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As the name suggests, the "sponge city" concept is an urban resilience strategy where city spaces are designed to absorb water through streetscapes and landscaping, functioning as a "sponge" to retain excess stormwater and filter it. This is achieved through "nature-based solutions" that help distribute water more effectively while improving drainage and storage systems.
We at Godwin Austen Johnson have been commissioned for a major master planning project in India, a country where the monsoon season often results in widespread urban flooding. In collaboration with landscape designers, we have proposed incorporating the "sponge city" concept into the development. This approach will not only aid in water conservation but also ensure that excess floodwater is effectively absorbed.
We are planning the following measures to reduce the number of hard surfaces, making the ground more absorbent and enhancing the overall well-being of users:
- Cluster parks and green spaces with bio-basins.
- Green rooftop spaces on large buildings.
- Rainwater harvesting ponds.
- Roads integrated with bio-swales.
- Land basins designed with xeriscaping.
- Water retention ponds.
At the masterplan level, pocket parks are strategically distributed across the township, serving as catchment areas that contribute to sustainability and help regulate the microclimate through shading and evapotranspiration. The diagram below illustrates how these parks are arranged as ribbons across the housing clusters. During rainfall, all local surface runoff water will be directed towards these green pockets, with any excess water transported to nearby retention ponds and soakaways.
The smaller parks will feature bio-basins, which will be activated during rainfall. On dry days, these areas will appear as low-lying xeriscape zones, designed to collect water during the rains and transform into rain gardens.
Green roofs, a well-established sustainability concept, are proposed to serve as water retention systems for large roofs. These roofs are not intended for villas, as they are maintained by individual villa owners; however, all shared facilities, such as clubhouses and community halls, will incorporate green roofs with water retention capacities. Traditionally, these roofs are hard surfaces, with all runoff water discharged onto the ground. With the addition of green roofs, the majority of rainwater will be absorbed, and any excess water will be directed to nearby soakaways.
Rainwater harvesting ponds have been proposed to channel all surface runoff from roads and hardscapes, thereby recharging the groundwater. These ponds also serve as landscaping features, and naturally become part of the urban fabric.
The roads and streets will be lined with bioswales, which not only enhance the landscaping design but also aid in collecting excess water. These bioswales are primarily trench-like features, with planting on top and a porous base containing rocks, allowing water to percolate through.
Since the introduction of large sponge city projects in China, the concept has been successfully adopted in many cities worldwide. It is crucial for designers around the globe to embrace this concept and implement it at various levels to transform cities into "water-smart cities." Municipalities worldwide can assess their respective regions and develop regulations to incorporate some of these features. Recently, the World Economic Forum introduced the concept of the "circular water city," which combines the principles of sponge cities with a comprehensive approach to recycling and reusing rain and stormwater runoff.
Researchers from various organisations have measured the extent to which seven major cities around the world are covered by "blue and green infrastructure," such as grass, trees, ponds, and lakes, compared to "grey infrastructure," including concrete and the hard elements of buildings. Satellite imagery, along with AI and machine learning, can be utilised to assess the urban fabric in terms of its capacity to store water. Most cities fall within the range of 25-30% sponge ratings, and as designers, we can work to increase this percentage, promoting greater water retention and helping to mitigate the risk of flooding.
