What Is a Sponge City? A New Approach to Urban Stormwater Management
The term "sponge city" was first put forward officially in China at the Central Urbanisation Work Conference held in December 2013. The General Office of the State Council then issued the Guiding Opinions on Promoting Sponge City Construction, which called for measures of infiltration, detention, storage, purification, use and drainage so that 70% of rainfall would be absorbed and used on site by 2020. [2]
On 31 December 2014, in line with the spirit of the important speeches on strengthening sponge city construction and the requirements of the recent Central Economic Work Conference, the Ministry of Finance, the Ministry of Housing and Urban-Rural Development and the Ministry of Water Resources decided to launch pilot projects for sponge city construction supported by central government finance, and issued the following notice.
1. Central government finance will provide earmarked subsidies for sponge city pilot projects over a fixed three-year period. The amount depends on city size: 600 million yuan per year for municipalities directly under the central government, 500 million yuan per year for provincial capitals and 400 million yuan per year for other cities. Where the public-private partnership (PPP) model reaches a certain share, a 10% bonus is added to the base amount.
2. Pilot cities are jointly nominated by the provincial finance, housing and urban-rural development, and water resources departments. A pilot city should be built into a "sponge" with the capacity to absorb, retain, purify and release water, improving flood control and waterlogging mitigation. The annual total runoff control rate should meet the requirements of the Ministry of Housing and Urban-Rural Development's Technical Guide for Sponge City Construction. Pilot cities prepare implementation plans in line with the three-year rolling budget; a separate guide for preparing the plans will be issued.
3. Pilot cities are selected through competitive review. The three ministries review the qualifications of applicant cities; qualifying cities then take part in a public defence session, an expert panel conducts on-site review, and the results are announced on the spot.
4. Performance of the pilots will be evaluated. The three ministries will organise regular performance evaluations and reward or penalise cities accordingly. Cities with good results receive a bonus of 10% of the central subsidy base; cities with poor results have part of the subsidy recovered. Detailed evaluation rules will be drawn up separately.
5. Local finance, housing and urban-rural development, and water resources departments should attach great importance to this work, plan actively, organise cities to prepare implementation plans and develop supporting policies. Further notice on applications will follow. [3]
Cities differ in character and advantage, so building a sponge city cannot mean mechanically copying the experience of others. Under scientific planning, each city should adopt locally appropriate measures that suit its own features if the sponge effect is to be fully realised, the urban ecological environment improved and residents' quality of life raised.
The Guiding Opinions state that sponge city construction coordinates natural ecological functions with engineered intervention to control stormwater runoff effectively and to achieve a model of urban development based on natural accumulation, natural infiltration and natural purification. This helps restore urban water ecology, conserve water resources, strengthen flood resilience, expand effective investment in public goods, improve the quality of new urbanisation and promote harmonious development between people and nature.
The Guiding Opinions specify that sponge city construction should minimise the ecological impact of urban development and construction and absorb and use 70% of rainfall on site. By 2020, more than 20% of the built-up area of cities should meet the target; by 2030, more than 80% should do so.
The Guiding Opinions set out ten specific measures in four areas: strengthening planning guidance, coordinating orderly construction, improving supporting policies and ensuring implementation. First, prepare plans scientifically, making the annual total runoff control rate a mandatory indicator in urban planning and establishing a regional stormwater discharge management system. Second, implement plans strictly, making sponge city requirements a precondition for planning permission and project approval and enforcing them at the stages of construction drawing review, construction permit and completion acceptance. Third, improve standards and codes, revising those relating to sponge city construction without delay. Fourth, advance sponge city construction in both new and old urban areas in a coordinated way. From 2015, new urban districts must fully apply sponge city requirements; old districts should combine shantytown and dilapidated housing renovation and organic renewal of older neighbourhoods, take action on urban waterlogging, rainwater harvesting and use, and treatment of black and odorous water bodies, and move towards area-wide treatment in which light rain leaves no ponding, heavy rain causes no waterlogging, water bodies are neither black nor odorous and the heat island effect is eased. A project reserve system should be established to avoid wholesale demolition and rebuilding. Fifth, promote sponge-type buildings and related