Sponge cities

Sanya Mangrove Park in China’s Hainan Province by Turenscape (Kongjian Yu).Photo from NZILA https://nzila.co.nz/news/2022/06/sponge-city-concept-la-to-present-to-2022-nzila-fi

Sponge cities is a term to describe a set of urban interventions designed to effectively manage rain and stormwater by mimicking the natural hydrological cycle. 

The term “sponge city” reflects the idea of cities behaving like sponges, able to absorb, store, and reuse rainwater to avoid flooding or runoff in undesirable areas. Songe city techniques utilise a combination of green, blue, and grey infrastructure to enhance water absorption, retention, and purification.

Sponge cities aim to address multiple urban water challenges, including flooding, water pollution, groundwater depletion, and urban heat island effects. By incorporating natural water management techniques into urban development, these cities can enhance resilience to climate change, improve water quality, reduce the risk of flooding, and create more sustainable and livable environments for residents (Li et al., 2017).

The concept originated with Chinese landscape architect Kongjian Yu. The concept was adopted as a national policy in China in 2013, prioritising large-scale nature-based infrastructures such as wetlands, greenways, parks, canopy tree and woodland protection, rain gardens, green roofs, permeable pavements, and bioswales. The concept is a response to China’s increasing urbanisation and the challenges posed by rapid urban development, including flooding and water pollution and scarcity (Li et al., 2017). China has been a pioneer in the development of sponge cities, with numerous pilot projects launched in cities across the country. The concept has gained interest and recognition globally as urban areas worldwide seek solutions to water-related challenges in the face of climate change and rapid urbanisation (Zevenbergen et al., 2018).

Kongjian Yu, in his Ted talk on sponge cities, explains the philosophy of sponge cities as retaining water at source, slowing down its flow, and then merging with larger water bodies at the end of the route (Yu, 2023). Ecological systems, like wetlands, mangroves, and other water-related NbS are deigned within and around the flow of water that is being slowed down. Key features of sponge cities include the incorporation of living elements such as green roofs, pervious surfaces, rainwater gardens, bioswales, bioretention systems, other nature-based stormwater management, and constructed wetlands. These features help to slow down stormwater runoff, increase infiltration into the ground, and promote natural water purification processes.

Integration of sustainable drainage systems (SuDS) techniques, which manage surface water in a way that mimics natural drainage processes is key to sponge cities. SuDS include features like ponds, swales, and infiltration trenches that capture and treat rainwater close to where it falls. 

Water-sensitive urban design (WSUD) is the adoption of design principles that prioritise water purity, management, and conservation in urban environments. This includes water-efficient landscaping, water recycling, and the restoration of natural watercourses. See: Rainwater harvesting, river rewilding, river daylighting, riparian restoration, and bioremediation / phytoremediation of water.

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Name of NbS

Sponge Cities

Type of NbS

Created or constructed living ecosystems; Engineered interventions (not using vegetation); Hybrid living/engineered interventions

Location

  • Urban

Case Study

Wynyard Quarter

Benjakitti Forest Park / Turenscape + Arsomslip Community and Environmental Architect. Image © Srirath Somsawat. From Archdaily https://www.archdaily.com/1008480/landscape-architect-kongjian-yu-pioneer-of-the-sponge-city-concept-wins-the-2023-oberlander-prize

Relationship to Indigenous knowledge

The relationship between sponge cities and Indigenous knowledge lies in the recognition of traditional wisdom and practices in water management that can inform and complement modern urban water strategies. Indigenous communities around the world, including in Te Moananui Oceania have developed intricate systems for managing water sustainably over centuries, often based on a deep understanding of local ecosystems and natural cycles. Indigenous water management practices often take a holistic approach, considering the interconnectedness of water, land, and communities. By integrating traditional knowledge into sponge city initiatives, planners can develop more comprehensive and culturally sensitive water management strategies.

‘Sponge cities’ is a collection of water-related strategies formed into interconnected urban systems. Specific Indigenous or local concepts of water management can be incorporated into sponge cities designs. Te Moananui Oceania examples that might be applicable, particularly related to whole water-shed management are outlined in customary resource management and slow-forming terraces.

Integrating Indigenous knowledge into sponge city initiatives can enrich the design process, implementation, and outcomes of urban water management projects, fostering greater resilience, sustainability, and cultural diversity in urban environments while supporting the celebration of identity.

Climate change benefits
  • Changes in rainfall
  • Flooding
  • Increased temperatures
  • Reduced air quality
  • Reduced water quality
  • Sea level rise
  • Urban heat island effect
  • Reduced freshwater availability

Sponge cities offer significant climate change benefits. The approach introduces softer infrastructure elements into the urban fabric (like vegetation, soil etc), enhancing the city’s ability to absorb and purify water during heavy rainfall events, thus mitigating the risks of extreme flooding. By integrating green infrastructure and various nature-based solutions, sponge cities help to reduce surface runoff and peak flow during heavy rainfall events, thereby decreasing the risk of flooding and erosion. Using rain gardens in urban systems can reduce surface runoff by 25-69% as well as reduce peak runoff by 12-71% for example  (Song, 2022). Using green roofs is also another technique for sponge cities. They can reduce the rate of rainfall-induced surface runoff by up to a delay of 20 minutes (Song, 2022), though this depends very much on the type and location of the green roof (Mackinnon et al., 2022). Sponge cities enhance water purification processes, improving the quality of stormwater runoff before it enters waterways.

