The People's Republic of China adopted the Sponge City initiative, motivated largely byː.

• - the failure of conventional gray infrastructure to control floods and manage stormwater; and.

• - the persistent efforts of Chinese green urban planners through letters and proposals sent to high-level authorities since early 2000.[13]​[14]​[15]​[16]​.

Although the concept had been published and practiced since the early 2000s, it was the Beijing flood of July 21, 2012, which caused 79 deaths, that prompted senior Chinese authorities to accept the sponge city concept and turn it into national policy.[15]​[17][9]​.

In 2015, China began a pilot initiative in 16 districts.[18]​[19][20]​ In the following years, additional pilot districts and cities were selected to continue implementing the sponge city design. For the year 2017, 4 lots were selected, made up of a total of 87 cities. The schedule for the sponge city pilot projects was as follows:[21]​.

• - 2015-2018: implement the design of sponge cities with small-scale urban pilot projects.

• - 2018-2020: Publish the standards, management and monitoring of the Sponge City initiative

• Recycle 70% of precipitation.

• - 2020-2030: Complete integration of sponge cities.

The country expects that 80% of its cities will collect their rainwater and 70% of the fallen liters will be reused. Building sponge cities does not necessarily require large investments, but this fact has been widely misinterpreted due to misleading media and because the term "sponge city" has often been incorrectly used by local governments, contractors and non-professional urban planners as a fashion brand and a slogan that actually has nothing to do with this nature-based solution.[22]​[23]​.

The main obstacles to achieving a sponge city are the traditional mentality of gray infrastructure engineering, ornamental gardening and conventional urban planning, as well as the regulations that have been established to defend these obsolete urban planning practices.[24]​ Financing sponge cities has also been a challenge.[25]​[26]​.

After achieving success in China, the sponge city model has attracted climatic zones with high risk of flooding such as Dhaka or Kenya, as well as large cities such as Berlin or Los Angeles.[27]​[28]​[29]​[30]​.

The philosophy of a sponge city is to retain the fallen rainwater, distribute it slowly, clean the water naturally (the rain is clean, but when it falls to the ground it becomes dirty with dust, oil and residue, so it must be cleaned before returning it to nature) and, when water accumulates in the sink areas, adapt to this accumulation (that is, design them so that, whether there is little or a lot of water, it does not cause problems).

This is in stark contrast to the conventional gray infrastructure solution, which consists of quickly collecting the water that falls on the city through sewage, centralizing it and accumulating it in storm tanks and, in the event that the water is discharged into rivers whose increasing flow represents a danger, defending it with taller and stronger retaining walls (levees) and dams.[10]​[31]​[32]​.

The sponge city theory underlines the basic principles of "nature-based", "source control" (the source of rainwater is the clouds, of course, but in this theory it is considered to be the ground where it falls), "local adaptation", nature protection, learning from nature, preserving urban ecological space as much as possible, restoring biodiversity, and creating a beautiful landscape environment.

All of this can be achieved through the natural absorption of rainwater, natural infiltration into the soil and natural purification.[10]​ These principles come from long-standing wisdom and strategies practiced throughout China for thousands of years, when it was necessary to work with water and avoid it instead of fighting it with gray infrastructure.[3][6] Similar techniques have been used in other latitudes for a long time as well (sowing and harvesting water).

The effects of the natural landscape (such as topography and landforms) on water infiltration, the purifying effect of vegetation and wetlands on water quality, and the combination of natural and artificial means allow the city to first absorb rainwater and then release it purified. Urban green spaces and water bodies in the city (constructed wetlands, rain gardens, green roofs, reclamation green spaces, grass ditches and ecological parks) are the central "sponge bodies".[33]​.

The development of these systems has 3 main facets: the protection of the original urban ecosystem, restorative management and low impact development.

• - Protection focuses on the city's original ecologically sensitive areas, such as rivers, lakes and ditches. Natural vegetation, soil and microorganisms are used to gradually treat the aquatic environment and restore the damaged urban ecosystem.

