EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
patterns and/or storm intensity, and long-term (secular) climatic variations are all examples of these longer-
term scales of motion. The effects of these phenomena on engineering and planning considerations are very
poorly understood at present. This is compounded by the fact that there does not even exist any real
consensus among atmospheric scientists as to what mechanism or mechanisms control these variations. This
may not diminish the importance of climatic variability but certainly detracts from the ability to treat it
objectively. As better information is collected over longer time intervals, these scales of motion will be better
understood.
(3) Temporal variability of wind speeds.
(a) Winds at any point on the earth represent a superposition of various atmospheric scales of motion,
all interacting to produce local weather phenomena. Each scale plays a specific role in the transfer of
momentum in the atmosphere. Due to the combination of different scales of motion, winds are rarely, if ever,
constant for any prolonged interval of time. Because of this, it is important to recognize the averaging
interval (explicit or implicit) of any data used in applications. For example, some winds represent "fastest
mile" estimates, some winds represent averages over small, fixed time intervals (typically from 1 to 30 min),
and some estimates (such as those derived from synoptic pressure fields) can even represent average winds
over intervals of several hours. Design and planning considerations require different averages for different
purposes. Individual gusts may contribute to the failure mode of some small structures or of certain structural
elements on larger structures. For other structures, 1-min (or even longer) average wind speeds may be more
related to critical structural forces.
(b) When dealing with wave generation in water bodies of differing sizes, different averaging intervals
may also be appropriate. In small lakes and reservoirs or in riverine areas, a 1- to 5-min wind speed may be
all that is required to attain a fetch-limited condition. In this case, the fastest 1- to 5-min wind speed will
produce the largest waves, and thus be the appropriate choice for design and planning considerations. In large
lakes and oceanic regions, the wave generation process tends to respond to average winds over a 15- to 30-
min interval. Consequently, it is important in all applications to be aware of and use the proper averaging
interval for all wind information.
(c) Figure II-2-1 shows the estimated ratio of winds of various durations to 1-hr average wind speeds.
The proper application of Figure II-2-1 would be in converting extremal estimates of wind speeds from one
averaging interval to another. For example, this graph shows that a 100-sec extreme wind speed is expected
to be 1.2 times as high as a 1-hr extreme wind speed. This means that the highest average wind speed in
36 samples of 100-sec duration is expected to be 1.2 times higher than the average for all 36 samples added
together.
(d) Occasionally, wind measurements are reported as fastest-mile wind speeds. The averaging time is
the time required for the wind to travel a distance of 1 mile. The averaging time, which varies with wind
speed, can be estimated from Figure II-2-2.
(e) Figure II-2-3 shows the estimated time to achieve fetch-limited conditions as a function of wind speed
and fetch length, based on the calculations of Resio and Vincent (1982). The proper averaging time for
design and planning considerations varies dramatically as a function of these parameters. At first, it might
not seem intuitive that the duration required to achieve fetch-limited conditions should be a function of wind
speed; however, this comes about naturally due to the nonlinear coupling among waves in a wind-generated
wave spectrum. The importance of nonlinear coupling is discussed further in the wave prediction section of
this chapter.
Meteorology and Wave Climate
II-2-3
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