Environment · Feature
Cities have to cope with heavy rainfall as well as longer dry spells. The sponge city approach seeks to retain, use or infiltrate water where it falls. This requires connected spaces and suitable soils. Water quality is also crucial: not all runoff can be allowed to enter the ground untreated.
From rapid drainage to deliberate distribution
Sealed surfaces often channel rain rapidly into sewers. During heavy rainfall, this can lead to high peak flows, while the water is then unavailable locally later on. Water-sensitive urban development therefore complements engineered drainage with retention, evaporation, use and suitable infiltration. Green roofs, swales, permeable paving and open spaces that can temporarily flood can all contribute. The US Environmental Protection Agency (EPA) describes such measures as green infrastructure that can complement existing systems. Connecting the individual elements is crucial. An isolated swale is of little help if kerbs or unsuitable ground levels prevent water from reaching it. Planning must therefore take actual flow paths into account and establish where excess water will go when the intended storage reaches its capacity or an exceptional rainfall event occurs.
Rainwater can carry pollutants
Rainfall is not automatically clean once it has flowed over a building or a surface used by traffic. Water can pick up various substances along the way. The German Environment Agency’s guide to sustainable rainwater management highlights, in particular, the potential for substances to leach from construction products. This makes the choice of materials itself part of protecting water bodies. The treatment required depends on the source and contamination of the runoff, as well as how it is to be managed. An infiltration area is not a treatment plant capable of handling unlimited loads. Groundwater conditions and possible legacy contamination must also be considered. The sponge city concept therefore does not mean letting as many drops as possible disappear into the soil without assessment. Rather, it calls for a coherent approach that assesses water quantity, water quality and the sensitivity of the particular site together, and brings them into alignment in a transparent way.
Public spaces take on additional roles
Road verges, squares and green spaces can temporarily hold water alongside their everyday uses. Such multiple uses are particularly attractive in densely built-up neighbourhoods, where additional space is scarce. However, they require clear safety and use plans. Not every area where people spend time can be allowed to become an uncontrolled depression during rain, and important access points must remain accessible. Maintenance and cleaning are also essential: sediment can block inlets or reduce the permeability of surfaces. Successful projects therefore plan operations as carefully as the physical design. Flood protection is not achieved on a single plot alone, either. How measures work together across the catchment remains crucial. The sponge city is thus less a particular construction product than a long-term way of working that incorporates water early in the design, redevelopment and maintenance of urban spaces.
Sources & further reading
- US EPA: About Green Infrastructureepa.gov
- German Environment Agency: Good Rainwater Managementumweltbundesamt.de
Written in our own words with AI assistance, based on the sources listed above. The text distinguishes research findings, technical prototypes and established knowledge. This feature is not a breaking-news report.