We have heard the term "500-year flood" often in the news these days, and yet these extremely rare events seem to occur too frequently. Just last week, Fargo, ND experienced record flooding only 12 years after the 500-year flood in 1997.

The problem is, people often have a misconception about flood-related terms, according to Jim Angel, state climatologist at the Illinois State Water Survey (ISWS), University of Illinois at Urbana-Champaign

"Whenever someone mentions a 100-year flood or 500-year flood, it implies that the occurrence is based on the calendar, in that a 500-year flood would happen only once in 500 years," Angel said. "In fact, what we're really talking about is probability."

A 100-year flood has a 1 percent chance of happening in any given year, and a 500-year flood has a 0.2 percent chance of occurrence. A 100-year flood could, in fact, occur in a particular location two years in a row.

"It's unlucky that this would happen, but not impossible," Angel said. "This misconception about flood terms can be dangerous, particularly when residents cancel their flood insurance or fail to prepare for the worst."

Flood terminology, while often giving the public a false sense of security, is used to classify floodplain locations for insurance and regulatory purposes. Historical data are analyzed as a basis to classify flood sizes.

The accuracy and timeliness of the data is all-important, according to Sally McConkey, Water Resources Engineer at the ISWS. Flood studies need to be updated as more data are collected or conditions change in a river basin, possibly due to urban development. With new information, scientists may re-evaluate flooding frequency.

The primary sources of public information on potential flooding are the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps, which depict the area expected to be flooded in the event of a 1 percent annual-chance flood.

"These maps show a line indicating the expected extent of flooding, but floods do not stop at a line on a map," McConkey said. "People need to understand that a flood slightly greater than the 1 percent annual-chance flood can cause extensive damage beyond that line."

With the changing land use in Illinois, studies used for estimating flood risk in floodplain areas have not kept pace, according to McConkey. FEMA has proposed a plan to continue to update and upgrade the engineering studies that back up the maps.

"We need a continuing cycle of review and updates so people are well informed," she said.