Urban Rewilding
Reducing Floods and Recharging Groundwater Reservoirs: The Concept of "Sponge Cities"

	Hari Srinivas
	Policy Analysis Series C-025

W hat is a Sponge City? A sponge city is an urban development concept that aims to manage and utilize water resources in a sustainable and resilient manner. It involves integrating various techniques and infrastructure to absorb, store, and purify rainwater, reducing the risk of floods and enhancing water availability during dry periods. Sponge cities typically incorporate features like green roofs, permeable pavements, constructed wetlands, and retention ponds to promote natural water infiltration and retention.

By mimicking the natural hydrological cycle, sponge cities help mitigate the impacts of urbanization on water resources, improve water quality, and enhance the overall ecological balance of urban areas. Sponge cities are a new urban planning model that aims to mimic the natural water cycle by absorbing, storing, and infiltrating rainwater, and recharging ground water reservoirs. This helps to reduce flooding, improve water quality, and create a more livable environment.

The main idea behind sponge cities is to mimic the natural water cycle by creating urban landscapes that can absorb, store, and purify rainwater instead of allowing it to quickly runoff and cause problems. The concept draws inspiration from natural sponges that can soak up and retain water.

By implementing various nature-based solutions, such as green roofs, permeable pavements, rain gardens, wetlands, and retention ponds, sponge cities aim to transform urban areas into more resilient, sustainable, and water-sensitive environments. Significance of Sponge Cities

The significance of sponge cities to nature restoration lies in their focus on integrating natural elements into urban spaces. By incorporating features like wetlands, green spaces, and vegetation, sponge cities provide habitats for plants, animals, and microorganisms. These elements help restore biodiversity and promote ecological balance within urban environments.

The restoration of natural water systems in sponge cities has several benefits for both humans and the environment. It helps reduce the risk of flooding by slowing down and retaining rainwater, minimizing the strain on drainage systems. By filtering and purifying water through natural processes, sponge cities also contribute to improving water quality. Additionally, these nature-based solutions can enhance urban aesthetics, create recreational spaces, and improve overall livability.

Sponge cities not only address water-related challenges but also provide opportunities for nature conservation and restoration. By incorporating ecological principles into urban planning, these cities aim to create a harmonious relationship between human development and the natural environment.

Benefits of a Sponge City

Sponge cities offer a number of benefits, including:

• Reduced flooding: Sponge cities can help to reduce flooding by absorbing and storing rainwater. This can help to protect people and property from flooding damage.

• Improved water quality: Sponge cities can help to improve water quality by filtering rainwater as it percolates through the ground. This can help to reduce the amount of pollutants that enter waterways.

• Enhanced biodiversity: Sponge cities can help to enhance biodiversity by providing habitat for plants and animals. This can help to improve the overall health of the environment.

Sponge cities are a promising approach to urban planning that can help to reduce flooding, improve water quality, and create a more livable environment. As cities around the world continue to grow and develop, sponge cities will become increasingly important.

Components of "Sponginess"

"Sponginess" in sponge cities is created through a combination of nature-based features and infrastructure that collectively mimic the water-absorbing and water-retaining properties of natural sponges. Here are some key elements commonly used to create sponginess in urban environments:

1. Permeable pavements: Replace conventional impermeable pavements with permeable surfaces that allow rainwater to infiltrate into the ground. These pavements are designed to enable water to pass through small gaps or porous materials, promoting groundwater recharge.

2. Green roofs: Install vegetated roofs on buildings, which consist of plants and a growing medium layered over a waterproof membrane. Green roofs absorb rainfall and release it slowly, reducing stormwater runoff and providing insulation for the building.

3. Rain gardens and bioswales: Construct shallow depressions or vegetated channels designed to capture and filter stormwater runoff. These features use native plants and specially designed soil mixes to absorb, retain, and purify rainwater, allowing it to gradually infiltrate into the ground.

4. Wetlands and retention ponds: Create or restore wetland areas and retention ponds that can temporarily store excess rainfall and runoff. Wetlands help absorb water, filter pollutants, and provide habitat for diverse plant and animal species.

5. Urban forests and green spaces: Develop and expand urban forests, parks, and green spaces that provide additional surface area for rainwater absorption. Trees and vegetation help intercept rainfall, reduce runoff, and enhance overall ecosystem health.

6. Rainwater harvesting systems: Implement rainwater collection and storage systems, such as rain barrels, cisterns, and underground tanks, to capture and store rainwater for later use in non-potable applications like irrigation or toilet flushing.

7. Sustainable drainage systems: Design and integrate sustainable drainage systems (SuDS) that manage stormwater at the source. SuDS utilize a combination of natural features and engineered solutions to slow down, filter, and store stormwater runoff, minimizing the impact on downstream areas.

