What are Sponge Cities and how they can prevent flooding

Cities around the world are facing an increasingly urgent challenge due to climate change: heavy rains they are transforming streets into real rivers, flooding houses, buildings and entire neighborhoods, often with tragic consequences, like those we saw in Rio Grande do Sul. Given this scenario, Kongjian Yu, renowned landscape architect and professor at Peking University, introduced the concept of sponge cities proposing an innovative approach: instead of resisting rainwater, he suggests that cities live with it, causing as little damage as possible.

Instead of simply building more drainage networks, erecting flood walls and channeling rivers between concrete embankments, as is traditionally done, Yu developed the concept that is based on slow and disperse the force of flood waters, allowing them to spread in a controlled manner. This approach is compared by Yu to “do tai chi with water”, a reference to the Chinese martial art in which the opponent's energy is fluidly and harmoniously redirected against himself, instead of being resisted in a brute way.

How sponge cities work

As sponge cities they are an innovative approach to dealing with urban flooding, an increasingly common and serious problem due to climate change. This approach, developed by landscape architect Kongjian Yu, proposes a new way of managing rainwater, in contrast to traditional drainage methods.

The concept of sponge city is based on the idea that rainwater should not be seen as a problem to be drained and removed outside the city, but rather as a resource to be managed intelligently. Instead of channeling all the water into underground drainage systems or rivers, sponge cities they seek to retain part of this water, allowing it to be absorbed by the soil, stored in reservoirs or redirected to green areas.

The concept of sponge cities has three main objectives:

• Retain rainwater as it falls from the sky. This involves reserving about 20% of the surface area for weir systems, preventing all the water from going straight into the main river.
• Reduce the speed of rivers. By reducing the speed of the water, it creates the opportunity for nature to absorb it.
• Adapt cities to have floodable areas. These areas are designed so that water can flow without causing damage, creating large natural structures that temporarily hold water and then slowly release it into the water table without flooding homes.

Therefore, one of the main characteristics of sponge cities is the extensive use of green areas such as parks, gardens and green roofs. These areas help absorb rainwater, reducing the amount of water entering drainage systems. Additionally, structures such as ponds and canals are designed to temporarily retain water from heavy rainfall, allowing it to be released in a controlled manner later.

The whole idea of ​​sponge cities is a philosophy: retain water in place. To keep water in the ground, because it has to do with the distribution of water resources, with the capture, whether from the roof or in your backyard and the distribution, and not the channeling of water, [the objective is not] to concentrate it in a duct system

Kongjian Yu

Another important aspect of sponge cities is the reduction in the use of impermeable surfaces, such as concrete and asphalt, which prevent the natural absorption of water by the soil. Instead, practices such as permeable pavements, which allow water to infiltrate into the ground, are encouraged. These pavers are designed with porous materials that allow rainwater to slowly infiltrate rather than collecting in puddles or rushing quickly into drainage systems. This helps recharge groundwater, reduce the load on drainage systems and prevent flooding.

Kongjian Yu developed innovative projects sponge cities na China, a country that faces serious challenges with urban flooding due to rapid urban development and climate change. In his interventions, Yu sought to integrate natural elements, such as lakes, rivers and green areas, with traditional urban infrastructure, such as roads and buildings. This not only helped control flooding, but also improved the quality of life of residents by providing more pleasant and healthy public spaces.

As sponge cities represent a more sustainable and integrated approach to urban stormwater management. Instead of simply trying to remove water, these cities seek to coexist harmoniously with the forces of nature, taking advantage of the benefits that water can bring and minimizing the negative impacts of floods.

Project results

A survey conducted by World Bank in 2020 revealed that the China is home to the majority of the world's population exposed to significant flood risks. According to the study, 329 million Chinese citizens, out of a total of 1,47 billion inhabitants globally, would be “directly exposed to substantial risks during floods that occur once every 100 years”.

Another study carried out in China, conducted in cities such as Shanghai, Zhousha, Suzhou and Xi'an, found that sponge city They can reduce surface runoff of rainwater by between 25% and 69%, in addition to reducing maximum runoff by up to 71%. On the other hand, green roofs can reduce the rate of rain runoff, with a maximum delay of 20 minutes.

After the 2012 Beijing floods, many of the original plans to implement sponge cities na China were put into practice. Kongjian Yu, dean and professor at Peking University College of Architecture, managed to gain support from the Chinese central government and draw attention to the concept. In 2014, the central government set the target that by 2020, in 20% of urban areas, 70% of rainwater runoff should be recycled. For 2030, this target will rise to 80%. Pilot projects of Sponge City began in cities such as Wuhan, Chongqing and Xiamen.

The implementation of sponge cities na China It has been a complex and challenging process, with mixed results in different regions of the country. The concept of sponge city it was adopted as a strategy to deal with increasing urban flooding and drainage problems caused by rapid urban development and climate change.

