Sponge Cities: Revolutionizing Surface Water Management in Urban Environments

The world is a rapidly urbanising place. Since 1950, the number of people living in cities has ballooned from 751 million to 4.2 billion and Asia is the epicentre of this growth with 54 percent of all city dwellers [1].

Urban environments present many environmental challenges such as air pollution and waste management that gain a lot of attention, but there is one area that is often overlooked – surface water management.

The construction of cities changes the natural process of water flow and this can have unintended negative consequences for both the environment and city residents. This is only exacerbated by the potential rise in sea levels and changing weather patterns caused by global warming [2].

Cities that do not pay close attention to surface water management risk catastrophic flooding events and some could even become completely uninhabitable in our lifetimes.

Already, cities like Bangkok in Thailand see yearly floods that cause massive losses to the local economy [3]. Jakarta, the Indonesian capital, has the dubious honour of fastest sinking city in the world and some areas will be uninhabitable in a few short years [4].

So what is the solution to the surface water problem in cities? Of course, there is no perfect answer, but there is promising work being done in the field of ‘Sponge Cities’ [5].

Before we evaluate the solution of Sponge Cities, let’s first take a closer look at the water management problems faced by modern cities.

The Water Problem in Modern Cities

It is safe to say that modern cities have a water management problem. Any construction on the scale of a modern city is going to impact natural water flows, but for too long city planners have ridden roughshod over nature and done little to mitigate this impact [6].

While many cities have the problem of too much water, others suffer from shortages. This water problem faced by modern cities around the world can have many causes, which will be outlined below.

Impermeable Surfaces

Cities are filled with impermeable surfaces, which are hard surfaces that water can not move through freely [7].

Soil in its natural state allows for water to infiltrate and this is an integral part of the water cycle. Rainfall filters through the soil, slowing its entry to waterways and replenishing groundwater supplies [8].

For human utility and comfort, cities are filled with hard surfaces that block this natural process. Our cities are ‘concrete jungles’, covered with roads, parking lots and footpaths made of impermeable asphalt and concrete [9].

We are stopping water from following its natural path and this is the fundamental problem that has many knock on effects. Such as flooding and the pollution of waterways.

Run-Off, Drainage and Pollution

One of the major problems linked to impermeable surfaces is surface runoff, drainage management and pollution.

Due to hard impermeable surfaces, water runs rapidly across the surface rather than slowly filtering into the soil. The water must go somewhere and this is typically into nearby waterways that are unable to cope with such a sudden rush of water, causing flooding [10].

However, if drainage is inadequate, it may even be difficult for the water to reach waterways. If man made drainage solutions are overloaded, the water can remain trapped in the urban environment causing economic and public health problems. This is a big problem in developing cities due to the expense of drainage [11].

The elephant in the room when it comes to surface run-off is pollution. Infiltration through soil acts as a natural filter, removing many pollutants from rain water. On impermeable surfaces, pollutants can build up and are simply swept straight into waterways by rain, causing massive environmental damage [12].

Destruction of Wetland Areas

Wetland areas in their natural state act as emergency water storage, absorbing excess water instead of letting it inundate surrounding areas [13].

Historically, humans have had a negative relationship with wetlands. They have been seen as useless or even harmful and are often ‘reclaimed’ and used to expand our ever growing urban areas [14].

This destruction of wetlands removes this natural ‘safety switch’ and often leads to more extreme flooding in urban areas.

These are just some of the varied problems caused by modern cities when it comes to water management. Are Sponge Cities the solution?

The ‘Sponge City’ Solution

The problem of urban water management has received increasing attention in recent years and many solutions have been offered, with the idea of ‘Sponge Cities’ receiving the most attention.

Sponge Cities are, as the name suggests, cities designed to absorb excess water in a more natural way. This reduces the need for expensive artificial drainage, reducing flooding and ensuring there is more clean water for the use of residents [15].

Changing a traditional city into a Sponge City is not as hard as you would imagine, but it does require a concerted effort and substantial resources. While many cities in the US are moving towards this model, it is in China that planners are pushing the limits of Sponge Cities and showing the world what is possible [16].

The easiest way to understand Sponge Cities and their benefits is to take a closer look at how they work.

How Does a Sponge City Work?

A Sponge City is not something that happens by accident – it takes conscious design and material choices to achieve.

There are many ways to achieve the ‘sponge’ effect in practice, but they all hinge on the following fundamentals.

Porous Design

The biggest change that must be made in cities is porous design. To allow natural natural infiltration, we must begin using more porous materials.

Porous concrete and asphalt that allow for higher infiltration rate are available for use on roadways, preventing excessive surface runoff [17].

For pedestrian paths, parks and plazas and even parking lots more natural materials such as natural stone aggregates can be used. When stabilizers, such as Organic-Lock™ are used in conjunction with these materials, it is possible to create hard usable surfaces with close to natural infiltration rates [18] all the while reducing erosion-based maintenance that is commonly found with unstabilized aggregates.

Maintaining natural infiltration with porous surfaces replenishes groundwater, keeps waterways healthier, stops our reliance on expensive drainage systems and reduces flooding in urban areas. For such a small change, using porous surfaces has a massive impact and this is a cornerstone of Sponge City design [19].

Open Green Spaces

City residents love open green spaces and these areas can also help with water management, leading to a great win-win situation.

Sponge City planning calls for maintaining natural, interconnected waterways throughout the city, which can include rivers, ponds, manmade channels and the preservation of wetland areas [20].

While it may seem counterintuitive to introduce more water when flooding is a concern, these natural waterways are the best drainage and water storage systems available and they have the side benefit of being more visually attractive than other solutions [21].

Maintaining green areas around these waterways also assists natural infiltration and provides sports and leisure areas for city residents.