8. Natural waterways and stream restoration: Restore or create natural waterways, streams, and river systems within urban areas. Natural waterways can absorb excess water during heavy rainfall events and contribute to improving overall water quality.

9. Rainwater recycling and reuse: Implement systems that collect and treat rainwater for various uses within urban environments, such as irrigation, industrial processes, or toilet flushing. This reduces the reliance on freshwater sources and minimizes the demand on the water supply.

It's important to note that the specific combination of elements implemented in a sponge city will vary depending on local conditions, available space, and the desired outcomes. The aim is to create a multifunctional urban environment that maximizes water absorption, storage, purification, and reuse, while also providing ecological benefits and enhancing the overall livability of the city.

Sponge Cities in Existing Urban Environments

Spnge Cty MEasures Protection measures focus 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. Restoration measures include identifying ecological patches, constructing ecological corridors, strengthening the connections between the patches, forming a network, and delineating the blue and green lines to restore the aquatic ecological environment. Mandatory measures apply to urban roads, urban green spaces, urban water systems, residential areas, and specific buildings to protect ecological patches, maintain their storage capacity, strengthen source control, and form ecological sponges of different scales.

Implementing sponge city concepts in existing urban environments requires careful planning, coordination, and a combination of technical and policy measures. Here are some key steps and strategies involved in setting up sponge cities in existing urban areas:

1. Comprehensive assessment: Conduct a thorough assessment of the existing urban environment, including water infrastructure, drainage systems, land use patterns, and environmental conditions. Identify areas prone to flooding, water scarcity, or pollution, and understand the specific challenges and opportunities in each location.

2. Integrated planning and design: Develop an integrated master plan that incorporates sponge city principles into urban design. This involves identifying suitable locations for nature-based features such as green roofs, permeable pavements, rain gardens, bioswales, wetlands, and retention ponds. Consider factors such as land availability, land use compatibility, and connectivity between different elements.

3. Retrofitting and redevelopment: Retrofit existing infrastructure to incorporate sponge city features wherever feasible. This may involve retrofitting existing buildings with green roofs, installing permeable pavements, or creating decentralized rainwater harvesting systems. In cases where redevelopment is possible, incorporate sponge city principles from the initial design stage.

4. Green space expansion: Identify opportunities to create or expand green spaces within the city. This can include converting unused or underutilized areas into parks, green corridors, or urban forests. Green spaces not only enhance the aesthetic appeal but also provide additional areas for rainwater infiltration and habitat creation.

5. Multi-functional land use: Encourage multi-functional land use planning that integrates water management with other urban functions. For example, designing parks and recreational areas to serve as flood retention basins during heavy rainfall. This approach maximizes the benefits of land use while optimizing water management.

6. Stormwater management: Implement decentralized stormwater management techniques to reduce runoff and enhance water infiltration. This can involve incorporating features like rain gardens, bioswales, and permeable pavements into the urban fabric to capture and treat stormwater close to the source.

7. Public participation and education: Involve local communities, stakeholders, and residents in the planning and implementation process. Raise awareness about the importance of sponge cities, their benefits, and the role individuals can play in water conservation and management. Promote community engagement in maintaining and preserving the sponge city features.

8. Policy and governance: Develop supportive policies, regulations, and guidelines that incentivize and enforce the adoption of sponge city practices. This can include incentives for developers to incorporate nature-based features, building codes that mandate green infrastructure, and stormwater management regulations.

9. Monitoring and evaluation: Establish a monitoring and evaluation framework to assess the effectiveness of sponge city interventions. Monitor the performance of green infrastructure, water quality, flood reduction, and other key indicators to continually improve the design and operation of sponge cities.

Setting up sponge cities in existing urban environments is a gradual and iterative process that requires long-term commitment and collaboration among various stakeholders, including government agencies, urban planners, engineers, architects, and the local community.

Creating a Sponge City

There are a number of different ways to create a sponge city. Some common techniques include:

• Using green infrastructure: This includes planting trees, creating rain gardens, and installing green roofs. These features help to absorb rainwater and reduce runoff.

• Reducing impervious surfaces: Impervious surfaces, such as concrete and asphalt, prevent rainwater from seeping into the ground. Reducing the amount of impervious surfaces in a city can help to improve water infiltration.

• Improving drainage systems: Sponge cities often include upgraded drainage systems that can handle larger volumes of rainwater. These systems can also help to prevent flooding by slowing down the flow of water.

Digital tools can play a significant role in creating sponge cities by aiding in planning, design, implementation, and monitoring of various water management strategies. Here are several ways in which digital tools can be leveraged to support the development of sponge cities: For example, digital tools can assist in collecting and analyzing data related to rainfall patterns, hydrological systems, land use, and infrastructure. This data can help urban planners and engineers make informed decisions regarding the placement and design of sponge city features. Geographic Information Systems (GIS) technologies enables the integration and analysis of spatial data, providing a visual representation of the urban environment. GIS can be used to identify suitable locations for green infrastructure, assess potential flood risks, and optimize the placement of different sponge city elements.

