The City as Sponge. Adapting to a Warmer, Wetter World

March 29, 2018


It’s physics. Warmer air holds more moisture.
Early climate models predicted, and now decades of observations confirm, many parts of the planet are now seeing rain come down in more intense downpours, straining infrastructure adapted to literally, another planet.

Even in my small midwestern town, the city council is currently awaiting a report from consultants on ideas to handle more frequent flooding events, which threaten property values in neighborhoods formerly not subject to flooding.

Cities need to adapt. Nature knows how.


Benjamin Franklin, Philadelphia’s favorite son, described his city’s stormwater problem well: By “covering a ground plot with buildings and pavements, which carry off most of the rain and prevent its soaking into the Earth and renewing and purifying the Springs … the water of wells must gradually grow worse, and in time be unfit for use as I find has happened in all old cities.”

When he wrote this in 1789, many of Philadelphia’s water sources, the scores of streams that ran into the Schuylkill and Delaware rivers, were already cesspools of household and industrial waste. As they became intolerable eyesores and miasmic health hazards, the city simply covered them with brick arches, turned the streams into sewers, and on top constructed new streets, an expanding impervious landscape that left the rains with even fewer places for “soaking into the Earth.”


A rain garden manages stormwater runoff in Philadelphia’s Germantown section. PHILADELPHIA WATER DEPARTMENT

Crude as it was, this network of underground-to-riverfront outfalls through ever-larger pipes was pretty much the way Philadelphia and other U.S. cities coped with their stormwater for the next 200 years. But Ben Franklin’s town has decided to take the lead in undoing this ever-more costly and outdated system that annually pours huge volumes of polluted stormwater runoff and untreated sewage into the Delaware and the Schuylkill. Instead of building more and bigger sewers and related infrastructure, Philadelphia has adopted a relatively new paradigm for urban stormwater: Rather than convey it, detain it — recreate in the urban streetscape the kinds of pervious places where, instead of running into surrounding waterways, rainfall and the contaminants it carries can once again soak into the earth.

The city is now in the seventh year of a 25-year project designed to fulfill an agreement with the U.S. Environmental Protection Agency (EPA) to reduce by 85 percent Philadelphia’s combined sewer overflows. These overflows occur when heavy rains overwhelm the capacity of the city’s sewage treatment plants to handle the flow from both storm and sanitary sewers, forcing the diversion of untreated effluent into the system’s river outfalls. But rather than spending an estimated $9.6 billion on a “gray” infrastructure program of ever-larger tunnels, the city is investing an estimated $2.4 billion in public funds — to be augmented by large expenditures from the private sector — to create a citywide mosaic of green stormwater infrastructure.


Integrated into the city’s green spaces, streetscapes, and public and private buildings, this green infrastructure ranges from simple home rain barrels and downspout planters to complex bioretention swales underlain by drains, filled with sandy soil, and planted with resilient species of grasses, perennials, shrubs, and trees. Along with rain gardens, tree trenches, green roofs, and urban wetlands, this infrastructure will, as one study put it, “optimize and engineer the landscape” to mimic and restore its natural hydrologic regime. In the end, Philadelphia hopes by the mid-2030s to create the largest green stormwater infrastructure in the United States.

Philadelphia began by adopting and adapting technologies developed on smaller scales in other locales. In the early 1990s, Prince George’s County, Maryland, for instance, explored the use of bioretention — directing stormwater into small depressed landscaped areas where it could infiltrate and be filtered through soil and plants. The success of what were then called “low-impact development” designs in detaining runoff and removing contaminants of all kinds inspired green infrastructure efforts in other places. These local governments eventually realized the advantages, both financial and environmental, of green stormwater infrastructure.

Portland, Oregon began its stormwater control efforts in 1993 with a “downspout disconnection program” that encouraged homeowners to redirect roof water to gardens and lawns. These efforts have since expanded into a Sustainable Stormwater Management Plan that links city agencies in an effort to control runoff. Green stormwater infrastructure is a major component of Milwaukee’s “ReFresh Milwaukee” sustainability plan, designed to reduce pollutant loads into Lake Michigan. By 2035, Milwaukee plans to capture the first half-inch of rainfall during any given storm on the city’s 43.7 square miles of impervious surface. In 2013, Seattle set a goal of using green stormwater infrastructure to control 700 million gallons of stormwater runoff and created a program called RainWise to encourage homeowners to install rain gardens and cisterns.


Parking lots in Chongqing are covered in gardens that allow water to be absorbed. (Wade Shepard)


Rain water is one of the world’s most wasted natural resources. Precipitation that falls upon cities typically gets expelled like a waste product, piped away via the standard complex of drainage systems.  Meanwhile cities go to great lengths to pump in water from rivers and lakes, or pump it up from aquifers that in many places are being depleted at a worrisome rate.

Three years ago, Chinese President Xi Jinping offered a new way to think about this. At China’s Central Government Conference on Urbanization in 2013, Xi announced that cities should act “like sponges.” This proclamation came with substantial funding to experiment with ways cities can absorb precipitation through permeable pavements, rain gardens and wetlands, or reuse the water locally for irrigation, parks or even for drinking. It also injected a new term into the global urban design vocabulary.

“Basically, ‘sponge city’ is a phrase that was invented in China,” says Austin Williams, an urban studies professor at Jiaotong-Liverpool University in Suzhou. “Nobody used this phrase until Xi Jinping announced it.”

