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between the wave crests and the isobaths at a given depth, c the speed
at that depth, and α 0 and c 0 the values for the same in deep water. The
result is that waves tend to become parallel to the isobaths, although,
where they start very obliquely, they remain to some degree oblique right
up to the shore. Irregular sea-floors give unexpected results, such as
concentrations of crests at certain points on the coast ( Fig. 1 A ). A
submarine valley perpendicular to the waves causes them to diverge
and thus disperse their energy. A submarine ridge perpendicular to the
waves causes a convergence of the waves and hence increases their
attack. Isolated swells in the sea-floor cause the waves to converge in
their lee.

Reflection is the reversal of a wave by some obstacle such as a jetty or
a strongly sloping foreshore ( Fig. 1 F ). As in optics, the angle of inci-
dence equals the angle of reflection. Reflection of swell approaching
perpendicularly causes a series of standing waves to be produced by the
interference of approaching and reflected waves (clapotis). If the waves
approach obliquely, the approaching and reflected waves form a grid
pattern. A vertical obstacle causes total reflection. The degree of re-
flection decreases with the angle of the obstacle and is negligible when
the slope is very gentle.

Diffraction ( Fig. 1 F ) occurs when a wave passes the end of an
obstacle. The waves change direction and rapidly die out. Thus, al-
though there is shelter in the lee of the obstacle, that shelter is not
complete.

Apart from the changes in direction, the wave undergoes a series
of other changes before it breaks. One characteristic, the period, remains
constant. The other characteristics vary with the ratio H/λ, where H
is the depth at any point and λ the wave-length in deep water. The
following changes occur:
Decrease of the wave-length.
Decrease of the speed of the wave.
Increase of the steepness of the wave (ratio between the height
of the wave and its length).
The wave system is simplified as short irregular waves which
often complicate the swell in deep water, die out or are greatly
reduced.
The orbital paths of the molecules are changed to ellipses: near
the bottom the elliptical motion virtually becomes a to-and-fro
movement.

-16-

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Publication Information: Book Title: Coastal and Submarine Morphology. Contributors: André Guilcher - author, B. W. Sparks - transltr, R. H. W. Kneese - transltr. Publisher: Methuen. Place of Publication: London. Publication Year: 1958. Page Number: 16.
    
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