Front. Earth Sci. Frontiers in Earth Science Front. Earth Sci. 2296-6463 Frontiers Media S.A. 1238203 10.3389/feart.2023.1238203 Earth Science Review Exploration of sponge city construction in China from the perspective of typical cases Shang et al. 10.3389/feart.2023.1238203 Shang Shouwei 1 Wang Leizhi 1 * Wang Yintang 1 Su Xin 1 Li Lingjie 1 Xia Xiaohan 2 1 Key Laboratory of Flood and Drought Disaster Defense, The Ministry of Water Resources, Nanjing Hydraulic Research Institute, Nanjing, China 2 School of Foreign Languages, Sun Yat-sen University, Guangzhou, China

Edited by: Xihui Gu, China University of Geosciences Wuhan, China

Reviewed by: Wei Si, Hohai University, China

Ting Zhang, Tianjin University, China

Yun Chen, China Three Gorges University, China

 *Correspondence: Leizhi Wang, wanglz@nhri.cn 21 07 2023 12 08 2026 2023 11 1238203 11 06 2023 13 07 2023 Copyright © 2023 Shang, Wang, Wang, Su, Li and Xia. 2023 Shang, Wang, Wang, Su, Li and Xia

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Night years have witnessed the construction of sponge city in China. In recent years, more frequent extreme rainstorm has put more pressure on urban flood control and waterlogging elimination, and the construction of sponge city has been facing many controversies. In response, this study discusses the concept, connotation, evaluation indexes, construction objectives, investment and benefits of sponge city construction. The key point is that the concept of the sponge city has expanded from the construction of low-impact development to a new model of urban development and construction, with an emphasis on improving the overall function of the city and enhancing the comprehensive response to urban water issues. Its construction benefits cannot be measured only in terms of flood control and waterlogging prevention. Besides, the biggest problem faced by cities is still flooding. The priority of building a sponge city is the prevention of waterlogging. Meanwhile, the integrity and systematic construction which integrates multiple sectors, industries and fields should be considered. The government should focus on the combination of ecological methods and engineering facilities, and scientifically promote the systematic management of urban water.

 sponge city urban water issue construction objectives construction benefits rainstorm waterlogging section-at-acceptance Hydrosphere 1 Introduction

Rapid urbanization in China has led to a series of “urban diseases”, such as urban heat island effect, water environment pollution and water ecological degradation (Yoshikoshi et al., 2009; Park et al., 2020). Moreover, climate change and human activities have brought frequent extreme hydrometeorological events, contributing to the worsening urban water situation in China. In particular, rainstorm waterlogging is quite serious, resulting in frequent “urban sea-watching phenomenon” (Jia, 2017) and tremendous economic losses in events such as the Beijing “7.21” flood in 2012, the Wuhan “7.6” flood in 2016, the Jinan “7.18” flood in 2016, the Nanning “3.25” flood in 2020 (Yuan et al., 2018), and the Zhengzhou “7.20” flood in 2021 (Li et al., 2022). For this reason, a large number of scholars have carried out continuous research on urban waterlogging panoramic simulation and risk management (Wu et al., 2021; Ge et al., 2022; Zhang H et al., 2023). However, the numbers of cities and people affected by floods in China are predicted to increase with climate change (Tellman et al., 2021). Hence, issues of flood control and drainage have become the focus of the management of urban water systems.

In recent years, thirty pilot cities have pumped more than 160 billion into sponge city construction (Ren et al., 2020); however, most pilot cities still suffer from serious urban waterlogging after extreme rainstorms, such as Zhengzhou, Nanning, and Wuhan. It is indicated that sponge city construction has achieved little in urban flood control and waterlogging elimination. At the same time, the construction of sponge city involves multiple sectors, industries and disciplines, while different industries and disciplines have different definitions of the sponge city. It triggered a series of debates and disagreements, mainly on the construction objectives, models and cost-benefit of sponge city (Hou et al., 2020).

