Les effets du changement climatique mondial risquent d'aggraver un environnement déjà fragile. La fréquence grandissant des prévisions de conditions météorologiques extrêmes causeront surement des dommages dévastateurs aux populations africaines vulnérables, car les inondations graves, la sécheresse et les maladies menacent la survie et la sécurité des populations. L'infrastructure grise conventionnelle, coûteuse et irréversible, qui soutient le développement dans les pays du Nord, a peu de chances de s'adapter à la situation climatique extrême à laquelle denombreuses villes africaines sont confrontées. Des infrastructures innovantes et adaptées au climat doivent êtreconçues, dessinées et mises en œuvre localement dans les pays africains en voiede développement. Des solutions basées sur la nature et la sagesse vernaculaire intégrées aux technologies modernes aiderontles pays africains à construire leurs villes de manière plus innovante et résiliente. L'expérience de la campagne de la ville éponge en Chine, évoquée dans cet article, pourrait inspirer les pairs africains.
The effects of global climate change are likely to worsen an already fragile environment. The predicted more frequent extreme weather conditions can be expected to cause devastating damage to vulnerable African populations as severe floods, drought, and disease threaten people's survival and security. The conventional and expensive, irreversible grey infrastructure that supports development in the Global North is unlikely to adapt to the extreme climate situations that many African cities face. Innovative and climate-adaptive infrastructure should be locally conceived, designed and implemented in developing African countries. Nature-based solutions and vernacular wisdom integrated with modern technologies could help African countries build their cities to be more innovative and resilient. The experience of the sponge city campaign in China, discussed in this article, might inspire African peers.
‘Sponge city’ is a relatively new term loosely defined as using natural landscapes to catch, store and clean water, but the concept has roots that run far back through the history of human adaptation to climate challenges, particularly in the monsoon world.
Turenscape, an award-winning landscape architecture and urbanism practice with more than 500professionals based in Beijing, is constantly developing and implementing nature-based techniques and solutions to address environmental issues across multiple scales, from eco-home design and ecological urbanism to national-level ecological planning.  It has become a major advocate of the sponge city concept, which has proved to be an effective nature-based adaptive solution to climate change and water-related urban issues.
For cultures that evolved with, and survived, the fluctuations of nature over thousands of years, including drought- and flood-adapted monsoon aquaculture, adapting to changing climates is nothing new. However, this kind of ancient wisdom has been buried under the conventions of so-called industrial civilisation. In the past half-century, little has been done to change the way we build cities. We still rely primarily on inflexible ‘grey infrastructures’ such as drainage channels, steel pipes and concrete flood walls for building and security, but climate change is challenging the way we live and the technologies we use to build. Grey infrastructure simply lacks resilience.
We need a paradigm shift in planning and designing our cities to adapt to the changing climate. We need to rethink single-minded, industrial technology-based engineering solutions and turn to nature-based and symbiotic solutions. Ultimately, we need to rediscover the ancient art of survival that has been evolving for as long as humans have been on the earth. 
It is from this mindset that the concept of sponge cities grew, with an emphasis on using landscape as a form of ecological infrastructure. In practice, the concept looks to provide holistic ecosystem services, including regulating water throughout seasons, cleaning contaminated water through biological processes and nourishing habitats for biodiversity. It is also means adapting to climate change and creating beauty and spiritual nourishment for local communities.
A sponge city requires a fundamental change in thinking about urban infrastructure and the city. It envisions the removal of concrete dams and dikes to be replaced by naturalised waterways, storm water pipes to be replaced by bio-swales and sewage treatment plants to be replaced by constructed wetlands, which use natural and biological processes to provide the same kind of services. Even if this green infrastructure approach does not ultimately sweep away all industrial technology-based grey infrastructure, it offers new options that can complement traditional infrastructure to tackle the climate crisis.
To create a sponge city based on this type of ecological infrastructure, two challenges must be addressed:
1. Finding effective nature-based techniques that can be used at a sufficiently extensive scale to fix the built environment and heal the ravaged planet, while being cost-effective.
