leaves the generating area and travels over some 2j000 miles of open 

 water before reaching a shoreo This 2,000 miles is called the decay- 

 distance ^ and studies show that at the end of this decay distance the 

 selected wave would have decreased in height to 2o5 feet^ would have 

 increased in length to 1300 feet and would have lengthened its period 

 to 16 seconds o The wave has now become a low swell j, which is the type 

 of wave usually noticed on our beaches during fair weather periods© 



It is to be noted that the l/H ratio of this wave has now become 

 52O0 M, large l/H ratio is characteristic of swells » In fact, this 

 L/H ratio is one method by which waves are classified o At the Beach 

 Erosion Board we usually consider that l/H ratios between about 10 and 

 35 characterize storm waves, between about 35 and 70 characterize inter- 

 mediate waves, and l/H ratios greater than 70 characterize well -devel- 

 oped swells 



Now let us study this 265-foot swell as it comes into shallow 

 watero So far as the wave is concerned, the water becomes shallow i^en 

 the depth is equal to or less than about l/2 the wave lengtho At this 

 depth we say that the wave wfeels"' the bottom, and the motion and foim 

 of the wave starts to adjust itself to this new conditiono Generally 

 speaking the effect is to increase the height and decrease the lengtho 

 Also, the internal motion of the water particles changes from circular 

 to elliptical with a noticeable increase in the velocities at the 

 bottomo In fact, these bottom velocities finally become of sufficient 

 magnitude to roll the sand particles back and forth on the bottom and 

 sand ripple formations resulto Some of the sand may be temporarily 

 thrown into suspension where any existing currents will cause a corre- 

 sponding movement of the sand particles o 



As the wave moves into even shallower water, we find that it fi- 

 nally reaches a depth where the wave crest can no longer be supported <> 

 At this point the wave combs over and breaks o jQiis breaking is accom- 

 panied isy great internal turbulence and causes large qixantities of 

 sand to be thrown into suspension This breaker zone is generally the 

 most active zone from the standpoint of action of the sand particles 

 forming the beacho Hydrodynamic relations show that our 2o5""foot 

 swell will increase in height to about 5 feet by the time it reaches 

 a water depth of 9 feet, at which depth it will breako 



After breaking, the wave, on gently sloping beaches, may reform 

 only to break again in even shallower watero On the steeper beaches, 

 the broken mass of water will rush up on the beach and return to the 

 sea as backwash, without reforming 



It woxild be well to note that waves in nature do not travel as 

 simple uniform wave trains o The picture is usually rather confused, 

 with what might be described as a spectrum of waves present, some hi^, 



11 