Over the past decade, the connotation of sponge city has changed from the low-impact development to the comprehensive enhancement of the fundamental functions of the urban ecological system. Meanwhile, the main goal of sponge city has also been developed from urban flood management to the comprehensive management of water environment, water ecology, water resources and water security. Sponge facilities can mitigate urban flooding and store storm water to some extent, but they cannot assume the main role of flood control and drainage (Xu et al., 2020). Therefore, this study identifies the concept in Section 2, discusses the objectives and evaluation indexes of sponge city in Section 3, and makes a statistical analysis of sponge city planning, construction and financial investment in each city in Section 4. As this study elaborates on the construction objectives, connotations and functions of sponge city, it is of great importance to the scientific understanding and effective construction of sponge city.

 2 Concept of sponge city

Highly urbanized cities may face aggravated urban water issues. Since the 1970s, western countries have tried to systematically solve a series of water issues concerning water resources and water environment in these cities. After nearly 50 years, a control system centered on urban stormwater management has been developed (Eckart et al., 2018).

With the increasingly prominent urban waterlogging in China, the concept of sponge city was put forward with the experience of foreign cities in urban rainwater management as a reference. In December 2013, China proposed to “build a sponge city with natural accumulation, natural infiltration and natural purification”. That was the first time that the concept of sponge city appeared in the national documents. In November 2014, the Ministry of Housing and Urban-Rural Development (MOHURD) of China issued the Sponge City Development Technical Guideline: Low Impact Development (hereinafter referred to as the Guideline). It can be seen that the concept of the sponge city was first associated with low-impact development. Subsequently, scholars from different disciplines and fields have explained this concept (Table 1). Since then, the connotations of the sponge city have been continuously expanded.

Comparison of sponge city concepts.

Scholars/Resources Date Concepts
The MOHURD November 2014 The city is constructed like a sponge, with good “resilience” in adapting to environmental changes and coping with natural disasters, which can absorb, store, seep and purify water during precipitation. It is ready for use, and is of low-impact
Qiu January 2015 The essence is to change the traditional concept of urban construction and follow a low-impact development model, that is, in harmony with nature. Sponge city construction is also known as low-impact design and low-impact development (Qiu, 2015)
Che et al April 2015 The “grey” and “green” infrastructures should be constructed as resilient infrastructures for rainwater. It can use water efficiently and enable cities to handle extreme rainstorm as well as climate change, maintaining urban ecosystems (Che et al., 2015)
Yu et al June 2015 The “sponge city”, as an ecological approach of urban water management, is to construct multi-scale hydro-ecological infrastructures (Yu et al., 2015)
The MOHURD October 2015 The sponge city construction should take the measures of “infiltration, detention, retention, purification, utilization and discharge” to minimize the impact of urban development and construction activities on the ecological environment
Wang September 2017 In the sponge city, there is no waterlogging, and the rainstorm-related pollution is under control. Rainwater can be reused. The fundamental philosophy is matching the sky area with the ground area, that is, absorbing rainwater over the local area, which emphasizes local adaptation (Wang et al., 2017)
Ren July 2017 As a new urban development construction concept and model, the sponge city is part of the comprehensive improvement of urban water resources and water environment, focusing on solving urban waterlogging and urban water environment deterioration (Ren et al., 2017)
Xia October 2017 The construction of sponge city requires a unified planning, clear interconnection and coordination of sponge measures and relationships among different departments (Xia et al., 2017)
The MOHURD December 2018 The sponge city construction follows the concept of “source reduction, process control and systematic remediation”, emphasizes the roles of urban water ecology and water environment function, and highlights the overall improvement of urban area function
The Ministry of Finance, jointly with the MOHURD and the Ministry of Water Resources of China April 2021 The sponge city coordinates the construction of drainage and flood control facilities, urban water environment improvement, urban ecological restoration, green space constructionetc.

In October 2015, the General Office of the State Council of China issued the Guiding Opinions on Promoting the Development of Sponge City (hereinafter referred to as the Opinion), presenting the comprehensive technical measures of “infiltration, detention, retention, purification, utilization and discharge” to minimize the impacts of urban development and construction activities on the ecological environment. The Opinion explicitly proposed to update the concept of urban planning and construction, and insisted on the combination of ecological methods and engineering facilities. In 2018, the MOHURD of China issued the Assessment Standard for Sponge City Development Effect (hereinafter referred to as the Standard), which put forward the sponge city concept of “source reduction, process control and systematic remediation”. It emphasizes the role and function of urban water ecology and water environment, and highlights the overall function improvement of urban areas. In April 2021, the Ministry of Finance, jointly with the MOHURD and the Ministry of Water Resources of China, issued the Notice on Systematic and Territorial Demonstration of Sponge City Construction (hereinafter referred to as the Notice). It clearly proposed that the construction of new urban areas should be goal-oriented, and that of the old urban areas should be problem-oriented to coordinate the construction of drainage and flood control facilities, urban water environment improvement, urban ecological restoration and green space construction.

