Edward J. Blakely is honorary professor of urban policy at the United States Studies Centre at the University of Sydney. He served as the Executive Director of Recovery in New Orleans after Hurricane Katrina. He is an international urban policy theorist and practitioner. His most recent book is "My Storm."

The city of Angkor Wat, Cambodia, was a vibrant, growing metropolis in the late 17th century. Angkor was the New York, Paris or Rome of its time. At its peak from the 9th to 17th centuries AD, no one could have imagined any threat to this Khmer city-state. Yet, Angkor collapsed almost totally in the 17th century, and the reasons behind its demise offer an important lesson for today’s cities.

Angkor was built on a vast transportation network: canals acted substantially like freeways. The metropolis grew by expanding its network of canals from the central city to form a vast complex of suburban satellites. As depicted below, this was a gigantic enterprise. Ankgor grew exponentially as internal wealth and power increased. The waterways allowed goods and people to move well beyond the central core of the city.

But as Angkor continued to grow, its waterways became more fragile and vulnerable. Rain and other small but severe weather changes occurred, and the system began to crumble. My colleague Roland Fletcher, a professor of architecture at the University of Sydney in Australia, describes this process as “low density metropolitan collapse.”...