My article on Atlanta’s flood of record in the January/February 2010 issue of Stormwater was submitted on October 16 of last year, and it makes clear that the local papers had misquoted Brian McCallum, assistant director of the US Geological Survey Water Science Center in Atlanta, as stating that the September 21 and 22 Atlanta area flood was a 500-year event when his September 24 press release had actually stated that there was “less than a 1 in 500 chance that parts of Cobb and Douglas counties were going to be hit with such an event.”
By November 4, USGS crews found out just how much less than a 1 in 500 chance the flood had of occurring, having completed surveys of high-water marks for the flood, and, reportedly, taken a few days to determine just how to articulate what the new flood mark data suggested: that the “annual chance of a flood of this magnitude was so significantly less than 1 in 500 that, given the relatively short length of streamgaging records (well less than 100 years), the US Geological Survey cannot accurately characterize the probability due to its extreme rarity.” (Emphasis added; see the full news release here). In the November 4 news release, McCallum went on to conceptually characterize a 500-year flood as a coffee cup’s worth compared to the Atlanta flood’s full pot. Do the math on an office or restaurant coffee dispenser; you’ll note the conceptual aptness and accuracy as the clever choice of metaphor couches the event in the 5,000- to 10,000-year flood range.
Enhancing the upgraded flood characterization of November 4, bringing it into even sharper focus, Ken Frantz of the National Weather Service states in the same news release, “Applying rainfall frequency calculators, we have determined that the chance of 10 inches or more occurring at any given point are less than one hundredth of one percent. This means that the chance of an event like this occurring is 1 in 10,000.”
Rain frequency does not equate to flood frequency per se, but with more than 20 inches of rain hitting some SW metro Atlanta areas on September 20 and 21, and with land significantly impervious from five days of soaking rain, a 10,000-year rain event swelled flood waters to record levels and a flood frequency so far beyond a 500-year flood that it is also expressible in some power of ten. I never thought I’d live, or at least work, long enough to see flood and rain frequencies as rare and high as—and numbers as big and menacing as—well, 3.7 or even 4.