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The capital city of South Korea grew up, like so many towns and cities, around a reliable source of water. During the Joseon Dynasty, the river flowing through what became modern-day Seoul was known as Gaecheon, which translates as ‘open stream’. In later years the name was changed to Cheonggyecheon, meaning ‘clean water stream’.

By the late 1960s, new technologies for abstracting, transporting and treating water meant that the stream was no longer considered an asset to the city. In 1968, an elevated motorway was built over the, by then dry, riverbed.

From the 1970s to the 1990s, this motorway brought tens of thousands of commuters into central Seoul daily. Air and noise pollution soared, and in the summer months the temperature in the districts around the highway was often several degrees higher than in other parts of the city due to severe heat-island effects generated by miles of hot concrete and tarmac. Businesses and people along this former river corridor began to suffer, and by the early 2000s, what was once considered a solution to a booming urban population was recognised as a serious liability. Soon, the city of Seoul decided to take action.

In 2005, after just 29 months of construction, the elevated motorway had been completely removed and the ‘clean water stream’ was flowing once again.��

The return of the Cheonggyecheon river is one of the most inspiring restoration projects in our modern urban history. It cost��a mere $281 million, a bargain for the economic, social and environmental benefits gained.

Habitats and species soon reappeared creating an artery for urban wildlife. The number of cars entering downtown immediately fell by 2.3% as people switched to rail, bus and bike. The choking pollution has abated and summer temperatures are now 2-5 degrees lower on any given day. The people of Seoul love the place too. In 2005, even before it became a popular destination for picnics and cultural events, a public survey showed respondents overwhelmingly noticed improvements in air and water quality, noise and smells. Businesses are booming once again along the stream sides and traffic congestion has reduced in the neighbouring streets.

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The return of the Cheonggyecheon river is one of the most inspiring restoration projects in our modern urban history. The cost of its transformation was a mere $281 million, a bargain for the economic, social and environmental benefits gained. Why then is this type of urban restoration project, which combines nature-based solutions with classic engineering, still relatively rare?

One reason is a lack of ecological awareness among those making decisions on the future shape and structure of cities.

The key to greening cities

Climate change-induced flooding and severe heatwaves already disproportionately affect urban and peri-urban areas. By 2050, climate change is likely to place at least 570 coastal cities and over 800 million people at risk from sea level rise and storm surges. The UN estimates that global economic damages of climate change under an “optimistic” 1.5-degree scenario will reach $54 trillion by the end of century, with much of this impact in cities.

Anticipating change now and making design decisions that build resilience into urban environments could help us cope better with climate shocks and help cities recover more quickly.

For a new wave of nature-based design solutions to be successful however, a fundamental rethink about how we perceive the physical form of cities is needed. All cities have traces - some more, some less - of the underlying natural ecosystem on which they were built. No city is truly separate or distinct from the geology, soils, water and natural habitats on which it was built. This is too often forgotten but is key to the ‘greening’ of cities.

Sculptors use the term armature for the framework around which a sculpture is moulded. For a city, its armature is its underlying ecosystem. An understanding of this armature is critical to making more informed decisions on how best to build urban resilience, improve human well-being and create prosperity.

All cities have traces - some more, some less - of the underlying natural ecosystem on which they were built. No city is truly separate or distinct from the geology, soils, water and natural habitats on which it was built.��

Towards a new ecological urbanism

In 2018, my colleague Ed Taylor and I looked into the relationship between the built and natural environment in cities with a view to defining some core principles of ‘ecological urbanism’. Our idea was that if an ecologist and an architect (Ed is the architect) could fuse together some empirically-based principles around what, on the one hand, makes a functional ecosystem, and on the other hand, a great urban neighbourhood, we might then be able to apply these principles in a very practical way to the urban sustainability challenge.

Environmentalists generally want bigger and more natural greenspaces, fewer cars and less pollution entering urban rivers. Meanwhile, planners, architects and urban designers are calling for inspiring buildings, more and better housing and amenities and more efficient transport solutions. Add to these the various demands from road engineers, water engineers, community interest groups, emergency services and varied business interests and the trade-offs soon become difficult to untangle.

Codifying all these competing needs into a list of infallible design solutions for each situation is probably not possible – but Ed and I attempted to at least codify a framework within which design decisions could be made. Urban ecosystems are comprised of different components from the micro (individual street trees or small gardens, for example) to the macro levels (such as large river corridors or major semi-natural parks). These different components can, if well designed, link together to form a connected system.