• - Restorative management or ecological restoration includes the identification of small ecological zones, the construction of ecological corridors, the strengthening of connections between these small zones, the formation of a network and the delimitation of blue and green lines (water and vegetation connections) to restore the aquatic ecological environment.

• - Low-impact development consists of design and construction standards for urban roads, green spaces, urban water systems, residential areas and specific buildings to protect ecological areas, maintain their water storage capacity, strengthen source control and form ecological sponges of different scales.[34]​.

These design principles can be applied at 3 different levels:

• - Macroscale: regional or watershed level for regional master plans.

• - Mesoscale: planning at the city, municipality and town level.

• - Microscale: individual “sponge units,” such as parks and neighborhoods.

Sponge city policies have been implemented more frequently in new construction than in redeveloped developments in recent decades of rapid urbanization. Xiamen's Yangfang residential area and Shanghai's Langang Park are new developments indicative of this trend.[35]​[36]​ Shougang Park, the former site of a steel factory that was redeveloped to include a 2022 Winter Olympics venue, Big Air Shougang, incorporates sponge city design concepts.[37]​.

In his speech at the Urbanization Work Conference on December 12, 2013, CPC General Secretary Xi Jinping said: "When modernizing the urban drainage system, priority should be given to rainwater retention and the power of nature to drain water. Build a sponge city with natural retention, natural infiltration and natural purification.”[38]​[39][40]​ To this end, in October 2014, the Ministry of Housing and Urban-Rural Development published the Technical Guidelines for the Construction of Sponge City, underscoring the importance of high-level design of rainwater management in cities, as well as planning to guide urban construction by prioritizing ecology.

In August 2015, the "Sponge City Construction Performance Evaluation Method" clarified the requirements for the use of state money and provided guidelines for the effectiveness of pilot city construction. In accordance with these guidelines and related regulations, the China Building Standards Research and Design Institute has initially established a standard system for sponge city construction, including newly built, expanded and reconstructed sponge buildings and communities, roads and squares, parks, green spaces and urban water systems.

The guideline of the General Office of the State Council on promoting the construction of sponge cities (Guobanfa [2015] No. 75) indicated that the construction of sponge cities occurs by strengthening the management of urban planning and construction, and fully utilizing the impact of buildings, roads, green spaces and water systems. Under this guideline, China's cities will collect and use 70% of rainwater. 20% of urban areas will reach this goal by 2020. By 2030, 80% will have achieved it.[41]​.

In 2015 and 2016, sponge city pilot projects were organized with the support of national policies. Sixteen cities, including Zhenjiang, Jiaxing and Xiamen, were selected as the first group of pilot cities to carry out sponge city construction in an orderly manner. Another 14, including Shenzhen, Shanghai, Tianjin and Beijing, were included in the second group. Additionally, the Central Ministry of Finance introduced a public-private partnership (PPP) model to increase support for financial policy.

However, this configuration may threaten the ability of local governments to finance these programs, which are estimated to require $230 billion through 2030 to achieve their goals.[42] The central state only plans to subsidize a fifth of the costs of implementing sponge city policies, and flooding in more than half of the pilot villages – such as Ningbo – since the program began could worry private investors.[42]​

China's central government has established that by 2030, 80% of the country's urban space must include sponge city adaptations and must recycle at least 70% of precipitation.[43]​.

In 2015, the following 16 cities were chosen for the first round of pilot projects:[44]​.

• - Baicheng[45]​.

• - Qian'an "Qian'an (Tangshan)")[46]​.

• - Jinan[47]​.

• - Hebi.

• - Xixian New Area[48]​.

• - Zhenjiang[49]​.

• - Jiaxing.

• - Chizhou.

• - Wuhan[50]​[51]​.

• - Changde[52]​.

• - Chongqing[53]​.

• - Suining.

• - Gui'an New Area[54]​.

• - Nanning[55]​.

• - Pingxiang[56]​.

• - Xiamen[57]​.

