EM 1110-2-1100 (Part II)
30 Apr 02
Table II-8-4
Probability Distribution Functions for Meteorological and Hydrodynamic Processes
Parameter
Representative Distribution Functions
Short-term Statistics
Surface elevations
Gaussian
Rayleigh: x=H; α=1/Hrms2
Individual wave heights
Rayleigh: x=T 2; α=1/T 4
Individual wave periods
Wave runup
Rayleigh
Long-term Statistics
Extreme wind
FT-I
Hs
Weibull: A= standard deviation of Hs; B= minimum value of Hs; k=1
Tp
Weibull: B= minimum value of Tp; A, k estimated from data
Extreme Hs
FT-I; Weibull
Water level
Log-Pearson Type III
b. Wind. Wind is discussed in detail in Part II-2.
(1) Design importance. Wind at a design site may be important for local wave generation, nearshore
current generation both inside and outside the surf zone, modification of nearshore breaking waves, nearshore
water level, nearshore sediment transport, subaerial sediment transport, intensification of runup, overtopping,
and flooding, sail forces on moored and moving boats, and harbor circulation and flushing.
(2) General climate. Winds at a coastal site are determined by some combination of large-scale weather
systems, smaller-scale systems, land-sea breeze circulations, land/water roughness differences, and orographic
effects. Winds can vary greatly over short distances along a coast. Thus, local measurements at the project
site are very helpful in establishing the climate. As with waves, 2-3 years of data are generally sufficient for
climatological purposes (i.e. annual or monthly mean and standard deviation of wind speed). Even one month
of data can be useful, though certainly not ideal, for estimating relationships between the project site and
winds at a long-term measurement station within the same region. Often wind climate information is
summarized in the form of wind roses, which can represent months, seasons, or years (Figure II-8-6).
(3) Storms. Storms are a natural part of the wind climate at a site. They can vary greatly in size and
intensity. Frequency of occurrence and intensity of storms are important concerns for functional design. The
occurrence of extreme storms is a necessary concern for structural design. The distribution of extreme wind
speeds has been modeled with FT-I, FT-II, and Weibull distribution functions (Figure II-8-2). The FT-I
distribution function seems to be the preferred choice, especially when the annual extremes do not include
rare, unusually powerful events arising from distinctly different meteorology (such as hurricanes) (Simiu and
Scanlan 1986).
II-8-14
Hydrodynamic Analysis and Design Conditions
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