Saturday, July 28, 2007

Wouldn't That Be a 21 Year Flood?

Sometimes, phrases are used in ways that show that they have completely lost their meaning. As an example, this statement appeared in a news report in today's Dallas Morning News:

Heavy rains have morphed the massive lake into a sprawling flood zone, and it will take about six weeks for the water to inch its way back to normal. It is the third 100-year flood since the lake opened in 1944.


Blogger Sarcastro said...

From the wiki:
A one-hundred year flood is calculated to be the maximum level of flood water expected to occur on average once every one hundred years. Sometimes, 500-year or 10,000-year floods are calculated (especially in low lying countries, such as the Netherlands). The 100-year flood is sometimes referred to as the 1% flood, since there is a 1% chance of it occurring in any year. Based on the expected water level, an expected area of inundation may be mapped out according to elevation above sea level. This area figures very importantly in building permits, environmental regulations, and flood insurance.

10-year floods have a 10% chance of occurring in any given year; 50-year flood, 2% chance; 100-year, 1%; 500-year, 0.2%; and 1000-year, 0.1%. The percent chance of an x-year flood can be calculated by dividing 100 by x.

1:54 PM  
Blogger MCO said...

Sarcastro, thanks for providing the dictionary definition. I would still wonder if there is not reason for re-calibrating the benchmarks, given that what is being called a 100 year flood has actually occurred at nearly a 5% pace in the 63 years that this particular lake has been in existence.

2:05 PM  

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