Sponge City Design: Can a City be a Sponge?

Sep 19

Yangtze River. 1931. China. The deadliest flood recorded in world history, with an estimated 3.7 million deaths. Water levels peaked at a staggering 28.28 meters in August 1931, along with a three-month (July-August 1931) submergence of the heart of the Wuhan. This resulted in more than 7 lakh refugees left homeless and cramped on higher ground. When the flood eventually subsided later in August, the recovery period lasted more than a year, hampered by disease outbreaks and agricultural failure. As a result, no one, neither China nor the world, wanted to see a repeat of what happened. The result? Sponge city.

As the name suggests, this type of infrastructure is based on the principle of absorbing and retaining water; just like soil. The process of engineering adopting from nature is called Biomimicry. In the period of 2008-2010, China experienced major floods and some of the highest flood damages and was only followed by the USA and India. This called for some major precautions to be taken as the loss of properties and human lives (nearly 57k people were evacuated and 79 died) was large and widespread. In the year of 2013, the concept of Sponge City was officially introduced by Chinese landscape architect, Dr. Kongjian Yu of Peking University. This plan was later adopted by the government in the same year.

The traditional concept of ‘gray’ infrastructure (concrete pipe and drains) is replaced with the concept of ‘green’ and blue infrastructure to store and purify rainwater. The first implementation is the permeable pavements. These pavements use porous materials like permeable asphalt or interlocking pavers. During rains, the water infiltrates the sub-base, reducing surface runoff and mitigating urban heat islands (UHI). The second innovation is the green roofs. The normally bare roofs would be covered with vegetation and soil layers. These mini rooftop gardens absorb rainfall, significantly slowing runoff volumes. They also help improve building insulation and air quality. Next is the storage. Rainwater is naturally occurring and hence needs only temporary storage units before being absorbed into the soil and naturally filtered by plants. To retain the excess water, Rain gardens, constructed wetlands, and retention ponds are integrated into public parks and residential areas. Last, but not least, is the smart drainage system. The new drainage systems rely on IoT sensors and real-time data to balance storage capacities across the city and prevent localized floods. This collected data is also used to open or close floodgates and valves to control the flow.

Despite the advantages of sponge city, there are drawbacks to it as well. These include high initial investment costs, limited space, maintenance requirements, and technical complexity. Sponge city design calls for an interdisciplinary collaboration between engineers, planners, and policymakers. Solving and addressing these problems requires long-term planning, supportive policies, and public participation.