Sponge cities-One way to solve urban floods

What are they made of? Do they actually work?

Sponge cities has been a popular topic lately. If this is the first time you heard of this word, you may laugh at its name. Yet being named “Sponge city” doesn’t mean such cities are full of sponges. Instead, sponge cities have this name because of how they act similarly to sponges—they can absorb and redirect large amount of rainfall to the underground, preventing a flood on the ground surface. Here you may wonder, why is there a need to redirect rain to the underground? Is there really that much rain? To explain this, we would have to start with the climate crisis and the water cycle.

There are vast amounts of water in the ocean. These water get heated up and evaporated by the shining sun. They then form water vapor and follow the flow of the wind to get distributed to different places on the Earth, where they cool down, and condense into rainfall. As you may know, the Earth is getting warmer and warmer. This means that more water is getting evaporated from the oceans, carried by the winds, and condensed into rainfall falling at various places on Earth. This is called the intensification of the water cycle, making wet places wetter(due to more rain), and dry places drier. In wet places, the increase in rainfall can easily cause floods. You may have heard of the 3 days flood at ZhengZhou, China in 2021. The flood was caused by a large rain with a precipitation rate of 201mm per hour.

[You may be wondering why the climate crisis causes wet places to get wetter and dry places to get drier, instead of making all places get wetter universally. This is because the wind on Earth flow according to a certain pattern, and carries the water vapor they contain according to a certain pattern, to predetermined regions in the world. This is why even before the start of the climate crisis, there already are wetter places and drier places on Earth. Some regions have a lot of winds carrying water vapor blow at them, while others do not. Now, the increasing of the Earth’s temperature do increase the amount of water vapor in the air, but it doesn’t change the way the air flows. This means that more water would be evaporated from the ocean, concentrated together in the wind, and get carried together to the same places less water would be carried to before the climate crisis. But why do dry places get drier then? Why don’t they just stay as they were? Other than evaporating the water from the ocean, the increasing temperature on Earth also evaporated more of the water in the waterbodies around the world, including the water in the rivers, lakes, and seas at the already dry regions. These water vapor would enter the air, and get blown to the already wet places too. Thus removing water from the dry places, while not adding water to them.]

As the possibility of a flood occurring increases, it becomes crucial to think of ways to protect the cities—where many live—from potential floods. The fact that cities are full of concrete and impermeable floors makes it easier for the rainwater to stay above ground and cause a massive flood. Floods can carry polluted substances and diseases, causing a health crisis and damaging infrastructure. A sponge city is needed to lead the rainwater underground, so that they don’t all accumulate above ground and form a flood.

Cities usually have drainage systems that treat polluted water and clarify them to a certain extent before dumping them into waterbodies again. However, when there are a large amount of rainfall the rain also floods into the drainage system, causing an overflow and letting the polluted water leak into the waterbodies directly without treatment. This pollutes the entire waterbody. This means that a sponge city is needed to absorb some of the rainfall and lead them underground, not letting them enter and overflow the drainage system.

So, what strategies do a sponge city use to redirect the rainwater to places other than the ground surface?

Firstly, sponge cities has a lot of open green spaces. Green spaces are gardens, parks, or places that are filled with vegetation and soil. Vegetation and their soil has the natural ability to retain water. Thus, letting the rain fall upon these vegetation instead of concrete, impermeable pavements can make sure that the rainwater is now flowing above ground by letting the water get temporarily held by the soil and the plant, or by letting the water seek down to the deeper grounds below the permeable soil layer, where they are redirected to the underground. One common type of green spaces are green roofs. These are gardens on the roofs of buildings. Planting vegetation on roofs could let the rainwater falling down get held by the vegetation on the green roofs. These water will then slowly leak out, and will get redirected into a pipeline inside the building, in which they are filtered, then released into more pipes connecting to the underground.

Secondly, sponge cities include many porous designs. Sponge cities can have roads that has little pores yet can still hold traffic. These pores on the roads makes the roads permeable, so that the rainwater can naturally flow underground instead of getting stuck on the Earth surface.

[You may have noticed that in this course we have mentioned the phrase “the underground” a lot of times. You may be wondering why the underground is such a good choice to store all the water and how the water is stored at the underground. These questions will be explained below. “The underground” refers to the store of groundwater below the ground surface. There is actually many layers of soil below the what we usually call “soil”. The layers start from the Humus—at the surface—and continues through the top soil, subsoil, parent rock, and the bed rock. In these layers of soil there exists many rocks, and in between these rocks frequently lay water. These water stored in the underground is what we call “groundwater”. Thus, by saying “redirect the water underground”, we are talking about letting the water leave the impermeable pavements of the city and go to the permeable soils, where they can seep down deep into the layers of soil below, where they are stored as groundwater underground, instead of causing floods above the ground.]

In this way, the sponge city successfully completes its mission of redirecting rainwater underground and preventing a flood. Yet, is sponge cities always this successful as imagined? Zhengzhou is a sponge city, yet it still suffered the 3-days flood mentioned at the beginning of the article. The climate crisis is intensifying, and extreme weathers will be more and more frequent. The sponge cities we have today may still need some improvement, and we may need to think of other method to work with the sponge city system in preventing urban floods. Yet, what we have done is a great first step. China has never given up the investigation and application of the sponge city concept, and with the cooperation of the entire globe, we humans shall find a way to protect ourselves from future floods.

Citation:

Team, ArchDaily. “What Is a Sponge City and How Does It Work?” ArchDaily, ArchDaily, 21 Apr. 2022, https://www.archdaily.com/979982/what-is-a-sponge-city-and-how-does-it-work.

Council, World Future. “Sponge Cities: What Is It All about?” World Future Council, World Future Council Https://Www.worldfuturecouncil.org/Wp-Content/Uploads/2020/10/Wfc-Logo-Rgb-Strapline-EN-1030x357.Png, 4 July 2016, https://www.worldfuturecouncil.org/sponge-cities-what-is-it-all-about/.