Therefore, the concept of sponge city has been constantly developing based on practice, evolving from “building a comprehensive rainwater management system with low-impact development” (building a low-impact development rainwater system) to “changing the concept of urban construction and development” (improving the overall function of urban areas). In essence, it consists of three aspects. First, the sponge city concerns the harmonious relationship between urbanization and the ecological environment. Second, the sponge city construction aims to make cities more resilient in adapting to environmental changes and coping with natural disasters. Third, it shifts the way to deal with flood prevention and control and advocates building a low-impact system of rainfall and flood management (Zhang, 2015). Therefore, at this stage, sponge city is essentially a change in the model of urban planning and construction.

 3 Objectives and content of sponge city construction

While the connotation of a sponge city is expanding, the evaluation indexes of sponge city construction in China are also being constantly improved. Table 2 presents the changes in the development of evaluation indexes of the sponge city in China. The Guideline issued by the MOHURD in 2014 marked the first time that China proposed a sponge city control index system at the government level. The Guideline emphasized the principles of low-impact development and ecological priority, and it pointed out that sponge city in China take urban rainwater and flood management as the control objectives, including planning control indexes of runoff volume, peak runoff and runoff pollution, and rainwater utilization. The Guideline divided China into five regions and clarified the application scope of indexes. However, these indexes are simple and imprecise, and they lack consideration of local conditions. In 2015, the MOHURD issued the Measures for Performance Evaluation and Assessment of Sponge City Development (Trial) (hereinafter referred to as the Measure), where the eighteen indexes fall into six categories covering water ecology, water environment, water resources, water security and display of effect. The Measure is used to systematically assess the effectiveness of sponge city construction. However, urban stormwater management is mainly judged by the total annual runoff volume capture ratio, rainwater resource utilization and urban stormwater flood control. The indexes in Measure are more specific than the Guideline, while the focus has changed, with more emphasis on water ecology and water environment. In August 2015, the Ministry of Water Resources issued the Guideline of the Ministry of Water Resources on Promoting the Water Conservancy Construction in Sponge City, which put forward the main indexes for water conservancy construction in sponge city. Compared with the Measure, it includes two new indexes of soil erosion control rate and flood control standards. The Standard promulgated by the MOHURD in 2018 is the first national standard on sponge city construction in China. It further described the indexes, clearly subdivided the annual runoff volume disposal into old urban built-up areas and newly-built areas, and made detailed provisions for the total annual runoff volume in different areas and places. The actual concentration abatement target was added to the water environment evaluation. Compared with the Measure, the Standard further improves and strengthens the control indexes, takes into account the differences in the urgency of different cities to achieve the central objectives, and emphasizes the localization and precision of the measures. The Notice issued in 2021 further stressed the display of effect and explicitly stated that every construction of an area should be effective, with an emphasis on the overall effect of sponge city construction.

Changes in sponge city evaluation indexes.

