EM 1110-2-1100 (Part II)
30 Apr 02
m. Water waves are considered oscillatory or nearly oscillatory if the motion described by the water
particles is circular orbits that are closed or nearly closed for each wave period. The linear theory represents
pure oscillatory waves. Waves defined by finite-amplitude wave theories are not pure oscillatory waves but
still periodic since the fluid is moved in the direction of wave advance by each successive wave. This motion
is termed mass transport of the waves. When water particles advance with the wave and do not return to their
original position, the wave is called a wave of translation. A solitary wave is an example of a wave of
translation.
n. It is important in coastal practice to differentiate between two types of surface waves. These are seas
and swells. Seas refer to short-period waves still being created by winds. Swells refer to waves that have
moved out of the generating area. In general, swells are more regular waves with well-defined long crests
and relatively long periods.
o. The growth of wind-generated oceanic waves is not indefinite. The point when waves stop growing
is termed a fully developed sea condition. Wind energy is imparted to the water leading to the growth of
waves; however, after a point, the energy imparted to the waters is dissipated by wave breaking. Seas are
short-crested and irregular and their periods are within the 3- to 25- sec range. Seas usually have shorter
periods and lengths, and their surface appears much more disturbed than for swells. Waves assume a more
orderly state with the appearance of definite crests and troughs when they are no longer under the influence
of winds (swell).
p. To an observer at a large distance from a storm, swells originating in a storm area will appear to be
almost unidirectional (i.e., they propagate in a predominant direction) and long-crested (i.e., they have well-
defined and distinctly separated crests). Although waves of different periods existed originally together in
the generation area (seas), in time the various wave components in the sea separate from one another. Longer
period waves move faster and reach distant sites first. Shorter period components may reach the site several
days later. In the wave generation area, energy is transferred from shorter period waves to the longer waves.
Waves can travel hundreds or thousands of kilometers without much loss of energy. However, some wave
energy is dissipated internally within the fluid, by interaction with the air above, by turbulence upon breaking,
and by percolation and friction with the seabed. Short-period components lose their energy more readily than
long-period components. As a consequence of these processes, the periods of swell waves tend to be
somewhat longer than seas. Swells typically have periods greater than 10 sec.
II-1-2. Regular Waves
a. Introduction. Wave theories are approximations to reality. They may describe some phenomena
well under certain conditions that satisfy the assumptions made in their derivation. They may fail to describe
other phenomena that violate those assumptions. In adopting a theory, care must be taken to ensure that the
wave phenomenon of interest is described reasonably well by the theory adopted, since shore protection
design depends on the ability to predict wave surface profiles and water motion, and on the accuracy of such
predictions.
Water Wave Mechanics
II-1-3
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