In 2016, another 15 were selected, of which 13 are listed below:[44]​.

• - Beijing[58]​.

• - Tianjin.

• - Dalian[59]​.

• - Qingyang.

• - Guyuan[60]​.

• - Xining.

• - Shanghai[61]​.

• - Ningbo[62]​.

• - Fuzhou.

• - Shenzhen[63]​[64]​.

• - Zhuhai[65]​.

• - Yuxi.

• - Sanya[66]​.

Sponge cities have been shown to achieve the following:.

• - groundwater recharge[67]​.

• - adapt to disaster/change[67]​.

• - environmental restoration[67]​.

• - less dependence on municipal pipelines[67]​.

• - improve the ecology of the city[67]​.

• - water purification using living plants[67]​.

• - prevention of sewer overflow[67]​.

• - reduce stormwater runoff[68]​.

• - flood mitigation.

• - provide ecosystem services.

• - Interconnection/communication between cities[67]​.

Studies have shown that sponge cities are very effective in reducing stormwater runoff and improving the quality of water that, after falling into the city, is returned to nature.[69]​ The effectiveness of sponge cities has been evaluated by modeling or by observation, as monitoring data is not widely available.[70]​[69]​[71]​ It has been difficult to obtain monitoring data due to the lack of monitoring data and evaluation standards published by the government.[72]​.

Currently, the impacts of flood mitigation are typically neighborhood-level, not city-wide, because planning and projects are done at the neighborhood scale.[73][71] Since the 30 cities were chosen as a pilot, 19 have experienced flooding.[73][21] However, specific neighborhoods or areas within those 30 cities that were retrofitted with the sponge city concept in mind have experienced little to no flooding following major flooding. storms.[73][71]​.

Benjakitti Forest Park

Benjakitti Forest Park is located in the center of Bangkok, the capital of Thailand. The land was previously vacant due to the construction of a tobacco factory. The area suffered subsidence and flooding due to urbanization. The 52.7 square hectometers of land were developed by Turenscape as a public green space in 2023, with 187,500 cubic meters of stormwater storage capacity. This capacity is prepared for the maximum rainfall in 10 years (flood systems are calculated according to the maximum fall in the last 10, 50 or 100 years). In the summer of 2023, much of Bangkok was flooded, but the park and its surroundings were not.[71][74] In addition to rainwater management, the park includes: sponge wetlands, a recreational boardwalk system, an amphitheater designed to be flood-safe, warehouses for sports and museums, and ecological environments for flora and fauna.[75]​ The project was created on a low budget ($20 per meter). square) and low maintenance in mind.[75]​ As the park retains water, the lush vegetation does not need watering during the dry monsoon season.

Luotian River

The Luotian River, with a length of about 8 kilometers, has adopted an ecological control method to regulate stormwater. As part of the Luotian Comprehensive Water Regulation Project, the river channel was widened to restore riparian areas and existing reservoirs were expanded into rainwater storage lakes. Rainwater was also used as a supply instead of diverting it to the river. The data revealed that the design of the sponge city allowed increasing water quality and reducing flooding.[76]​.

Gui-an New District

One of the pilot cities, Gui-an New District, was established in 2014. US$1 billion was invested in its design as a sponge city, with an emphasis on permeable pavements, as well as 70 monitoring stations. However, in 2022, much of the sponge city's design was still not in place, including the monitoring stations. Permeable concrete had been placed, but often not maintained (to maintain its permeability, it needs to be cleaned periodically).[77]​ The modeling revealed that the risk of flooding is lower, but has not been reduced to zero. A sponge city will avoid flooding only if the amount of rain is less than the design amount.[70]​ For example, if it is designed not to flood even if 600 liters per square meter fall in a day, and 800 fall, it will flood.

Sanya city, Hainan island

The coastal city of Sanya was part of the second group of pilot cities. It has suffered flooding and habitat degradation (coastal cities are more prone to flooding, because at high tide it is easier for the swollen river to overflow its banks). A mangrove park and a wetland park were established to mitigate these impacts and restore the ecological environment.

