Introduction to Sponge Cities

Sponge City is a response to the failing modern concrete and steel stormwater management systems especially during heavy unexpected rains. Sponge City is an alternative resilient stormwater management approach that was conceived as early as the 1970s by Peter Cook but was shaped by the constant efforts of the Beijing-based landscape architect Dr.Kongjian Yu and gained popularity when applied by the Chinese government in 2013. 

Yu argued that identical to sponges that allow slow infiltration, retention, and release of water, natural wetlands also allow retention of rainwater during floods and recharge water during drought, explaining the flood-mitigating capacity of natural wetlands. Upon revealing the absorption, retention, and flood-control potential of nature, Yu advocated the use of nature-based ecological infrastructure as a green sponge in developing a flood-proof resilient city which he termed the ‘Sponge City’.

Subsequently, the six principles that constitute a sponge city are infiltration, detention, storage, purification, reuse, and discharge of stormwater with the help of green roofs, green walls, bio-swales, rain gardens, urban parks, ponds, etc. The concept of a sponge city is being applied in various parts of the world but under different names like Water Sensitive Urban Design (WSUD) in Australia and the Middle East, Sustainable Urban Drainage Systems (SuDS) in the UK, and Natural Drainage Systems in Seattle.

Standard Techniques for Sponge City Development

The implementation of sponge city developments involves realizing various concepts and employing distinct techniques. Green infrastructure is at the heart of sponge cities. Bio-retention systems and green roofs are important green infrastructures in a sponge city that control stormwater runoff, enhance infiltration, recharge the groundwater, alleviate the urban heat island effect, reduce energy consumption, promote bio-diversity, and serve as public spaces.

Permeable pavements are another technique that reduces the water runoff with improved infiltration. There are many permeable surfaces including permeable grass, permeable concrete, permeable asphalt, and permeable interlocking concrete pavers. 

Rainwater resourcing is one such important technique that enhances the existing system of rainwater harvesting to combat water scarcity and the lack of water resources. While rainwater harvesting only accounts for storing rainwater for later use, sponge city developments realize the potential of rainwater harvesting in mitigating urban water runoff.

Hence, the sponge city rainwater resourcing systems are designed to take the hydrological characteristics of the area (including water surface runoff, flow time, discharge, and peak time) into account to prevent urban flooding. Natural or artificial wetland ecosystems are used for rainwater resourcing in sponge cities.

Over the years, with extensive industrialization and urbanization, the water quality has deteriorated due to improper chemical discharge and water pollution. Sponge city developments aim to reverse the quality of water by natural means, creating an environment to facilitate the self-purifying ability of water. This is termed ecological water management. Soil, plants, and microorganisms are active contributors, improving the quality of water in artificial or natural water systems. The four discussed techniques; green infrastructure, permeable pavements, rainwater resourcing, and ecological water management, all aim to fulfill the six principles of Sponge City.

Sponge City Development at 3 Urban Scales

Sponge city development primarily involves greening of the urban fabric at micro, macro, and meso levels as discussed below.  

Micro-Scale – Individual houses and Neighborhood Level

By incorporating blue and green infrastructure like green roofs and green walls, rain gardens, small bioswales, open vegetated trenches, and other small filtration basins into individual houses or a neighborhood, sponge city principles could be achieved at a primitive scale. However, the benefits of such developments are often overlooked. In addition to nature-based solutions, some engineering like permeable concrete pavements, rain barrels for rainwater harvesting, and wastewater treatment plants for individual household purposes could help in dealing with water scarcity issues and ensure constant availability of water at a micro level.

Meso Scale – District Level

Achieving the sponge city principles becomes tricky at the district level, as the urban fabric is composed of linear elements like roads, pedestrian paths, bicycle routes, and spread-out residential districts. To turn these linear urban elements into eco corridors and sponge streets, linear greening infrastructure like permeable surfaces and vegetated filtration channels, bioswales, and buffer strips of street greenery along the edge of the streets, sloping downwards. While hydrophytic ponds, wet rain gardens, small surface and underground retention basins, underground stormwater drainage systems, underground root boxes, green walls, green roofs, and a system of urban eco-farms are used to enhance the sponge effect at the district level.

Macro Scale – City Level

On a city scale, the sponge effect is accomplished by integrating the systems of the meso and micro-scale and ensuring water availability throughout the year while reducing water runoff and preventing floods. Another objective of macro-scale sponge city planning is to not meddle with natural water processes and not harm the ecological water systems which is ensured by creating an ecological security pattern (ESP).

A Brief History of Sponge Cities in China  

Though the concept of ‘Sponge City’ was first applied in 2000 in the Zhongguancun Life Science Park project in Beijing designed by Turenscape led by Kongjian Yu, it was only in 2013 that the Chinese government showed interest in implementing Yu’s sponge city idea after the catastrophic floods of 2012 hit Beijing killing civilians. In 2015, a total of 16 cities were subjected to sponge city pilot construction followed by another 14 cities in 2016.

