Interpretation and application of Sponge City guidelines in China

Abstract

‘Sponge City’ is the term used to describe the Chinese government's approach to urban surface water management. The concept was conceived in 2014 in response to an increasing incidence of urban flooding or water-logging in Chinese cities. While ambitious and far-reaching in its aim (of decreasing national flood risk, increasing water supply and improving water quality), the initiative must be implemented by individual subprovincial or municipal-level government entities. Thus, while the concept is similar to sustainable drainage systems (SuDS) in the UK (or low-impact development (LID) in the USA), it is developing with different regional characteristics, and during continuing rapid urbanization. Indeed, the increasing use of national rather than international examples of best practice reflects a growing body of knowledge that has evolved since the start of the Sponge City initiative. In this paper, interpretation and development of the national Sponge City guidelines are assessed for the Ningbo Municipality, an affluent and rapidly expanding city on China's low-lying east coast. While climate, geology and socio-economic factors can all be seen to influence the way that national guidelines are implemented, project financing, integration and assessment are found to be of increasing influence.

This article is part of the theme issue ‘Urban flood resilience’.

1. Introduction

It is widely recognized that surface water management in Chinese cities is undergoing rapid development which, while challenging, is presenting multiple opportunities for improvements to city infrastructure [1–4]. This paper reviews the development of surface water management as represented by the Sponge City concept and associated national government guidelines [5]. In addition to reviewing current national policy and guidelines, examples of local implementation in Ningbo Municipality are used to illustrate their different possible interpretation and impacts. Ongoing challenges for implementation of the concept across China are then discussed.

(a). Background

Xi Jinping (China's President) first raised the concept of a Sponge City in 2012. Subsequently, in 2013, the Office of State Flood Control and Drought Relief Headquarters identified 641 cities in danger of flooding in China, with more than 100 cities experiencing flooding each year [6]. In response, the concept of Eco-Sponge City was forwarded by the Xi Jinping in December 2013 at a central urbanization work conference. The concept envisages drainage infrastructure designed to act like a sponge, such that, during rainstorms, surfaces throughout the city absorb as much water as possible (either into natural soils and geology or purpose-built storage areas). Semi-natural or purpose-built bio-reactors can then be used to attenuate pollutants in infiltrated water before it can reach natural water courses [7,8].

The concept was sufficiently developed to be presented as a realistic response to the country's increased risk of urban flooding, and as an opportunity to upgrade failing urban drainage systems throughout the country. Indeed, the stated aim of the Eco-Sponge City initiative was to develop urban water management systems to be better able to handle increased urban flood risk, and to capture, purify and store more rainwater for use in grey-water activities.

In 2014, the State Council of China published the Sponge City construction guidance [5], which described the design and construction criteria for a range of urban infrastructure. The guide advocated utilization of natural drainage wherever possible (i.e. soil infiltration and filtration capacity), to reduce flood risk, improve surface water quality and conserve water. To further promote the implementation of the Sponge City approach, Li Keqiang (China's Prime Minister) called for the rational adoption of the new national guidance by all new city developments [9].

It can be argued that the scale and ambition of the Sponge City initiative is greater in scope and ambition than comparable international initiatives that have preceded it (LID in the USA; SUDS in the UK; or WSUD in Australia [10]). This is reflected in the objectives of the initiative which were described by short, medium and long-term development goals [5] as follows.

• —Short term (2015–2018): Introduction of Sponge City construction and development concepts, and promotion of small-scale urban pilot projects.
• —Medium term (2018–2020): Establishment of Sponge City standards, management systems, and monitoring and early warning systems by 2020; with greater than 20% of municipal areas able to recycle 70% of incident rainfall.
• —Long term (2020–2030): Complete integration of the Sponge City concept in urban development, planning and construction management by 2030; with greater than 80% of municipal areas able to recycle 70% of incident rainfall.

To demonstrate how the above goals may be achieved, in 2015, the Ministry of Finance (MOF), Ministry of Housing and Urban and Rural Development (MOHURD) and Ministry of Water Resources (MWR) identified 16 out of 130 candidate Sponge City pilot schemes (table 1). To qualify, each city had to have a strong case for urban drainage infrastructure renewal, plus the potential to achieve the targets within the projected life of the initiative. Demonstration areas within pilot cities needed to be greater than 15 km² and have an annual average rainfall of over 400 mm [5]. High priority was given to projects that involved reconstruction of existing water and management systems, the use of green landscape and green building designs that considered social and cultural needs (though these were not specifically defined).

Table 1.