Aside from the sheer volume of greenspace, there are three key ecological principles that determine the degree to which urban ecosystems function effectively and, by extension, support the health, wellbeing and related benefits for people:

• Degree of connectivity – the extent to which large and small green-blue spaces and freshwater systems are joined up, physically and functionally
• Degree of naturalness – how close the flora, fauna, soils and freshwater systems are to what was there before the city was built
• Degree of structural diversity – the structural complexity and variety of the flora, fauna, soils and freshwater systems present

But what of the built dimension of ‘ecological urbanism’? Ed and I began to realise that these three ecological principles had neat parallels, or coequals, when applied to the grey infrastructure components of the city. When flipping from green to grey, proven ecological principles become well-established urbanism principles. So:

• Degree of connected sustainable transport networks equates with connectivity
• Degree of locally adapted design styles and building materials equates with naturalness
• Degree of vibrant, medium density, mixed-use neighbourhoods equates with structural diversity

Urban ecosystem health

Ecologically, there is good scientific basis for selecting connectivity, naturalness and structural diversity as the most important factors determining urban ecosystem health.

In general, the smaller and more isolated an area of green-blue space, the fewer species it can support and the less likely those species are able to move to other green-blue patches in the city. In effect, individual greenspaces act like islands in a sea of built development. This is the basis for the Theory of Island Biogeography, first described in 1967 by ecologists Robert MacArthur and Edward Wilson. Simply put, the theory holds that larger and less isolated islands support more species in a more stable equilibrium than smaller, more isolated islands. Hence by linking up isolated patches of green-blue space, they begin to behave like one larger, more stable patch, with more species that are less likely to become locally extinct.

But connectivity of green-blue space is also important for people. Large areas of unbroken grey infrastructure are not only impenetrable to nature but can lead to a disconnection of the urban population from nature. This has well studied physical and psychological impacts on health and wellbeing. A nation-wide study conducted in Denmark in 2018 and covering over 900,000 people concluded that children who grow up with very low levels of greenspace had up to 55% higher risk of developing a psychiatric disorder, independent from effects of other known risk factors.

When it comes to naturalness, many studies have shown that native plants support the lifecycles of between 10 and 100 times more insect species than non-native plants. These in turn support more species of birds and mammals. Higher levels of species and habitat diversity are particularly important in stabilising ecosystems with inherently low levels of diversity, such as cities.

Similarly, the greater the structural diversity within stands of vegetation and habitats, the more ecological niches are available to support a greater diversity of species. Evidence from research shows that through designing large and small greenspaces that are connected to each other, are rich in native species, and have diverse habitat structure, it is possible to significantly increase the health of the urban ecosystem and its capacity to provide benefits to both people and wildlife.

Grey and green together

Urban environments are complex modified ecosystems in which the grey and green infrastructure are so inextricably bound together that they need to be managed together, as a system.

Positive cascade effects from improving the quality and connectedness of the green-blue infrastructure might include increased walking and cycling that in turn decreases car use and increases the walkability and socialisation of neighbourhoods. This in turn might lead to a greater community sense of security and reduce crime. This may then attract new entrepreneurial talent to live and work in the area and encourage businesses to invest, leading to better care of green-blue infrastructure and further improvements in air quality and the health of the local population and so on.

Urban environments are complex modified ecosystems in which the grey and green infrastructure are so inextricably bound together that they need to be managed together, as a system.

Clearly these relationships are not linear, but rather comprise a highly complex network of connections similar to the web of life in nature. Ecological urbanism seeks to support the functioning of this system by combining nature-based design with excellence in architecture, planning and placemaking. It is a very practical approach and will, I hope, improve ecological literacy among urban decision makers.

I invite all interested ������ϲʿ�������ֳ�ֱ�� Member organisations and Commission members, but also city planners and policy makers in urban areas, to connect with the ������ϲʿ�������ֳ�ֱ�� Urban Alliance, which offers a platform for debate and knowledge exchange on urban dimensions of nature conservation and seeks to catalyse collaboration and partnership for nature-positive urban development. The Urban Alliance is currently developing a new Urban Nature Index which aims to help cities evaluate their natural capital, set targets for improvement, and monitor progress. Together, I hope we will pave the way for inspiring projects like the Cheonggyecheon to become as commonplace as building a city block.

Find out more about the ������ϲʿ�������ֳ�ֱ�� Urban Alliance at: .