On Sat, 22 May 2004 14:58:47 GMT, "Steve Young"
wrote:
"Torsten Brinch" wrote in message
.. .
On Sat, 22 May 2004 05:55:55 GMT, "Steve Young"
topposted, with no attribution of quoted lines:

I think there might be a problem when you assume a 100 year return is
actually a 1 00 year return

Huh?


"They" say things like, That was a 100 year flood, expected to occur only
every 100 years. (or snow, drought, whatever) But I have doubts about how
they calculate that.

We do not need to get into the detail, but from a solid observation
series (preferably many hundreds of years :-) one can of course make a
good statistical estimate of what magnitude the event must have to
have only a 1 % chance of occurring in any given year. The problem
with some estimated 100 year events is that they do not have that kind
of solid base in observation. Say, if you have only 20 years worth of
data, you are thrust into making an extrapolation resting on a set of
assumptions, which reality may very well see fit to overturn in time.

Example, where I live, a few years ago we had a "100
year" storm with flooding. But if you look at newspapers from the 1920's,
there were floods EVERY YEAR!. As more people build in river flood plains,
wouldn't you expect more flooding, people are moving to where the water is.
As more concrete if poured, there is less soil to absorb the water. It has
to go somewhere, and that's what we call flooding, when it goes where we
don't want it..

I've been in three different "100 year floods. I guess that makes me 300
years old

Oh, so that's what you meant. That's the misconception that if
something has a 1 % probability of occuring each year (= a 100 year
event), then the probablity of it occuring within a 100 year period
is 1 (unity). The same type of misconception would lead one to believe
that if one cast a die 6 times then each of the faces will come up
once :-)