26 HARBOUR CONSTRUCTION. 



and more intensified translation; and the particles are, in a 

 constantly increasing degree, being carried forward by each 

 successive wave. Finally, the bottom friction so retards the 

 lower particles of the wave that the crest overruns the face 

 slope, and the wave " breaks." At this moment, i.e. just as the 

 wave is commencing to break, the forward motion of the par- 

 ticles is equal to the velocity of the wave, and the wave-stroke 

 is delivered with its most destructive effect. 



Although the length of such waves, from crest to crest, 

 diminishes as they approach the shore, their period l remains 

 constant. Therefore, seeing that a lesser distance is traversed 

 in the same time, it follows that the velocity of waves dimin- 

 ishes with their length. Since, then, the velocity of waves is 

 that which most affects their impact, long waves are more to be 



< Direction of propagation of wave. 



FIG. 1 Breaking wave. The dotted lines indicate the successive forms which a wave assumes prior 



to breaking. 



feared than shorter ones, notwithstanding that the latter may 

 be of greater height. 



Waves whose length is small compared with the depth of 

 water traversed by them, may, for all practical purposes, be con- 

 sidered as belonging to the class called " Oscillatory." 



From what has been said, it will be seen that in water of a 

 given depth small waves may be, or at any rate may closely 

 approximate to, simple waves of oscillation ; whereas in water of 

 the same depth storm waves may assume the character of waves 

 of translation, in consequence of their being so much more 

 affected by bottom friction. 



I mention this here as having reference to the effect of 

 long rubble foreshores such as those in the mound type of 



1 The period of a wave is the time occupied in its traversing a distance equal 

 to its length. It is also equal to the time which the respective particles composing 

 the wave take in performing one complete revolution. 



