CHAPTER XX 
COAST DEFENCE, COASTAL WORKS, AND 
RECLAMATION 
Waves AND WavE Action—The remorseless attack of the 
sea has devoured wide tracts of coastland by marine abrasion, 
which is mainly the work of the waves. Their power de- 
pends on their size and speed. The length of a wave is the 
distance from crest to crest; the height is the difference in 
level between the crest and the bottom of the adjacent 
trough ; the amplitude is the height of the crest above the 
average level of the water. A wave appears to be an ad- 
vancing ridge, but that aspect is often as delusive as with 
the waves that sweep across a wheatfield as the stems sway 
before the wind. In waves of oscillation the particles of 
water revolve around a stationary point, and do not move 
forward; in waves of translation the particles ‘advance as 
well as revolve. Waves of oscillation occur in the open sea, 
where the depth of water is greater than the length of the 
wave. Where the depth is the less, the movement on the 
lower side is retarded by friction with the floor, and the 
particles move forward in the upper part of the circuit further 
than they go back in the lower part, and the water advances 
as a wave of translation. It has been objected, as by B. 
Cunningham (Harbour Engineering, 1918, p. 164), that in 
practice the distinction between waves of oscillation and of 
translation is artificial, as all sea waves cause some advance 
of the water. When the wave reaches shallow water the 
crest advances more rapidly than the base, and the front 
is fed with water more slowly than the back ; hence the wave 
curls over, and falls as a breaker. Waves on a beach are 
waves of translation; and the backward and forward move- 
ment gives the water its power of attack. 
The greatest oscillatory waves are in the Southern Ocean 
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