100 Year Storm

We hear about them in public meetings and read about them in newspapers around
the United States. From every small town and hamlet to the biggest cities
engineers talk about 100-year storms.
Even the larger storms, like hurricanes can come in the 100-year variety. These
storms are so called because their force is so powerful that they are suppose to
occur only once every hundred years.
There are other levels of storms as well. 10 year storms, 20 year storms and
50 year storms. These are all mixed in with the regularly occurring rain
showers we all experience.
The 10, 20, 50 and 100 year storms come in varieties as well. There 10 year
wind storms and 10 year rain storms. Hail seems to be a special case and could
occur at any time.
A fellow by the name of Jafar Korloo has been designing offshore platforms for
Unocal. He has this to say about 100 year storms, "We're seeing more 100-year
events happening more often, even every few years."
To a reasonable human being that would mean they aren’t 100 year events.
What is not explained at public meetings or in the newspapers is why these 10,
20, 50 or 100 year events are mentioned at all.
Well, they are used by engineers in order to inform an uninformed public about
the costs for certain structures. They can also be used by insurance adjustors
to determine insurance rates.
The scales for determining weather are different all over the country.
The University of Minnesota has this to say about 100-year storms, “A "100-year
storm" drops rainfall totals that had a one percent probability of occurring at
that location that year. Encountering a "100-year storm" on one day does nothing
to change your chances of seeing the same amount of precipitation the very next
day.”
The Scottsdale Arizona Road Drainage Master Plan tells us this about 100-year
storms, “A 100-year storm is an event that statistically has a one percent
chance of occurring in any one given year. If protection from a 100-year storm
could not be attained for reasons of cost, impact, available land, etc., then a
10-year level of protection could be used in the design of flood control
facilities. A 10-year storm has a ten percent chance of occurring in a given
year.”
It would follow then that if a 10 year storm has a 10 percent chance of
occurring in a given year that a 20 year storm would have a 20 percent chance.
Let us follow the logic therefore up to 30 year storms – that would be a 30
percent chance of occurrence. Now 40, 50, 60, 70 and 80 – that would provide 40
percent, 50 percent, 60 percent, 70 percent and 80 percent.
Let’s go all the way with it then and go for 90 and 100. If a 10 year storm
has a 10 percent chance of occurring in one year then a 90-year storm would have
a 90 percent chance of occurring and a 100 year storm would have a 100 percent
chance of occurring.
But we just read that it only has a 1 percent chance of occurrence.
Let’s move on to waves at sea. There is also a scale for waves – they come in
all the levels that storms come in – from 10-year waves to 100-year waves.
The problem with the present scale is that is has been discovered in the last
two years that waves at sea are much higher than previously thought on a regular
basis.
Will these problems be worked out soon or will we continue to leave them to ill trained and corrupt bureaucrats?