1.2.6 Refraction and diffraction
As waves begin to feel the bottom, a phenomenon called
refraction may occur. When waves enter water of transitional
depth, if they are not travelling perpendicular to
the depth contours, the part of the wave in deeper water
moves more rapidly than the part in shallower water,
according to Equation 1.11, causing the crest to turn
parallel to the bottom contours. Some examples of
refraction patterns are shown in Figures 1.6, 1.7 and 1.8.
Generally, any change in the wave speed, for
instance due to gradients of surface currents, may lead
to refraction, irrespective of the water depth. In Section
4.5.1 a few examples are given to illustrate refraction
under simplified conditions. A more complete description
of methods for the analysis of refraction and
diffraction can be found in Sections 7.3 and 7.4, and in
CERC (1984).
Finally, the phenomenon of wave diffraction should
be mentioned. It most commonly occurs in the lee of
obstructions such as breakwaters. The obstruction causes
energy to be transformed along a wave crest. This transfer
of energy means that waves can affect the water in
the lee of the structure, although their heights are much
reduced. An example is illustrated in the photograph in
Figure 1.9.