In some Chinese cities, such as Shanghai, Suzhou and Xi'an, security measures sponge city have been successful in reducing the impacts of flooding. The construction of rain gardens, green roofs and other green infrastructure has helped increase water absorption and retention capacity, reducing surface runoff and relieving pressure on drainage systems.

One of Yu's best-known projects is the expansion of Hongshan Park in Wuhan, which transformed a flood-prone area into an ecological park with retention lakes and permeable green areas. These interventions not only significantly reduced the risk of flooding in the region, but also created a leisure and recreation space for the local community. Another project Sponge City in Chongqing is a notable example of a successful pilot: by 2020, 24,2% of the city's urban area had been transformed. The expectation is that by 2025, more than 45% of the city will comply with Sponge City requirements.

However, in other areas, the implementation of sponge cities encountered significant challenges. In some cases, a lack of adequate planning and inadequate infrastructure resulted in projects failing to achieve their objectives. Furthermore, resistance from local authorities and the population's lack of awareness about the importance of sustainable water management were also obstacles to the success of these initiatives.

Check out the video below to see how Chinese sponge citieshow they work:

Examples of sponge cities around the world

As sponge cities, designed to address the challenges of urban flooding and stormwater management, have gained prominence in various parts of the world as an innovative and sustainable approach.

Em Singapore, underground reservoirs, known as “water tanks”, were built to store rainwater for later use. Additionally, the city has invested in green roofs and natural absorption areas, such as parks and gardens, to help reduce surface runoff and increase water infiltration into the soil.

Na Europe, the city of Copenhagen, the Denmark, is another example of a city that adopted the concept of sponge city. Copenhagen has implemented a range of stormwater management measures, including the use of permeable pavements, sustainable drainage systems and the creation of urban green spaces. These measures have helped to significantly reduce flooding problems in the city.

Berlin, the Germany, is also one of the cities in Europe with extensive construction projects Sponge City. Unlike many other cities in the country, Berlin does not pipe water from springs or other sources. All of the city's drinking water comes from groundwater within its boundaries. This means that water management in Berlin needs to be careful, as there is the potential for contamination of groundwater and other bodies of water by wastewater and rainwater.

Most of the water in Berlin flows through the Spree, the main river that runs through the city. However, the flow of water is slow, which means that during heavy storms, water cannot drain quickly through piping and drainage systems, resulting in flooding. After the flash floods in 2017, there was a political commitment for Sponge City concepts to be applied by law in “every new development” in Berlin.

No to Canada, the city of Vancouver has also been praised for its sponge city. The city has implemented a comprehensive stormwater management plan, which includes the use of green roofs, rain gardens and infiltration areas. These measures have helped reduce flooding problems and improve water quality in urban areas.

In Tokyo, in Japan, the project Greening Tokyo includes the construction of green roofs, floodable parks and permeable pavements to reduce the risk of flooding in the city. These measures also contribute to improving air quality and creating urban green spaces.

Na Australia, the city of Melbourne implemented a stormwater management program that includes the use of rain gardens, rainwater storage tanks and permeable streets. These measures helped to reduce the impact of flooding and increase the city's water sustainability.

Em Rotterdam, the The Netherlands, known for its innovations in water management, parks and public spaces were built to act as temporary reservoirs during heavy rains. Additionally, the city adopted a “green disconnection” plan, which involves disconnecting impervious areas, such as parking lots and roads, from the drainage system, allowing rainwater to be absorbed into the ground.

Challenges in implementing green infrastructure

The implementation of green infrastructure, as part of stormwater management strategies, faces several challenges, and it is important to highlight that it alone is not capable of preventing catastrophes such as floods in the Rio Grande do Sul. Although the sponge cities are an effective measure to reduce the impacts of floods, it is essential to adopt an integrated approach that combines different techniques and policies.

One of the main challenges in implementing green infrastructure is the need for available space. Not all urban areas have enough space to create parks, rain gardens and other green solutions. Furthermore, maintaining these areas can be expensive and require specialized human resources.

Another challenge is the resistance of authorities and the population to changes in the urban landscape. Implementing green infrastructure often requires the removal of existing pavements and buildings, which can generate resistance and controversy. Furthermore, green infrastructure may not be sufficient to deal with extreme rainfall events, especially in densely populated urban areas. In these cases, additional measures are necessary, such as more efficient drainage systems and water reservoirs.

To address these challenges, it is essential to adopt a holistic approach to stormwater management that includes not only green infrastructure, but also measures such as land use control, sustainable urban planning, environmental education and community engagement. Only with an integrated and collaborative approach will it be possible to effectively reduce flood risks and create cities that are more resilient to climate change.

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Sources: NY Times, The Conversation e The Urbanist

reviewed by Glaucon Vital in 14 / 5 / 24.