2. Breaking through the business-as-usual inertia that favour's grey infrastructure.
A major challenge in creating ecological infrastructure is developing sharable techniques that can be executed easily and inexpensively at an extensive scale. For inspiration, Turenscape turned to the global –and particularly Chinese – ancient knowledge of water management in farming and settlement-building. For centuries, farmers employed simple cut-and-fill techniques to dramatically transform the surface of the earth and successfully adapt to various types of forbidding environments, using nothing more complex than a hoe.
In south-west China’s mountainous areas, for example, rice paddy terraces were built across vast territories. As long as 2,000 years ago, Chinese farmers knew that 20% of their cultivated land needed to be set aside and used for ponds to regulate water through the rainy and dry seasons. In China’s Pearl River Delta, a pond-and-dike system was developed to transform the marshy land into one of the most productive and populated landscapes in the world. In what is now Mexico, the Aztecs created floating islands to turn otherwise uninhabitable lakes for agriculture and settlement use as early as the twelfth century.
Some of these basic, time-tested landscape techniques, such as terracing, ponding, ponding-and diking and islanding are affordable, scalable methods that can be used today as we encounter floods, droughts and extreme weather caused by climate change.
Beyond the technical aspects of nature-based solutions, it is no less important to build consensus among leaders and citizens about the need to break through the inertia of grey infrastructure, which can require a political campaign of sorts. For more than 20 years, the author has:
- written open letters to key decision-makers, including top national authorities
- published books and articles targeted at mayors throughout China.
- given 350 face-to-face presentations to urban decisionmakers
- spread the message publicly via channels such as China Central Television.
Typically, the practice is commissioned to design a project only after spending a fair amount of effort convincing municipal-level decision-makers of the potential of nature-based solutions and ecological aesthetics.
In many cases, even after Turenscape has been commissioned, the practice will return to explain the underlying philosophy behind the project in order to build understanding and support among city government officials before they approve the specific design.
Several projects at different scales on China’s Hainan Island illustrate how Turenscape has turned sponge city thinking into real, on-the-ground results.
Under the influence of a monsoon climate and now global climate change, China’s Hainan Island, located off the southern coast near Vietnam, frequently suffers severe floods and urban inundation, together with other water-associated problems such as pollution and habitat loss. Those challenges made the island a good demonstration site for China’s national Sponge City campaign, launched in 2014 by the central government to test and promote sponge city thinking to address the overwhelming urban flooding issues that confront more than two-thirds of Chinese cities. In 2015, local governments in Hainan, along with the Chinese Ministry of Housing and Urban and Rural Construction, launched an island-wide Sponge City campaign focused on the two most densely populated cities, Haikou with a population of 2.3 million and Sanya with a population of 0.64 million, and we were commissioned to carry out some major projects in these cities.
Taking advantage of the top-down political system, we gave six major presentations on the sponge city concept and ecological restoration that were made mandatory for government officials of all ranks on Hainan Island; altogether, more than 3,000 officials attended.
Meanwhile, public mass media was extensively used and more than 50 hours of television time was devoted to rebroadcasting the presentation series, which helped build broad community-level support.
A water-based ecological infrastructure plan was developed for both cities. Each plan integrated areas at high risk of flooding, wetlands, ponds, rice paddies, parks and coastal habitats into a holistic sponge system to retain, clean and recycle water. The plans called for more space for flood accommodation, which necessitated the removal of some illegal buildings in the floodplain. Concrete flood walls that had channeled and reduced the resilience of rivers were replaced with an eco-friendly embankment using earth work and flush vegetation.
The design identified areas for ecological restoration and wetland construction along the coastline and waterways to increase flood resilience and, in urban areas, the creation of bio-swales along major roads to adapt to the monsoon climate and alleviate waterlogging. The ecological infrastructure also integrated an interconnected system of pedestrian and bicycle paths. Overall, the process offered a chance to turn the challenges of climate change into opportunities for urban renewal and a transformation of the two cities.
Given the challenges of executing a master sponge city plan within any reasonable timeframe, it is far more feasible to incrementally carry out projects in individual sub-drainages of the larger watershed.
