A saturated sponge may appear dry, but when squeezed it releases copious amounts of water.

According to United Nations predictions, a future of record-setting rainfall, droughts, and heat waves awaits us. In response, scientists have proposed the idea that urban development should be adjusted to make cities resemble a sponge, absorbing rainwater from heavy storms, and utilizing the stored-up water during dry spells. This would not only help mitigate flooding and runoff, but it could also help resolve droughts.

The concept has gained legs internationally. After an afternoon thundershower in 2011 flooded Beijing, Chinese authorities resolved to take action, establishing the Sponge City Construction Technology Innovation Alliance to systematically develop porous sponge cities across the country. Ahead of the curve, Singapore was the first to widely pave the city-state with porous asphalt.

In Taiwan, there is a remarkable road in New Taipei's Xizhi District that is said to "breathe." Not only can it prevent flooding, resist droughts, and reduce atmospheric carbon dioxide, but it can also regulate temperature, replenish substrata moisture, and even prevent stream and river pollution.

Located near Xizhi's green corridor in the Limen neighborhood, the road was originally developed for the purpose of watering plants. Several years ago, Limen neighborhood chief Li Chao-fa and his wife planted all over the neighborhood to green and beautify it, but the area was often plagued by water shortages. In order to water the community's plants and grass, they were forced to pump water from a ditch several meters deep. Apart from the dangers involved, even after extended efforts they were often unable to extract more than a small storage pool's worth of water.

New Taipei City deputy mayor Chen Shen-hsien, then minister of the Water Resources Agency, suggested that the couple use the NT$3 million top prize the Limen neighborhood had previously won in a conservation contest held by the Environmental Protection Agency to finance the installation of Taiwan's first complete breathable road.

Stores Water, Prevents Flooding

At first glance, there is nothing unusual about the red square brick tiles. However, upon closer inspection, each is covered with tiny pores, and it is these pores that make the road come to life.

The official name of this road that breathes is "JW Permeable Pavement." Inventor Chen Rui-wen explains that the tiny holes in the road surface connect to water pipes made from recycled plastic and reinforced with concrete. A bed of gravel beneath acts to filter water, which flows into a storage tank, much like a hidden river under the road.

Chen Shen-hsien gives the breathing road his seal of approval, noting that it "has just as much load-bearing capacity as regular pavement road surfaces." Since regular macadam roads cannot absorb water, puddles often form on them, which not only impacts travel but can lead to flooding. In contrast, permeable pavement can absorb up to 200 milliliters of water per minute, equipping it to handle almost any level of rainfall.

Li Chao-fa excitedly borrows a hose to demonstrate. Even exposed to a full blast, water flows no more than two or three meters on the surface before being completely absorbed.

Li then opens another valve, and stored water spurts out in a constant stream. His performance a success, he relates with a hint of self-satisfaction that during the installation the storage capacity was estimated to be 70 tons; however, last year after a full month of no rainfall, over 100 tons of water was extracted with plenty to spare.

Even more impressive is that the water can be recycled. The water collected from underneath the breathable road used to water flowers and vegetation that is not absorbed by the plants goes back inside the road, so nothing goes to waste.

Eco-Friendly, Money-Saving

Another remarkable aspect of the breathable road is its capacity for water filtration. The key to this is the gravel layer underneath the pavement. As Chen Rui-wen explains, since the gravel has not been compressed like a regular road surface, it is far more permeable, giving microorganisms a good environment in which to break down pollutants and purify the water.

"Some people take the 2.9 E. coli index of the Chiatung Creek's mountain spring water, which is used for brewing fine tea, as a standard," relates Chen. "The E. coli index of the breathing road is only 1.8, and all other indices are also safe. In fact, it's safe to drink after just basic filtration."

"If breathing roads were used everywhere in Taiwan," Chen asks confidently, "would there ever be water shortages?"

Cars traveling over the breathable road, which can even absorb their emissions, do not stir up dust. Chen Rui-wen explains that the working principle is that the permeable tubes are wide at the bottom and narrow at the top to create a chimney effect, which naturally pulls exhaust and particulates into the lower layer for cleansing, while soaking up CO2 at the same time.

Air convection can also cool a hot surface and raise the temperature of a cold one. During the summer months the breathable road's surface is nearly 20 degrees Celsius lower than nearby asphalt. While the ground surrounding a small breathable road Chen made at the Beijing Olympic venue was covered with a blanket of snow, the road surface accumulated none of the white stuff.

Though it costs NT$2.7 million to make a 120-meter breathable road, Chen Rui-wen says the price is cheap in the long term, explaining that the earliest breathable road was completed a decade ago, and other than clearing off silt it has needed no maintenance at all.

"Cement and plastic both last for 100 years (without decomposing), but the combination of these two materials, not known for being environmentally friendly, is actually quite environmentally friendly," Chen says with a chuckle.

The unique design saves drainage system costs while also performing water storage and purification functions. In addition, it mitigates the urban heat island effect and purifies the air, thus representing significant savings compared to conventional asphalt roads, which require frequent maintenance and harm the environment.

Chen Rui-wen boldly predicts that, in the span of less than 10 years, "sponge cities" will become the norm around the world.

Translated from the Chinese by David Toman