The first thing you
need to know about the concept of a 100-year floodplain
is that it is based on a statistical probability needed
by the insurance industry as a standard upon which to base
policies. Both the federal government and the private sector
assist the insurance industry in gathering scientific measurements
that are then used to generate a “*best guess”
of stream flow peaks over a time. All this information goes
into a formula/statistical model that generates elevations
on tracts of land throughout a watershed that have “one-in-one
hundred chance (1 percent) of occurrence of flooding in
any given year, or a "return period" of once every
100 years.”
100-year floodplains are not arbitrary but they
are:
1. Limited to the “best information at the time"
2. Not a determination of where and how frequently actual
flood damage will occur.
3. Subject to change
*We say “best guess” for several
reasons, chiefly that streamflow data has only been collected
for a maximum period of 150 years (much less in many areas)
which is a small sampling in the context of regional weather
patterns and actual flood events.
Read more
The "100-Year Flood" USGS Fact Sheet: http://water.usgs.gov/pubs/FS/FS-229-96/
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