Storm Classifications

Floodplains are defined as any area that floods as a result of a specific type of storm, most commonly a 100 year storm event. This storm event is not simply defined as a storm that occurs within 100 years, rather it is a probability description. In essence, a 100 year storm event has a 1% chance of occurring in any given year while a 50 year storm event has a 2% chance and a 10 year storm has a 10% chance. A more detailed explanation can be found at http://water.usgs.gov/edu/100yearflood.html. As a matter of fact, the Chicagoland area may experience multiple 100 year storm events within one year. In order to qualify as a 100 year storm event, our area must receive approximately 7.5 inches of rain within a 24 hour time period. It also qualifies if we receive approximately 4.5 inches within 2 hours, or 2.8 inches within 30 minutes.