Building “Sponge Cities” to Restore Urban Aquatic Ecosystems
Release Time:
2016-05-20
Source:
Building “Sponge Cities” to Restore Urban Aquatic Ecosystems
In past urban development, rainwater management has been guided by the fundamental principles of “rapid drainage” and “centralized discharge at the downstream end.” Yet these very approaches have been major contributors to urban flooding. According to Chen Jining, President of Tsinghua University, large volumes of rainwater are typically conveyed away through rudimentary pipe networks, which collect runoff and channel it for centralized discharge at the system’s terminus. During heavy rainfall, substantial stormwater runoff—and the pollutants it carries—are rapidly transported to the downstream end of the drainage network within a short time, thereby increasing the risk of flooding in downstream areas. In fact, if most of the rainwater were retained rather than discharged, it would be possible to maximize its storage, infiltration, and natural purification within the urban environment, thus promoting the sustainable use of rainwater resources and the protection of the ecological environment. Retaining more rainwater and discharging less—this is precisely the core principle underlying the “sponge city” concept.
A “sponge city” is a metaphor for a city that, like a sponge, can “absorb, store, infiltrate, and purify” runoff rainwater on-site or nearby when it rains, thereby recharging groundwater and regulating the water cycle. During dry periods when water is scarce, the stored water can be “released” and put to use, making the movement of water within the city more “natural.” In other words, a sponge city holds the promise of pre-emptively managing large volumes of rainwater that would otherwise be rapidly discharged to the end of the drainage network by means of absorption, storage, and infiltration, thus providing an effective solution to the problem of urban flooding whenever it rains. Once a sponge city is completed, the ratio of rainwater that is drained away versus that which is retained will be reversed: whereas previously 70% of a city’s rainfall was quickly conveyed through the drainage system and discharged, a sponge city aims to retain 70% of that rainfall.
How should rainwater be managed? Sponge city development emphasizes the prioritization of “source-based, decentralized” control measures. At the source—on urban buildings and residential communities, roads, green spaces, and other sites—large-scale implementation of ecological infrastructure such as rain gardens, sunken green spaces, vegetated swales, and permeable pavements enables initial infiltration and storage of rainwater, thereby effectively controlling the pollutant load carried by stormwater runoff and reducing the total volume of runoff discharged. The conventional urban development model of single, “rapid drainage” must be transformed into a multi-objective, full-process, integrated management approach that incorporates infiltration, detention, storage, purification, reuse, and discharge.
Relevant authorities have pointed out that clear control objectives and performance indicators should be established at the urban planning stage. For buildings and residential communities, this means greening urban rooftops; where conditions permit, green spaces in residential areas should be designed as “sunken” landscapes, allowing rooftop and pavement runoff to first infiltrate, store, and purify in these sunken green areas, among other measures. Building “sponge cities” to restore urban aquatic ecosystems is aimed at creating a better living environment and is also a key component of sustainable development. Sound conceptual frameworks require robust institutional support, and the vigorous promotion by the government in recent years has provided precisely such support for the development of sponge cities.
