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 insurance policies. Both the federal government and the private
sector assist the insurance industry in gathering the scientific measurements
that exist 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 what is termed a "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
With urbanization, better calculations and the lessons from recent flood events, many communities (like Boulder) are realizing they have more households in harms way from floods.
*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.
Source: The Flood Safety Project