When we think of the city of Shenzhen, it is the epitome and symbol of China’s opening up to the outside world, it has created a miracle in the history of industrialization, modernization and urbanization development in the world, and it always ventures to lead the world.

After forty-year high-speed development, the 1997 km² land carries a GDP of over 2.2 trillion RMB and a population of 20 million, which means that the demand for water resources far exceeds its carrying capacity; moreover, with an average annual rainfall of over 1900 mm and more frequent extreme weather in recent years, it seems that the rivers are too short and the environmental capacity too weak; though it is known as an “international garden city” and “Ecological Garden City”, Shenzhen still suffers from pressing water problems: flood control and water shortage.

Background

Climate and Hydrogeology

The city of Shenzhen is situated in the southeast of China, to the north of Hong Kong and south of Dongguan and Huizhou. It has a warm, monsoon-influenced, humid subtropical climate; the rainy season lasts until late September to early October. The monsoon reaches its peak intensity in the summer months; the region is prone to torrential rain as well. The groundwater sub-basins coincide with the surface water catchments throughout the study area (Lancia et al., 2019). The regional groundwater table follows the morphology, with gentler gradients. From high points, groundwater flows radially away, and feeds streams and rivers or directly discharges into the sea.

Figure 1. Hydrogeological map and sections of Shenzhen

Water Supply

Shenzhen relies strongly on the external water supply. By 2020, more than 88% of Shenzhen’s water needs to be brought in from outside the province, supplied by the Dongshen Water Supply Project and the Dongjiang Water Source Project. (Water Resources Bureau of Shenzhen Municipality #)

Among them, the Dongshen Water Supply Project’s water transmission line is 68 kilometers long, with an annual water supply capacity of 2.423bn cubic meters per year, and the amount of water allocated to Shenzhen is 8.73bn cubic meters; the total length of the water transmission line of the Dongjiang Water Source Project is 106 kilometers, all of which supply water to Shenzhen, with an annual water supply capacity of 720mn cubic meters/year. In total, water supply from surface water sources was 189.41bn m³, accounting for 93.66% of the total water supply, while the percentage for groundwater sources is 0.13% and for non-conventional sources is 6.21%. (Water Resources Bureau of Shenzhen Municipality #)

Figure 2. (a) Map of the Dongjiang River Basin (b) Distribution of the Reservoirs, Waterworks, Pumping Stations, Gates and Water Supply Pipelines in Shenzhen

Major Challenges

The rapid growth in Shenzhen means that demand often outstrips supply. As a result, many water challenges now plague the city. It has been estimated that Shenzhen will face water shortages of 690 and 890 million cubic meters by 2020 and 2030 respectively. (De Vette) 

The more alarming issue is that, due to both higher hazard intensity and socioeconomic exposure, Shenzhen ranks at the top of waterlogging risk in Eastern China cities, (Sun et al. #) according to an indicator system for waterlogging risk assessment based on hazard, exposure and adaptability factors.

 Figure 3. Map of Risk Rating of Waterlogging Risk of Perfect-level Cities in Eastern China

In short, the main challenges for water management in Shenzhen are flooding and future water shortages, the causes of which seem hardly related to each other. The causes of flooding are classified as the following fives: meteorological changes, deterioration of the natural environment, inefficient infrastructure, lack of construction planning, and ineffective urban management. As for the water shortage, the main causes are the excessive dependence on external water sources and the continuous high growth of water demand.

Based on the previous analysis, it seems impossible to solve both problems at the same time, until one realizes the very essence of them – flooding means too much water, and water scarcity means too little water. So, there must be some way to store the excess water from floods and reuse it when droughts come.

Figure 4. Mind Map of Major Challenges for Water Management in Shenzhen

Sponge City Strategy

Based on the above-mentioned problems and challenges currently faced by Shenzhen, the Shenzhen government officially declared itself as the second batch of national pilot cities for sponge city construction in April 2016. The aim was to construct an international sponge city, to systematically solve Shenzhen’s water problems, and to make Shenzhen the most dominant city in China for sustainable water development. Moreover, after understanding and analyzing the special planning and implementation plan for sponge city construction proposed by the Shenzhen government, the authors will also propose a renovation strategy that is conducive to sponge city construction for the current high-density residential communities unique to Shenzhen.

Opportunities

The opportunities that Shenzhen currently possesses became the determinant for the government and the authors to propose sponge city strategies for different scales of urban construction. They are:

1. Shenzhen’s abundant rainfall throughout the year will hopefully become a supply of non-conventional water resources (stormwater, seawater, recycled water, etc.) for the city, solving the problem of water shortage and over-dependence on unlocal water resources due to Shenzhen’s explosive population growth while solving the city’s extreme vulnerability to flooding.
2. Existing urban environmental problems (e.g. water, but also many other problems such as air quality) are so urgent that the government has provided significant investment resources to attempt to find solutions to these problems.
3. In order to become a leading sustainable city both domestically and internationally, Shenzhen has the ambition to take a leading role in science and technology. As a result, the government has significantly increased its budget for sustainable development construction and has increased its involvement in international research.

Design Strategies

The authors propose the Shenzhen Sponge City Design Strategies System based on the “Shenzhen Sponge City Construction-Special Plan and Implementation Plan (2016)” released by the Shenzhen government, including macro, meso, and micro levels (Figure 1).