infrastructure: sponge buildings and residential areas, sponge roads and squares, urban drainage and waterlogging facilities and the upgrading of flood-prone points, separation of stormwater and sewage, and rational siting of stormwater storage facilities. Sixth, promote park green space and ecological restoration: sponge parks and green spaces that absorb their own rainwater and provide space to store and detain rainwater from surrounding areas, together with protection and ecological restoration of urban ponds, rivers, lakes and wetlands. Seventh, innovate construction and operating mechanisms, encouraging private capital to invest in and operate sponge city projects, encouraging technology companies to work with financial capital, and using turnkey contracting to capture integrated benefits. Eighth, increase government input: central finance should actively guide sponge city construction and local governments at all levels should increase funding. Ninth, improve financing support: encourage financial institutions to expand credit support, include sponge city projects in the scope of special construction funds, and support eligible enterprises in issuing bonds. Tenth, ensure implementation: municipal governments are responsible for sponge city construction, and the Ministry of Housing and Urban-Rural Development, together with the National Development and Reform Commission, the Ministry of Finance and the Ministry of Water Resources, guides and supervises local work. [4]
Sponge city construction should follow the principle of giving priority to ecology, combining natural approaches with engineered measures, and, on the premise of safe urban drainage and flood control, maximising the accumulation, infiltration and purification of rainwater in urban areas so as to promote the use of rainwater resources and ecological protection. Building a sponge city is not a matter of starting over or replacing conventional drainage systems, but of relieving the load on them and supplementing them so that the city itself works as well as it can. The process should treat natural precipitation, surface water and groundwater as a system, coordinate every link in the water cycle from supply to drainage, and take account of its complexity and long-term nature.
As an important marker of a shift in urban development philosophy and construction methods, China's sponge city timetable is now fixed and can only move forward, never back. More than 130 cities nationwide have drawn up sponge city construction plans.
The core target is that sponge city construction should enable 70% of rainfall to be absorbed and used on site. The timetable for this target is that by 2020, 20% of a city's built-up area should meet it. If a city has 100 square kilometres of built-up area, at least 20 square kilometres should meet the requirement in 2020. By 2030, 80% of the built-up area should meet it. [5]
Building a sponge city begins with a change of mindset. Conventional urban development paves everything over. When heavy rain comes, drainage relies mainly on "grey" facilities such as pipes, channels and pumping stations, with "rapid removal" and "end-of-pipe concentration" as the governing design concepts, which all too often means flooding whenever it rains and abrupt swings between drought and waterlogging. Under the Technical Guide for Sponge City Construction, urban drainage should give priority to "green" measures such as grassed swales, rain gardens and sunken green spaces, with "slow release" and "distributed source control" as the governing concepts.
A sponge city needs "sponges". Urban sponges include water systems such as rivers, lakes and ponds as well as urban facilities such as green spaces, gardens and permeable pavements. Rainwater infiltrates, is detained and purified and is reused through these sponges, and the remaining runoff is discharged through pipes and pumping stations, which effectively raises the standard of the urban drainage system and relieves pressure from urban waterlogging.
Building a sponge city depends on continually increasing the scale and quality of these sponges. In the past, urban development sought flat, level land and often filled in lakes and levelled hollows. Under the Technical Guide, every locality should protect existing sponges such as rivers, lakes, wetlands, ponds and ditches from the impact of development; damaged sponges should be gradually restored through physical, biological and ecological means, and a certain proportion of ecological space should be maintained.
Where conditions allow, new sponges of a certain scale should also be built. According to the Technical Guide, sponge city construction uses urban buildings, residential areas, roads, green spaces and squares as carriers. City roofs, for example, can be greened: a green roof retains rainwater while also saving energy, cutting emissions and easing the heat island effect. Roads and squares can use permeable paving, and green spaces in particular should be sunken.
On 2 April 2015 the list of sponge city pilot cities was officially announced. Ranked by regional score, the top 16 cities were (in administrative order): Qian'an, Baicheng, Zhenjiang, Jiaxing, Chizhou, Xiamen, Pingxiang, Jinan, Hebi, Wuhan, Changde, Nanning, Chongqing, Suining, Gui'an New Area and Xixian New Area. On 10 June the Ministry of Housing and Urban-Rural Development issued a document adding Sanya to the pilot cities for urban "dual repair", underground utility tunnels and sponge cities.