First batch of nominated pilot Sponge Cities and relevant sources.

	city	city level	province	environment	hydrological issues	citation/source (in Chinese)
1	Chongqing	municipality	—	confluence on the Yangtze and Jialing Rivers	water-logging pluvial flooding	Chongqing planning and design institute (2018) Guidelines for planning and design of Sponge City in Chongqing (http://www.cqghy.com.cn/index.php?s=/articles/174.html)
2	Jinan	subprovincial	Shandong	Yellow Riverfloodplain	water-loggingpluvial flooding	Jinan Government (2015) Guidelines for planning and design of Sponge City in Jinan (http://www.jinan.gov.cn/art/2015/11/12/art_1862_230391.html)
3	Wuhan	subprovincial	Hubei	Yangtze River floodplain	water-loggingpluvial flooding	Wuhan Planning and design institute (2017) Guidelines for planning and design of Sponge City in Wuhan (http://www.wpdi.cn/academic-research-i_11398.htm)
4	Xiamen	subprovincial	Fujian	coastal, humid subtropical	water-loggingpluvial floodingcoastal flooding	Xiamen Government (2018) Guidelines for planning and design of Sponge City in Xiamen (http://www.xm.gov.cn/zwgk/flfg/sfbwj/201808/t20180801_2081407.htm)
5	Baicheng	prefecture	Jilin	Mongolian steppe, Tao'er River floodplain	drought and the only city to be selected for the Sponge City programmr in the NE China	Jilin Government (2018) Guidelines for planning and design of Sponge City in Baicheng (http://www.jl.gov.cn/hd/zxft/szfzxft/zxft2018/20180224ft_123315/ftzy/201811/t20181109_5228720.html)
6	Zhenjiang	prefecture	Jiangsu	Southern bank of Yangtze River	water-loggingpluvial floodingcoastal flooding	Zhenjiang Government (2017) Guidelines for planning and design of Sponge City in Zhenjiang (http://jsj.zhenjiang.gov.cn/xxgk/zcfg/dfxfg/201708/t20170816_1879146.htm)
7	Jiaxing	prefecture	Zhejiang	drained land	water-logging	Jiaxing Government (2018) Guidelines for planning and design of Sponge City in Jiaxing (http://www.njszgl.com/szgh/257.html)
8	Chizhou	prefecture	Anhui	Southern bank of Yangtze River	pluvial flooding	Anhui Government (2017) Guidelines for planning and design of Sponge City in Chizhou (http://www.ah.gov.cn/UserData/DocHtml/1/2017/6/13/6972547274419.html)
9	Hebi	prefecture	Henan	Qi river flood plain	pluvial flooding	Hebi Government (2017) Guidelines for planning and design of Sponge City in Hebi (https://fgw.hebi.gov.cn/zghb/436364/436368/2029120/index.html)
10	Pingxiang	prefecture	Jiangxi	mountainous floodplain	pluvial flooding	Pingxiang Government (2018) Guidelines for planning and design of Sponge City in Pingxiang (http://xxgk.pingxiang.gov.cn/fgwj_1/qtygwj/201805/t20180514_1726051.htm)
11	Changde	prefecture	Hunan	Yuan River floodplain	pluvial flooding	Changde Government (2017) Guidelines for planning and design of Sponge City in Changde (http://zfjsw.changde.gov.cn/art/2017/3/13/art_5254_1034945.html)
12	Nanning	prefecture	Guangxi	Yong River floodplain	pluvial flooding	Nanning Government (2018) Guidelines for planning and design of Sponge City in Nanning (http://www.nanning.gov.cn/xxgk/xxgkml/jcxxgk/zcwj/zfwj/t756426.html)
13	Suining	prefecture	Sichuan	Fu River Basin	pluvial flooding	Suining Government (2016) Guidelines for planning and design of Sponge City in Suining (http://gk.suining.gov.cn/t.aspx?i=20170104163744-845752-00-000)
14	Qian'an	subprefecture	Hebei	Luan River Floodplain	drought	Tangshan Government (2016) Guidelines for planning and design of Sponge City in Qian'an (http://www.tangshan.gov.cn/zhuzhan/zwgkgcztbzbgg/20161130/368216.html)
15	Gui'an	new area	Guizhou	Mountainous	pluvial flooding	Guian Government (2019) Guidelines for planning and design of Sponge City in Gui'an (http://www.gaxq.gov.cn/zwgk/xxgkml/zdlyxxgk/cxgh/201901/t20190115_2214467.html)
16	Xixian	new Area	Shannxi	Mountainous	pluvial flooding	Xixian Government (2016) Guidelines for planning and design of Sponge City in Xixian (http://www.fcfx.gov.cn/xwzx/mbjj/20706.htm)

Table 1 illustrates that the first batch of pilot cities consisted of one municipality, three subprovincial cities and 12 prefecture-level cities (figure 1). The distribution of cities across the different climatic regions and landscapes of China meant that a range of different hydrological conditions and risks could be represented (mountainous, coastal, lowland, dryland temperate and tropical).

Figure 1.

Geographical location of the first (dark circles) and second (light circles) batch of Sponge City pilot cities. (Online version in colour.)

Each pilot city was given a central government subsidy for the purchase of raw materials, and initial engineering and construction costs: US$ 85 million per year (for 3 years) for provincial capital cities; US$ 71 million for municipalities directly under the control of central government; and US$ 57 million for all other cities. Design and evaluation of criteria for proposed drainage systems varied from city to city and was determined by local climate and hydrology [7]. Cities evaluated to be performing well were awarded an additional 10% subsidy, whereas poorly performing cities had their subsidies withdrawn.

