<<Technically speaking, a 100-year flood means that the chance of a flood of
<<<that level occuring in that spot in any given year is 1%.
Yes, that seems to be the definition which renders the 100 year thing a bit
confusing. I suppose if one had many sequences of 10,000 year periods the
average occurrences of such a flood would be 100 times.
Howsomever, what really is the probability of a 1% risk flood occurring any
time in 100 years? Nearly certain? Not really. it is a Bernouli process and it
is MUCH easier to answer how often it will never occur:
0.99**100 or about 0.366---so that leaves about a 63.4% of a "100 year flood"
any time in a hundred years.
As I understand it for many areas there is a paucity of flood data and the
risk is in flux. Uncle Sam got involved after Hurricane Betsy in 1965
("Billion Dollar Betsy"). There was very little private insurance available as
the actuaries had too little data to assess risk and know what to charge and
how much they needed to have in reserve to cover losses. If they can't
measure anything, they can't offer it. Whether Uncle Sam should offer insurance
multiple times to rebuild in flood plains is another question.
Happy to be about 80 ft above sea level, Mike
--
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