As the water depth decreases to several times the wave height, the 

 time variation of velocity changes from approximately sinusoidal motion 

 to a high shoreward velocity associated with the brief passage of the wave 

 crest and a lower seaward velocity associated with the longer time 

 interval occupied by the passage of the trough. As the shoreward 

 velocity associated with a passing wave crest slows down and begins to 

 reverse direction over a ripple, a cloud of sand is lifted upward from 

 the lee (landward) side of the ripple crest. The cloud of sand then 

 drifts seaward with the ebbing flow under the trough. 



The water stress on the bottom due to turbulence and wave -induced 

 velocity gradients moves sediment in the surf zone with each passing 

 breaker crest. Sediment motion is by both bedload and suspended load 

 transport. The high velocities under breaking waves entrain sediments 

 so that they can be easily transported along the shoreline by alongshore 

 currents . 



The length of time sediment particles remain entrained in the water 

 column depends on their fall velocity, which is a function of sediment 

 size, shape, composition (density), and characteristics of the fluid and 

 flow. While in the water column, particles will respond to any currents 

 present and will be displaced from their origin. Since waves generally 

 approach the coast at some angle, a part of the wave energy is directed 

 alongshore driving the longshore current. This current, coupled with 

 the entrainment capacity of shoaling waves, is the major force moving 

 sediments at the coast (Johnson and Eagleson, 1966; Kennedy and Locher, 

 1972). 



VII. BEACH EROSION-EFFECT OF STRUCTURES 



As discussed previously, waves and wave -generated currents entrain 

 sediment and transport it alongshore. Waves, through mass transport, 

 also move sediment onshore and offshore (Johnson and Eagleson, 1966). 

 At any particular beach, erosion or accretion is the net effect of all 

 forces acting upon that beach. A beach area erodes when the total amount 

 of material leaving the area exceeds the total amount of material arriving, 

 irrespective of the amount of material transported along the beach. 

 Conversely, a beach will accrete if the total amount of material arriving 

 exceeds the amount of material leaving, again irrespective of the amount 

 of material in transport. 



In the case of a beach or coastal sector containing a structure such 

 as a jetty or groin, when waves from the open sea can approach the beach 

 unimpeded, the turbulence generated by the waves will be nearly as great 

 on the downstream side of the obstruction of flow as they would be in the 

 absence of the obstruction. A longshore current will be developed in the 

 downstream direction ivhich can carry any sedimentary material picked up, 

 but the supply of sand from the upstream direction will have been cut off. 

 Thus a net erosion will occur, not because the turbulence intensity on the 

 beaches has been increased, but because the supply of sand for that beach 



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