For the first 16 pilot cities, a total investment of US$ 12.2 billion was planned over the first 3 years (for an estimated construction area of 450 km² (average investment of US$ 14–21 million km⁻²). To meet the development goals of the project, however, more than 20% of the country's 658 urban areas also needed to meet the defined standards by 2020, which would equate to an annual total investment of more than US$ 56 billion. By 2030, when more than 80% of the urban area is required to achieve the same standards, some US$ 0.23 trillion will be needed—representing a significant challenge in terms of financing.

A second batch of pilot cities was identified in February 2016 (table 2). These were chosen to represent an even wider range of environmental, and climatic diversity than the first ( figure 1), with only the hyper-arid or mountainous regions in western China not represented. A further two groups (of 20 and 37) were selected for urban planning pilot cities in 2017 [11].

Table 2.

Second batch of nominated pilot Sponge Cities.

	city	city level	province	environment	hydrological issues	citation/source (in Chinese)
1	Beijing	municipality	—	capital mega-city	pluvial flooding	Beijing Government (2018) Guidelines for planning and design of Sponge City in Beijing (http://www.ccgp.gov.cn/cggg/dfgg/zbgg/201812/t20181206_11284932.htm)
2	Tianjin	municipality	—	coastal mega-city	pluvial floodingcoastal flooding	Tianjin Government (2019) Guidelines for planning and design of Sponge City in Gui'an (http://zfcxjs.tj.gov.cn/)
3	Shanghai	municipality	—	coastal mega-city	water-loggingpluvial floodingcoastal flooding	Shanghai Government (2018) Guidelines for planning and design of Sponge City in Shanghai (http://www.shanghai.gov.cn/nw2/nw2314/nw2319/nw12344/u26aw56510.html)
4	Dalian	subprovincial	Liaoning	coastal peninsular	pluvial floodingcoastal flooding	Dalian Government (2018) Guidelines for planning and design of Sponge City in Dalian (http://www.dl.gov.cn/gov/detail/file.vm?diid=101D05000180910260518093009&lid=3_4)
5	Ningbo	subprovincial	Zhejiang	coastal floodplain	water-loggingpluvial floodingcoastal flooding	Ningbo Government (2018) Guidelines for planning and design of Sponge City in Ningbo (http://zfxx.ningbo.gov.cn/art/2018/7/12/art_2447_2350366.html)
6	Qingdao	subprovincial	Shandong	coastal city	waterloggingpluvial floodingcoastal flooding	Qingdao Government (2018) Guidelines for planning and design of Sponge City in Qingdao (http://www.qingdao.gov.cn/n172/n24624151/n24626395/n24626409/n24626423/180524152516531751.html)
7	Shenzhen	subprovincial	—	coastal floodplain	water-loggingpluvial floodingcoastal flooding	Shenzhen Government (2018) Guidelines for planning and design of Sponge City in Shenzhen (http://www.sz.gov.cn/sswj/ztzl/bmzdgz/hmcsjs/zhyw/201808/P020180820364500062887.pdf)
8	Fuzhou	prefecture	Fujian	coastal floodplain	waterloggingpluvial floodingcoastal flooding	Fuzhou Government (2018) Guidelines for planning and design of Sponge City in Fuzhou (http://fzjw.fuzhou.gov.cn/zz/zwgk/tzgg/201804/t20180428_2189253.htm)
9	Zhuhai	prefecture	Guangdong	coastal city	pluvial floodingcoastal flooding	Zhuhai Government (2018) Guidelines for planning and design of Sponge City in Zhuhai (http://www.zhzgj.gov.cn/xxgk/tzgg/201801/t20180105_25444461.html)
10	Sanya	prefecture	Hainan	coastal city	pluvial floodingcoastal flooding	Hainan Government (2018) Guidelines for planning and design of Sponge City in Sanya (http://www.sanya.gov.cn/sanyasite/fgwj/201609/715518bb75694b46ad12297dfdcb8fd2.shtml)
11	Yuxi	prefecture	Yunnan	mountainous floodplain	pluvial flooding	Yuxi Government (2018) Guidelines for planning and design of Sponge City in Yuxi (http://www.yuxi.gov.cn/zxdt/20170525/571123.html)
12	Qingyang	prefecture	Gansu	mountain plateau	pluvial flooding	Gansu Government (2018) Guidelines for planning and design of Sponge City in Qingyang (http://www.gansu.gov.cn/art/2018/3/28/art_48_359212.html)
13	Xining	prefecture	Qinghai	Mountain floodplain	pluvial flooding	Xining Government (2018) Guidelines for planning and design of Sponge City in Xining (http://www.xining.gov.cn/html/171/373685.html)
14	Guyuan	prefecture	Ningxia	Mountainous	pluvial flooding	Guyuan Government (2018) Guidelines for planning and design of Sponge City in Guyuan (http://www.nxgy.gov.cn/zwgk/szfwj/201708/t20170808_384666.html)

2. National policy and guidance

As the implementation of Sponge City concepts is conducted at local government level, central government will coordinate guidance and assistance from planning and development departments from three ministries. MOHURD is responsible for designing and issuing related guidance and standards to assist in the delivery of supported projects; MOF allocates and manages investments; and MOWR is responsible for monitoring and guidance of new projects. The National Development and Reform Commission are responsible for the interpretation of national-scale policy and standards of delivery at a regional and local scale, and approve and evaluate all Sponge City developments.

