What are sponge cities and why are they the future of urban design?

Chapman Taylor has been involved in the design of a number of major projects in China using sponge city principles. In this paper, Yichun Xu from our Shanghai studio explains what sponge cities are, why they are being created across China and how Chapman Taylor is well placed to bring its sponge city design experience to urban environments across the world.

What is a sponge city?

A sponge city is an urban area which has been designed to cope with excess rainfall using a variety of techniques. Existing urban areas often have to deal with flooding caused by heavy rain, high tides or swollen rivers, and sponge city design can mitigate or prevent such events by providing the area with the ability to naturally absorb the water.

Simply reducing the number of hard surfaces and increasing the amount of absorbent land, particularly green space, can make a significant difference in reducing the severity and frequency of flooding events. Supplementing this approach with efficient channelling and storage systems can help to counter the frequency of water shortages, which can be particularly acute in large cities.

Other measures can include:

• The provision of rooftop green spaces.
• The storage and harvesting of rainwater.
• Building roads with porous surface materials.
• The use of water-intensive plants and trees.
• Creating land basins to hold excess water.
• Introducing more ponds and lakes which can hold some of the excess water.

Nature based sponge city infrastructure performs much the same function as traditional infrastructure which is aimed at drainage and re-use only, whereas a natural system provides many additional environmental and socio-economic benefits.

A well considered sponge city design will reduce the frequency and severity of floods, improve water quality and reduce water waste while also improving air quality and reducing the urban heat island effect. In addition, the green spaces and water bodies incorporated can provide relaxing amenity spaces for people as well as havens for wildlife, and we know that abundant green space is a key tool for boosting mental and physical wellbeing.

Why are sponge city strategies necessary?

Global climate change and an increasing world population both pose great challenges in two interlinked respects – an increased frequency of extreme flooding events in many areas and a shortage of clean, drinkable water, often in those same places.

Urban areas, particularly low-lying ones, are prone to flooding during storms because there is nowhere for the water to go; as concrete and other hard surfaces will cause the water to pool and flood. Flood barriers and storm drains can be sufficient for normal rainfall but can be overwhelmed by more intense storms, especially when a major contributory factor is rising water levels in local rivers.

A sponge city is designed to create a much more absorbent urban environment so that the groundwater from such events can be drained away more effectively and collected in aquifers for later cleaning and re-use. A good sponge city strategy also reduces or eliminates the risks from polluted runoff.

Following major flooding in Beijing in 2012, the Chinese government put in place a programme of sponge city urban developments across China, which has now expanded to over 30 locations, including Shanghai and Beijing. Drawing upon the research of Harvard-based Landscape Architecture specialist Professor Kongjian Yu, the authorities are rolling out the strategy to such an extent that it is fast becoming the norm for urban design in China. Other countries, especially India, the USA and Russia, have been watching this process closely with a view to following suit in their own flood-prone areas.

Context-driven design

When we masterplan major new developments in China, natural methods of water conservation and flood prevention are a key element of our approach. Our designs all provide large areas of green space, including green rooftops, roadside planting, ponds and lakes, among other absorbent design elements.

There is no one-size-fits-all approach to the design and implementation of sponge city strategies. Each urban design must be context-specific, taking account of the area’s topography, climate, seasons, meteorological history, geology, layout, materiality and existing infrastructure. What may be a perfect solution for a city in a continental mountain area may be of little use when designing for example a maritime coastal plain.

Where appropriate, we have created artificial wetlands in the centre of a development site, which serves both to hold excess rainwater and to cleanse the groundwater as it filters underground, using a carefully selected range of plants to aid the process. We link the ponds and lakes, whether artificial or natural, with the nearby rivers and create above-ground and underground water systems for the drainage and collection of excess water.

At our successful Xiong’an masterplan development, a seven-tier central park will form the basis of our sponge city strategy, with the lowest two levels combining as a wetlands system to capture excess rainfall in storm season via a network of streams. The groundwater will be naturally filtered and stored in underground aquifers or in ponds. The system will be completely organic, with no need for hard artificial surfaces or infrastructure.

We have also been working on a sponge city masterplan at Nanchong in Sichuan province, but the topography is very different – it’s a mountainous region and the city is in a valley. We took advantage of the topography, using gravity to guide runoff towards a series of wells, including many placed along the city’s streets, and ultimately towards artificial wetland aquifers where the cleansed water is stored for later use. This wetland environment will form the basis for the city’s central park in future years.

For our Qilong Innovation Park project, we worked in collaboration with landscape architects Martha Schwartz Partners to create two linear parks within the masterplan site, which intersect at a large central parkland area that serves as part of the wetlands system. The intersection meets a natural river, while several artificial sinks collect excess groundwater for reuse, doubling as small ponds for people to enjoy in the rainy season while, during the dry season, the spaces can be used as lawns for events, sports, picnics and relaxation.

A cost effective longterm solution

Implementing an effective sponge city strategy requires major investment, particularly when retrofitting existing urban areas. However, compared with the costs involved in repairing the damage caused by major flooding events a one-off investment in natural sponge city systems is common sense, potentially producing massive savings in the long run. There are also many associated benefits, including reduced pollution and the boost to mental and physical health that comes with adding more green space and water bodies in the city.

Sponge city systems are being implemented in cities across China, but the strategy affects the entire watershed area beyond, which often also includes other cities or towns. When floods occur, they tend to have a negative impact upon connected areas, whether urban or rural, within the wider natural water system. If the watershed area is able to cope with excess water, this benefits the entire network, and the easier it is to provide for water needs across that region in drier seasons.

The Chinese Government is investing heavily in sponge cities and it is now a core element of urban design schemes across the country. However, the approach could be applied around the world. There are very few countries which do not have to cope with regular flooding events or water shortages, and the frequency and severity of both will only increase as climate change accelerates.

We therefore need to consider sponge city strategies as a matter of course in the urban design process, as one of a range of tools for protecting the homes, livelihoods and lives of many millions of people. Chapman Taylor has been creating sponge city designs collaboratively with engineers, ecologists, local authorities and other stakeholders in cities across China and is well placed, as an international design group, to use that knowledge to implement effective and bespoke sponge city strategies around the globe.