(Wisdom)2
Memo
To:
Ari Rooney
From:
Katie Wisdom
Date:
2/6/2016
Re:
Technical Definition
Audience
This definition is directed at the people of Moorhead, MN, whose town has been hit by several 100 year
floods since 1997.
Purpose
This definition is intended to inform the public of the meaning of this term. 100 Year Flood is a term
utilized by hydrologists and meteorologists and often misunderstood by the public. This definition is
intended to bridge the gap between these two groups and prevent confusion and frustration for the
public.
Problem
Moorhead experienced a 100 year flood’ in 2009 and another in 2011. In 2009, the public
misinterpreted this term to mean that another flood of the same magnitude could not occur for another
100 years. In 2011, they assumed that another large scale flood would not occur. Instead, rapid snow
melt and a rainy spring led to another 100 year flood. In the resulting confusion, residents began to
question the wisdom of scientists regarding flooding in their area.
Placement
This definition was written for inclusion in a letter to the editor of Moorhead’s local paper, The Forum.
The remainder of the letter focuses on background for the definition and urges the town to better
prepare for yearly flooding.
Extended Definition
A 100 year flood is a weather event with a return period of 100 years. The return period is based on the
probability that a flood’s magnitude will exceed the area’s water capabilities. For a 100 year flood, the
likelihood of snow melt and/or precipitation exceeding flood magnitude is 1/100, or 1% per year. Similar
calculations can be done for all flood return periods, but 100 year floods are commonly used in city
planning and building design and are discussed the most frequently.
Storm and flood data from years past is analyzed to determine the likelihood of a disturbance occurring
that will cause a flood in a given area. As a result, the magnitude of an area like Moorhead’s 100 year
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flood may change over time as new data is accumulated for analysis. Several consecutive years with
light precipitation and no flooding can lower the probability of larger floods occurring, reducing the
magnitude of a flood required for 100 year classification. Multiple years of large storms increase the
chances of larger floods. This results in larger floods having lower return periods.
Probabilities are based on the assumption that events are independent. Last year’s weather does not
impact this year’s weather, which is why places like Moorhead may experience two unlikely floods in a
short period of time.
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