Cities are made of impervious materials. As streets and buildings replace natural materials that absorb and hold water, rainfall is diverted to storm sewers and the like, which divert the water to a treatment plant and then into rivers, lakes, and oceans.

Sounds like a good idea, but for the fact that these systems are routinely overwhelmed by heavy rains, which leads to flooding and the release of untreated sewage into waterways.

One alternative involves creating cities with more permeable and absorbent areas, creating “sponge cities.” Unlike traditional development known as gray infrastructure, sponge cities use methods such as permeable paving and wetlands to capture and hold water. This not only lessens and/or prevents flooding, but also allows the water to be used for irrigation, toilet flushing, and even drinking, after treatment.

China has embraced sponge cities with gusto. Ironically, many of China’s cities face both water shortages and flooding, partially caused by inadequate water management and by gray infrastructure that is overwhelmed by extreme rains. More than half of the 657 cities evaluated by the Chinese government are classified as water scarce or severely water scarce, according to UN standards.  

The Chinese government has selected 16 cities for the sponge cities pilot program, and will help fund the program with up to 600 million yuan ($94 million US) per city. The project aims to minimize discharge into rivers by letting at least 60 per cent of water soak into the ground. Methods that encourage infiltration include permeable paving for roads and squares, and residential areas with wetlands, filtration pools, and storage ponds.

Other methods for building sponge cities include a variety of different projects, including green roofs, bioswales, wetlands, curb inlets, and restored floodplains. According to an article published by E&E Publishing, Changde City in central China has replaced 15 per cent of impervious areas with bioswales. These vegetated gullies retain stormwater rather than channeling it straight into the sewer system, and cut by half the upgrades needed for the rest of the drainage network, according to Peng Chiyan, an advisor on the project.

In cities with stressed drinking water supplies, green roofs and bioswales can be used to capture water and fill cisterns. In the Xixian New Area, extreme drought has led to the adoption of green roofs for water collection. Engineer Deng Chaoxian said the project will collect rainwater and drain it into filtration tanks. That water will be pumped into wells for later use.  

Another city suffering through drought is Los Angeles, California. As the second-largest city in the US, Los Angeles has experienced robust growth for the last century or so, with impervious surfaces proliferating. With the state now four years into an historic drought, flood control might not seem to be a priority, but it remains a key factor as the city looks to revitalize the river.

After devastating floods in 1938, the city took steps to mitigate flooding on the Los Angeles River, a 51-mile waterway that runs from the San Fernando Valley to the Pacific Ocean. Most of the river is now concrete from one riverbank to the other, with fencing cutting off the channel from the public. The concrete-channel approach has been effective in minimizing flooding, but it has cut residents off from a natural resource.

A new vision for the river involves restoring much of the channel with parks, bike and walking paths, and greenery. In addition to recreation areas, the project, which now includes architect Frank Gehry, could also include methods for reclaiming millions of gallons of water that flow, sporadically, down the concrete channel to the Pacific Ocean.