What are Sponge Cities and how can they save us from floods?

As countries around the world face the increasing challenges of devastating floods and unusually heavy rainfall, the idea of 'sponge city' has gained greater traction in recent years.

In China, floods are a frequent occurance, with a recent one in the Sichuan province destroying homes and killing several people. Water in general has been an issue for decades in the country, with northern provinces suffering droughts and southern ones being prone to floods.

The concept of a "sponge city" was first proposed in 2000 by an architect named Yu Kongjian. The idea was to create urban areas that could retain, clean and reuse stormwater, effectively collecting and recycling rainwater and redirecting it to areas in need of water. But it wasn’t until 2014 when this concept was officially promoted and established by the Chinese government to handle urban floods and management. 

The concept can help cities build climate resilience, according to Dr Faith Chan, associate professor at the University of Nottingham Ningbo China, who penned multiple studies about sponge cities in China. 

"The sponge city concept can deliver multiple benefits on building climate resilience, such as expanding urban greening  [and reducing] heat-island effects and carbon [budgets] to relieve greenhouse effects [in cities] that have already addressed heat waves or urban heat effects," Dr Chan told FairPlanet, adding that the concept can also address urban stormwater issues.

Case study: Hong Kong

Dr Chan further noted that the sponge city construction guidance in China has shown that the stormwater protection provided by this method can achieve a 30-year return period. While the Hong Kong programme differs from China's approach, it has still managed to achieve some success in preventing floods over the years.

Hong Kong, located in southern China, receives an average annual rainfall of 2,400 millimeters, a figure that is expected to increase due to the impacts of climate change. In addition, the city is also vulnerable to more frequent and severe tropical cyclones, further exacerbating the risk of flooding. To address this issue, the Hong Kong government has implemented a series of measures to prevent flooding since 1989.

The sponge city concept has been included in the city’s sustainability report in 2016-17 as part of its climate mitigation and adaptation efforts. 

"The design elements of the 'Sponge City' concept includes porous pavements, attenuation and treatment ponds, storage tanks, retention lakes and greening facilities," Liu Chun-san, Secretary for Development, said at a Legislative Council meeting in 2020. 

The Hong Kong government started encouraging the adoption of the sponge city concept in the drainage industry and has undertaken several projects to implement this approach. These projects include the construction of a storage facility in Happy Valley capable of handling 60,000 cubic meters of water, which is equivalent to 24 swimming pools. The facility has a 50-year return period and is designed to mitigate the risk of flooding in the area. Additionally, the government has completed three other sponge city projects.

The harvested rainwater, the government said, is used for irrigation, toilet flushing and cleaning. 

The government also announced that it would incorporate the Sponge City concept in public housing sites under development by - among other measures - building a river park near the Tung Chung to restore ecology, adding treatment ponds along the river to collect and filter stormwater runoff and building porous pavements to absorb rainwater. Scattering plants across the roofs of drainage facilities can further absorb stormwater runoff, the government stated. 

Chan referenced a wetland park in Hong Kong’s northwest New Territories as a success story. "That is one of the successful cases as the wetland park is not only functioned for stormwater storage, but the [its] vegetation also enhances the stormwater pollutants retention (e.g. dust and particulates)."

Officials in Hong Kong claim the Yuen Long Bypass Floodway wetland has been able to intercept 40 per cent of the runoff in the district. "I think Hong Kong has done well and provide good experiences for the mainland cities to learn."

The challenges ahead

Dr Chan has pointed out that the main challenges to implementing the sponge city concept in Hong Kong are related to financial constraints and limited urban land space. Hong Kong has one of the most expensive property markets in the world, and as a result, land use has been highly competitive over the past few decades.

"The technology and skills are very good, but just those two challenges are quite difficult to be solved. Hong Kong should learn from the mainland, particularly on the effectiveness of the promotion and construction of green infrastructure," Dr Chan suggested.

To achieve greater impact, Hong Kong should consider cooperating with the neighbouring Chinese city of Shenzhen, the sponge city expert suggested. 

"Hong Kong should work closely with the GBA [the Greater Bay Area - consisting of nine southern Chinese cities] on climate preparation and modelling and establish a joint GBA disaster management centre, as typhoons and storms are not [only] hitting Hong Kong, but the whole region," he explained. 

The Sponge City concept has meanwhile been spread across the globe, as cities are scrambling to boost their climate resilience in urban design and architecture.

Germany’s capital city Berlin, for instance, encourages the use of water-permeable surfaces and the construction of ponds and urban wetlands to avoid floods. 

According to design company Arup, several cities in Africa have the potential to adopt the sponge city concept. Arup's analysis indicates that Cairo, Durban, Kigali, Lagos and Nairobi have a certain level of "sponginess," which is calculated based on the amount of urban space covered by trees and water resources compared to buildings.

Image by Ruslan Bardash.