Green Roofs

A large portion of the ground in a city is covered by buildings and roofing, making the management of this run-off important in Sponge Cities.

The solution is Green Roofs, which are designed to retain and filter rainwater before slowly releasing it back into the natural environment. This can involve water storage on roofs, or even rooftop gardens [22].

Using green roofs can improve city aesthetics, but it can also be an important source of water in cities that suffer from shortages.
Water Saving and Recycling
Saving water may not seem like the most important thing when flooding is an issue, but it is important for Sponge Cities.

The waste water from city residents has to be treated and released back into the system and this massive amount of water can be the nail in the coffin for the already overloaded natural systems [23].

Sponge City initiatives call for increased water saving and recycling, particularly at the individual building level to reduce wastage and the overall impact of humans on the natural water cycle.

Examples of Sponge Cities

As mentioned previously, China is a centre of Sponge City innovation, however there are promising examples of Sponge City initiatives around the world that can show us what is possible when water management is at the forefront of urban design.

Lingang District, Shanghai

At more than 26 million people, Shanghai is one of the most important cities in China and a major global commercial hub [24]. It is also a coastal city built at the mouth of the mighty Yangtze river and has perennial problems of flooding and water management.

The Lingang district has been chosen as a model area for water management in the city and results have been promising. Due to little available land for new parks and green areas, the focus has been on the creation of Green Roofs.

Before the changes, as little as 20% of rainwater was able to infiltrate in the district. With the new initiatives that see rainwater captured and stored by Green Roofs, as much as 70% of rainwater will be reused before reintroduction to the system, reducing the load on drainage significantly [25].

With a modest $120 million investment, this project highlights the massive changes that can be achieved with just a small change.

City of Philadelphia, Pennsylvania

Philadelphia is one of the first cities in the US to recognize the problem of urban water management and take significant steps to address the issue.

The combined sewage and stormwater system in Philly risks being overloaded in large rainfall events, causing untreated water to be released. It was noted that limiting flows into the stormwater system during heavy rainfall could solve this issue [26].

Permeable surfaces, such as stabilised aggregates and porous concrete/asphalt were identified as the ideal solution. Hard surfaces across the city were retrofitted with these porous materials and the results were amazing [27].

Drexel University, a private research University in Philadelphia, is a shining example of using porous surfaces on campus. Using a combination of permeable concrete pavers, as well as Organic-Lock^™stabilized aggregates, allows for the functionality of hard surfaces while still maintaining permeability of the water into the water table below.

Stormwater runoff is thought to have been reduced by up to 80% in areas with porous surfaces, easing the strain on the cities drainage infrastructure.

Taipei, Taiwan

In 2013, the Taipei government began its ‘Permeable City’ initiative, with the aim of reducing run-off rates and promoting natural infiltration [28].

They set up a demonstration zone and implemented new, stricter building codes to encourage the construction of more permeable building and green areas.

Impervious pavement was replaced with more porous options and water retention and recycling at the individual building level was introduced.

A number of green areas were preserved and additional wetlands created to make the cities drainage system more resilient and able to cope with storm events, while also improving the lives of the cities residents [29].

Urban Planning for the Future

With changing weather patterns globally, rising sea levels and more intense storm events, water management in our growing metropolises is more important than ever.

City planners and urban designers need to more carefully consider the flow of water when creating new spaces. Without a doubt, the best way to manage water flows is to work with natural systems, rather than against them like we have been.

The Sponge City concept is the answer. Cities that make the effort now to manage water run-off – have a bright future, free from disastrous floods while allowing residents to live healthier lives more in tune with nature.

References

[1] Phys.Org (2019). UN: 68 percent of world population will live in urban areas by 2050. Available from: https://phys.org/news/2018-05-percent-world-population-urban-areas.html

[2] R. R. Brown, N. Keath, T. H. F. Wong; Urban water management in cities: historical, current and future regimes. Water Sci Technol 1 March 2009; 59 (5): 847–855. doi: https://doi.org/10.2166/wst.2009.029

[3] Agence France-Presse (2018). Bangkok is sinking. How will Thailand’s capital cope when flooding disaster strikes again? South China Morning Post. https://www.scmp.com/news/asia/southeast-asia/article/2162409/bangkok-sinking-how-will-thailands-capital-cope-when

[4] Mei Lin, M., Hidayat, R. (2018). Jakarta, the fastest-sinking city in the world. BBC News. https://www.bbc.com/news/world-asia-44636934

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[6] OECD (2014) Managing Water for Future Cities. https://www.oecd.org/environment/resources/Policy-Perspectives-Managing-Water-For-Future-Cities.pdf

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[16] Biswas, A.K., Hartley, K. (2018). China’s ‘sponge cities’ aim to re-use 70% of rainwater. CNN Future Cities. https://edition.cnn.com/2017/09/17/asia/china-sponge-cities/index.html

[18] [19] Jia, H., Wang, Z., Zhen, X. et al. (2017). China’s sponge city construction: A discussion on technical approaches. Frontiers of Environmental Science & Engineering. https://link.springer.com/article/10.1007/s11783-017-0984-9#citeas

[22] [23] Howell, C., Drake, J., Margolis, L. (2017). How green roofs can help cities sponge away excess stormwater. Phys.Org. https://phys.org/news/2017-08-green-roofs-cities-sponge-excess.html

[24] [25] Beach, G. (2018). Shanghai’s sponge districts fight flooding with green space. Inhabitiat. https://inhabitat.com/shanghais-sponge-districts-fight-flooding-with-green-space/

[26] [27] Lazos, P.J. (2018). Spong Cities. Global Water Alliance. https://www.globalwateralliance.net/sponge-cities/

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