Three regions may be distinguished in the figure: (1) the region consisting
of incident and reflected waves (A), (2) a region in the shadow of the
breakwater in which the crests form circular arcs (B) and (3) a region in
which the incident waves progress undisturbed (C). As the waves spread
behind the structure, their amplitudes decrease. The ratio of the diffracted
wave height to the height of the incident wave is termed the diffraction
coefficient. For the specific case of diffraction with incident waves at
various angles at a long breakwater, values of diffraction coefficients have
been tabulated by Muir Wood and Fleming (1969).
The diffraction pattern through a breakwater gap is usually obtained
assuming the gaps to be small or large. The solution for a large gap is
obtained by a superposition of the two solutions for the two breakwaters
for normal incidence of the waves. For graphical solutions for a number of
cases, the reader is referred to Wiegel (1964) and the Shore Protection
Manual(US Army, 1984).
14.9 Wave prediction
14.9.1 General
The energy of waves is provided by the wind blowing over the ocean. The
onset of wave formation is influenced by surface tension; although import-
ant in model experiments this is of no consequence in engineering prac-
tice. Waves are generated on the initially flat water surface by the motion
of the pressure-producing patterns of turbulence of the air stream; in the
later stages of development of the waves, the energy required for the wave
growth comes directly from the mean motion of the wind.
Waves generated by wind have a wide range of frequencies and
wavelengths. The longest waves are those the celerity of which is equal to
the wind speed. The steepest waves are determined by the breaking con-
dition in deep water. The heights and periods generated depend on the
wind speed, U, the distance or fetch, F, over which the wind blows and the
duration,Tw, of the wind.
There is a certain interaction between the wind and the isobar
spacing as given in the meteorological charts. In meteorological practice,
isobars are spaced at 4 mbar in the UK and at 3 mbar in the USA. The
wind direction is parallel to the lines of isobars but is modified by friction
over the water surface. The pressure distribution normal to the isobars is
determined by the Coriolis force resulting from the Earth’s rotation and
the centripetal force due to the curvature of the moving air masses. The
resultant wind is called the gradient wind. When the isobars are parallel
and straight, only the Coriolis force is important and the wind is called
geostrophic wind. The equation governing the motion of the geostrophic
594 WAVES AND OFFSHORE ENGINEERING