Sponge cities contribute to climate resilience by providing options for water storage and retention, which can help mitigate water scarcity during periods of drought (Qi et al., 2020). These initiatives contribute to building climate-resilient cities that are better equipped to withstand the challenges of a changing climate while enhancing the overall quality of life for urban residents (Nguyen et al., 2019).

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Societal / socio-cultural benefits
  • Disaster risk reduction
  • Water security and quality
  • Waste management and hygiene

By incorporating green infrastructure, such as rain gardens and green roofs, sponge cities in Oceania can reduce the urban heat island effect, improving both local microclimates and overall air quality. This creates a more pleasant and liveable urban environment for residents, promoting physical and mental well-being. Additionally, green spaces and natural water features contribute to a sense of connection to nature and enhance the aesthetic appeal of urban areas.
The social and environmental benefits of the sponge city model, as demonstrated in Wuhan, China, include reduced carbon emissions, improved public health, enhanced natural cooling, and increased biodiversity (Oates, L et al., 2020). Much of this relates to the increase in urban blue-green and green space that is associated with sponge cities (Nguyen et al., 2019).

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Ecological and biodiversity benefits
  • Climate regulation
  • Fixation of solar energy
  • Food production (for humans)
  • Habitat provision
  • Medicinal resources
  • Species maintenance

Green walls support biodiversity by providing habitats for insects, birds, and plants in urban areas addressing loss of biodiversity and habitat fragmentation due to urbanisation and land use changes (Mayrand and Clergeau, 2018). They promote plant diversity, contribute to urban green spaces, and enhance ecological connectivity. Vertical greenery in walls offers nesting sites, shelter, and food sources for diverse species. Different plant species attract pollinators, birds, and beneficial insects, creating ecological niches and supporting urban biodiversity (Mayrand and Clergeau, 2018). 

Green walls support plant growth, reproduction, and survival, creating microhabitats for different species. They attract pollinators, birds, and beneficial insects, promoting ecological interactions and species diversity and therefore supporting ecosystem functioning, genetic diversity, and resilience to environmental changes (Mayrand and Clergeau, 2018). 

Solar energy fixation benefits plants by optimising photosynthesis and growth conditions. It also supports wildlife by creating cooler microhabitats and reducing thermal stress in urban landscapes.

Green walls can incorporate medicinal species known for their healing properties, such as herbs used in traditional medicine (Tablada and Kosorić, 2022). They also preserve traditional knowledge about medicinal plants and their cultural significance.

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Technical requirements

To implementation of sponge cities, wider-scale integrated watershed design and planning is needed to ensure the connection between different water management strategies (Nguyen et al., 2019). A thorough understanding of local hydrological conditions, including rainfall patterns, soil characteristics, and existing drainage infrastructure is required. 

Green infrastructure elements in sponge cities require regular maintenance to ensure their effectiveness and longevity. This includes pruning vegetation, cleaning filtration systems, and repairing infrastructure damage.

Schematic diagram of the sponge city concept. Image by Ryanlovelondon, 2012.

Issues and Barriers

Barriers to the implementation of sponge cities in Oceania stem from several challenges. Rapid urbanisation and climate change have led to underground drainage structures becoming less effective in some places. This may pose technical complexities and increase costs for integrating green infrastructure components (Zevenbergen, C et al., 2018).

Implementing sponge city projects requires substantial investments in hybrid and green infrastructure, and ongoing maintenance, making it challenging to secure sufficient funding and financing mechanisms, especially in regions with limited financial resources or competing priorities.

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Opportunities

There are some opportunities for the implementation of sponge cities in Te Moananui Oceania, particularly for small island nations. These nations may be able to leverage their relatively small land mass to better connect nature-based solutions into sponge cities systems, and effectively incorporate restoration of ecosystems where appropriate.

Because many towns and cities are rapidly urbanising in the region, existing hard infrastructure and ineffective water channelling may not exist. These cities can be designed and built on sponge city principles rather than needing to retrofit pre-exisiting grey infrastructure. 

This approach aligns with the principles of sponge cities, capitalises on the unique biodiversity of Te Moananaui Oceania’s island nations, and likely better aligns with traditional Indigenous knowledge and watershed management practices.

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Financial case

While the initial investment in sponge city infrastructure may be significant, the long-term financial benefits often outweigh the costs. Sponge cities reduce the need for expensive traditional infrastructure such as stormwater drains and sewage systems. By incorporating green infrastructure elements instead, sponge cities can effectively manage stormwater runoff, mitigate flooding, and improve water quality at a lower cost (Song, 2022).

Sponge cities promote environmental sustainability by enhancing biodiversity, reducing energy consumption, and mitigating the urban heat island effect, leading to long-term cost savings and improved public health outcomes. 

Sponge cities may have the potential to attract investment, stimulate economic growth, and enhance property values by creating more attractive and resilient urban environments.

References
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Further resources