Documents Categories Evaluation indexes Differences
Sponge City Development Technical Guideline: Low Impact Development Total annual runoff volume capture ratio, peak runoff and runoff pollution control targets, rainwater utilization
Measures for Performance Evaluation and Assessment of Sponge City Development (Trial) Water ecology Total annual runoff volume capture ratio, ecological shoreline restoration, groundwater level, urban heat island effect The 18 evaluation indexes of the sponge city are subdivided into six categories, focusing on the water environment and water ecology
Water environment Water environment quality, urban surface source pollution control
Water resources Wastewater recycling rate, rainwater resource utilization rate, pipe network leakage control
Water security Urban rainstorm water-logging disaster prevention and control, drinking water safety
System construction and implementation Planning and construction control system, blue and green line delineation and protection, technical specifications and standards construction, investment and financing mechanism construction, performance assessment and incentive mechanism, industrialization
Effect Multi-regional demonstration effect, more than 60% areas meeting the standard
Guideline of the Ministry of Water Resources on Promoting the Water Conservancy Construction in Sponge City Flood control standard, rainfall retention rate, water area rate, surface water quality standard-reaching rate, rainwater resource utilization rate, recycled water utilization rate, levee standard-reaching rate, flood discharge standard-reaching rate, river and lake ecological protection ratio, groundwater depth, soil erosion control rate Two new indexes have been added, which are soil erosion control rate and flood control standard
Assessment Standard for Sponge City Development Effect Total annual runoff capture ratio New construction and alterations: shall not be lower than the lower limit in the area on the total annual runoff control zoning map Based on the water Provisions of water ecology, water environment, and water security are made clear in the Measures for Performance Evaluation and Assessment of Sponge City Construction (Trial)
Source reduction Total annual runoff pollutants (Suspended solids) abatement rate ≥70% for new buildings, complexes, roads and squares, and ≥40% for reconstruction; total park runoff volume capture ratio ≥90% for green spaces
Waterlogging and flood control Reasonable connection of grey and green infrastructures, no ponding or waterlogging with respect to precipitation under the design return period
Ecological shoreline protection of water bodies Water area should not be reduced, and the proportion of ecological shoreline is no less than 70%
Environmental quality of urban water bodies There is no direct discharge of sewage or wastewater in dry days. The annual overflow volume control rate of the combined sewer overflow discharge outlet in rainy days is ≥50%, and the water body is not black or odorous
Trends in groundwater burial depth, urban heat island effect mitigation
Notice on Systematic and Territorial Demonstration of Sponge City Construction Display of effect Successful construction of each area Emphasis on the successful construction of each area

The changes in evaluation indexes show that the construction of sponge city in China is not limited to low-impact development, and it emphasizes more on the improvement of overall urban functions. It is clear that these evaluation indexes have been improved. However, there are still some problems, such as the disconnection among multiple disciplines and fields, lack of correspondence between objectives and indexes, insufficient display of effect, and fragmented construction.

 4 Typical cases of sponge city planning and construction 4.1 Construction objectives and targets of typical cities

In 2015 and 2016, a total of thirty cities including Qian’an, Baicheng, Fuzhou, Beijing and Xining were selected as pilot sponge cities in China, and another 20 cities including Guangzhou, Wuxi, and Suqian were selected as national demonstration cities in 2021 to systematically promote the whole construction of sponge city. Table 3 shows the distribution of these cities. The climate zone and city scale of pilot cities show that the construction of sponge city is not limited to the highly urbanized areas in East China, and they are also distributed in the arid areas in the hinterland of China. In addition to these pilot cities, more than three hundred cities, such as Nanjing, Zhongshan, Chengdu, and Zhuzhou, are also constructing sponge city, and China has entered a period that the construction of sponge city is undergoing throughout the country.

Distribution of pilot sponge cities.

Classification Grading Number of pilot cities Number of demonstration cities promoting construction in the whole region
Climate zone Humid 18 13
Semi-humid 12 7
City scale Large cities 14 4
Medium cities 9 10
Small cities 7 6

Table 4 presents the control indexes of sponge city planning for large, medium and small cities in China in four aspects: water ecology, water security, water environment and water resources. The indexes include total annual runoff volume capture ratio, waterlogging prevention and control standard, rainwater pipe and drainage design standard, flood control standard, surface source pollution control and rainwater resource utilization rate, etc. Among the cities in the statistics, four are coastal cities; five are northern cities; four are inland cities; five are southern cities. Nine of the above cities are national pilot cities and two are national demonstrations, so these cities are represented in terms of spatial distribution.

Main controlling indicators of typical cities.