Figure 5. Sponge City Strategies System

• Urban Scale

The “Shenzhen Sponge City Construction-Special Plan and Implementation Plan” provided a unified sponge city strategies design for the whole city of Shenzhen. Based on the current situation of groundwater level in Shenzhen (Figure 6) and the analysis of sponge bases such as mountains, water, forests, fields and lakes, the current spatial layout and characteristics of sponge bases were identified, and the sponge ecological sensitivity zone was finally determined (Figure 7), which further guided different zones to adopt specific sponge city strategies:

1. highly sensitive zones, accounting for 26% of the city’s total area, where sponge city construction should be based on ecological protection.

2. medium-high sensitive zones, accounting for about 15% of the city’s total area, where sponge city construction should focus on ecological protection and restoration.

3. medium sensitive zones, accounting for 23.8% of the city’s total area, where sponge city construction should focus on ecological restoration and soil and water conservation.

4. medium-low sensitive zones and low sensitive zones, accounting for 17.5% and 17.7% of the city’s total area, respectively. These zones are the main space for urban construction and are also the space that need to be focused on during the construction of the sponge city.

Figure 6. The current situation of groundwater level

Figure 7. The sponge ecological sensitivity zone

Based on the above research, combined with the sponge ecological pattern, water system pattern and green space pattern in the central city, the “Shenzhen Sponge City Construction-Special Plan and Implementation Plan” has built a spatial structure of the sponge city based on landscape, with blue and green corridors connecting urban space and achieving multi-point distribution (Figure 8).

Figure 8. The spatial structure of the sponge city

• Building Scale

In this one section, the authors focus on ecologically less sensitive zones and low sensitive ecological zones, that is, the construction of sponge cities in the main spaces of urban construction. High-density residential communities in Shenzhen are selected as case studies, aiming to specifically integrate sponge city strategies into the construction or renovation of high-density residential communities (Figure 9).

First, the building is built with a green roof and green walls, allowing rainwater to be stored on the roof or in the walls. Rainwater that exceeds the collection capacity of the green roof and walls will be discharged to underground water cisterns where it is filtered and disinfected. This water is then piped to the three-stage stormwater treatment plants, where some of the treated water is discharged directly into a constructed rainwater wetland, some into natural water sources (surface water like rivers, lakes, oceans, etc. ), and some is sent to the building for recycling as non-potable water. At the same time, the ground is permeable, and some of the rainwater that reaches the ground will infiltrate into the soil, while some of the rainwater will be filtered and purified through settling basing and then entering the pipes that lead to the stormwater treatment system. The grass trenches along roadsides and constructed rainwater wetlands within the community can further assist in rainwater collection and infiltration.

In the event of extreme weather, when stormwater exceeds the storage capacity of the water cisterns and settling basing, some of the excess stormwater is discharged into the soil and the remainder enters “excess stormwater runoff systems” such as stormwater regulating tanks, deep tunnels, etc., which then drain directly into the stormwater wetlands or natural water sources.

Figure 9. The application of sponge city strategies in high-density residential communities

Goals

According to the requirements of the Shenzhen Sponge City Construction Special Plan and Implementation Plan and other related policies, and with reference to relevant researches and planning in Shenzhen, the Shenzhen government has determined a total of 13 goals in four major categories for the construction of the sponge city (Table 1).

Combined with the Shenzhen government’s goals and the author’s renovation of Shenzhen’s high-density residential areas, the goals of total annual runoff control rate, surface water quality standards, CSO overflow contamination control, rainwater resource utilization rate, waterlogging prevention and control standards, and urban flood control standards will be further enhanced.

Table 1. The Construction of the sponge city

Citations

De Vette, Kirsten. “Shenzhen.” International Water Association, https://iwa-network.org/city/shenzhen/. Accessed 11 December 2021.

Sun, Shao, et al. “Urban waterlogging risk assessment in well-developed region of Eastern China.” Physics and Chemistry of the Earth, vol. 115, no. ISSN 1474-7065, 2020, https://doi.org/10.1016/j.pce.2019.102824.

Water Resources Bureau of Shenzhen Municipality. “2018 SHENZHEN WATER RESOUR€ES BULLETIN.” no. Water Resources Bureau of Shenzhen Municiple, 2018.

Zevenbergen, C.; Fu, D.; Pathirana, A. Transitioning to Sponge Cities: Challenges and Opportunities to Address Urban Water Problems in China. Water 2018, 10, 1230. https://doi.org/10.3390/w10091230

Michele Lancia, Chunmiao Zheng, Xin He, David N. Lerner, Charles Andrews, Yong Tian, Hydrogeological constraints and opportunities for “Sponge City” development: Shenzhen, southern China, Journal of Hydrology: Regional Studies, Volume 28, 2020, 100679, ISSN 2214-5818, https://doi.org/10.1016/j.ejrh.2020.100679.

“Shenzhen sponge city construction special planning and implementation plan”. Shenzhen Planning and Natural Resources Bureau. Shenzhen Urban Planning and Design Institute Co. 2019

孙波. 基于海绵城市理念的南方沿海城市内涝防治策略研究[D].南华大学,2020.DOI:10.27234/d.cnki.gnhuu.2020.001049. [Sun Bo. Research on flood prevention and control strategies of southern coastal cities based on the concept of sponge city [D]. South China University, 2020. doi:10.27234/d.cnki.gnhuu.2020.001049.]

李晓君，黄垚洇，马倩倩，等.共建模式下高密度老旧城区海绵城市建设实施路径研究——以深圳市罗湖区为例[J].中国防汛抗旱，2021，31（5）：1-6 .LI Xiaojun，HUANG Yao⁃yin，MA Qianqian，et al.Research on the implementation path of sponge city construction in high-density old town districts under co-construction model—A case study of Luohu District, Shenzhen[J].China Flood & Drought Management，2021，31（5）：1-6（. in Chinese）