Tables 3 and 4 summarize the key policies and standards related to implementation of the Sponge City concepts. The core document, the Sponge City construction guidance [5], for example, deals with planning controls, design approach, engineering standards, risk management and maintenance regimes. Infrastructure types described include stormwater control systems, road drainage systems, for both green and community spaces. Section 3a of the guide [5] indicates that drainage of surface water runoff within Sponge City sites should also follow the Code of Design of Outdoor Wastewater Engineering (GB 50014-2006) [12]. Other relevant urban drainage, water-logging, and planning and design standards include the Urban Drainage Facility Planning Code (GB50318-2000); the Outdoors Drainage Design Code (GB50014-2016) and the Urban Flood Control Facility Design Code (CJJ50-92).

Table 3.

National planning and policy relevant to implementation of Sponge City guidelines.

national policy	aims	source (in Chinese)
The Communist Party of China's State Council policy on urban drainage storm water drainage facility construction notice (April 2013)	—transform water drainage and sewage control systems (by 2023) —promote sustainable drainage construction methods —permeable surfaces area ratio minimum of 40% in new developments —convert impermeable surfaces to permeable surfaces, to store rainwater and reduce hydrograph peak	The State Council of the Peoples’ Republic of China (2013) The Communist Party of China's State Council policy on urban drainage stormwater drainage facility construction notice. Notice No. 23 of the State Council (http://www.gov.cn/zhengce/content/2013-04/01/content_5066.htm)
The Communist Party of China's State Council policy on promoting the Sponge City-building guidance and aims (October 2015)	—increase implementation of Sponge City approach —rehabilitate urban water ecology —conserve urban water resources —increase storm water drainage capacity —increase public investment in drainage projects —promote human development in harmony with nature —reduce impact of urban development on ecology —infiltrate 70% of rainfall within the development areas —by 2020, 20% of urban areas should achieve the above objectives —by 2030, 80% of urban areas should achieve the above objectives	The State Council of the Peoples’ Republic of China (2015) The Communist Party of China's State Council policy on promoting the Sponge City-building guidance andAims. Notice 75 of the State Council (http://www.gov.cn/zhengce/content/2015-10/16/content_10228.htm)
The Communist Party of China's State Council on further strengthening the urban planning and construction management (February 2016)	—draft Sponge City construction guidance —use natural landscape: topography, wetlands, farmlands, woodlands, grasslands, and existing rivers and lakes —develop synergistic eco-spaces to promote water conservation; recycling; flood and water-logging resilience —encourage households to install rainwater collection devices —significantly reduce in urban impermeable surfaces	The State Council of the Peoples' Republic of China (2016) The Communist Party of China's State Council on further strengthening the urban planning and construction management. 6 February 2016 (http://www.gov.cn/zhengce/2016-02/21/content_5044367.htm)

Table 4.

National standards for prevention of water-logging.

national standards	specification	source/citation (in Chinese)
Outdoor drainage design code (GB50014-2016)	—guidance on rainwater harvesting, transportation, storage, discharge, processing and utilization of natural and artificial facilities, and related management measures —medium-sized cities and small urban water-logging prevention schemes to design for return periods of 10–20 years —no more than 15 cm of runoff depth on road surfaces —engineering and non-engineering measures —used to prepare for and respond to urban water-logging —post-development conditions not to exceed original runoff	Ministry of housing and urban rural development of the People's Republic of China (2016). Code for design of outdoor wastewater engineering (GB 50014-2006), 2016 edition. China Planning Press, Beijing, China.
Sponge City construction technology guide—Low-impact development storm water systems construction	—volume runoff objectives:total annual runoff rate to be limited to 15–20%annual runoff control of 80–85% of incident rainfall —peak runoff objectives:city storm sewer and pumping stations to be designed with respect to outdoor drainage design code (GB50014-2016) —stream pollution control targets:classification system for runoff pollutant and to control the frequency and total amount of combined sewer overflow	Ministry of Housing and Urban-Rural Development (2014). The construction guideline of Sponge City in China. Ministry of Housing and Urban-Rural Development, China Planning Press, Beijing, China.
The city flood control engineering design code (GBT50805-2012)	—relates to flood control, including storm surge, flood tide and related impacts —flood control and engineering design standards —river flood engineering for 200-year events —urban flood design for 20-year events	Ministry of housing and urban rural development of the People's Republic of China (2012). ‘Code for design of urban flood control project (GB/T 50805-2012)’, 2012 edition, China Planning Press, Beijing, China.

