So I've been watching the coverage of the major flooding in the Mississippi River basin (CNN, of course, they're my network of choice) and I'm noting there seems to be some confusion as to the terms 100-year flood and 500-year flood. This is just a short diary and it has a call to action at the end. Jump!
The terms 100-year flood and 500-year flood are not statistical recurrance times. Recurrance interval are the average times that events are expected to occur (such as say, a category 3 hurricane hitting Cape Cod. They may hit every 80 years or two may hit in back to back years. It's an average, not an exact time clock.) I found this link pretty good at explaining recurrance intervals and so on.
A 100 Year Flood, however is different. The USGS writes:
The term "100-year flood" is misleading because it leads people to believe that it happens only once every 100 years. The truth is that an uncommonly big flood can happen any year. The term "100-year flood" is really a statistical designation, and there is a 1-in-100 chance that a flood this size will happen during any year. Perhaps a better term would be the "1-in-100 chance flood."
The actual number of years between floods of any given size varies a lot. Big floods happen irregularly because the climate naturally varies over many years. We sometimes get big floods in successive or nearly successive years with several very wet years in a row.
Or, at FEMA:
The term "100-year flood" is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1- percent chance of being equaled or exceeded each year. Thus, the 100-year flood could occur more than once in a relatively short period of time. The 100-year flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program (NFIP) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage.
link
This page is a pretty good write up on 100-year floods.
A 500-year flood is the same thing. Jeff Masters writes:
The U.S. Geological Survey has preliminary data showing that this month's floods on four of Iowa's rivers--the Cedar, Iowa, Shell Rock, and Wapsipinicon--were 500-year floods. Back in 1993, many rivers in the Midwest also experienced 500-year floods, so the region has endured two 500-year floods in the past 15 years. How can this be? First of all a definition--a 500-year flood is an event that has only a 0.2% chance of occurring in a given year, based on available river flow data. Of course, reliable data only goes back a century at most, so designation of a 500-year flood event is somewhat subjective.
All of these, however are moot to people who have lost their homes. So pony up and donate to the Red Cross or your organization of choice.
Red Cross