You may have heard an extreme weather event, or a flood, called a one-hundred-year event. However, this name is misleading because it does not mean that this event only happens 1 time every one hundred years. The name actually means that the event has a 1/100, or 1%, chance of occurring during any given year. A five-hundred-year event has a 1/500 or 0.2% chance of happening in any given year; whereas, a one-thousand-year even has a 1/1000 or 0.1% chance of occurring in any given year.
The probability of one or more events occurring during any given year is known as the probability of exceedance. This value describes the percent chance of natural, inherent, or hydrologic risk. For example the probability that one or more one-hundred-year events will impact a location during the next 100 years is greater than a 50/50 chance (i.e. it is approximately a 63.4% chance).
So what does this all mean? It means, despite a misleading name, your location may be at a greater risk for experiencing a very powerful event.
For example, from August 2015- August 2016 the United States National Weather Service recorded eight (8) one-thousand-year flood events.