The above guidance documents describe the development of infrastructure to handle up to 1-in-30-year (24 h) rainfall events, compared to previous guidance which accounted only for 1-in-1 to 1-in-5-year (24 h) events [7]. With this improvement, new towns and development areas will move closer to levels of flood protection seen in other Asian cities, such as Tokyo, Hong Kong and Singapore (which design for 1-in-50-year events) [13].

In December 2018, MOHURD published Assessment standards for Sponge City construction (GB/T 51345-2018) [14], which all pilot cities were required to adhere to by August 2019. The standards provide assessment guidelines for (i) volume control of urban runoff; (ii) stormwater source control and implementation effectiveness; (iii) road surface flood control; (iv) urban water quality; (iv) ecological conservation and eco-system services; (vi) groundwater depth and condition; and (vii) urban heat island effect reduction. These new standards allowed MOHURD to address issues omitted from the earlier 2014 guidance, such as benchmarking the effectiveness of new infrastructure on stormwater control; guidance for improving the eco-system services and ecology conservation; description of groundwater effects; and reducing urban heat island effects.

While technical developments that support implementation of Sponge City guidelines are increasingly in place, financing for such schemes will continue to be a challenge. Indeed, MOHURD estimated the construction cost of Sponge City developments will be in the order of US$ 0.14 million km⁻². To roll-out the Sponge City concept to other cities across China then, the total central government investment will be significant. Even with central government under-writing cities by up to 20%, the remaining required capital will need to be met by local government or private investors.

3. Local-scale implementation: Ningbo, Zhejiang Province

Sponge City guidelines are relevant at local-scale planning, design, construction and maintenance stages, with each stage requiring technical, implementation and assessment guidance. The case of Ningbo City is used to illustrate how Sponge City guidelines [5] are being applied locally, and how intra-city variability in environmental conditions necessitates flexibility in how the guidelines are interpreted.

By the time Ningbo was included in the second batch of Sponge City pilot areas, it was already a pilot city for the World Bank Climate Resilient Cities (CRC) programme, and had been ranked as one of the most vulnerable cities in the world by the OECD [15]. It is also a nationally recognized ‘water-saving city’, ‘garden city’ and ‘ecological city’ [16].

As a coastal city surrounded by mountains (figure 2), Ningbo has always been at risk from both pluvial and coastal flooding [17]. However, this risk has increased in recent years as urban development has replaced farmland, marshland, parkland and lakes that previously absorbed much of the runoff produced within and around the city boundaries. As the city's perimeter has expanded, the density of urban development therein has also increased. As a result, many older urban drainage systems that feed into the Yong River (and thence into the sea) have been unable to contain runoff produced from modern sidewalks and road networks. Older pumping stations have struggled to reduce water levels quickly enough from the city's canals, so that surface water flooding occurs more frequently [18]. Adoption of Sponge City guidelines, and seeking to increase stormwater storage and infiltration capacity should, therefore, yield measurable benefits.

Figure 2.

Location of Ningbo and its Sponge City projects (A: New East City, B: Ci Cheng:, C: Xiaojia, D: Yinzhou, E: Nan Tang Laojie). (Online version in colour.)

In order to achieve the objectives described in tables 3 and 4, the national guidelines [5] were refined relative to relevant municipal-level planning, and policy and standards [19]. While planning documentation was compiled by the Ningbo Urban Construction Design and Research Institute Co., Ltd, other organizations involved in the interpretation of guidance included the Ningbo Construction Committee; Ningbo Bureau of Finance; Ningbo Planning Bureau; and the Urban Management Law Enforcement Bureau.

Tables 5 and 6 illustrate Ningbo's municipal interpretation of the national standards. These policies were adopted for specific development or redevelopment areas, as determined by the municipal government master planning for the city. As such, this represented a major step towards city-scale planning and integration of Sponge City objectives. Subsequent municipal guidance documentation is planned to facilitate future developments within the city.

Table 5.

Ningbo Municipality policy relevant to implementation of Sponge City.

