Using Multi-Objective Management to Reduce Flood Losses in Your

Watersheds vary in size. They can be divided into smaller subwatersheds.
The important thing is that the stream and its watershed are connected. What
happens in the watershed−even in the higher elevations up near the divide –
affects things both in the low-lying lands near the stream or lake and also
farther downstream.
The floodplain
The size of a river depends on how much water it collects from the watershed.
Most of the time, a river stays within its banks. However, after heavy storms or
snowmelt, the extra amount of water can be too much for the channel, causing it
to overflow onto normally dry land.
A floodplain can be defined according to the frequency of the flooding that
WHAT ARE THE ODDS OF A
100-YEAR FLOOD
The term “100-year flood” has caused much
confusion for people not familiar with statistics.
Another way of looking at it is to think of the
Odds that a 100-year flood will happen
sometime during the life of a 30-year mortgage
(26% chance).
Chance of Flooding over a Period of Years
Time
Flood Size
Period 10-year 25-year 50-year 100-year
1 year 10% 4% 2% 1%
10 years 65% 34% 18% 10%
20 years 88% 56% 33% 18%
30 years 96% 71% 45% 26%
50 years 99% 87% 64% 39%
Even these numbers do not convey the true
flood risk because they focus on the larger,
less frequent, floods. If a house is low
enough, it may be subject to the 10- or
25-year flood. During the proverbial 30-year
mortgage, it may have a 26% chance of being
hit by the 100-year flood, but the odds are
96% (nearly guaranteed) that it will be hit by
a 10-year flood. Compare those odds to the
only 5% chance that the house will catch fire
during the same 30-year mortgage.
covers it. An area that is flooded every
year (a 1-year floodplain) is smaller than
the floodplain that is inundated less
frequently (a 15-year, 75-year, or even
larger floodplain).
The base flood−The national
standard for floodplain management is the
base or one percent chance floodplain.
This is the floodplain that has at least one
chance in 100 of being flooded in any
given year. It is also called the 100-year
floodplain. This is the area shown on the
Flood Insurance Rate Map (FIRM) for
your town issued by the Federal
Emergency Management Agency.
The term “100-year flood “ has
caused a lot of confusion. Some folks
think hat it will only happen once in 100
years, but that is definitely not true.
Instead, “100-year flood “ is a statistical
term that refers to the odds of a flood of
that size happening in any given year (see
box). It is possible to have 100-year floods
two years in a row.
The floodway−Another term you
will see is floodway. This is the river
channel and the overbank area near the
channel that carry the deeper and fastermoving
flood waters. Some maps show a
regulatory floodway, an area where construction regulations require special
provisions to account for this extra hazard.
Flood problems
As noted above, flooding is a natural occurrence but flood problems are the result
of human development. Floods may be acts of God, but flood damage results from
human actions. Flood problems result from:
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