104 



BREAKERS AND SURF 



view. It is perhaps of equal interest that the relationship between 

 lengths and velocities that applies to waves in deep water is altered 

 as they run in over a shoaling bottom, with the result that waves that 

 ditt'er widely in length tend to run at similar velocities as they near 

 the breaker line. This is illustrated in figure 21, from which the pro- 

 portional alteration in length and in velocity of a wave can be read 

 directly at different points in its advance over shoaling bottom. The 

 relationship is also summarized in table 29. 



Table 29. — Decrease in lengths and velocities of ivaves of different dimensions as they 

 advance over a shoaling bottom 



Perhaps the most interesting rule illustrated by table 29 is that 

 longer waves are slowed much more, relatively, as they advance shore- 

 ward, than shorter ones are, by the time they reach the surf zone. If, 

 for example, this zone were along the 5- or 6-foot contour, as it often is 

 in moderate weather, waves 50 feet long would be still advancing at 

 about TO percent of their original velocity when they broke, but waves 

 2( 10 feet long at only about one-fourth the initial velocity, i. e., at about 

 5 knots. It is for this reason that breakers, caused by a long swell, so 

 often seem to hang almost stationary for the few instants before their 

 crests fall forward. 



The fact that waves are slowed down by the effect of the bottom can 

 be used to great advantage in determining water depths off an in- 

 accessible coast. Overlapping aerial photographs are taken at short, 

 accurately known intervals of time. From these, the rate of advance, 

 or velocity, of several wave crests is measured. By applying the rela- 

 tionships between wave velocity, period, and depth of water, the depth 

 is then found. 



It is pointed out, on page .32, that the simple relationship between 

 velocity, length, and period holds only for waves that are very low, 

 relative to their lengths, and that steep waves travel a little faster than 

 would be indicated by the simplified equation (See footnote 7, p. 32.) 

 When waves run into shoaling water and steepen further, the increase 

 in their heights seems to oppose their tendency to shorten, the result 