local guidelines	aims	source/citation (in Chinese)
The Ningbo city centre drainage and storm water drainage planning guidelines (March 2015)	—improve existing stormwater drainage systems —establish water-storage systems —promote low-impact development and construction including buildings, community rainwater collection, permeable areas —support strengthening of flood risk assessments in new urban developments in accordance with planned construction sequence	Ministry of housing and urban rural development of the People's Republic of China (2017) ‘Code for Urban wastewater and stormwater engineering planning' GB50318-2017, 2017 edition. China Planning Press, Beijing, China.
Ningbo city—Five water treatments programme (May 2015)	—aims to improve the water environment, protect water resources and improve people's livelihood —speed up the development of ecological civilization, to aid sustainable economic and social development —secure fortification of city flood protection measures to meet the state standards (50 years events) by 2020; urban built-up area drainage standards (20 year events)	Ningbo Municipal Bureau of Ecology and Environment (2015) Ningbo city—Five water treatments programme. Ningbo Municipal Bureau of Ecology and Environment, Ningbo, China (http://sthjj.ningbo.gov.cn/art/2015/3/13/art_19571_3095282.html)
Ningbo Municipal People's Government: Implementation of upgrading urban and rural quality to build a beautiful Ningbo—action plan guidelines (September 2015)	—short term (2015–2018): combined with ‘Ningbo City promotion, beautiful quality construction’ project to promote Sponge City construction projects (2015–2018) —medium term (2018–2020): establish Sponge City support standards, management systems and monitoring, and early warning system of the city —long term (2020–2030): urban planning, construction and management and implementation of the sponge construction concept to 80% of the city's built-up area	Ningbo Government (2015) Ningbo Municipal People's Government: Implementation of upgrading urban and rural quality to build a beautiful Ningbo—action plan guidelines (October 2015). Ningbo Government, Ningbo China (http://gtog.ningbo.gov.cn/art/2015/10/20/art_692_297911.html)

Table 6.

Ningbo Municipality standards relevant to the implementation of Sponge City guidelines.

municipal standards	specifications	citation/source (in Chinese)
Eastern Ningbo city planning area	—prevent flooding for 20-year 24 h eventsin the riverbasin planning standards and planning conditions, new town planning in the region in the East River 20-year design flood water-level for 2.6 cm (Yellow Sea datum), 100-year flood water-level 2.95 cm —drainage system: using rainwater diversion system —1. discharged into the river nearby, but not into planned Rivers —2. Network design parameters: storm intensity. Design return period 1 year, the initial concentration time: 10–15 min; the composite runoff coefficient 0.4–0.6	—Ningbo Planning Bureau (2017) Master plan of Ningbo East New Town (July 2017). The Eastern New Town (NET), Ningbo Government, Ningbo, China (http://www.nbent.cn/site/detail/id/95550/catId/481)
Sponge City Ningbo city planning guide (2016–2020) (March 2016 first published and updated May 2019)	—by construction guidelines relating to development of the city water system using ‘Green–grey design combination’ and below-ground water-storage strategies; integrated objectives for ecology, water environment and water resources —aim to reduce downtown total annual runoff control rate by 75% for design rainfall event; in 80% of the urban area by 2030	—Ningbo Municipal Bureau of Housing and Urban Rural Development (2019) ‘Ningbo urban planning and design guideline for Sponge City’ Code 2019 DX-08. Ningbo Municipal Bureau of Housing and Urban Rural Development, Ningbo, China.

The Municipal Construction Committee's aim in adopting Sponge City guidelines was not only to increase flood resilience, but also to enhance the quality of the city's existing waterways and therefore to improve the quality of eco-system services, and people's lives and social well-being. Interestingly, the location of developments that were supported in Ningbo ranged from city centre to peri-urban areas. The examples outlined below (locations shown in figure 2) illustrate the range of environments to which the new guidelines have been applied. They also illustrate the range of agencies involved in each development.

(a). New East City

The highest profile new development in Ningbo is the 205-acre New East City development area which hosts a range of urban parklands that use natural landscape features of topography, hydrology and vegetation. The construction zone includes a 3.3 km long ‘Eco-corridor’ which hosts a range of wetlands and ponds and land-based plant assemblages designed to promote bio-filtering of surface water runoff [18,19]. In addition, slow flow areas within the canal system promote natural settlement, aeration and bio-processing of transported sediment and pollutants with the aim of reducing eutrophication caused by excess nutrients.

The area is typical of new urban environments found in Chinese cities but has been built around part of the city's old canal system (as is most of Ningbo), which has been used to improve the aesthetic and amenity value of the development. The area is a mix of new and retrofit design, such that the self-proclaimed new city centre will represent both old and new Ningbo. The centre-piece of the development is a 2803 m² lagoon that will provide storage capacity for runoff during flood events. Under normal flow conditions, the area will also support local ecosystems and provide amenity space for local communities (cycling, fishing, boating, etc.). In this way, the development meets multiple design criteria (flood control, water quality and amenity value) of national guidelines [5] for the construction of Sponge Cities.

(b). Cicheng New District

Cicheng New District is located in the northwest of central Ningbo, where even before Ningbo had become a pilot city, developers had started to adopt compatible design principles. Rainwater infiltration and storage areas have formed a core part of the urban drainage design since 2004. The project is the result of collaboration between Ningbo Housing and Urban Construction Committee, Ningbo Urban Construction Design Institute, Beijing Construction Engineering Design and Research Institute and the Beijing Architecture University [20].

The ‘hybrid core’ design of the site links surface water drainage systems with wetlands and pre-existing ecological areas. Surface runoff water is diverted to roadside infiltration zones and filtered before it reaches the shallow groundwater. The residence time of the infiltration zones is calibrated to optimize bio-absorption of nutrients and removal of soluble and particulate contaminants. Drainage water from residential districts is diverted to bio-swales underlain with drainage pipes and secondary filtering before discharging downstream. Constructed wetlands are positioned between the residential areas and the central area to filter runoff before it reaches the central lake. Water from the lake will be used for ground irrigation, toilet flushing and road cleaning.

