Turning Jakarta into a giant sponge is surprisingly one way to save it from sinking.
Source: Tom Fisk 
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Jakarta — All coastal cities worldwide are battling the effects of rising sea levels, but some will be hit harder than others for one particular reason: land subsidence.
In cities unlucky enough to be hit by both phenomena, the risk of flooding is relatively high. Take Jakarta for example. Early this year, the megacity was hit by flash floods . At least 60 people died, and more than 170,000 had to be evacuated.
It's not just Indonesia. In China, roughly 641 of its 654 cities are affected by regular flooding, especially sprawling megacities on the coast such as Shanghai . To tackle this, in 2015, China launched the sponge city initiative in 16 pilot cities before being expanded to 30, including Shanghai, Wuhan and Xiamen . Through this initiative, China aims for 80% of urban areas to absorb and re-use 70% of rainwater by 2030, therefore reducing flood risk and increasing water supply and security at the same time.
What is a sponge city anyway?
Source: World Economic Forum 
Extensive urbanization has essentially created impermeable cities . Impervious roads, pavements, roofs and parking lots impede the natural water cycle, making it hard for rainwater to be filtered through the urban soil and reach the aquifers deep underground.
Another key issue is the fact that many wastewater treatment plants are often unable to accommodate all the water—rainwater and wastewater—that our drainage system carries, thus increasing the level of pollution in our local water bodies.
By contrast, sponge cities aim to construct, or in many cases, reconstruct a city in a way that could help it soak as much as excess rainwater as possible, primarily by enhancing its absorption capacities using a combination of artificial ponds, permeable pavements, green roofs, scenic wetlands and rain gardens . These green infrastructures can facilitate the natural water absorption process, which would consequently recharge the aquifers or let any excess water be stored for future use, thus increasing the city’s water availability and defence against flood risks.
“We can make friends with water,” said Profesor Kongjian Yu, a leading sponge city architect and the founder of the planning and design office Turenscape in Beijing. “Sponge cities allow the natural flow to come back. We use a wetland system, a spongy system to retain the water instead of draining it away.”.
In essence, the sponge city concept presents a promising approach to mitigate subsidence and also its twin challenge of rising sea levels propagated by climate change.
Is it actually working?
Source: Zhang Kaiyv 
With the ambition of becoming China’s largest sponge city, the Lingang city government has renovated a total of 36 kilometers of roads with permeable pavements and transformed a nearby park into a “sponge park” with the absorption capacity of 158 cubic meters of rainwater and 190 cubic meters in ponds and wetlands .
More than that, the city government also developed Internet of Things sensors, big data analysis and cloud computing technologies in efforts to monitor the amount of rainwater, which could potentially reduce flood risks.
Evidently, these newly-added green infrastructures successfully helped the city resisted super typhoon Lekima last August . Similarly, sponge cities in Xiamen and Wuhan were reported to have performed effectively during heavy rainfall .
That’s nice, but is Jakarta ready for this?
Source: Afif Kusuma 
Retrofitting an existing city is not without its challenges. Often times, sprawling megacities like Jakarta lack the space to support any additional construction or reconstruction. Moreover, existing drainage systems may be outdated and worn out. In the case of Jakarta, this issue is further exacerbated by poor waste management system where municipal waste often clogs waterways, thus increasing flood risks as a result .
But perhaps the biggest issue is none other than cost. Launching projects to the likes of sponge cities can be highly expensive. In the case of China, central government subsidies for its sponge city projects are only set to last until 2020 so scaling up the scheme to cover 80% of the city by 2030 can be a huge financial burden unless the local government can persuade private investors to be involved in these projects .
“One possible way is to involve real estate developers because such sponge sites do help increase the value of the land,” says Dr Faith Chan, assistant professor in geographical sciences at the University of Nottingham Ningbo China 
While these challenges need to be taken into consideration, it is wise for Jakarta to start thinking up ways to solve its flooding problem because it's not waiting to be solved. The future, if we plan to be there, depends on what we do now, and seeing how effective sponge cities have been performing in China, there is a big reason for us to stay optimistic and even a bigger reason for Jakarta to become the next sponge city in Asia, if not, the world.
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