Answer:
The
term "100-year flood," is used to describe the recurrence
interval of floods. As the table below shows, the
"100-year recurrence interval" means that a flood of that
magnitude has a one percent chance of occurring in any
given year. In other words, the chances that a river will
flow as high as the 100-year flood stage this year is 1 in
100. Statistically, each year begins with the same
1-percent chance that a 100-year event will occur.
|
Recurrence interval,
in years |
Probability of
occurrence in any given year |
Percent chance of
occurrence in any given year |
|
|
|
1 in 100 |
|
1 in 50 |
|
1 in 25 |
|
1 in 10 |
|
1 in 5 |
|
1 in 2 |
|
|
But, just because a 100-year flood happened last year
doesn't mean that it won't happen this year, too. In other
words, future rainfall and floods don't depend on the
rainfall and floods that happened in the past. The past
records are mainly used to show what kind of river flows
can be expected. So, when you hear about a 100-year flood,
at least you have a general idea that it does mean a BIG
flood, and if you hear of a 200-year flood you know that
it means one even BIGGER! As an example, in July of 1994,
some places in south Georgia received more than 20 inches
of rainfall in a few days -- the floods they produced were
tremendous... way over the 100-year flood. At Senoia, Ga.,
the maximum amount of water flowing by the Line Creek gage
was 2.4 times greater than the 100-year flood level.