A 10-hectare area on the bank of the Linchun River was restored and transformed into a mangrove swamp to improve flood resilience. Previously, concrete walls had been built there against the avenues, thus degrading the natural habitat of the mangrove. The area was converted into a mangrove park through the restoration of riparian habitats. An interlocking finger-like terrain design (ecotones) was used to reduce the force of ocean tides and storm surges. Terraces were also built from the city streets to the river elevation with sustainable drainage to capture and filter runoff. After 3 years of construction, ecological restoration and flood mitigation have been successful. The mangrove is still healthy and growing as a result of flood mitigation and improvements in water quality, attracting biodiversity and ecotourism.[78][79]​.

Located in central Sanya, the 68-hectare park was previously contaminated by illegal dumping and overgrown with invasive species.[79][80] Systems of ponds and dikes were established to capture water and filter runoff. A forested wetland was also created in the center of the park, with recreational areas on the edges. The wetland is designed to store 830,000 cubic meters of stormwater, which will reduce the risk of flooding.[79] Surrounding communities have experienced less urban flooding and contaminated runoff. The park has attracted biodiversity and recreation to the area.[80]​.

• - Water management.

Urbanization encourages the construction of gray infrastructure (the classic one, characterized by using cement, and so called to differentiate it from green infrastructure) in cities. Overuse of this gray infrastructure can lead to water shortages, flooding, pollution, and overall degradation of water ecosystem services. Current urban architectural planning also creates a large number of buildings, while limiting cities' green spaces, drainage and rainwater collection capacity. Precipitation thus causes numerous problems in modern cities.[10]​.

Meanwhile, high-intensity artificial constructions, such as buildings, roads and public squares, waterproof the top layer of soil, altering hydrological characteristics. Due to this, surface flow (runoff) increases from 10 to 60%, while infiltration is drastically reduced, even to zero.[11]​ According to research carried out by the Ministry of Housing and Urban-Rural Development in 2010, 62% of 351 cities throughout China suffered floods between 2008 and 2010 (it should be taken into account that much of the country is subject to very intense and continuous monsoon rain in summer and very scarce in winter); 137 cities were flooded more than 3 times during that period.

These frequent urban floods make more and more people recognize the importance of aquatic ecosystems and urban ecological infrastructure. The simple concept of rapid discharge (into some nearby river, or into the sea), a traditional gray water management model, is no longer sufficient to address the problem of rainwater combining with rapid urbanization. To cope, China is attaching increasing importance to urban flood management and water ecosystem services and is vigorously promoting the sponge city idea, which is seen as a method of adaptation to climate change.[12]​.

The People's Republic of China adopted the Sponge City initiative, motivated largely byː.

• - the failure of conventional gray infrastructure to control floods and manage stormwater; and.

• - the persistent efforts of Chinese green urban planners through letters and proposals sent to high-level authorities since early 2000.[13]​[14]​[15]​[16]​.

Although the concept had been published and practiced since the early 2000s, it was the Beijing flood of July 21, 2012, which caused 79 deaths, that prompted senior Chinese authorities to accept the sponge city concept and turn it into national policy.[15]​[17][9]​.

In 2015, China began a pilot initiative in 16 districts.[18]​[19][20]​ In the following years, additional pilot districts and cities were selected to continue implementing the sponge city design. For the year 2017, 4 lots were selected, made up of a total of 87 cities. The schedule for the sponge city pilot projects was as follows:[21]​.

• - 2015-2018: implement the design of sponge cities with small-scale urban pilot projects.

• - 2018-2020: Publish the standards, management and monitoring of the Sponge City initiative

• Recycle 70% of precipitation.

• - 2020-2030: Complete integration of sponge cities.