In using a constructed wetland for both stormwater storage and water purification, the project represents a significant showcase for Sponge City projects and is developing best practice for adoption in projects in Ningbo and China. A detailed implementation plan, technical guidance and policy system is being developed.

(c). Xiaojia River District

Xiaojia River in the northeast of Ningbo is a similar development to Cicheng. The project is led by a Sino-Dutch collaboration between the Ningbo Planning & Design Institute, Kuiper Compagnons and the Connecting Cities Network. The development uses the existing waterways as a foundation to guide future, ecological, social, economic and touristic development. The project integrates urban planning and design, water management and will form part of a green corridor around the city. The 22 km² Xiaojia River District will become an integrated part of Ningbo's larger municipal-scale flood management strategy.

(d). Yinzhou Central River

The Yingzhou Central River Transformation project [21] demonstrates an alternative approach to urban waterfronts in China and addresses the challenge of minimal land availability and flood control. The project transformed an urban concrete channel into an environmentally friendly garden landscape capable of providing multiple eco-system services. The waterfront space previously consisted of a 50–80 m wide strip between the street and the water with a narrow green strip for trees adjacent to such channels. While flood control regulations required that the drainage capacity of the channel should not be reduced, the upper part of the riverbank was removed to provide a softer transition between land and water. The elevation of the riverbank was also reduced so that a wetland could be created. The surrounding areas were developed with courtyards, paving areas, native grasses and framed bamboo enclosures that would withstand occasional flooding or inundation. The project represents an effective approach to improving amenity space in the central development area while also improving flood resilience.

(e). Nan Tang Lao Jie (Nan Tang Old Street)

While not strictly falling under the umbrella of Sponge City construction style and methods, Nan Tang Lao Jie is worth mentioning because it has adopted the traditional architecture style that was used parallel to many historic inland waterways in Ningbo's central area [22]. As such, it has set a trend for subsequent mixed-use riverside developments in the city. The building density is low by modern standards and there is a high percentage of green or amenity space. By combining modern concepts of sustainable urban development with aesthetic and functional riverside design, both flood resilience and high commercial value has been achieved [18]. The development is in contrast with many of Ningbo's peri-urban waterways which are still awaiting development and are in a semi-derelict state of repair (figure 3).

Figure 3.

Peri-urban development on Ningbo's historic waterways. (Online version in colour.)

4. Discussion

While all of the developments described have resulted in improvements in flood resilience and demonstrate effective implementation and workability of Sponge City concepts at a local scale, such designs will be limited in their impact if considered in isolation of city-scale surface water and flooding management plans. In 2017, for example, severe city-wide flooding in Ningbo resulted from a combination of heavy prolonged rainfall and high tides caused by Typhoon Fitow [23]. Local flood water storage areas overflowed when floodwaters flowing into the city from the mountains (after 100-year return period rainfall), simply had nowhere else to drain to (as the Yong River was backed-up by the incoming high-tide).

Indeed, the above event underlined the need for both urgent drainage infrastructure renewal and upgrade, and larger flood protection measures to be prioritized in the city. While current feasibility studies for a coastal flood protection barrage in the lower tidal-reaches of the Yong River are currently being completed, until such measures are integrated with local flood management, the control of such large floods will remain outside the remit of the current Sponge City guidelines.

This lack of hydrological connectivity between new development areas and their boundary conditions (surrounding catchments) has emerged as a common area of concern for the Sponge City initiative. While the national guidelines recommend that flood modelling should be integrated with catchment-scale flood and water resource models, support from central government is focused on urban development, such that integration with the larger catchment planning processes is under-represented.

While the public generally supports government investment to reduce flood impacts and risks, the fact that 19 of the 30 pilot cities have experienced flooding since 2014 [24] is not an encouraging signal to send to potential investors. Water-logging or flooding has occurred in 10 of the first 16 pilot sponge cities, and 9 of the 14 later pilot cities [25]. These included Beijing, Tianjin, Chongqing and the capital cities of Fuzhou, Wuhan, Jinan and Nanning. While Beijing, and many other cities, have since revised urban drainage designs to accommodate larger return period events [26], large flood events will continue to blight the perceived performance of the Sponge City initiative until the percentage of associated infrastructure increases and is better integrated with larger flood control engineering structures. In addition, expectations of green infrastructure can be better managed through the provision of better public information.

It would be unfair to exclusively use contemporary flood events to measure the success of the Sponge City programme. However, as the pilot studies are completed and the concepts are extended to cover 80% of the country by 2030, flooding from upstream areas or coastal storm surges will inevitably continue to influence city-wide flood risk. Such floods occurred in Shenzhen and Guangzhou after Typhoons Hato (2017) and Mangkhut (2018), respectively, despite significant investment in local flood infrastructure in those cities [13]. In those cases, increased flood risk resulted from land-use changes in upstream and coastal catchment areas.

