A 100-year flood refers not only to the size of a flood, but also to the odds of it occurring. A 100-year flood has a one percent (or 1 in 100) chance of occurring in any given year. It has no relationship to the frequency of occurrence. The magnitude of such a flood is relative to the frequency of occurrence, so a 100-year flood is much larger than any run-of-the-mill annual flood. A 500-year flood has only a one in 500 (0.2 percent) chance of occurring in any given year and would be much larger and more devastating than a 100-year flood.