The collapse of the ancient Cambodian city of Angkor holds lessons for urban resilience today

The ruins of the Ta Phrom temple, Angkor. Image: Diego Delso/Wikimedia Commons.

A series of floods that hit the ancient city of Angkor would have overwhelmed and destroyed its vast water network, according to a new study that provides an explanation for the downfall of the world’s biggest pre-industrial city.

Our research, published in Science Advances, explains how the damage to this vital network would have triggered a series of “cascading failures” that ultimately toppled the entire city. And it holds lessons for today’s cities about the danger posed when crucial infrastructure is overwhelmed.

Angkor, in modern-day Cambodia, was founded in AD802 and abandoned during the 15th century. Its demise coincided with a period of highly variable rainfall in the late 14th and early 15th centuries, with prolonged droughts and extremely wet years.

We know Angkor’s water distribution network was heavily damaged by flooding during that period. But we didn’t have an explanation of how this triggered the city’s eventual collapse and abandonment.

Flooding fate

Angkor is an unusual archaeological site because the remains of the city can still be seen on the ground and, particularly, from the air. It is thus possible to map precisely the constructed features that made up its urban fabric and, from this, to interpret the function and flow of the living city.

We used existing archaeological maps of Angkor to chart the city’s water distribution network, which was made up of hundreds of excavated canals and embankments, temple moats, reservoirs, natural river channels, and other features. This sprawling network, covering more than 1,000km2, provided both irrigation and flood defence.

We then used a computer model to simulate the effects of flooding, such as would have occurred during huge monsoonal rains, to see how the system would have coped with the biggest deluges.

We found that large floods would have been channelled into just a few major pathways, which would have suffered significant erosion as a result. Other parts of the network, meanwhile, would have had less water flow and would have begun to fill up with sediment.

The resulting feedback loop would have caused damage to cascade through the network, ultimately fragmenting Angkor’s water infrastructure.

A watery end. Image: Alcyon/Wikimedia Commons.

There are two main messages from our research. First, it demonstrates how climatic variability in the 14th and 15th centuries could have triggered the demise of the city.

Second, it shows how Angkor’s fate resonates with today’s concerns about the resilience of our own urban infrastructure – not just to extreme weather (although that is important), but also to other potentially damaging events such as terrorism.

Angkor was once the largest city on Earth. But its huge growth made it unworkable, unwieldy, and ultimately irreparable. Its critical urban infrastructure was both complex and interdependent, meaning that a seemingly small disruption (such as a flood) could fracture the entire network and bring down an entire city.

Ancient Angkor, it seems, experienced the same challenges as modern urban networks. As we move further into a period characterised by extreme weather events, the resilience of our urban infrastructure will be tested.


As cities grow, their infrastructure becomes more complex. Eventually, networks such as roads, water infrastructure or electricity grids reach a critical state that is neither predicted nor designed by those that operate them. In these networks, small errors or outages in one part of the network can quickly propagate to become a much larger failure. One example would be an electrical fault that triggers a wide-scale blackout.

Government agencies around the world have developed or are developing strategies to deal with threats to critical infrastructure, including from terrorism, natural disasters and, increasingly, extreme weather events related to climate change. Resilience can be built into infrastructural networks by increasing redundancy (or alternative flow paths) and emphasising modularity, so that cascading failures, if they occur, can be localised while maintaining the function of the wider network.

Our research on the demise of Angkor’s infrastructure sounds a warning from history about the dangers of the complex urban environments in which most humans now live, and the urgent need to prepare for a more variable future.

The Conversation

Dan Penny, Associate Professor, University of Sydney.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

 
 
 
 

Green roofs improve cities – so why don’t all buildings have them?

The green roof at the Kennedy Centre, Washington DC. Image: Getty.

Rooftops covered with grass, vegetable gardens and lush foliage are now a common sight in many cities around the world. More and more private companies and city authorities are investing in green roofs, drawn to their wide-ranging benefits which include savings on energy costs, mitigating the risk from floods, creating habitats for urban wildlife, tackling air pollution and urban heat and even producing food.