The country expects that 80% of its cities will collect their rainwater and 70% of the fallen liters will be reused. Building sponge cities does not necessarily require large investments, but this fact has been widely misinterpreted due to misleading media and because the term "sponge city" has often been incorrectly used by local governments, contractors and non-professional urban planners as a fashion brand and a slogan that actually has nothing to do with this nature-based solution.[22]​[23]​.

The main obstacles to achieving a sponge city are the traditional mentality of gray infrastructure engineering, ornamental gardening and conventional urban planning, as well as the regulations that have been established to defend these obsolete urban planning practices.[24]​ Financing sponge cities has also been a challenge.[25]​[26]​.

After achieving success in China, the sponge city model has attracted climatic zones with high risk of flooding such as Dhaka or Kenya, as well as large cities such as Berlin or Los Angeles.[27]​[28]​[29]​[30]​.

The philosophy of a sponge city is to retain the fallen rainwater, distribute it slowly, clean the water naturally (the rain is clean, but when it falls to the ground it becomes dirty with dust, oil and residue, so it must be cleaned before returning it to nature) and, when water accumulates in the sink areas, adapt to this accumulation (that is, design them so that, whether there is little or a lot of water, it does not cause problems).

This is in stark contrast to the conventional gray infrastructure solution, which consists of quickly collecting the water that falls on the city through sewage, centralizing it and accumulating it in storm tanks and, in the event that the water is discharged into rivers whose increasing flow represents a danger, defending it with taller and stronger retaining walls (levees) and dams.[10]​[31]​[32]​.

The sponge city theory underlines the basic principles of "nature-based", "source control" (the source of rainwater is the clouds, of course, but in this theory it is considered to be the ground where it falls), "local adaptation", nature protection, learning from nature, preserving urban ecological space as much as possible, restoring biodiversity, and creating a beautiful landscape environment.

All of this can be achieved through the natural absorption of rainwater, natural infiltration into the soil and natural purification.[10]​ These principles come from long-standing wisdom and strategies practiced throughout China for thousands of years, when it was necessary to work with water and avoid it instead of fighting it with gray infrastructure.[3][6] Similar techniques have been used in other latitudes for a long time as well (sowing and harvesting water).

The effects of the natural landscape (such as topography and landforms) on water infiltration, the purifying effect of vegetation and wetlands on water quality, and the combination of natural and artificial means allow the city to first absorb rainwater and then release it purified. Urban green spaces and water bodies in the city (constructed wetlands, rain gardens, green roofs, reclamation green spaces, grass ditches and ecological parks) are the central "sponge bodies".[33]​.

The development of these systems has 3 main facets: the protection of the original urban ecosystem, restorative management and low impact development.

• - Protection focuses on the city's original ecologically sensitive areas, such as rivers, lakes and ditches. Natural vegetation, soil and microorganisms are used to gradually treat the aquatic environment and restore the damaged urban ecosystem.

• - Restorative management or ecological restoration includes the identification of small ecological zones, the construction of ecological corridors, the strengthening of connections between these small zones, the formation of a network and the delimitation of blue and green lines (water and vegetation connections) to restore the aquatic ecological environment.

• - Low-impact development consists of design and construction standards for urban roads, green spaces, urban water systems, residential areas and specific buildings to protect ecological areas, maintain their water storage capacity, strengthen source control and form ecological sponges of different scales.[34]​.

These design principles can be applied at 3 different levels:

• - Macroscale: regional or watershed level for regional master plans.

• - Mesoscale: planning at the city, municipality and town level.

• - Microscale: individual “sponge units,” such as parks and neighborhoods.

Sponge city policies have been implemented more frequently in new construction than in redeveloped developments in recent decades of rapid urbanization. Xiamen's Yangfang residential area and Shanghai's Langang Park are new developments indicative of this trend.[35]​[36]​ Shougang Park, the former site of a steel factory that was redeveloped to include a 2022 Winter Olympics venue, Big Air Shougang, incorporates sponge city design concepts.[37]​.

