 3 p.m.. Without the sponge city facilities, the accumulated water would have started flowing from the area about 10 to 15 minutes after the rainstorm, but our monitoring found that the water only obviously began flowing out after 11:30 a.m.," Peng Bo, an official with the Wuhan commission of housing and urban-rural development, told China Newsweek.

How sponge cities work

There are various mitigation techniques that a sponge city might use.

Green roofs can keep rain from ever reaching the ground and overwhelming sewer or drainage systems. By catching water at the tops of buildings, and storing it in soil (and tanks for later use in the garden when it's dry), water can be used to keep green spaces verdant, provide an outdoor area for the people who live and work in the building, and even be used to grow food.

Swales are permeable spaces between sidewalks, roads or other paved surfaces that can absorb runoff. Picture a stream-sized depression between pathways, filled with water-loving plants. This looks more attractive than a whole paved area, and water can be utilized by the plants that live in the swales, and also absorbed into the earth beneath — some of which will eventually make its way into aquifers below, allowing them to recharge. They also can naturally filter local pollutants out of the water without expensive mechanical filtration systems.

Parks and green spaces, even smaller ones, can make a difference when it comes to flooding, especially in industrial areas that are entirely paved over. Learn more in this example:

By placing some green spaces in areas that are now semi-empty paved lots that aren't even being fully used, local wildlife will have a spot to re-establish itself, and water that makes its way through them will be naturally detoxified.

Of course larger parks featuring local plants can make an even bigger difference. A great example of that is the Hudson River Park that runs along Manhattan's West Side. Formerly just a paved-over walking track along the river, it now features plenty of room for cyclists and runners, but also includes buffers planted with native grasses, flowers and trees. This both protects the city from flooding from the Hudson River, cleans the water going into the river from the city, and is much more pleasant (and in some spots downright beautiful) than the previous incarnation of the park.

Permeable pavement is simply a porous version of what we are all familiar with. It allows water through it (filtering it at the same time) so it can soak into the earth, and make its way to aquifers below. In addition to asphalt that lets water through, there are also special gravels, pavers and bricks that are decorative as well as porous, so they can be used in public places.

There are other ways to de-pave, too: In Montpelier, Vermont, the state capital is reverting back to "dirt" roads made up of soil and ground-up asphalt, that's "reinforced with geotextile, a type of durable and permeable fabric used to bolster soil stability, prevent erosion and help with drainage," according to MNN's Matt Hickman. In places that deal with ice, porous pavements and gravels can also have the advantage of icing over less frequently, since water doesn't pool and freeze on top of concrete, increasing safety for drivers or walkers.

Urban wetlands come in a variety of shapes and sizes; even a relatively small wetland can absorb a lot of extra water. Ideally located in naturally low-lying areas, they have many benefits, including giving homes to water-plants, and a place for birds and other local wildlife to gather. Other wetlands might be added to already existent ponds and pools. (Instead of a hard edge at a city park's pond, it could have a more natural edge filled with cattails, reeds, and other water plants.)

Wetlands in cities could also be larger, maybe even reintroduced to where they once existed along coasts, but were torn up during development. These wetlands might keep ocean or river water from infiltrating into the city, while also naturally cleaning and filtering water that flows out into the body of water between them.