It's doubtful that we think of natural elements, green infrastructure or absorbent materials (let alone a sponge).

But the concept of “sponge cities” may provide us with a better way of dealing with extreme weather events such as droughts and floods, as well as enabling sustainable water recycling practices. In fact, in some parts of the world this concept has become a reality.

What are Sponge Cities?

Needless to say, sponge cities aren't cities literally made out of sponge. Rather they are an innovation in urban design that aims to make the urban environment more sustainable and compatible with local ecosystems. They involve a redesign of buildings and the urban environment in order to more effectively collect, store, treat and ultimately reuse rainwater. Sponge cities offer an alternative to traditional flood defence and drainage systems and could provide a far more effective barrier against floods. With extreme floods and heavy rains rising 50% globally over the last decade, the sponge city concept could play an important role in urban risk management in coming years. Moreover, by retaining water more effectively, drought can be overcome without endangering water supply.

The main problem with concrete cities is that their impenetrable surfaces don't allow water to return to the ground. The drainage systems easily overflow, creating storm water runoff which can end up polluted with general trash and pathogens before flowing into waterways.

Sponge cities allow rainwater to filter through the soil where it can reach urban aquifers before being extracted from the ground for immediate reuse. Local retention in buffer systems or ponds allow for efficient local reuse as well. Typical features of a sponge city are:

• an abundance of biodiverse greenspace including waterways. This may involve the creation of “storm water parks” with ponds and wildlife

• green roofs on buildings that are at least partially covered with vegetation and allow for the absorption and filtration of rainwater
• porous roads and pavements that can soak up water or allow it to travel down to a filtration point
• systems that can harvest the collected rainwater for reuse in a range of household and commercial purposes, e.g. irrigation to even drinking water production

Benefits of Sponge Cities

Transitioning to sponge cities brings with it a range of potential advantages, including:

• More sustainable and readily available water supply – With the overuse of freshwater supplies heavily contributing towards risks of future global water shortages, sponge cities could offer a timely solution by providing an abundant supply of recycled water.
• Less Pollution – Sponge cities can reduce storm water runoff by around 70%, which will in effect reduce the pollution of urban waterways.
• Reduced Flood Risk – By absorbing and treating water rather than allowing it to fill drainage systems and gather as storm water, sponge cities are better designed to deal with flood risks.
• Increased Biodiversity – Existing city designs have led to a decrease in urban biodiversity through reduced green spaces and the creation of hotter microclimates. Sponge cities can help redress this imbalance.

The sponge city concept naturally comes with some drawbacks, including costs associated with the urban redesign and the implications of an oil spill were to occur on a more porous road surface. However, costs are likely to come down if the technology becomes more mainstream and measures can be put in place to deal with any potential disasters.

Sponge Cities in Action

China is the world leader when it comes to sponge cities. Following severe floods in Beijing in 2012, the country launched a “sponge city initiative” in 2015 which started with 16 model sponge cities before extending to 30 in total. Today, there are 250 sponge city initiatives across China as well as plans to develop programs in countries including the US, Russia, and Indonesia.

Contact Water Experts to find out more about the latest sustainable water technology solutions and to learn how your environment can be turned into a sponge.