Guangming New District in Shenzhen, Guangdong Province, is actively applying to be among the first national sponge city pilots. This follows its selection as a national demonstration area for low-impact development and rainwater utilisation and as a national-level green ecological demonstration district, and marks another push towards national pilot status for new green ecological construction. [6]
Sixteen cities, including Nanning and Jinan, will receive more than one billion yuan in central government subsidies for sponge city construction. The subsidy stems from the Ministry of Finance notice of 31 December 2014 on launching pilot projects for sponge city construction supported by central finance.
On 4 March 2015 the three ministries confirmed that 22 cities would take part in the competitive review defence for national sponge city pilot status, and 16 were eventually selected.
On the evening of 2 April 2015, according to the Ministry of Finance website, the list of sponge city pilot cities was officially announced. Ranked by competitive review score, the top 16 cities were (in administrative order): Qian'an, Baicheng, Zhenjiang, Jiaxing, Chizhou, Xiamen, Pingxiang, Jinan, Hebi, Wuhan, Changde, Nanning, Chongqing, Suining, Gui'an New Area and Xixian New Area.
In October 2015 the General Office of the State Council issued the Guiding Opinions on Promoting Sponge City Construction, setting out arrangements for the work.
Thanks to a well-developed underground pipe network, advanced rainwater utilisation technology and rationally planned urban green space, Germany's "sponge city" approach has been notably effective.
The extent and capacity of German urban sewer networks are among the best in the world. German cities all have modern drainage facilities that not only drain water and sewage efficiently but also help balance the urban ecosystem. In Berlin, the sewers total about 9,646 km, some of them nearly 140 years old. Pipes in the city centre are mostly combined systems that handle sewage and stormwater together, which saves underground space and avoids obstructing the metro and other underground services. In the suburbs, separate systems prevail, with sewage and stormwater treated in different pipes, which improves the targeting and efficiency of treatment.
From the end of the 20th century Switzerland promoted a nationwide "rainwater programme", a low-cost, high-yield and highly practical rainwater utilisation scheme. Normally, urban buildings have rainwater pipes running from the roof to the sewer, and the rainwater flows through them into the sewer and then into rivers and lakes. Switzerland works at the level of individual households, making small alterations to existing houses: a small hole is drilled in the wall, a pipe leads rainwater into an indoor storage tank, and a small pump sends the collected rainwater to various parts of the house. Switzerland is known as the garden country, with little wind-blown dust and almost no smoking industry, so its rainwater is relatively clean. Households pump up the settled and filtered rainwater to flush toilets, scrub floors, water plants and even wash clothes, fruit and vegetables.
Today many buildings and homes in Switzerland have dedicated rainwater pipes outside and storage tanks inside, and the rainwater is used after treatment. For ordinary users this rainwater system covers essentially all domestic water use apart from drinking. The Swiss government also uses tax relief and subsidies to encourage people to build such energy-saving houses, so that rainwater is recycled and considerable water is saved.
In Swiss urban development the finest infrastructure is the comprehensive citywide water supply and drainage network and the domestic sewage treatment plants. Simple open drains exposed on the road surface already existed in Switzerland in the 17th century, and these old drains can still be seen in the old town of Geneva. From 1860 the sewer was regarded as an important part of the public system, and Swiss city builders began constructing underground drainage to the needs of the time. Today's Swiss sewer system was mainly built after the Second World War, when large-scale urbanisation produced many satellite towns. In that period Switzerland enacted water use and water treatment legislation and began implementing sewer construction plans.
Singapore is a tropical island country with abundant rainfall, and although its maximum annual rainfall has risen steadily over the past 30 years, urban waterlogging is rare. Everything rests on a scientifically designed and rationally distributed rainwater collection and urban drainage system: first, planning the drainage system in advance; second, strengthening rainwater conveyance and building large storage reservoirs; third, establishing strict drainage standards for surface buildings.
Most American cities follow the traditional engineering approach of storing rainwater in the suburbs, carrying it into the city through channels and discharging sewage through underground conduits. This approach, which in Western accounts dates from Roman times, still prevails, even in California, where it does not suit local ecology.
For many years Los Angeles sent its rainwater into a river channel and on to the sea. In the 1940s the Los Angeles River was turned into a concrete channel to carry floodwater in the rainy season. It is a river in name only, effectively a 51-mile bathtub lying between the city and the sea. Before it was converted into a flood channel it frequently overflowed and flooded riverside towns; once the channel was lined with concrete the flood threat disappeared and the banks were taken over by the city. Conditions have now changed a great deal: people no longer worry about floods but regret that the rainwater simply runs away. [7]