A recent report in the UK suggested that the green roof market there is expanding at a rate of 17 per cent each year. The world’s largest rooftop farm will open in Paris in 2020, superseding similar schemes in New York City and Chicago. Stuttgart, in Germany, is thought of as “the green roof capital of Europe”, while Singapore is even installing green roofs on buses.

These increasingly radical urban designs can help cities adapt to the monumental challenges they face, such as access to resources and a lack of green space due to development. But buy-in from city authorities, businesses and other institutions is crucial to ensuring their success – as is research investigating different options to suit the variety of rooftop spaces found in cities.

A growing trend

The UK is relatively new to developing green roofs, and governments and institutions are playing a major role in spreading the practice. London is home to much of the UK’s green roof market, mainly due to forward-thinking policies such as the 2008 London Plan, which paved the way to more than double the area of green roofs in the capital.

Although London has led the way, there are now “living labs” at the Universities of Sheffield and Salford which are helping to establish the precedent elsewhere. The IGNITION project – led by the Greater Manchester Combined Authority – involves the development of a living lab at the University of Salford, with the aim of uncovering ways to convince developers and investors to adopt green roofs.

Ongoing research is showcasing how green roofs can integrate with living walls and sustainable drainage systems on the ground, such as street trees, to better manage water and make the built environment more sustainable.

Research is also demonstrating the social value of green roofs. Doctors are increasingly prescribing time spent gardening outdoors for patients dealiong with anxiety and depression. And research has found that access to even the most basic green spaces can provide a better quality of life for dementia sufferers and help prevent obesity.

An edible roof at Fenway Park, stadium of the Boston Red Sox. Image: Michael Hardman/author provided.

In North America, green roofs have become mainstream, with a wide array of expansive, accessible and food-producing roofs installed in buildings. Again, city leaders and authorities have helped push the movement forward – only recently, San Francisco created a policy requiring new buildings to have green roofs. Toronto has policies dating from the 1990s, encouraging the development of urban farms on rooftops.

These countries also benefit from having newer buildings, which make it easier to install green roofs. Being able to store and distribute water right across the rooftop is crucial to maintaining the plants on any green roof – especially on “edible roofs” which farm fruit and vegetables. And it’s much easier to create this capacity in newer buildings, which can typically hold greater weight, than retro-fit old ones. Having a stronger roof also makes it easier to grow a greater variety of plants, since the soil can be deeper.


The new normal?

For green roofs to become the norm for new developments, there needs to be buy-in from public authorities and private actors. Those responsible for maintaining buildings may have to acquire new skills, such as landscaping, and in some cases volunteers may be needed to help out. Other considerations include installing drainage paths, meeting health and safety requirements and perhaps allowing access for the public, as well as planning restrictions and disruption from regular ativities in and around the buildings during installation.

To convince investors and developers that installing green roofs is worthwhile, economic arguments are still the most important. The term “natural capital” has been developed to explain the economic value of nature; for example, measuring the money saved by installing natural solutions to protect against flood damage, adapt to climate change or help people lead healthier and happier lives.

As the expertise about green roofs grows, official standards have been developed to ensure that they are designed, built and maintained properly, and function well. Improvements in the science and technology underpinning green roof development have also led to new variations on the concept.

For example, “blue roofs” increase the capacity of buildings to hold water over longer periods of time, rather than drain away quickly – crucial in times of heavier rainfall. There are also combinations of green roofs with solar panels, and “brown roofs” which are wilder in nature and maximise biodiversity.

If the trend continues, it could create new jobs and a more vibrant and sustainable local food economy – alongside many other benefits. There are still barriers to overcome, but the evidence so far indicates that green roofs have the potential to transform cities and help them function sustainably long into the future. The success stories need to be studied and replicated elsewhere, to make green, blue, brown and food-producing roofs the norm in cities around the world.

Michael Hardman, Senior Lecturer in Urban Geography, University of Salford and Nick Davies, Research Fellow, University of Salford.

This article is republished from The Conversation under a Creative Commons license. Read the original article.