In his speech at the Urbanization Work Conference on December 12, 2013, CPC General Secretary Xi Jinping said: "When modernizing the urban drainage system, priority should be given to rainwater retention and the power of nature to drain water. Build a sponge city with natural retention, natural infiltration and natural purification.”[38]​[39][40]​ To this end, in October 2014, the Ministry of Housing and Urban-Rural Development published the Technical Guidelines for the Construction of Sponge City, underscoring the importance of high-level design of rainwater management in cities, as well as planning to guide urban construction by prioritizing ecology.

In August 2015, the "Sponge City Construction Performance Evaluation Method" clarified the requirements for the use of state money and provided guidelines for the effectiveness of pilot city construction. In accordance with these guidelines and related regulations, the China Building Standards Research and Design Institute has initially established a standard system for sponge city construction, including newly built, expanded and reconstructed sponge buildings and communities, roads and squares, parks, green spaces and urban water systems.

The guideline of the General Office of the State Council on promoting the construction of sponge cities (Guobanfa [2015] No. 75) indicated that the construction of sponge cities occurs by strengthening the management of urban planning and construction, and fully utilizing the impact of buildings, roads, green spaces and water systems. Under this guideline, China's cities will collect and use 70% of rainwater. 20% of urban areas will reach this goal by 2020. By 2030, 80% will have achieved it.[41]​.

In 2015 and 2016, sponge city pilot projects were organized with the support of national policies. Sixteen cities, including Zhenjiang, Jiaxing and Xiamen, were selected as the first group of pilot cities to carry out sponge city construction in an orderly manner. Another 14, including Shenzhen, Shanghai, Tianjin and Beijing, were included in the second group. Additionally, the Central Ministry of Finance introduced a public-private partnership (PPP) model to increase support for financial policy.

However, this configuration may threaten the ability of local governments to finance these programs, which are estimated to require $230 billion through 2030 to achieve their goals.[42] The central state only plans to subsidize a fifth of the costs of implementing sponge city policies, and flooding in more than half of the pilot villages – such as Ningbo – since the program began could worry private investors.[42]​

China's central government has established that by 2030, 80% of the country's urban space must include sponge city adaptations and must recycle at least 70% of precipitation.[43]​.

In 2015, the following 16 cities were chosen for the first round of pilot projects:[44]​.

• - Baicheng[45]​.

• - Qian'an "Qian'an (Tangshan)")[46]​.

• - Jinan[47]​.

• - Hebi.

• - Xixian New Area[48]​.

• - Zhenjiang[49]​.

• - Jiaxing.

• - Chizhou.

• - Wuhan[50]​[51]​.

• - Changde[52]​.

• - Chongqing[53]​.

• - Suining.

• - Gui'an New Area[54]​.

• - Nanning[55]​.

• - Pingxiang[56]​.

• - Xiamen[57]​.

In 2016, another 15 were selected, of which 13 are listed below:[44]​.

• - Beijing[58]​.

• - Tianjin.

• - Dalian[59]​.

• - Qingyang.

• - Guyuan[60]​.

• - Xining.

• - Shanghai[61]​.

• - Ningbo[62]​.

• - Fuzhou.

• - Shenzhen[63]​[64]​.

• - Zhuhai[65]​.

• - Yuxi.

• - Sanya[66]​.

Sponge cities have been shown to achieve the following:.

• - groundwater recharge[67]​.

• - adapt to disaster/change[67]​.

• - environmental restoration[67]​.

• - less dependence on municipal pipelines[67]​.

• - improve the ecology of the city[67]​.

• - water purification using living plants[67]​.

• - prevention of sewer overflow[67]​.

• - reduce stormwater runoff[68]​.

• - flood mitigation.

• - provide ecosystem services.

• - Interconnection/communication between cities[67]​.

Studies have shown that sponge cities are very effective in reducing stormwater runoff and improving the quality of water that, after falling into the city, is returned to nature.[69]​ The effectiveness of sponge cities has been evaluated by modeling or by observation, as monitoring data is not widely available.[70]​[69]​[71]​ It has been difficult to obtain monitoring data due to the lack of monitoring data and evaluation standards published by the government.[72]​.