City scale City Water ecology Water security Water environment Water resources
Total annual runoff volume capture rate Flood control standard Stormwater drainage design standard Flood control standard (central city area) Surface source pollution control Rainwater resource utilization
Large Cities Shanghai ≥75% Once in 100 years ≥5 years in the main urban area, 30 years for sunken buildings Meeting planning requirements ≥75% ≥5%
Shenzhen 70% Once in 50 years 3–5 years, ≥10 years for especially important areas Once in 200 years 100% Short-term: 1.5%
Long-term: 3%
Guangzhou 70% Once in 50 or more years 5–10 years, 30–50 years for sunken buildings Once in 200 years New construction areas: >50% >3%
Rebuilding areas: >40%
Zhengzhou 75% Once in 20–50 years 3–5 years, 30 years for sunken buildings Once in 200 years >50% >5%
Shijiazhuang Old urban areas: 70% Once in 20–50 years 3–5 years Once in 200 years ≥50% 7.5%
In new urban areas: 75%
Wuhan 60%–85% Once in 50–100 years 10 years for trunk roads, 20 years for expresswaysetc. Once in 200 years ≥50% ≥5%
Qingdao 75% Once in 50 years Once in 100 years 2020: 50% 2020: 5%
2030: 65% 2030: 8%
Nanjing Built-up areas: 70%–75% Once in 50 years 3–10 years, 30–50 years for sunken buildings Once in 200 years 50% 2020: 2%
New construction areas: 80%–85% 2030: 5%
Beijing 72.6% Once in 50–100 years 20–100 years Once in 200 or more years ≥60% 2020: 8%
2035: 10%
Chongqing ≥70% Once in 50 years 5–10 years, 50 years for sunken buildings Once in 100 years ≥50% localization
Medium cities Xining ≥85% Once in 50 years 2–5 years Once in 50–100 years ≥60% ≥2%
Huai’an 70% Once in 30 years 3–5 years Once in 100 years ≥45% ≥3%
Suqian 70% Once in 30 years 2–5 years, 20 years for sunken buildings Once in 100 years 40% ≥3%
Qinzhou 70% Once in 50 years Once in 100 years 40% 0.9%
Xiangtan 75% Once in 30 years 2–5 years, 20–30 years for sunken buildings Once in 100 years 55%
Zhuhai 80% Once in 30–50 years 3–5 years, 20 years for sunken buildings Once in 100 years Short-term: 35% Short-term: 5%
Long-term: 40% Long-term: 10%
Zhuzhou 80% Once in 30 years 3–10 years, 20–30 years for underground buildings Once in 100 years Short-term: 45% ≥4%
Long-term: 60%
Small cities Pingxiang 75% Once in 50 years 2017: 50% 2017: 12%
2030: 75% 2030: 15%
Hebi 70% Once in 30 years ≥2 years Once in 20–100 years 70% 1.1%
Yiyang Short-term: ≥75% Once in 30 years 2–5 years, 20 years for sunken buildings Once in 50–100 years 45% Short-term: 1%
Long-term: ≥80% Long-term: 3%

From the main control indexes of each city, the construction of the sponge city in different cities focuses on water security, water ecology, water environment and the function of water resources. Besides, some cities take into account the water sight and water culture, such as Zhengzhou, Guangzhou, Yiyang, Zhuhai, Zhuzhou, and Pingxiang. Among the nineteen cities in the statistics, seven cities, including Shenzhen, Beijing, Pingxiang, and Zhuhai, have proposed detailed near-term and long-term implementation targets, while Shijiazhuang, Nanjing and Guangzhou have further proposed construction targets for different areas on this basis, considering the differences between new and old urban areas. In addition, each city’s Special Plan has proposed a clear time node, that is, the total annual runoff in 20% (80%) of the built-up area should meet the control rate requirements by 2020 (2030). In short, at this stage, the sponge city constructions have not yet formed a whole. Sponge infrastructure is scattered, and the fragmented construction is serious. Therefore, the construction cannot play the overall effect of sponge infrastructure, and the effect of sponge city construction is not as expected.

From the main control indexes of each city mentioned above, we can find that sponge city construction has different focuses in different cities, and each city considers regional differences in terms of control indexes as well as construction objectives. At the same time, water culture-related content is further incorporated into sponge city planning, so the connotation of the sponge city is constantly expanded during the construction.

