The term “100-year flood event” can be misleading. It is not an event that happens once in 100 years.  Rather, it is the amount of precipitation needed to achieve a flood elevation, and this event has a 1% chance of being equaled or exceeded in any given year. In this area, it is the elevation based on a 24-hour rainfall of 5.9-6.0 inches. Encountering a "100-year storm" on one day does not decrease the chance of a second 100-year storm occurring in that same year or any year to follow.

More recently, FEMA has tried to clarify the probability of a 100-year flood by saying that it has about a 26 percent chance of occurring during the life of a 30-year home mortgage. That's about triple the risk of a fire during that same period.  There’s also been talk about changing the terminology and/or metric for measuring flood risk.  However, key players -- such as FEMA, the Army Corps of Engineers and the National Weather Service -- have not agreed on a new metric.  Until then, it will be up to engineers, hydrologists and flood agencies to inform the public as to the terminology used.

In Minnesota, most municipalities require storm sewers to be designed for a 10-year storm event.  Larger events create ponding at catch basins until the water reaches the emergency overflow elevation (EOF).  The EOF is typically designed to be 1’-2’ lower than adjacent buildings.  Stormwater ponds are designed for the 100-year, 24-hour storm event.  However, there are many factors that can contribute to flooding.  Among other factors, frozen ground, saturated soil, high river/lake levels, silted catch basins and an intense rainfall (flash flood event) can exacerbate flooding.