Currently, the impacts of flood mitigation are typically neighborhood-level, not city-wide, because planning and projects are done at the neighborhood scale.[73][71] Since the 30 cities were chosen as a pilot, 19 have experienced flooding.[73][21] However, specific neighborhoods or areas within those 30 cities that were retrofitted with the sponge city concept in mind have experienced little to no flooding following major flooding. storms.[73][71]​.

Benjakitti Forest Park

Benjakitti Forest Park is located in the center of Bangkok, the capital of Thailand. The land was previously vacant due to the construction of a tobacco factory. The area suffered subsidence and flooding due to urbanization. The 52.7 square hectometers of land were developed by Turenscape as a public green space in 2023, with 187,500 cubic meters of stormwater storage capacity. This capacity is prepared for the maximum rainfall in 10 years (flood systems are calculated according to the maximum fall in the last 10, 50 or 100 years). In the summer of 2023, much of Bangkok was flooded, but the park and its surroundings were not.[71][74] In addition to rainwater management, the park includes: sponge wetlands, a recreational boardwalk system, an amphitheater designed to be flood-safe, warehouses for sports and museums, and ecological environments for flora and fauna.[75]​ The project was created on a low budget ($20 per meter). square) and low maintenance in mind.[75]​ As the park retains water, the lush vegetation does not need watering during the dry monsoon season.

Luotian River

The Luotian River, with a length of about 8 kilometers, has adopted an ecological control method to regulate stormwater. As part of the Luotian Comprehensive Water Regulation Project, the river channel was widened to restore riparian areas and existing reservoirs were expanded into rainwater storage lakes. Rainwater was also used as a supply instead of diverting it to the river. The data revealed that the design of the sponge city allowed increasing water quality and reducing flooding.[76]​.

Gui-an New District

One of the pilot cities, Gui-an New District, was established in 2014. US$1 billion was invested in its design as a sponge city, with an emphasis on permeable pavements, as well as 70 monitoring stations. However, in 2022, much of the sponge city's design was still not in place, including the monitoring stations. Permeable concrete had been placed, but often not maintained (to maintain its permeability, it needs to be cleaned periodically).[77]​ The modeling revealed that the risk of flooding is lower, but has not been reduced to zero. A sponge city will avoid flooding only if the amount of rain is less than the design amount.[70]​ For example, if it is designed not to flood even if 600 liters per square meter fall in a day, and 800 fall, it will flood.

Sanya city, Hainan island

The coastal city of Sanya was part of the second group of pilot cities. It has suffered flooding and habitat degradation (coastal cities are more prone to flooding, because at high tide it is easier for the swollen river to overflow its banks). A mangrove park and a wetland park were established to mitigate these impacts and restore the ecological environment.

A 10-hectare area on the bank of the Linchun River was restored and transformed into a mangrove swamp to improve flood resilience. Previously, concrete walls had been built there against the avenues, thus degrading the natural habitat of the mangrove. The area was converted into a mangrove park through the restoration of riparian habitats. An interlocking finger-like terrain design (ecotones) was used to reduce the force of ocean tides and storm surges. Terraces were also built from the city streets to the river elevation with sustainable drainage to capture and filter runoff. After 3 years of construction, ecological restoration and flood mitigation have been successful. The mangrove is still healthy and growing as a result of flood mitigation and improvements in water quality, attracting biodiversity and ecotourism.[78][79]​.

Located in central Sanya, the 68-hectare park was previously contaminated by illegal dumping and overgrown with invasive species.[79][80] Systems of ponds and dikes were established to capture water and filter runoff. A forested wetland was also created in the center of the park, with recreational areas on the edges. The wetland is designed to store 830,000 cubic meters of stormwater, which will reduce the risk of flooding.[79] Surrounding communities have experienced less urban flooding and contaminated runoff. The park has attracted biodiversity and recreation to the area.[80]​.

• - Water management.