The Meishe River corridor reconstruction was an important demonstration of such a drainage-based project in Haikou City.
The 23-km-long Meishe River is the main river of Haikou city on Hainan’s northernmost tip. For decades, piecemeal solutions to the river’s problems were attempted, including building walls and locks to control floods and tides, dredging, growing flowers and laying lawns at the riverbank, and channeling polluted tributaries. However, those measures only exacerbated flooding, pollution and other problems. The flood control walls turned the river into a lifeless concrete channel which most residents turned their backs on and treated as a sewage dump. In 2016,Turenscape was commissioned to reconceive the Meishe as a type of ecological infrastructure to improve flood control and water quality, restore natural habitat and create much needed public spaces.
A green sponge system was planned by integrating the river with all its tributaries, wetlands and potential greenspaces, and to separate storm water from sewage flows. We also designed an interconnected pedestrian and recreational network. Concrete flood walls and locks were removed. The blocked river was reconnected to the ocean so that tides could once again enter the city. Wetlands and shallow river margins were reconstructed so that mangroves could be restored.
A terraced mosaic of wetlands along the banks of the river was designed as water cleansing facilities that filter urban runoff –and, in some cases, primary treated sewage from local ‘urban villages’ that are currently unable to access the central sewage treatment system. The wetland can clean 6,000 tons of wastewater effluent daily from grade V to III(i.e. from the poorest surface water quality to non-portable clean water). It is tested regularly to ensure that the water quality is safe and sufficiently clean for public use. The biomass from the wetland is harvested and regularly decomposed into fertilisers for use in the landscaping, creating a circular process. The project has proven a success.
Haikou is now more resilient to monsoon floods, the river water is clean again, mangroves are re-established, fish and birds have returned, and tens of thousands of people have visited the new landscape. Even more significantly, the nature-based solutions showcased in this project are replicable elsewhere.
To reduce the risk of climate change-driven flooding on the tropical island of Hainan, it is critical to restore mangroves along the waterways and coastal shoreline. One of the key challenges is finding an efficient and cost-effective method to restore mangrove habitat at a broad scale.
Site challenges and objectives
The 10-ha site on the bank of the Sanya River, on the southern tip of Hainan Island, is critical in terms of ecological relationships between the sea and the inland coast, where the daily tides meet with the river’s fresh water. High concrete walls enclosed the site and allowed it to become choked with urban debris. The design objectives were to rehabilitate the mangroves and make the site a park. Several challenges had to be addressed, including:
- the mangrove community had to be protected from both storm surge and monsoon-driven floods originating upstream, which can wash young mangroves away
- sensitive mangrove seedlings needed protection from polluted urban runoff
- habitat restoration needed to be balanced with public accessibility.
Design strategies: form follows process
Materials composed of urban construction debris and concrete from the demolition of the flood wall was recycled onsite. Cut-and-fill techniques were subsequently used to create a gradient of different riparian habitats, which are for diverse fauna and flora particularly different species of mangroves. An interlocking finger design was used to lead ocean tides into the waterways, while also attenuating the impact of both tropical storm surge and flash floods originating in the urban and upland area upstream, both of which can harm the establishment of mangroves. This also maximised habitat diversity and edge effects, which increased the interface between plants and water and which, in turn, enhanced ecological processes, such as nutrient removal from the water. The water depth varies from 0 m to 1.5 m, creating a dynamic aquatic environment that follows the rise and fall of tides and provides several aquatic species with the daily water-level fluctuation they need for survival. Terraces are staggered along the 9-m-high gradient from city streets to the river and have been augmented with bio-swales to catch and filter urban stormwater runoff. The terraces also provide public recreation spaces.
Just three years after its construction, the mangroves are well-established and fish and birds are abundant.
The park has become a showcase of ecological restoration that not only benefits the natural environment but public wellbeing as well.
The approach tested in this project has been implemented in other mangrove restoration projects at a large scale throughout the region.