By 2017, the sponge city concept made it to the “Government Work Report” and by 2020, 20% of the built-up urban area in China possessed the ability to absorb rainwater. The authorities are determined and working towards a goal that by 2030, 80% of the built-up urban areas in China will be permeable utilizing 70% of the local rainfall. 

Adoption of Sponge City Development in Other Countries

While the success of sponge cities in China, made it a top priority for the Chinese government, the idea of sponge cities garnered international attention too. The accomplishment of the first stage of the Kaban Lake project in Russia in 2018 is a testimony to it. Subsequently, the ‘Rain City strategy’ of Vancouver City in Canada, launched in 2019, depends on natural infrastructure and ecological processes to mitigate urban flooding, reduce pollutants in urban water runoff, enhance water quality, strengthen biodiversity, and alleviate heat.

Vancouver City has employed permeable landscaping, rainwater tree trenches, and wetlands, has restored streams, and captured rainwater as close as possible to where it falls to realize its vision. 

Last year, Mayor Valérie Plante announced that the Canadian city of Montreal has planned to develop 30 sponge parks and 400 sponge sidewalks to combat heavy downpours by slowing down runoff and enhancing infiltration to reduce the load on the sewer system. Interestingly, even before China adopted the sponge city model, the Canadian City of Toronto passed a law in 2009, that stated green roofs are required to be developed on new projects with a certain area requirement.

The City of Amsterdam in the Netherlands is depending on blue-green roofs while the City of Los Angeles is working with sponge street ideas of green buffers along the edges of the streets, all cities are working with sponge city concepts in one way or the other to befriend rainwater.

India is no exception to the devastating effects of flash floods and the Indian cities have also begun adopting sponge city principles. Researchers at Arup measured the sponginess of seven cities around the world and concluded that Mumbai along with Singapore and Newyork with 30% are better than the cities of Shangai and London with 28% and 22% respectively.

Amdavad Municipal Corporation (AMC)has planned to adopt sponge city principles to address the issue of water logging in certain areas in the city of Ahmedabad. Interlinking lakes, developing eco-bloc sponge parks and infiltration tank systems in key waterlogging areas, and developing a real-time flood warning system are some of the significant components of the Ahmedabad urban flood management plan. Similarly, the Greater Chennai Corporation is also working on developing 42 sponge parks across the city of Chennai to combat the downpours of the northeast monsoon and to harvest rainwater.

Why Sponge Cities are topical today?

What grabbed the headlines last month? The heavy rainfall and flash floods in the Persian Gulf region, particularly in Dubai. Despite the cause of this disaster, this incident rather alarming accident has proved the fact that even global cities like Dubai with one of the best urban engineering have failed the ‘drainage test’ of climate change.

Well, it has also taught the global community, to ‘expect the unexpected’ due to climate change.  And Dubai is not alone in this. Established cities like New York, the cities of Portugal, and even developing countries like India and Bangladesh also face the threat of an inadequate or inefficient drainage system in combatting unannounced intense rainfall and flash floods. 

Upon analysis, the erasure of natural water-absorbing surfaces and extensive impermeable concrete paving is deemed the main culprit leaving the others like maintenance of the rainwater drains, improper waste management, and the increase in people residing in urban areas, less accountable. On the other hand, sponge cities depend intensively on green infrastructure rather than gray infrastructure not only to address the issue of urban flooding but also to solve water scarcity during dry seasons. In the long run, upon creating a sponge planet, there is a possibility to even reverse climate change. 

On the grounds of unexplainable extreme weather fluctuations, recurring heavy downpours, flash floods, severe droughts, and failing conventional stormwater management systems, and the rising reflections on the potential of sponge city developments, don’t the sponge cities seem like a viable solution?

FAQ 

1. What are bioswales?

Bioswales are relatively depressed channels loaded with vegetation, mulch, and soil that help to absorb and treat stormwater before it enters the local water system. Bioswales are helpful in infiltration of stormwater and removal of pollutants from water runoff. Bioswales are usually linear with length to width ratio higher than 2:1. Bioswales, a form of bioretention facility, are one of the sustainable passive stormwater management systems.

2. What are the benefits of sponge cities?

Sponge cities implemented on right scales in the long run ensure increased water infiltration, availability of clean groundwater and river water, and reuse of stormwater. In addition to promoting biodiversity and alleviating urban heat island effects, the green projects of sponge cities also act as public places and recreational places promoting social interaction, physical and mental well being of the residents, and community development. The parallel sponge city developments reduce load on the conventional sewer system and also lead to economic benefits like reduced water and electricity bills.

3. What are the limitations of sponge cities?

Sponge cities require very high upfront costs and the very act of developing a sponge city may lead to inconveniences in the urban life during the construction phase. Moreover, studies suggest that the sponge cities can handle only up to 200 millimeters of rainfall per day. There is not enough standardized system or precedents to look up to, in order to predict the success of sponge cities, especially with adverse effects of climate change in the scene.