How often does a 100-year flood occur?

From the New York Times:

 

"We have had two almost 100-year floods and two almost 500-year floods in a 35-year period," said Nicholas Pinter, a professor who specializes in flood hydrology at Southern Illinois University, Carbondale. "Flood levels all along this stretch of the Mississippi have climbed upward, not just by inches but by 8, 10, 12 feet — up to 18 feet over historical 100-year flood conditions. So the simple answer is that floods are higher and more frequent.

"But the underlying theme to everything that's going on," Dr. Pinter added, "is that the current estimates for flood frequency and intensity appear to be grossly underestimated."