Although the terminology is new, the idea behind sponge cities has been around for a while. Researchers have long looked for ways to deploy “green infrastructure” that can give entire cities the virtues of a rain garden. Programs in the United States as well as strategies in the United Kingdom like “SuDS” have aimed to develop new ways to handle and utilize runoff.

City leaders have begun putting some of these ideas into practice. Singapore has installed a network of gutters, cisterns, and reservoirs across half the city to capture rainwater that is used for everything from flushing toilets to — once purified — drinking water. And Portland, in the U. S. state of Oregon, is installing rooftop gardens, bioswales, permeable pavement, retention ponds and large amounts of surface vegetation.



At first glance, the square known as Tåsinge Plads doesn’t look much different from other parks in Copenhagen. A young couple lounges on a small hill surrounded by newly planted trees and wildflowers. Children laugh and play. Old women sit chatting on benches under the shade of tall sculptures shaped like upside-down umbrellas.

But there are hidden features that make Tåsinge Plads part of this seaside city’s plan to survive the effects of climate change.

During heavy rains, the flowerbeds fill with water and wait to drain until the storm runoff subsides. The upside-down umbrellas collect water to be used later to nourish the plantings. And clever landscaping directs stormwater down into large underground water storage tanks. Above those tanks are bouncy floor panels that children love to jump on — when they do, the energy from their feet pumps water through the pipes below.

Just a few years ago, this square was paved with asphalt and dominated by parked cars — a small grassy area was used more as a toilet for dogs than as a park. Now, it’s the cornerstone of a plan to make the surrounding area of Saint Kjelds into what planners here are calling the world’s first “climate-resilient neighborhood.”

The tarmac has been torn up and the greenery reduces the urban-heat island effect. More parks like it are being built to purposefully turn into small ponds during heavy rains, allowing them to capture and retain water on site until the drainage system has capacity to handle it. During the worst deluges, certain streets with raised sidewalks will become “cloudburst boulevards,” creating a Venice-like cityscape of water channeled safely through the city until it can empty into the harbor.


9 Responses to “The City as Sponge. Adapting to a Warmer, Wetter World”

  1. rhymeswithgoalie Says:


    “[G]reen roofs absorb water, which helps reduce storm run-off. ”

    I do worry about putting greenspace on top of buildings, unless they are explicitly designed to take the weight of sodden or overgrown (i.e., neglected) growth.

    • L RACINE Says:

      Historically the reason humans have drained standing water is because of the association with disease vectors… that is not addressed in the above article.

      To start to change/modify existing infrastructure to may not be feasible. How much of an expenditure of green house gases will it take to make these changes versus the cost of abandoning existing structures and starting over.

      IMO much of the current infrastructure will have to be written off (think Miami) it will not be suitable for purpose when climate change reality starts to hit home. When the calamity of what is to come becomes a reality the resources to implement these concepts will be scarce.

      At the end of the day it will be about the rate of change and our ability to adapt (which to date has been dismal).

      • greenman3610 Says:

        “standing water” is much more likely to be an issue where hydrology is not considered
        in urban design.

  2. L RACINE Says:

    IMHO this site tends to over states the inroads of renewables and fails to take into account the total green house gas emissions for the build out/construction and the on going operation and maintenance of these renewable generation facilitates and/or new technologies. Tesla cars are a perfect example… what is the total carbon foot print of these cars? It is interesting that no one discusses the on going maintenance of these vehicles and the associated carbon foot print of said maintenance/operations and “last but not least… ” what is the usable life span??? It is called a CARBON FOOT PRINT COST BENEFIT ANALYSIS!!!

    The laws of physics cannot be broken and there are limits. It takes time to scale up new technologies and put them into reliable cost and green house gas neutral production (Tesla is cutting their teeth, but it is by no way “proven/reliable tech” or that at the end of the day it reaches it stated goal of reducing carbon emissions, has it stands today it does NOT.) It is very naive and unrealistic to think otherwise.

    Our current situation is horrific, to not face the facts of just how grim the situation is IMHO extremely counter productive.

    This is an interesting well cited article by Kurt Cobb, “The Troubling Realities of Our Energy Transition”.

    • greenman3610 Says:

      nice try, but this is a standard bullshit talking point – “what about the carbon footprint of solar panels”.
      This has been answered many times. The idea that switching to carbon free energy increases carbon out put is the rhetorical equivelant of a perpetual motion machine.

  3. dumboldguy Says:

    “Nature knows how”. And how Nature works is explained by science. But too many humans are fighting a war against science.

    So, this small effort in a few citie is nice but is not going to make a difference—–it addresses the local effects of AGW without addressing global root causes. Window dressing and “feel good” eyewash

  4. redskylite Says:

    This is interesting and an attempt to mitigate some of the effects of warming. Down under in New Zealand there is also thought given to Climate Change adaption in new building enterprises and I was surprised to see extensive “Rain Gardens” included in a nearby new urban development.

  5. redskylite Says:

    Preparing for climate changes is not a new idea. . .but let’s hope we can suceed, where others failed.

    “Faced With Drought, the Pharaohs Tried (and Failed) to Adapt

    In a study published in this year’s edition of the journal Egypt and the Levant, the researchers pieced together ancient evidence — including flint and bone records from the fallen city of Megiddo, fossilized pollen data from the Sea of Galilee and ancient cattle DNA — to shed light on how Bronze Age Egyptians used careful planning and policies to adapt to a drought that lasted from around 1250 B.C. to 1100 B.C., while their ancient counterparts appeared to be less well prepared.

    Even with preparation, however, the Egyptian empire ultimately collapsed. But the study shows how recognizing and preparing for climate disaster can make societies more resilient.”

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