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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteCities can reduce some flood risk by managing rain close to where it lands: slowing runoff, storing it temporarily, letting some soak into the ground, and filtering or reusing some of it. A “sponge city” is not one device or a promise that floods will disappear. It is a coordinated mix of green features and conventional drainage, designed around local conditions.
What does “sponge city” mean?
Roofs, roads, and parking lots are largely impervious, so rain runs off them faster and in greater volumes than it would across permeable ground. A sponge-city approach tries to keep more of that water near its point of origin rather than sending all of it immediately into storm drains.
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Hong Kong’s Drainage Services Department describes the idea in terms of absorbing, storing, infiltrating, and cleaning stormwater, then releasing or using it as needed. The U.S. Environmental Protection Agency (EPA) uses the related term green infrastructure for measures that capture rainwater “where it falls.” In practice, water may soak into soil, sit in storage for a time, be reused for irrigation, or pass more slowly into the drainage network.
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That makes “slurp up rain” a useful image, but not a literal description: the water does not vanish. The goal is to change where it goes and how quickly it moves.
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How does a city manage rain this way?
Imagine rain falling on a roof or street. Instead of flowing straight to a drain, it may enter a planted garden, run through a vegetated swale, pass through permeable pavement, or be directed to a tank or pond. Depending on the design and site, the water can infiltrate, be stored temporarily, have pollutants filtered, or be collected for later non-drinking uses. Pipes and drains still carry water that the green features cannot manage.
Measures that soak in or filter runoff
- Rain gardens and bioretention cells: planted depressions that collect runoff and allow it to filter through soil and vegetation.
- Bioswales and vegetated swales: planted channels that slow and guide runoff while helping filter it.
- Permeable pavement and infiltration trenches: surfaces or below-ground features that route water into surrounding or underlying soil, where site conditions allow.
- Green roofs, planter boxes, and urban tree canopies: vegetation that helps intercept and manage rain before it reaches drains.
Measures that store or reuse water
- Rain barrels and cisterns: collect roof runoff for later nonpotable uses such as irrigation. A barrel is a home-scale example, not a substitute for city drainage works.
- Ponds, lakes, wetlands, and restored water bodies: can provide space to hold runoff and, depending on the design, support filtration and other ecological functions.
- Underground storage tanks: hold stormwater temporarily and can release it after downstream drainage has receded.
EPA describes green infrastructure as most effective as part of a broader stormwater strategy that also uses gray infrastructure—such as pipes, drains, and storage facilities. The mix matters: no single feature can handle every site or storm.
What can sponge-city measures do—and what can’t they do?
Green infrastructure can reduce runoff and pooling in streets, slow water moving toward drains, store some stormwater temporarily, filter pollutants, and ease pressure on drainage pipes. Captured water may also be reused for purposes such as irrigation. EPA emphasizes benefits for localized flooding and smaller, frequently occurring storms; it does not claim that green infrastructure prevents all flooding.
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Localized street flooding and river flooding are different problems. Managing rainfall runoff can help with the first, but riverine risk also calls for measures such as preserving floodplains and open space. A city can combine these approaches with drainage improvements, but it should not treat one as a replacement for the others.
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Why results vary from one place to another
Performance depends on how quickly local soils absorb water, how much space is available, the intensity and volume of rainfall, the storage capacity and placement of each feature, the condition of existing sewers, and ongoing maintenance. A feature that works in one neighborhood may perform differently elsewhere, or may need to be paired with drains, pipes, or storage.
EPA recommends hydrologic and hydraulic modeling to identify combinations of green and gray infrastructure that fit a community’s flood-reduction and water-quality goals. When comparing options, ask what kind of flooding they target, what storm volume they are designed for, whether they infiltrate, store, filter, or reuse water, and how their performance was measured under conditions comparable to the proposed site.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What do reported projects show?
Historical project examples illustrate both the potential of these measures and why results should not be generalized. They are not guarantees of performance in another city.
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The EPA reported results for a project in Ramsey County as of 2010: infiltration trenches reduced runoff volumes by 77 percent, rain gardens by 88 percent, an underground storage and infiltration system by 100 percent, and a regional pond by 5 percent. These figures describe that project, not typical or universal reduction rates.
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- Convenient Spigot: Easy-to-use plastic spigot for quick and efficient water retrieval.
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EPA also reported an estimated $2 million cost for the green-infrastructure project, compared with an estimated $2.5 million for a new 60-inch storm sewer alternative. Those were project-specific estimates, not current bids or a general cost comparison.
Hong Kong: a network of measures
Hong Kong’s Drainage Services Department reported that identified flooding blackspots fell from 90 in 1995 to 7 in 2017 under a three-pronged flood-prevention approach: stormwater interception, flood storage, and drainage improvements. The department’s report does not isolate how much of that change came from sponge-city measures alone.
The same 2017 report described green roofs and porous pavement to increase infiltration, recreation areas and lakes for detention and flood storage, harvesting for reuse, and underground tanks to hold water temporarily. It gave the Happy Valley Underground Stormwater Storage Tank a capacity of 60,000 cubic metres. That is the capacity of one facility, not a measure of citywide flood reduction. The department also reported average annual rainfall of about 2,400 millimetres for Hong Kong; that figure is local context, not a global benchmark.
How should residents assess a proposal?
A useful proposal explains which local flooding problem it addresses and how the measures will work with existing drainage. Look for a clear account of site conditions, intended storm scale, storage or infiltration capacity, maintenance responsibilities, and monitoring. For large projects, modeled estimates and measured results from comparable local conditions are more informative than a broad claim that a city will become “flood-proof.”
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