Concrete Jungles Going Green
While the U.N. and U.S. Census Bureau disagree on when the human population will reach eight billion, there is no argument that our population is growing, and fast. As total global populace grows, so do urban populations through natural growth and immigration into cities. In fact, over 50% of the world’s inhabitants live in cities. While these urbanites make up only half the population, they consume 75% of energy produced and emit 80% of total greenhouse gases . This raises areas of concern within city limits.
First, there is carbon emissions. While many cities have implemented low-carbon development systems to try and limit carbon emissions, more than just carbon reduction is required. We need carbon to be taken out of the atmosphere and stored in what can be referred to as “carbon sinks.” Terrestrial carbon sinks can be soil, forests, and artificial areas that passively sequester carbon from the air.
Second, the presence of more airborne particles paired with copious impervious, non-reflective surfaces including streets and buildings leads to cities becoming heat islands. According to the EPA, a heat island occurs when a “built up” location is hotter than surrounding rural locations. In a city with more than one million people, about the size of San Jose in California, the temperature difference caused by this phenomenon can cause city’s annual mean temperatures to be 1.8–5.4°F warmer than its surroundings, reaching a difference as high as 22°F at night . Luckily, there is a solution that will tackle both issues at once: sponge cities.
Sponge Cities: Retaining Stormwater and Containing Pollutants
Sponge Cities are those that maximize the abundance of green systems to help regulate stormwater while taking advantage of different engineering and construction developments. Some features that sponge cities frequently have include:
- More trees than the average city
- Porous pavement
- Living rooftops
- Urban wetlands
These features bring many benefits to the cities they reside in. Retaining water instead of diverting it allows for evaporation to occur within the city, cooling it. Think about your own experience: When you get hot, you sweat. This sweat evaporates off your skin and cools down your body. Or when you step out of pool on a warm summer day, the air suddenly feels cool. This is not because the air’s temperature changed, but rather because the water on your skin is evaporating in the sun. The process is the same from a small scale, like your body, to the size of a city. If water retained and left to evaporate within the city, it can act as natural air conditioning. With city temperatures lower, this leads to a decrease in overall electrical consumption. Of course, there is less of a need for air conditioning, but the cooler temperatures also encourage residents to go outside instead of remaining indoors with lights on, using electronic devices as entertainment. In fact, some sponge cities have seen energy reduction by up to 30% . With less energy being consumed, less carbon is emitted. Even as green energy is gaining popularity and feasibility, a majority of global electricity generation is through fossil fuels with the two highest energy sources being oil at 32% and coal at 26% . This means that a reduction in energy consumption is a reduction in carbon emissions.
China has been the largest adopter of this new city design. They have implemented two rounds of sponge city developments, one in 2015 with the construction of the first 16 cities and another in 2016 when 14 more cities were developed. These cities have roof greening, permeable paving, swales, rainwater storage ponds, and river and lake wetlands constructed to primarily address water pollution and the urban heat island effect .
Xiamen was one of the first 16 cities developed in 2015. The sponge city construction focused on six aspects: Construction of ecological water systems, drainage and flood control system, garden spaces, roads, sponge residential communities, as well as pollution prevention and control. Because of their efforts, the city has been able to control 70% of total annual runoff and it is estimated that Xiamen saw carbon reduction of 66,266.70 tons per year. This is still increasing as the city continues building to reach its goal of 7,475.79 new hectares of green land by 2020 .
Germany has taken on the sponge city model as well, turning their capital Berlin into a sponge city. They are so confident in the model that Berlin has no traditional stormwater management system! No pipes whatsoever, only porous surfaces. Since the sponge city implementation, all new development within Berlin is required to have stormwater managed on-site . Their application isn’t perfect, and climate mitigation measures still need to be developed. This was most apparent in the summer of 2017, when the city was flooded after what was said to be the heaviest rainfall in over a century. Berlin sees on average 580 liters of rain every year. During the 2017 flood, 150 liters of water fell per square meter in just one day . However, the government and citizens of Berlin are confident in their sponge city.
Maintenance Required to Live and Let Live
Like any stormwater system, a maintenance strategy is needed to keep sponge cities fully functional. For sponge cities specifically, it is vital that all hydraulic and substance retaining capability is available. If the porous city is no longer porous, that would equate to a total system failure. Routine inspection and maintenance are typically enough between significant rainfall events. However, once a large volume of water is deposited on the city immediate inspections, cleaning and possible repairs are required . Maintenance can change if the frequency of rainfall decreases. If a system is left to dry out for a long period of time the vegetation can suffer and the soil can become hydrophobic, making it difficult to rewet and therefore failing to reach its intended purpose of absorbing water. These are city-wide systems and must be maintained city wide. A single aspect left unmaintained can lead to the sponge city being unable to absorb and retain water poured onto the city. Building a system is just the first step, maintaining that system is where stormwater management gets its value.
The world is changing through increased development, population growth, and climate change. As a global society we must also change and adapt to our new environment. Implementing ecological and sustainable lifestyle changes such as sponge cities is a great place to start.
Learn more about different stormwater management features, including many of those used in sponge cities.
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||S. Köster, "Maintenance and Safety of Sponge City Infrastructure," in Urban Water Management for Future Cities, Springer, Cham, 2019.