Sponge Cities – A Solution To Urban Flooding?

As global warming intensifies, the world is getting warmer and wetter. Flood control is therefore becoming a much higher priority, especially for low-lying cities. Hence the emergence of a new urban planning concept: the "sponge city". How do sponge cities prevent urban flooding? What are their main features? We explain all you need to know.
Sponge City , green roof designs to climate-proof city.
An example of a green roof in Germany. Germans use the term Stadtschwamm, meaning sponge city. Image: © efb-greenroof

Green Infrastructure

This article shows how sponge cities offer a viable solution to urban floods, which is becoming progressively worse as climate change increases humidity and rainfall levels. It also shows how sponge cities can help lessen the effects of global warming on humans living in high density areas.

It’s all part of the new climate-proof approach to city planning, pioneered by the Dutch, known as green infrastructure, which is based on the idea of building with nature in order to create a more sustainable environment. 1 Watch this idea become a central feature of climate action plans around the globe.

What is a Sponge City?

A sponge city is a city designed with ecologically friendly alternatives to traditional flood defences and drainage systems. The basic idea of a sponge city is to replicate the natural water cycle by keeping water where it lands.

Rather than building storm drains to carry away heavy rainfall, you allow soil and plants to act as ‘sponges’ by soaking up excess water.

Following the example of urban areas in The Netherlands, like Amsterdam, Rotterdam and Utrecht, 2 cities like Wuhan in China and Berlin in Germany are adopting this concept as part of their climate change adaptation strategy. 3

The term sponge city is similar to other urban design concepts involving low-impact development, nature-based solutions and water-sensitive cities.

How Do They Work?

Modern cities are made of concrete, glass and steel. They look and act unnatural – absorbing heat and repelling water. The idea behind a sponge city, is to imitate the natural environment as much as possible. In the process, sponge cities can even act as a carbon capture solution by absorbing carbon dioxide from the atmosphere, one of the main runaway greenhouse gases driving global warming.

Sponge city designs include:

1. Permeable Pavements

In rural areas, rainwater naturally infiltrates the ground. But in cities, this process is disrupted by concrete buildings and paving. Typically, a city is 50 percent impervious, meaning water can’t seep into the ground and this figure increases to 70 percent in megacities like Shanghai and Beijing. 4  

One solution is permeable pavements, also known as ‘green pavements’, which have a porous surface due to the building materials used.  Permeable paving is particularly useful in low traffic areas such as sidewalks, parking lots and driveways.

Permeable pavements are more environmentally friendly because they can be built from recycled materials and by-products such as slag cement from iron manufacturers. Recycling of materials will lead to fewer landfills and methane gas emissions.

Another benefit is that porous pavements don’t freeze over because rainfall doesn’t stay on the surface. This makes them much safer. In summer, they stay cool because the air can circulate more freely.

2. Swales

Swales are ditches lined with grass or other vegetation, which slow and control the flow of water preventing puddling and erosion. In Rummelsburg, Berlin, excess water from roads and pavements flow into swales which are so effective, the district did not have to install a large public drainage system.

3. Open Green Spaces

Sponge city planning calls for the creation of more open green spaces, including rivers, ponds and wetlands. These act not only as porous surfaces but also as natural storage solutions during heavy rainfall.

4. Rooftop Gardens

Planting trees, shrubs and grasses on top of buildings offers a multitude of benefits. The soil soaks up excess water, and slowly releases it back into the environment. Rainfall can also be saved and recycled by residents, offering a solution to water shortages in heatwaves and periods of low rainfall. 5. It also acts as a natural air conditioning system because plant respiration has a cooling effect. Residential areas with high proportions of green roofs are usually cooler on hot days.

What Are The Benefits of Sponge Cities?

The key benefits include:

Flood Prevention & Water Recycling

Rainwater is absorbed naturally, and can even be captured and repurposed for irrigation and home use. In China, the country plans for 80 percent of its urban cities to harvest and reuse 70 percent of rainwater.

Pollution Reduction

If there is a sudden downpour, rain water can overload a city’s stormwater retention system. Excess runoff can become mixed up with the sewage system, polluting drinking water and recreational areas. This is of particular concern for large cities with high levels of ground level ozone, as studies now link air pollution to increased likelihood of rainfall. 6

The incorporation of green rooftops and lots of green spaces with plants and shrubs as well as grasses, also helps to filter the air and neutralize certain chemicals in smog and acid rain.

The introduction of electric vehicles (EVs) is another essential step towards pollution control in urban areas.

