EM 1110-2-1100 (Part II)
30 Apr 02
Figure II-1-23. Measured sea surface velocity in the entrance to San Francisco Bay
If a recorder were to measure waves at a fixed location on the ocean, a non-repeating wave profile would be
seen and the wave surface record would be rather irregular and random (Figure II-1-23). Although individual
waves can be identified, there is significant variability in height and period from wave to wave.
Consequently, definitions of wave height, period, and duration must be statistical and simply indicate the
severity of wave conditions.
(4) Wave profiles are depicted in Figure II-1-24 for different sea conditions. Figure II-1-25 shows a
typical wave surface elevation time series measured for an irregular sea state. Important features of the field-
recorded waves and wave parameters to be used in describing irregular waves later in this section are defined
in Figures II-1-26 and II-1-27. We note that the sea state in nature during a storm is always short-crested and
irregular. Waves that have traveled far from the region of generation are called swells. These waves have
much more limited range of variability sometimes appearing monochromatic and long-crested.
(5) This part of Part II-1 will develop methods for describing and analyzing natural sea states. The
concept of significant wave height, which has been found to be a very useful index to characterize the heights
of the waves on the sea surface, will be introduced. Peak period and mean wave direction which characterize
the dominant periodicity and direction of the waves, will be defined. However, these parameterizations of
the sea surface in some sense only index how big some of the waves are. When using irregular wave heights
in engineering, the engineer must always recognize that larger and smaller (also longer and shorter) waves
II-1-62
Water Wave Mechanics
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