Q: With all the rain in metro Atlanta, I read several times about 100- and 500-year floods. How do meteorologists determine how often a major flood is likely to happen? — Frank Manfre,|
Grayson

The Atlanta Journal-Constitution

Q: With all the rain in metro Atlanta, I read several times about 100- and 500-year floods. How do meteorologists determine how often a major flood is likely to happen?

— Frank Manfre,|
Grayson

A: The term 100-year flood does not mean that a flood should be expected only once in a lifetime or once every 100 years, said WSB Radio meteorologist Kirk Mellish. It means that a level of flooding has a 1 percent chance of occurring in any given year and therefore is a 1 /100 year event — statistically speaking, it should occur once in every 100 years. The formula can be complicated because it has its roots in hydrology engineering, statistical analysis and probability theory, but Mellish provides the following explanation. The total record of known rainfall and data from past floods are combined with knowledge of the flood plain in question, the topography, land use, vegetation, soil types, elevation and land slope of the river basin. All that data are put into mathematical formulas to determine what amounts of rain, over what area, over what period of time will lead to what level of flooding in various locations. Mother nature does not follow statistics and can give us three 100-year floods in one year or two 500-year floods in a single decade, or anything she wants, he said.

Lori Johnston wrote this column. Do you have a question about the news? Call 404-222-2002 or e-mail q&a@ajc.com (include name, phone and city).