 4.2 Construction content of typical sponge cities

The construction programs for water security, water resources, water ecology and water environment in the Special Plan of each city are shown in Table 5. For water security, the sponge city construction in each city contains three major approaches of ecological protection, ecological restoration, and low-impact development. The construction of rainwater drainage channels and the improvement of urban drainage systems are the priorities of construction. Meanwhile, the integration of gray and green infrastructures is emphasized. Zhengzhou and Xiangtan proposed to build flood protection dikes and consolidate river embankments based on their needs. Guangzhou, Zhuhai, and Yiyang focus on the management of waterlogging-prone spots in urban area. At the same time, these cities are restoring inland water systems, building natural water storage systems in wetland parks, and enhancing rainwater use and control as non-engineering measures to ensure water security. For water resources, engineering measures (such as improving and expanding the urban water supply system and building rainwater storage tanks) and non-engineering measures (such as adjusting the water source structure and optimizing water resource allocation) are the main construction content, aiming to improve the urban water resource security rate. For water ecology, the cities focus on the “integration of gray and green infrastructures”. Shenzhen, Shijiazhuang, Beijing and other cities combine the construction of ecological riverbanks and sunken green areas with the strengthening of water system protection, regional runoff control and the construction of green space systems to jointly safeguard urban water ecology. The eleven cities in the statistics have taken measures to improve the quality of the urban water environment and achieve the water environment construction objectives by using rainwater and sewage diversion renovation, building and expanding sewage recycling and treatment systems, conducting water treatment, restoring ecological wetlands and replenishing ecological water.

Engineering and non-engineering measures for the construction of the typical sponge cities.

City Water security Water resources Water ecology Water environment
Engineering measures Non-engineering measures Engineering measures Non-engineering measures Engineering measures Non-engineering measures Engineering measures Non-engineering measures
Shenzhen Floodway construction River cross-section regulation, rainwater drainage system construction New storm sewers and reservoirs Unconventional water resources utilization, adjustment of water source structure Construction of ecological river cross-sections Conservation of green space systems and water system protection Construction of drainage pipelines, diversion of rain and sewage Wetland purification and runoff control
Zhengzhou Strengthening river embankments, building flood storage and detention areas and building drainage facilities 191 floodways are planned Construction of water supply pipe network Optimization of the allocation of water resources and the use of unconventional water resources \ \ Rainwater reservoirs, sewage treatment plants Comprehensive river course regulation and revitalized water flow project
Shijiazhuang Construction of flood control facilities and improvement of drainage systems Restoration of inland water systems, optimization of drainage zoning, and strengthening of Blue Line management \ \ Construction of ecological river cross-sections Regional runoff control, water system protection Storm sewage diversion project Malodorous black water treatment
Guangzhou Construction of drainage facilities and treatment of waterlogging-prone spots Sponge and wetland park construction Renovation and expansion of water plants and reclaimed water plants Ecological wetlands, ecological corridors \ River ecological remediation Construction or expansion of a sewage treatment plant River remediation, constructed wetlands
Xicheng District, Beijing Permeable pavement renovation, renovation and expansion of drainage systems Stormwater control and utilization Renovation of water supply pipe network and improvement of reclaimed water system Optimization of the allocation of water resources Shoreline landscape construction Strengthening the management of blue lines of rivers and lakes, preventing and controlling soil erosion comprehensive improvement of old residential areas, renovation of shanty towns Control of non-point source pollution
Huai’an Construction of flood control and drainage facilities Strengthening the rainwater regulation and storage, and restoring the inland river system Construction of rainwater harvesting facilities Improving the utilization of unconventional water resources Construction of ecological river cross-sections Strengthening the water system protection and regional runoff control Construction of rainwater sewage diversion system and sewage pipeline network Smooth flow of running water, malodorous and black water treatment
Xiangtan Construction of flood control dikes, drainage facilities, and floodway construction Adjusting floodways and inland river connecting and dredging Construction of sewage treatment plants and rainwater utilization facilities \ Building ecological barges and sunken green spaces Comprehensive improvement of river courses and restoration of bank zones New rainwater sewage diversion pipe network and sewage pipe network Ecological restoration, comprehensive improvement of river courses
Zhuhai Construction of embankments and waterlogged spot treatment, and renovation of the drainage pipe network Establishing water source protection zones and building coastal ecological coastlines Renovation and expansion of green space systems and rainwater harvesting systems Utilization of unconventional water resources New reservoirs and regulating ponds Construction of forest for water source conservation and restoration of wetlands Construction of sewage interception ditches River system dredging, malodorous and black water body remediation
Zhuzhou waterlogging-prone spots rectification, pipe network upgrading, 34 sewage pump stations construction Comprehensive treatment of flood control water system Construction of reclaimed water facilities and rainwater storage facilities Utilization of unconventional water resources Sunken green space, permeable paving, storage facilities \ Storm sewage diversion retrofit Remediation of malodorous and black water body, whole-process control of rainwater pollution
Lishui Construction of the drainage system and rainwater runoff drainage system exceeding the standard Multi-level rainwater storage and source regulation Water supply pipe network, secondary water supply renovation Unconventional water utilization, control of pipe network leakage, strengthening the technical archives management Construction of urban artificial lakes and dredging of river courses Returning fishing to lakes, returning farmland to lakes and transforming ecological shorelines Construction of rainwater sewage diversion, sewage treatment facilities, and rainwater storage tanks Street dredging, ecological restoration, water diversion, and replenishment
Yiyang Floodway construction, renovation of drainage systems, and treatment of waterlogging points River course rehabilitation Construction of a reclaimed water reuse system Establishing water source protection areas and emergency water source areas \ \ Construction of sewage treatment plants, rainwater and sewage diversion Remediation of malodorous and black water body, wetland construction