Urban inundation is one climate change-driven problem that directly affects people’s lives. A tropical storm can easily bring more than 200 mm of precipitation in a single day, which can easily overwhelm conventional drainage systems. Creating a green sponge in the centre of the urban environment is an essential adaptation strategy for increasing resilience to climate change.
Site challenges and design objectives
The heavily polluted 68-ha site lies along the Sanya River corridor, and was filled with non-permitted buildings and illegally dumped urban debris, which caused severe flooding and a noxious odour in the surrounding communities. Meanwhile, public spaces were badly needed.
Design strategies: ponding, diking and islanding
Inspired by the ancient pond-and-dike systems and islanding techniques in the Pearl River Delta, a necklace of ponds and dikes were created along the periphery of the park that catch and filter urban runoff from the surrounding communities using simple cut-and-fill methods. The dikes are connected to form a pedestrian network and are dotted with recreation areas. In the central part of the park, dirt and fill were used to create islands that are planted with banyan trees to create a forested wetland. Both ponding and islanding dramatically increase the water-retention capacity of the park and increase the ecotones between water and land to speed up the removal of nutrients.
The constructed wetland can accommodate 830,000 cubic metres of storm water, dramatically reducing the risk of urban inundation. The property value in the surrounding area has increased from less than US$1,000 per square meter to about US$4,000.
It has become a model project for the national Sponge City campaign and has been visited by more than 200 mayors seeking new ideas.
Climate change is real, and it is only exacerbating a host of serious problems that the world is already facing, ranging from floods, droughts and air and water pollution to loss of habitat and biodiversity. While the urban elite in developed countries are worried about how their standard of living will be affected by the drastic reduction in greenhouse gas emissions needed to head off the climate crisis, people in other regions are struggling for basic survival – as they have for generations.
Globally, over 85% of sewage in urban areas – mainly in developing countries – is untreated and pollutes rivers, lakes and oceans and worsens existing water shortages. While centralised sewage treatment systems are economically infeasible for some isolated settlements, nature-based solutions can play an important role in improving water quality and softening the impact of floods, as well as creating public spaces that fulfil important cultural and social needs. In addition, 3% of energy in developed countries is used for wastewater treatment; that number could be cut by 75% if nature-based water cleansing processes are integrated with traditional primary wastewater treatment. Doing so would be a tremendous contribution to the effort to stop climate change.
In the past three decades, China consumed about half of the world’s cement every year, yet still only managed to pave less than 3% of the country’s total surface area. In the name of preventing floods, China channeled most of the rivers running through its urban areas – but now, perversely, more than 400 of its cities flood every year. If this model is adopted more widely, how much more cement will it take to pave all the developing countries in the monsoon world? And how many cities will be built of concrete without any resilience?
We need to rethink the way we build our cities. Nature-based solutions are not only affordable but also generate far fewer greenhouse gas emissions. As a result, they are an important way to not only help communities adapt to climate change, but to actually tackle the emissions problem itself.
Humans have adapted to harsh and unpredictable climates for millennia. It is only recently that we have become caught in the trap of the so-called technology of high expense, which also comes at high cost in terms of greenhouse gas emissions. Not only that, but it has ensnared us in a Sisyphean cycle of perpetually trying to fix specific parts of the system while ultimately only worsening the environment as a whole.
The shift from grey infrastructure to green is challenging traditional design practices technically, aesthetically and ethically. It will revolutionise the moral criteria and values that professionals are compelled to follow in practice. It will require design languages and designed forms that are not only climate positive but also aesthetically inspiring. It is already demanding innovative green technologies that are replicable and can be inexpensively implemented at the massive scale needed to halt this truly global threat – as our examples have shown. Turenscape is striving to meet these challenges to create deep forms through designed ecologies that will simultaneously make the built environment climate resilient and reduce the emissions that drive climate change itself.
The threat of climate change gives us an opportunity to rediscover hard-won ancient knowledge and bring it to bear on a challenge the world has never seen before. Even more important, it is helping us not just rediscover but actually strengthen the connection between man and nature.
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All images and diagrams by Turenscape