Metro Atlanta flood points and impervious surfaces

From 1999 to 2009, metro Atlanta added 91, 000 acres of impervious surface — the equivalent of nearly 69,000 football fields. The growth was most dramatic in the suburbs in Paulding, Cherokee, Forsyth and Gwinnett counties, maps show. As more streets, parking lots and buildings cover the ground, nearby streams swell more quickly when it rains.

Scroll the timeline below to see how impervious surfaces in Atlanta have grown over the past 10 years. Roll over the circles to view the flash index, a measure of change in stream flow from day to day, for each stream gage.

A "flashy" stream will have higher peak flows during rainstorms and lower normal base flow, and is thought to be more susceptible to flash flooding. For Metro Atlanta streams in 2009, the flashiness index ranged from 0.05 to 1.13, with an average of 0.52. measurements for each year.

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