EM 1110-2-1100 (Part II)
30 Apr 02
(5) Providing a more comprehensive assessment of processes over an area than is generally possible with
point measurements.
Physical models, properly scaled, often provide a helpful representation of the hydrodynamics of a complex
project site, including wave shoaling, wave breaking, wave reflection, and wave-current interaction. They
are typically considered for:
(1) Transformation of waves, tides, and/or currents from offshore to a complex project site.
(2) Design forces and overtopping of breakwaters (Part VI).
II-8-4. Statistical Methods - Short-Term
a. Introduction.
(1) Hydrodynamic conditions in a coastal area at any instant in time may be viewed as a sea state, that
is, a state in which conditions are relatively constant over some short time period. That time period is
typically from 1 to 6 hr before the sea transitions to some significantly different state of waves and, in the
nearshore area, water level and currents. Since waves are typically the most intensely varying factor outside
the nearshore area, the term sea state is often intended to mean waves. Sea states change in response to
changing local and offshore winds, tides, and other factors. Thus wave measurement and hindcasting
programs often gather wave conditions at 1-hr to 6-hr intervals to adequately sample the range of sea states.
The constancy of a sea state is best defined in statistical terms because components of the sea state,
particularly wind waves and swell, have strong variations over time periods of seconds. By contrast, wave
theories and many physical model tests performed more than about 10 years ago represent a sea state as a
regular wave (e.g., Part II-1). This deterministic representation masks some important aspects of wave
behavior.
(2) Most design is based on long-term hydrodynamic statistics representing many years of record.
However, statistical variations within a sea state, referred to as short-term variations, can also be critical for
design applications in which damage is a highly sensitive response to individual extreme waves rather than
an integrated response to the overall sea state. For example, damage to a pier or platform deck will occur only
if a wave crest is high enough to hit it. Damage in this example results from a combination of extremes in
long-term statistics (extreme combination of Hs and water level) and short-term statistics (extreme value of
individual wave height). Another example in which both long and short-term statistics are important is the
case of waves overtopping a seawall. Only the individual waves that run up over the seawall crest will cause
damage behind the wall. Distinctions between short-term and long-term statistics are summarized in
Table II-8-1.
b. Probability distribution functions. Short-term statistics are discussed in earlier chapters of Part II.
Rayleigh, Gaussian, and normal distribution functions are useful tools. The normal distribution is a
normalized version of the Gaussian distribution in which the mean is zero and the standard deviation is one.
Characteristics of distribution functions for various short-term statistics are summarized in Figure II-8-1.
Hydrodynamic Analysis and Design Conditions
II-8-3
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