action to effectively generate waves. Brief gusts reaching high velocities do not last long enough to
cause wave growth. The fastest-mile is a convenient way to characterize both wind speed and duration.
The maximum fastest-mile is the highest speed that occurs with a sufficient duration for the wind to
travel one mile. In other words, a 60-mile/hour wind must last for one minute to travel one mile, whereas
a 30-mile/hour wind must last for two minutes. Figures 15 and 16 are maps of the continental United
States, including Alaska, which displays the maximum fastest-mile wind speed contours for 10- and 25-
year return periods. For example, the 10-year fastest-mile wind speed at New York City is 60 mph, and
at Charleston, South Carolina it is 75 mph.
A brief review of the concept of return period is needed because the public tends to be confused
about its meaning. For instance, when told that a device will withstand the 10-year wave at a site, most
people will probably conclude that the structure will be safe for the next ten years. Or, if design wave
conditions have occurred recently, they may assume that these will not occur again for another ten years.
Neither of these perceptions is correct. What is really meant can be illustrated by an example. For
instance, if over a long time (e.g., 100 years), 10 episodes with waves of a certain size were observed, the
return period for that wave height, based on the available statistics, would be 10 years (100 years of
record/10 observed episodes).
Return periods can be used to assess the risk involved in a particular decision. The probability,
P, that a particular event with return period, Tr., will occur during a period of time, I , is given by,
P = [1 (1 1/Tr)I] * 100
(4)
Table 4 contains probabilities of occurrence (percentages) for events with 10- or 25-year return periods
as a function of various project durations. For example, consider an individual who wishes to protect his
shoreline for 10 years. Using methods explained later, the designer chooses a 10-year design wave. The
chance of experiencing the design wave during a structure's 10-year life is 65 percent. If this is an
unacceptable level of risk, the designer may then provide protection against a larger design wave, say 25
years. In that case, there is a 34-percent chance of the 25-year wave occurring during the structure's 10-
year life. If this is an acceptable level of risk, the design can proceed on that basis.
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