CAUSES OF SURF 



103 



The reflection back of storm seas from a cliff or breakwater against 

 the next incoming waves may so increase the heights and steepness 

 of the latter as to cause them to break heavily, some distance out from 

 the barrier. But the counterwaves may prevent heavy breakers from 

 developing there at all, if the weather is moderate or the wind off- 

 shore. It has been stated that small boats can lie in safety in the 

 zone of confused but low wave action that results next to the shore 

 line under these circumstances. We can only comment, in this regard, 

 that we have never seen a situation of this sort, except on a very minor 

 scale and when the waves were so small that they were not dangerous 

 in any case. 



Alteration in length and in velocity over a shoaling bottom. — A 

 wave first begins to show measurable deformation when it reaches 



olo°. 



h a: 



< Id 



o *- 



LI Z 

 > LI 



8i 



.ill 



•oi 



(9 



O.IO 

 DEPTH. 



0.15 0.20 0.25 0.30 0.35 0.4O 0.45 



RELATIVE TO LENGTH OF WAVE IN DEEP WATER 



Figube 21. — Theoretical alteration in the dimensions of waves as they advance 



over a shoaling bottom. 



a point over a shoaling bottom where the depth of the water (measured 

 below undisturbed sea level) is about one-half the wave length from 

 crest to crest. It has long been known that the lengths of waves are 

 progressively reduced in their further progress from this point on- 

 ward, so that they are telescoped together, as it were, as they near 

 the shore. 



The reduction that takes place in the lengths and in the velocities 

 of waves over a shoaling bottom is usually given as proportional to 

 the square root of the depth of water. Recent observational as well 

 as theoretical studies, however, have shown that the situation is much 

 more complex than this, and especially that the decrease in the length 

 of a wave, and hence the increase in its steepness, is much more abrupt 

 as the wave approaches the shore line than was held by the older 



