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Shoaling

 
  
 
The energy that is being expended in producing this phenomenon is the energy that was given to the sea surface  when  the  wind  developed  the  sea  waves.  This energy is diminished as the swell waves move from the fetch area to the area of occurrence of the surf. The surf zone is the extent from the water up-rush on the shore to the most seaward breaker. It will be within this area that the forecast will be prepared. When waves enter an area where the depth of the bottom  reaches  half  their  wave  length,  the  waves  are said to “feel bottom.” This means that the wave is no longer traveling through the water unaltered, but is entering intermediate water where changes in wave length,  speed,  direction,  and  energy  will  occur.  There will be no change in period. These changes are known as shoaling and refraction. Shoaling affects the height of the waves, but not direction, while refraction effects both. Both shoaling and refraction result from a change in  wave  speed  in  shallow  water.  Now  let’s  look  at shoaling and refraction in more detail. Shoaling The shoaling effect is caused by two factors. The first is a result of the shortening of the wave length. Wave length is shortened as the wave slows down and the  crests  move  closer  together.  Since  the  energy between crests remains constant the wave height must increase if this energy is to be carried in a shorter length of water surface. Thus, waves become higher near shore than they were in deep water. This is particularly true with swell since it has along wavelength in deep water and travels fast. As the swell speed decreases when approaching shore, the wave length shortens, and along swell that was barely perceptible in deep water may reach a height of several feet in shallow water. The second factor in shoaling has an opposite effect (decreasing wave height) and is due to the slowing down of the wave velocity until it reaches the group velocity. AS the group velocity represents the speed that the energy  of  the  wave  is  moving,  the  height  of  the individual  wave  will  decrease  with  its  decreasing  speed until the wave and group velocity are equal. The second factor predominates when the wave first feels bottom, decreasing  the  wave  height  to  about  90  percent  of  its deep water height by the time the depth is one-sixth of the wave length. Beyond that point, the effect of the decreased distance between crests dominates so that the wave height increases to quite large values close to shore. Refraction When  waves  arrive  from  a  direction  that  is perpendicular to a straight beach, the wave crests will parallel the beach. If the waves are arriving from a direction  other  than  perpendicular  or  the  beach  is  not straight, the waves will bend, trying to conform to the bottom  contours.  This  bending  of  the  waves  is  known as refraction and results from the inshore portion of the wave having a slower speed than the portion still in deep water. This refraction will cause a change in both height and direction in shallow water. Surf  Development When a wave enters water that is shallower than half its wave length, the motion of the water near the bottom is retarded by friction. This causes the bottom of the wave  to  slow  down.  As  the  water  becomes  more shallow the wave speed decreases, the wave length becomes shorter, and the wave crest increases in height. This continues until the crest of the wave becomes too high and is moving too fast. At this point the crest of the wave becomes unstable and crashes down into the preceding wave trough; when this happens the wave is said to be breaking. The type of breaker (that is, whether spilling, plunging, or surging) is determined by the steepness of the wave in deep water and the slope of the beach. Figure 6-9 depicts the general characteristics of the three types of breakers. SPILLING BREAKER.— Spilling breakers occur with shallow beach slopes. The water at the crest of a wave may create foam as it spills down the face of the wave.  Spilling  breakers  also  occur  more  frequently when  deepwater  sea  waves  approach  the  beach.  This  is because the shorter wavelength of a sea wave means that the wave is steeper in the deep water and that the water spills from the crest as the waves begin to feel bottom. Because the water constantly spills from the crest in shorter wavelength (shorter period) waves, the height of spilling waves rarely increases as dramatically when the wave  feels  bottom,  as  do  the  longer  period  waves forming at the crest and expanding down the face of the breaker. PLUNGING  BREAKER.—  Plunging  breakers occur with a moderately steep bottom. In this type of breaker, a large quantity of water at the crest of a wave curls out ahead of the wave crest, temporarily forming a  tube  of  water  on  the  wave  face  before  the  water plunges down the face of the wave in a violent tumbling action. Plunging breakers are characterized by a loud, explosive sound made when the air trapped in the curl 6-13


   


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