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BREAKWATER DESIGN.
119
the period of the wave, calling this period P and the length of the wave in
feet L,—we have
or, fairly approximately,
T _ P2?.
L” 27’
L=5 P2.
The length of the wave in conjunetion with the depth of water détermines
the speed of movement of the wave and, conjointly, the velocity of the
particles of which it is composed. The relationship existing between these
elements will be discussed a little later.
Breaking ’ Waves.—We have now to consider the manner in which a
wave acts upon any fixed obstacle in its path, whether it be the beach
upon which it is spent or an artificial barrier which causes its abrupt
collapse.
Dealing first with the oscillatory wave, and assuming that it reaches a
wall or other obstruction having an abrupt, vertical face, we find that it is
reflected in the manner indicated by fig. 99. The particles of water in
contact with the wall (A) move up and down <;^
through a height which is twice the height of >^\ b
the original wave, as also do the particles in
the trough (C) half a wave-length distant. At ____________________________
a point (B) midway between the trough and 'Z^^^^^^g
the wall, that is, one quarter of a wave-length_____________________Fio. 99.
from either, the particles move horizontally
backwards and forwards, while at intermediate points the path of the particles
is inclined at varions angles. The whole motion, in fact, is the inverse of
that which occurs in the unobstructed wave.
When, on the other hand, without meeting with any abrupt obstacle, the
wave advances into rapidly shoaling water, its energy is communicated to
smaller and successively decreasing masses. Consequently there is a tendency
to produce in those masses an agitation of inereasing violence. But this
effect is generally diminished, and sometimes entirely counteracted, by
the loss of energy due to friction along the bottom, and to surging.
On the other hand, the influence of concentration arising from funnel-
shaped inlets is clearly to intensify the agitation, and the same effect
is producible by submerged rocks with deep narrow gorges between, in
passing through which the water is heaped up into masses of considérable
volume.
When, however, the bottom friction has produced the necessary retarda-
tion, the crest of the wave falls forward, as has already been explained, and
impact takes place at the precise stage at which the forward motion of the
particles has become equal to the velocity of the wave, so that the stroke of
the latter is delivered with maximum effect.
Taking all these diverse phenomena into considération, it is evident that
breaking waves result in the generation of four separate and distinct forces