Using digital modeling tools, urban planners can create virtual representations of sponge city designs using 3D models and visualizations. These models help stakeholders visualize and understand the proposed changes, allowing for better communication and collaboration during the planning and design stages. Similarly, hydrological modeling simulate the movement of water in urban areas, taking into account various factors such as rainfall intensity, surface characteristics, and drainage systems. These models can assist in evaluating the performance of sponge city features, predicting the impact of different scenarios, and optimizing the design for water management.

Digital decision support systems combine data analysis and modeling techniques to assist in decision-making processes. These tools can help identify the most effective combination of sponge city elements, evaluate the cost-benefit ratio, and prioritize interventions based on specific objectives and constraints.

Sensor networks and real-time monitoring throughout the urban environment can provide real-time data on rainfall, water levels, and water quality. This information enables timely responses to changing conditions, optimizing the operation of sponge city infrastructure, and enhancing flood control and water management strategies.

Examples of Sponge Cities

The sponge city concept was developed in China in response to the country's increasing urban flooding problems. Since then, it has been adopted by cities around the world, including in India, the United States, and Australia.

Here are some examples of cities that have adopted the concept:

Shanghai, China: Shanghai has embraced the sponge city concept to address its water management challenges. It has implemented measures such as constructing green roofs, rain gardens, and permeable pavements to absorb and retain rainwater, reducing flooding risks and improving water quality.

Copenhagen, Denmark: Copenhagen is known for its innovative approaches to sustainable urban development. The city has incorporated sponge city principles by creating green infrastructure, including green roofs, wetlands, and permeable surfaces, to manage stormwater and prevent urban flooding.

New York City, United States: In response to the increasing frequency of intense storms and flooding, New York City has adopted the sponge city concept as part of its resilience strategy. The city has implemented measures like green infrastructure, bioswales, and rain gardens to enhance stormwater absorption, reduce runoff, and protect against flood-related damages.

Singapore: Singapore has implemented the sponge city concept as part of its holistic approach to water management. Given its limited land area and vulnerability to intense rainfall, the city-state has integrated various techniques to enhance water absorption and retention. These include features like bioswales, rain gardens, permeable pavements, and underground storage systems. Singapore's commitment to becoming a sponge city has helped mitigate the impact of urbanization on water resources, reduced flood risks, and improved water quality in urban areas.

The above cities' commitment to becoming a sponge city has helped mitigate the impact of urbanization on water resources, reduced flood risks, and improved water quality in urban areas.

References

• Chen, Y., Zhan, H., Li, S., & Du, X. (Eds.). (2020). Sponge City: Water Management in Chinese Cities. Springer.
• Yu, X., & Zhang, W. (2019). The Sponge City Concept and Its Implementation in China: A Narrative Review. Environmental Science & Policy, 93, 101-109.
• Wang, C., Zhang, L., Li, M., & Ren, Y. (2021). Sponge Cities and Their Role in Sustainable Urban Water Management: A Review. Journal of Cleaner Production, 303, 126768.
• Li, M., Zhang, L., & Wang, C. (2020). Key Principles and Design Strategies for Sponge City Construction: A Review. Journal of Cleaner Production, 256, 120375.
• Ministry of Housing and Urban-Rural Development of the People's Republic of China. (2014). Sponge City Construction Technology Guidelines (Trial Version).
• World Bank. (2018). Sponge Cities: Creating Resilient Cities in China.
• C40 Cities, Arup, & Tongji University. (2018). The Sponge City Index: Assessing Chinese Cities' Resilience to Flooding and Drought.
• China Water Risk. (2018). Sponge City: China's Urban Water Revolution. [Blog post]. Retrieved from https://www.chinawaterrisk.org/opinions/sponge-city-chinas-urban-water-revolution/
• Global Water Forum. (2018). The Sponge City Concept: An Innovative Approach to Urban Water Management. [Blog post]. Retrieved from https://www.globalwaterforum.org/2018/09/18/sponge-city-concept-innovative-approach-urban-water-management/
• Wong, T. H., Lam, K. P., & Lee, J. H. (2019). Building Sustainable Cities: Lessons from Low Carbon and Sponge Cities. Sustainable Cities and Society, 48, 101532.
• Yu, X., Zhang, Y., & Wang, W. (2019). Sponge City Planning and Design: An Analytical Framework. Sustainable Cities and Society, 44, 800-811.
• Gou, Z., Wu, X., Zhang, H., & Jiang, G. (2020). Sustainable and Adaptive Water Management in Sponge City Planning: A Case Study in China. Journal of Cleaner Production, 262, 121360.