How urban infrastructure falls apart: a medieval cautionary climate tale

The downfall of the medieval city of Angkor contains lessons about the vulnerability of modern cities to extreme weather events – the kind that are likely to increase in frequency and intensity with climate change. An analysis published last week in Science Advances connects the dots between past and present.

Angkor, in present-day Cambodia, was the largest city in the pre-industrial world, covering more than 1,000 square kilometers at its peak. The new study suggests that flood damage to the city’s complex water distribution system touched off a cascading series of failures that likely played a role in Angkor’s decline and near-abandonment during the 15th century.

Of course, climate change has played a role in the abandonment of other cities and even the fall of civilizations. But this is the first study to show specifically how urban infrastructure is involved in such a collapse. That’s especially relevant today since the majority of the world’s population lives in cities.

In the study, researchers developed a computer model based on archaeological maps of Angkor’s water distribution network in the mid-14th century. This complex system of canals, reservoirs, dikes, and moats was developed over the course of more than 500 years. It provided flood control in Angkor’s urban core and irrigation water for agriculture in the outlying areas of the sprawling, low-density city.

In the late 14th century, the climate of Southeast Asia abruptly became much rainier than it had been before. Scientists have suspected for a while that changing climate and rainfall patterns had something to do with Angkor’s decline. The new study provides a quantitative analysis of how things went wrong.

The researchers used their computer model, which is composed of 1,013 canals and other structures that direct the flow of water and 617 connections between them, to compute how flooding could damage different parts of the water network.

They found that water flows above a certain threshold will cause erosion in some canals. Erosion begets erosion because the now-wider canals take on a greater proportion of the water from upstream. In turn, flow is reduced in adjacent parts of the network, producing sedimentation in the low-flow channels.

“Very large floods will therefore quickly result in systemic instability in the water distribution network,” the researchers write. The bigger the flood, the greater the likelihood and extent of these cascading failures.

The model predicts that the upstream, northern and western parts of Angkor’s water network will be more prone to damage than the central urban core and southeastern region. This fits pretty well with the archaeological data known so far.

Angkor’s history illustrates the importance of building resilience into modern cities’ infrastructure, the researchers say. And it suggests there’s a danger in building only for past climates, without looking ahead to potential future risks. Angkor’s water network and agricultural practices were likely adapted to periods of prolonged drought that occurred during the 13th and 14th centuries. So when the climate regime shifted, the city’s infrastructure was vulnerable.

“The basic pathology of Angkor’s collapse is analogous to the challenges faced by networked urban infrastructures in the modern world,” the researchers write. “This was not an exotic catastrophe with no modern analog.”

Source: Penny D. et al. “The demise of Angkor: Systemic vulnerability of urban infrastructure to climatic variations. Science Advances. 2018.

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