Submerged Cities

Professor Peter Groffman (GC/Brooklyn, Earth and Environmental Sciences, pictured below), who is on the faculty of the Environmental Sciences Initiative at the Advanced Science Research Center at the Graduate Center, recently spoke to the GC about urbanization and storm-related flooding, topics that have taken on a new urgency after multiple hurricanes in the last month alone caused extensive damage to urban areas.
 
Groffman is the author of a recent study on urban ecosystem homogenization, and is one of the lead investigators, along with Professor William D. Solecki (GC/Hunter, Earth and Environmental Sciences), of a National Science Foundation-funded sustainability research network project on "Urban Resilience to Extreme Events."

To what extent was the flooding in Houston following Hurricane Harvey due to overdevelopment?
Certainly urban development makes the flooding associated with rainfall events much worse. And Houston, which is flat and low lying, is particularly vulnerable to flooding.

However, 50 inches of rain is a lot of rain! The extraordinary magnitude of the event was by far the big driver of the flooding in Houston.

And what about Puerto Rico? How did paving and urbanization affect the floods that have wreaked havoc on the island after Hurricane Maria?
Again, the extraordinary nature of the event is the big driver of the flooding in Puerto Rico. Paving and urbanization have the effect of turning ordinary rainfall events into extraordinary floods. It is important to keep this distinction in mind: flooding induced by urbanization is one big problem, and flooding induced by extraordinary rainfall events is something else.

Urbanization is not going away, so how can we protect cities from similar disasters? What, in your mind, are the most important preventative steps? 
Cities across the world have been taking steps to reduce the impacts of flooding reduced by urbanization. The development of new approaches to "green infrastructure" that use natural plant and soil processes to absorb floodwater has been one of the most exciting areas of advancement in environmental science over the past 20 years or so. 

What other urban areas are especially vulnerable to flooding?
Vulnerability to flooding is surprisingly widespread. All coastal areas are subject to storm-surge flooding. Anywhere rainfall is high is vulnerable. Even arid areas are vulnerable because even though it doesn't rain a lot, the rain often comes in very intense events that cause flooding. We like to point out that flooding associated with urbanization is the second-oldest environmental problem that humans have faced. When the first cave person put a roof up, they created the first storm water problem.

Do you see any true successes out there, in terms of urban areas that are prepared for floods?
One of my favorite examples is in a major stream that runs through the Phoenix metropolitan area (Indian Bend Wash). Instead of digging a deep concrete trench with steep concrete walls, they instead let this stream run onto its floodplain a few days a year when they get big rainfall events. So on most days, the floodplain is a heavily used public park and for just a few days it is flooded. This is an example of a more adaptive, flexible approach to flooding, often called "safe to fail," that is a marked contrast to the heavily engineered "fail safe" approaches that have been taken in the past.

Is New York City more or less vulnerable after Hurricane Sandy? How does it compare to Houston?
In some ways we are less vulnerable than we were following Sandy. Emergency generators have been moved out of basements, evacuation protocols have been improved, and some homes have been raised. In other ways, we are still quite vulnerable to a large storm surge such as produced by Sandy. Building resilience to extreme events is a long process, one that the city has moved on pretty aggressively. But it is a long process and it is easy for our focus to fade along with our memories of the storm.