Marshall Shepherd on how roads and roofs help cause urban floods

A June 2010 study blames impervious surfaces – roofs, streets, and sidewalks – for recent major floods in Atlanta, Nashville and Oklahoma City.

Heavy rains were not the only cause of recent urban flooding, according to University of Georgia research meteorologist Marshall Shepherd. His June 2010 study also blames what he call ‘impervious cover’ – that is, roofs, streets, and sidewalks – for major flooding in Atlanta in 2009 and Nashville and Oklahoma City in 2010.

Marshall Shepherd: What we found with the Atlanta floods, and indeed many of the recent floods in 2010, is that it was a perfect convergence of several meteorological factors. But one of the other interesting aspects of this flooding event is the real signature of the impervious surfaces of the urban land cover.

Impervious surfaces, like roofs of buildings, sidewalks and parking lots, don’t absorb the rainwater the way soil does. The water runs off.

Marshall Shepherd: Coupled with streams and lakes that were already nearing capacity in terms of soil moisture, when you dump a lot of rainfall on urban impervious surfaces, it runs off in volumes quite rapidly. And it really had nowhere to go.

Dr. Shepherd said that humans are playing a bigger role than before in creating conditions that are ripe for floods in cities.

Marshall Shepherd: What we’re clearly seeing is that the role of human beings and their impervious surface is as much a part of the hydrological water cycle system in cities as the natural environment itself. These go hand in hand.

Dr. Marshall spoke more about the recent urban flooding.

Marshall Shepherd: The Atlanta floods of 2009 were really historic floods. We’re talking about 100 to 500 year flood events in many cases. And with the recent flooding in 2010 and places like Nashville and Oklahoma City, we were really interested in the context of that flood, it’s really historic nature and whether there are any possible implications as it relates to climate change and meteorological hazards in general.

Looking forward, Dr. Shepherd voiced concern over elevated risk to cities from flooding.

Marshall Shepherd: With each of the flooding events that we’ve seen recently in 2009 and 2010, there are certainly meteorological explanations. But what is important to note is that two things are happening. Urban impervious surfaces are increasing, which exacerbates these flooding events even more than they would have even a hundred years ago. And secondly, the Intergovernmental Panel on Climate Change has noted that the frequency and/or the intensity of extreme flooding events is expected to increase under a globally warmer climate. Now it’s certainly irresponsible at this point to suggest that these recent flooding events are directly linked to climate change, although these types of events are certainly consistent with what we would expect as our climate warms. So, really the message that we want to convey in this work is that the combination of increased impervious surfaces and the specter of increasing events like this under changing climate scenarios should certainly cause us to think about how we deal with these types of flooding events going forward.

Jorge Salazar