Søgning i bogen
Den bedste måde at søge i bogen er ved at downloade PDF'en og søge i den.
Derved får du fremhævet ordene visuelt direkte på billedet af siden.
Digitaliseret bog
Bogens tekst er maskinlæst, så der kan være en del fejl og mangler.
BREAKWATER DESIGN.
midpoint between these extremes, it is purely vertical. Below the surface
level the circular paths diminish rapidly to an insensible minuteness. At a
depth equal to the length of the wave, the displacement of the water particles
Is TJX of that of the surface particles, and at double the depth the ratio is
reduced to 000,000*
In shallow water of uniform depth, that is, in water the depth of which is
less than the length of the wave, the orbit of the water particles is approxi-
mately elliptical with the major axis horizontal, as shown in fig. 95. The
centre of the orbit lies slightly above the position of rest. With this exception,
the same dispositions hold good as in the previous case as regards the move-
ment of the particles. The ellipses of movement become flatter as the
distance below the surface increases, until finally at the bottom there is
horizontal motion only. In water which has a depth of only one-tenth of the
length of the wave, the ratio of the elliptical axes at the surface is about ^,
and at nine-tenths of the depth it is y^.
When, instead of remaining uniform in depth, the water in which a wave
is travelling becomes increasingly shallow (fig. 96), the orbits of the particles
of water become correspondingly distorted. Owing to the friction exerted
along the bottom, the major axis of revolution acquires an inclination to the
horizontal, which is continually augmented. The wave ceases to be purely
oscillatory : it undergoes a graduai transformation. The front of it becomes
steeper than the back, the crest gaining more and more upon the trough until
it actually overhangs. Then it falls forward and breaks into surf. At this
point the wave is altogether a wave of translation, and the forward motion of
the particles is exactly equal to the velocity of the wave. It is in this phase
that waves possess their most formidable potency.
Any sudden change in the level of the ground over which a wave is
travelling is capable of producing the disruption of the wave. This effect is
not confined to shallow reaches, but extends to depths as great as 16 to 20
fathoms, or even more, in the open sea. Thus, on the Herreca reef, seven
miles from land, breakers are apparent in tempestuous weather in a depth of
90 feet of water.
Height of Waves. —The inception of waves being due to the wind, their
development manifestly depends upon the extent of surface acted upon.
Waves generated without restriction are capable, under propitious circum-
stances, of attaining a very high degree of development, both as regards height
and length. On the Lake of Geneva, for instance, storm waves are stated to
reach a height of 10 feet; in the German Ocean, from 12 to 15 feet; in the