The above analyses and practices show that the current measures of sponge city construction focus on green infrastructures like ecological restoration and water environment management, and very few engineering measures such as drainage system construction are involved. Therefore, the construction of sponge city deviates from the original objective of flood control and waterlogging elimination, and is more inclined to restore the urban water environment and water ecology. However, the urgent issue that needs to be tackled in the current urban development process is the urban flooding issue. The “urban sea-watching” is essentially a failure of the urban flood control and drainage system (ZHANG C et al., 2023). The current sponge city construction focuses on the source control system. Most of these constructions are “green infrastructure”, while urban drainage and de-risking rely more on the construction of “gray infrastructure” such as drainage networks. In addition, the flood control capacity of the source control system is limited (Feng et al., 2021), and the construction of sponge city in part area (with the old urban areas focusing on the management of waterlogging points) leads to the fragmentation of sponge city construction and planning (Ren, 2018). The overall urban flood control and drainage system cannot be changed. Thus, the current sponge city construction can only solve the waterlogging caused by poor drainage in local areas, while it is difficult to cope with the urban flooding under extreme rainfall. Therefore, at this stage, we should steadily and scientifically promote the construction of sponge city, reasonably plan the construction content and continuously optimize the configuration of “sponge infrastructures”.

The investment in sponge city mainly involves seven aspects, including road traffic system, urban water system, building district renovation, garden and green space system, water supply and drainage system, capacity building and related projects outside the built-up areas. The investment of each city is shown in Table 6. It is found that except for Shijiazhuang, other cities mainly invest in road traffic system, urban water system and garden and green space system, accounting for more than 75% of the total investment. The proportion of investment in road traffic system exceeds 50% in Jinan and exceeds 25% in Zhengzhou, indicating that the sponge city construction of these two cities is mainly in urban infrastructure construction. Shijiazhuang’s investment is mainly concentrated in the renovation of building districts and water supply and drainage facilities, accounting for 69.3% of the total investment. Then, the investment in sponge city construction is mainly concentrated in “sponge facilities”, while the investment in disaster prevention and mitigation capabilities such as monitoring and early warning accounts for less than 1% of the total funds. At the same time, the “sponge facilities” are mainly concentrated in three aspects of green space square, urban roads and urban water system, where 48 of the 68 sponge projects in Hebi and 38 of 58 projects in Xixian New Area are green space, roads, and urban water systems. There is basically no project related to capability building. Therefore, the proportion of investment in sponge city construction is unbalanced, and there is a lack of financial support in the forecasting and early-warning operation and the urban hydrological system research that facilitate the development and construction of sponge city, which is precisely the basis for understanding the urban hydrological cycle and enhancing source control (Bell et al., 2016; Pande and Sivapalan, 2016).

