Meteorologist on 500/1000 year flood claims: ‘We are talking about billions of years of climate & usually just a hundred years of actual, observational data’
100, 500 and 1000 year floods can be confusing. The flooding from Harvey is being called a 500 year flood. What does that mean?
Well, the US Government in the early 1970s wanted to improve the National Flood Insurance Program. So, flood plains were drawn estimating worst case scenario floods to determine premiums for home owner flood insurance. A 500 year flood does not mean that an area will see a flood of that magnitude once in 500 years. It means that in any given year there is a .2% chance of a 500 year flood and likewise a 1% chance every year for a 100 year flood. Think of it like this. If I flip a coin and it lands heads 5 times in a row the 6th time I flip the coin the odds are still 50/50. The odds are always 50/50.
There have been ‘100 year’ floods in Houston in 1929, 1935, Allison in 2001 and Ike in 2008. Folks are probably scratching their heads wondering what in the world is going on. There are several explanations. Firstly, the maps drawn to depict these floods were educated guesses. Remember, we are talking about billions of years of climate and usually just a hundred years of actual, observational data. Secondly, urban development reduces the surface of the ground that allows the rain to permeate into the ground. Adding parking lots, more roads and driveways create more runoff. Thirdly, at least in the case of Houston 1000s of homes have been built close to streams, creeks and bayous that should have never been built in the first place. Houston is the fourth largest city in the U.S.