Native Plants, Habitat Restoration, and Other Science Snippets from Athens, Georgia

Thursday: 24 September 2009

Perfect Storm  -  @ 08:27:55
You can't have missed hearing of the flooding that occurred in northwest Georgia this past week. The traditional media concentrated on the drama and tragedy, and little was said about the cause or extent, other than that there was a lotta rain. I'm sure there is discussion about this in wonkier policy and related venues.

So out of curiosity I used the CoCoRaHS rain reports for counties in north Georgia to determine the extent of rainfall from last Friday through Monday. I averaged the reports and colored in the counties (note that the rain actually started the Tuesday before). In five counties there were individual reports that were in the 15-20" range, so I marked the county with a red circle. (The white counties were either not done or didn't have a cocorahs observer.)

That wasn't enough, though, so at this website I found population density data for 2008. Again I colored in the counties by their ranking - the six most densely populated counties in red, and so forth. That's the inset at the lower left.

You can't really read the county names, and that's ok. I also didn't bother to indicate the location of Atlanta - you can see exactly where it is in northwest Georgia - that large multicolored bullseye with red at the center.

The coincidence of most of the heaviest rainfall and the most densely populated counties is unmistakeable. As you can see, this wasn't just any rainfall event that covered a large region that happened to include the Atlanta metro area - the most intense rainfall practically targetted the Atlanta area.

Atlanta proper is a small portion of that colored area - the metro area is highly suburbanized and over 5 million people live there. Atlanta is a driving city - there is the Metro, but most people commute by car to work, not a few by a couple of hours one way. So with all the housing, the commercial establishments and their parking lots, and the enormous network of highways to get you from one to the other there is a lot of soil covered by impervious concrete and asphalt.

One figure puts the average soil impermeability at 10% of the surface area of the metro region. Within those red counties that figure goes up. Here, for instance, a set of case studies done on urbanization included one for the Peachtree Creek watershed area. An incredible figure of 35% imperviousness in 1968 is indicated - who knows what it is now, four decades later, and how much farther outward from the center a high degree of impermeability extends.

So it's not too surprising that there was flooding - there is simply little soil surface to absorb the rainfall, so not only must storm systems accomodate most of it but what is accomodated isn't socked away locally but dumped (along with attendant pollutants) into area creeks and rivers as a gift to downstream. It didn't help that, as indicated by one report, at least one storm system was plugged up and you can predict what happened because of that.

I suppose there is at least one additional factor that led to the results you saw: continually increasing population pressure has led to what must be at least nominally illegal and certainly inadvisable developments constructed on creek and river floodplains. That's just asking for exactly the kind of trouble that transpired.

Anyone who has followed the drought of the last few years has probably come to the conclusion that the Atlanta area has not done the best job in management of its water resources. Rampant population growth outstripped the water supply then and now, and the same thing led directly and indirectly to last week's problems. It will be interesting to see if they'll have any effect.

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