It has been suggested that continued flooding in high-profile cities may have contributed to reports of continued weak interest in Sponge City Public–Private Partnership (PPP) schemes. However, investor disinterest may also be due to the difficulty in ascertaining the performance of such investments, as drainage infrastructure development yields as many intangible as tangible benefits. Zhang et al. [27] found that inadequate supervision, government intervention, immature law and regulation, project fragmentation and unclear catchment boundaries were the main risk factors affecting PPP schemes under the Sponge City remit and which have led to a loss of confidence in the model.

The growth of PPP interest was initially motivated by high demand for drainage upgrade and development, combined with and shortage of capital. However, little attention has been paid to identifying or allocating critical risks to such projects which has since also acted as a disincentive to contractors. Attracting financing to form effective PPPs may require integration of objectives from multiple sectors (housing, sanitation, health, etc.), which could help meet Sponge City targets by facilitating innovation in project management, urban planning and governance, and technological support (e.g. infrastructure design, building material development, etc.). Indeed, each of these areas have seen sustained growth within China in recent years [3,17,26].

The funding ratio used for developments in Ningbo was 2:1:1 (central : city : district financing). To date, this includes approximately US$ 0.17 billion from central finance, and US$ 0.08 billion from city and district governments. An additional US$ 0.6 billion has been sourced from PPP or engineering, procurement and construction (EPC) contracts (though notably only one PPP project had been approved by July 2018) [28].

The continued development of the Sponge City approach will inevitably be impacted by the performance of pilot cities in the first 3 years (2015–2018). Future financing of such projects will require interest from investors (e.g. private developers, funders, etc.) that have the same belief in the potential lifestyle and thus economic benefits of this type of investment. This will need to be obtained for initial construction costs; subsequent maintenance of infrastructure; and to establish long-term goals that will benefit all parties. This will be made difficult by the continuing rise in real estate prices in China [29]. Also, current public participation and stakeholders' involvement in the Sponge City planning process is still minimal. This has resulted from a lack of public awareness and understandings about what Sponge City pilot city's and projects are really trying to achieve (though there has been much press coverage) [24,26].

5. Conclusion

To some extent, the need for the development of a Sponge City concept started with the acceleration of China's urban development process and the associated conversion of farm and park lands for city development. This led to a loss of water resources, as increased impermeable surfaces led to more rainfall runoff and less water conservation. Increased runoff generation meant that the conveyance capacity of channels in pre-existing drainage systems (rivers, lakes and canals) was exceeded more frequently. Thus it is that the Sponge City initiative is now seeking to restore surface water storage capacity with infrastructure that will improve surface water infiltration.

Insufficient conveyance capacity, the integration of new and old drainage networks, and the integrated management large upstream catchments [30–32] are just some of the issues that need to be addressed in China. While the Sponge City guidelines address some of these issues, it is predominantly locally scaled with consideration of regional and national legislation. The role of municipal or subprovincial government in creating the link between local-scale drainage schemes and municipal-scale infrastructure will therefore be critical in addressing larger catchment considerations.

In this paper, developments in Ningbo have been used to illustrate how national guidelines are being interpreted and applied. Early indications are encouraging, as municipal and subprovincial governments appear to be taking the opportunity to perform thorough reviews of existing drainage design, and engineering standards and practices. Significant barriers remain, however, before the aims of the Sponge City initiative can be fully met. In particular, the transformation of old building stock and associated drainage infrastructure [33] will be hindered by planning requirements for land modernization, pressure from land scarcity, land price inflation and unresolved ownership issues.

A sign that progress is being made in sustainable urban drainage in China is the fact that while overseas expertise in LID, SUDS and WSUD is frequently referenced, examples of best practice are increasingly being drawn from national rather than international examples [34]. This is particularly true in the area of infrastructure performance measurement and cost–benefit analysis—all of which bode well for the encouragement of future investment needed to sustain the initiative into the future. Indeed, as the PPP model of investment becomes less assured, new models for financing new development will be key in the delivery of national Sponge City aims.

Data accessibility

The article has no additional data.

Authors' contributions

All authors contributed to the development and design of the research described in this article. All authors contributed to the preparation of this article. J.G., F.K.S.C. and M.S. undertook the initial review of Sponge City guidance and related legislation. They were assisted in interpretation and description of guidance documents and case studies by F.Z. J.G. and D.L.H. drafted the discussion and conclusion. All authors contributed to the final preparation of the article.

Competing interests

We declare we have no competing interests.

Funding

This work was supported by funding from the Ningbo Social Development Project, Ningbo Science and Technology Bureau, China (grant no. 2014C50011/201401C5008005), the National Natural Science Foundation of China (NSFC) (grant no. 41850410497) and the Research grant on the Strategy and Practice of the Development of Sponge City in Ningbo New Eastern Town funded by the Ningbo Housing and Urban-Rural Development Bureau.