Sponge city construction investment (Unit: 100 million yuan).

Cities Road traffic system Urban water system Building district renovation Garden and green space system Capacity building Related projects outside the built-up areas Water supply and drainage system Total
Jinan 76.40 35.00 8.00 2.80 2.60 16.00 140.80
Zhengzhou 137.10 138.10 57.50 139.90 1.00 61.20 534.80
Shijiazhuang 5.70 2.87 30.63 2.88 0.54 11.71 54.33
Xicheng District, Beijing 62.08 0.60 4.10 66.78
Wuhan 24.90 85.60 27.80 8.50 13.00 159.80
Suqian 7.31 81.64 24.96 0.89 21.13 135.93

The construction of sponge city is mainly based on water environment governance and water ecological restoration. Its content, such as water system management and green park construction, is mainly to improve the overall function of the city, and the construction of urban flood control and waterlogging elimination system (drainage facilities) accounts for a very low proportion of investment. Therefore, the effectiveness of sponge city construction cannot be assessed only from urban waterlogging elimination. In addition, sponge city construction overlaps with other urban planning. Most of the investment in current urban construction is categorized as the investment in sponge city construction. For example, the road traffic system investment accounts for more than 50% in Jinan and more than 25% in Zhengzhou, and the renovation of old residential areas in Wuhan accounts for more than 50%. Whether the construction of all these infrastructures can be counted as an investment in sponge city remains controversial. Therefore, we cannot just consider the investment in sponge city and a specific aspect when assessing the benefits of sponge city. Sponge infrastructure and ecological rivers play an important role in building new urban water systems, and sponge facilities and ecological restoration involve benefits from environmental, economic as well as social aspects. Therefore, when measuring the effectiveness of sponge city, we cannot just consider flood control and waterlogging elimination or deny the benefits of sponge city just because waterlogging occurs in the city.

 5 Conclusion and suggestions 5.1 Conclusion

By reviewing the development history of sponge city concept in China and combining the construction content, measures and investment of typical sponge city, this paper expounds on the essence of sponge city in China, and draws the following two conclusions:

First, a scientific understanding of the sponge city is essential. Sponge city is not the same as low-impact development, and its concept has developed from low-impact rainwater system construction into new urban construction. It is a new model of integrated urban water system management, which is designed to promote the harmonious development of urbanization, water resources and water ecology, aiming to further enhance the overall function of the city and make the city “resilient” to coping with natural disasters.

Second, the construction of sponge city is to systematically tackle the water issues in China’s cities and to integrate rainwater management into the whole urban development and construction. And it can effectively combine flood and waterlogging control, water resources utilization, water environment protection and water ecological restoration to further enhance the comprehensive response to urban water issues.

 5.2 Suggestions

Throughout the development of China’s sponge city, the construction of sponge city has achieved remarkable results in the urban water ecological environment. However, there are still some problems in urban waterlogging control, multi-department and multi-level coordinated promotion. Based on this, two suggestions are put forward on promoting sponge city.

First, at the current stage, the construction goals, indexes, content and investment in sponge city are relatively broad, and are easily confused with other municipal construction content, thus leading to the “fragmentation” of sponge city system construction. Hence, we should scientifically promote the construction of sponge city, give priority to solving urban flooding, reasonably lay out the “sponge facilities” and give full play to their overall advantages.

Second, the fundamental problem of sponge city construction is the systematic governance of water. It not only involves the city itself, but also the watershed and the region. Therefore, in sponge city construction, we should emphasize the overall concept, coordinate different sectors,

 Author contributions

SS: Information collecting, Writing—original draft. LW: Conceptualization, resources, writing—original draft, research design. YW: Resources, writing—review and editing. XS: Writing—review and editing. LL: Writing—review and editing. XX: Writing—review and editing. All authors contributed to the article and approved the submitted version.

 Funding

The research was funded by National Key R&D Program of China (2021YF3000101-02), the National Natural Science Foundation of China (52109028, 52239008), the Natural Science Foundation of Jiangsu Province (BK20210042).

 Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

 Correction note

This article has been corrected with minor changes. These changes do not impact the scientific content of the article.

 Publisher’s note

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