Heat Reduction

Cities tend to be hotter than the surrounding area. This is partly due to the widespread use of concrete and asphalt that absorb and re-radiate the sun’s energy. This warming effect is exacerbated by the release of heat from heated buildings as well as vehicles.

Petrol or diesel-powered cars emit black carbon particles and a host of chemical gases and particulate matter that coalesce in the air, forming heat-trapping smog and ozone. All this contributes to a phenomenon known as the urban heat island effect. See also: Health Effects of Air Pollution.

Through their incorporation of natural features, sponge city designs help to reduce this excess heat. Tree-planting is a highly effective method of improving a city’s microclimate. Tree leaves absorb sunlight, provide shade and cool the surrounding air through their transpiration of water vapor.

This heat reduction benefit is likely to become a lot more important if global temperature projections prove to be true.

Lower Emissions

Sponge cities also help to limit global warming through their use of natural material. The reduction in the amount of stone and concrete used in urban infrastructure, leads to an automatic reduction in the amount of cement CO2 emissions incurred. Cement accounts for roughly 8 percent of all man-made greenhouse gas emissions, and any reduction is welcome.

Aesthetically Beautiful Cities

Grass swales, artificial ponds, wetlands and rain gardens all add to the visual appeal and calming effect of urban development.

Where Are Sponge Cities Located?

China is the main driving force behind the development of sponge cities, mainly driven by the rapid urbanisation of their population. According to the Chinese Ministry of Housing and Urban-Rural Development, more than 10 million Chinese migrate from rural areas to the cities every year.

As cities expand, natural vegetation is lost and is replaced by concrete, steel and asphalt. A massive flood in 2012 in Beijing provided a wake-up call.  At least 37 people were killed in the heaviest rains the Chinese capital has seen since records began more than 60 years ago.

In response, in 2015 the government announced that they are piloting the first batch of sponge cities, to include the cities of Wuhan, Shanghai, Chongqing and Xiame. This number is to extend to 30 by 2030.

In Wuhan, for example, stormwater management is integrated into the city design as a whole. The aim is for the city to retain 70 percent of all stormwater by the end of 2020. Wuhan’s Xinyuexie Park has been converted into a beautiful green space with rain gardens, permeable pavements, and storage ponds for rainwater to be recycled across the city. The park also includes homes, office, shops and recreational areas. It’s like creating a new type of urban ecosystem to help climate-proof our lives./

The city of Harbin in northern China also aims to preserve and improve how it copes with storm water. It has created its own 34-hectare sponge city stormwater park. The park is multi-functional, providing multiple ecosystem services. It collects, cleanses and stores rainwater and allows it to percolate into the aquifers. At the same time it preserves the native natural habitats and offers an aesthetically appealing space for public recreation.

The Netherlands, Germany, the United States, India and Russia are among the countries that are also investing in water-sensitive cities.

Is it Expensive to Create a Sponge City?

Yes. Sponge cities come at a cost. The cost of retrofitting a city, it’s public spaces, schools and residential areas with absorbent features, is astronomical and runs into billions of dollars. The Chinese government estimates it will cost about 100 to 150 million yuan ($14 million to $22 million USD) per km2. 7. Without strong political will and public support, such large scale projects are difficult to get off the ground.

Sponge Cities: Conclusion

As Earth’s climate crisis worsens, increased rainfall and rising sea levels will make life in urban cities increasingly more challenging. However, what concepts like sponge city show, is that we can make friends with floods. We can retrofit our cities to solve water problems, instead of creating them. In the long run, there is little doubt that water-sensitive cities will also help to reduce carbon emissions and help fight climate change.

References

  1. What is Green Infrastructure?” EPA. []
  2. Rainproof cities in the Netherlands: approaches in Dutch water governance to climate-adaptive urban planning.” Liping Dai, Rebecca Worner & Helena F. M. W. van Rijswick. Published 26 Sep 2017 []
  3. Inside China’s leading ‘sponge city’: Wuhan’s war with water. 2019, The Guardian []
  4. “Urban Permeable Pavement System Design Based on “Sponge City” Concept”. M M Yu et al. 2017 []
  5. The Impact of the Construction of Sponge Cities on the Surface Runoff in Watersheds, China” 2018, Guoqiang Dong et al []
  6. Catastrophic Chinese floods triggered by air pollution“. 2015 , Science Magazine []
  7. Transitioning to Sponge Cities: Challenges and Opportunities to Address Urban Water Problems in China“. Chris Zevenbergen et al, 2018 []
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