Global warming is increasing the urban threats of both flooding and drought. While flooding is a major issue in China (in 2013, more than 230 Chinese cities experienced floods) more than half of China’s cities are considered water scarce. A further 400 rely on groundwater sources for drinking water, even though 60 % of groundwater qualifies as relatively or very polluted.
Urban flooding poses serious safety risks and causes tremendous economic losses. Water scarcity could hamper many of China’s most pressing policy objectives, including continuing urban growth and achieving food self-sufficiency. Such problems are not unique to China and represent a growing global threat.
China’s response is the “sponge city” programme which is designed to offer a flexible solution for both water-scarce and water-logged cites.
Unlike traditional cities, where impermeable roadways, buildings, and sidewalks interfere with the natural water cycle, sponge cities mimic and support the natural water cycle. They use nature-based solutions, such as rain gardens, green roofs, constructed wetlands, and permeable pavement, to naturally capture, slow down, and filter stormwater. That water can then replenish groundwater aquifers or be stored for future use.
Sponge city technologies could also support sludge-to-energy development by preventing excess stormwater from diluting sewage water so much that it cannot be used to generate electricity for powering treatment plants.
Sponge-like cities should naturally accumulate, filter, and purify rainwater.
A pilot programme is underway to test out the best approached. 30 pilot cities have been selected including Beijing, Shenzhen, Chongqing, Fuzhou, Dalian and Shanghai.
Beijing, for example, has announced plans to develop more than 70 new green spaces throughout the city to collect excess stormwater, and Shanghai set a goal of planting 400,000 square meters of new rooftop gardens in 2016.
In October 2015, the State Council issued guidelines dictating that by 2020, 20 % of urban areas should locally store and reuse 70 % of all rainwater. By 2030, 80% of urban areas should meet that target. Achieving this goal will mean retrofitting existing urban areas and designing new development to incorporate sponge city features.
The pilot projects in Xiamen and Wuhan have already achieved positive outcomes and efficiently managed heavy storms on June 11, 2016. The reconstructed areas of Xiamen, a coastal city in southern China that is developing 236 projects costing over 7.2 billion RMB, endured Typhoon Nepartak without waterlogging.
Around the world, green infrastructure options perform many of the same functions as their traditional (or “gray”) infrastructure alternatives, and in many cases, they are more cost-effective. Gray infrastructure—which includes drains, concrete channels, piped drainage systems, pumps, and treatment plants—tends to be single purpose, whereas green infrastructure provides a variety of environmental, social, and economic co-benefits.
For example, in 2010, New York City released a city-wide green infrastructure plan that integrates new rain gardens and green roofs into existing stormwater systems to improve overall performance at a projected cost of $5.3 billion, $1.5 billion less than a comparable gray infrastructure approach.
The green infrastructure approach is also expected to reduce energy demand and CO2 emissions, improve air quality, and increase property values. One key objective of the plan is to reduce combined sewer overflows. While implementation of the plan is ongoing, in 2016, the New York Department of Environmental Protection reported that the project had already reduced annual overflows by 507 million gallons.