What is a 100 year flood?

[Collinsville]

This was a funny one that was brought to my attention. Some company is trying to build a gravel quarry on a fairly steep hillside in North Granby pretty close to the East Hartland border. Locals are trying to fight it and at the initial town meeting the company said they have sufficient drainage for a "100 year flood".  When asked what a 100 year flood is one of the guys reads from a piece of paper "5.70 inches in 24 hours". When I was told this I of course burst out laughing as its a ludicrous figure. I've topped that threshold 4 times already in just 13 years of keeping rainfall, with two other events that are really close. Irene dumped 9.39" in less than 24 hours. Did it twice in October 2005 just 7 days apart! Floyd in 1999 crushed this number as well which makes 5 in 14 years.

 

So what is a "100 year flood" for North Granby in the foothills wnw of BDL?

 

You can't even use BDL/Hartford numbers as there is a definite upslope component for many events like Irene.

 

[NeffsvilleWx]

This was a funny one that was brought to my attention. Some company is trying to build a gravel quarry on a fairly steep hillside in North Granby pretty close to the East Hartland border. Locals are trying to fight it and at the initial town meeting the company said they have sufficient drainage for a "100 year flood".  When asked what a 100 year flood is one of the guys reads from a piece of paper "5.70 inches in 24 hours". When I was told this I of course burst out laughing as its a ludicrous figure. I've topped that threshold 4 times already in just 13 years of keeping rainfall, with two other events that are really close. Irene dumped 9.39" in less than 24 hours. Did it twice in October 2005 just 7 days apart! Floyd in 1999 crushed this number as well which makes 5 in 14 years.

 

So what is a "100 year flood" for North Granby in the foothills wnw of BDL?

 

You can't even use BDL/Hartford numbers as there is a definite upslope component for many events like Irene.

 

100 year flood has nothing to do with the amount of precip.

 

http://en.wikipedia.org/wiki/100-year_flood

 

The link below shows the areas that are considered "at risk" for 100 year type floods. This data is often used by insurance companies. Many times, property owners in these areas are required to purchase flood insurance.

 

http://www.arcgis.com/explorer/?open=95be6f4e30e14df498e9bd2ae0582553