of the beach adjacent to the jetty. The beach on the far left side of the 

 figure is assumed to be outside the area of influence of the structures, and 

 therefore its position remains fixed. 



78. Waves arrive at the site as shown in Figures 9b, c, and d. In 

 these figures, the longshore distributions of the breaking wave height and 

 breaking wave angle are displayed in graphic form above the related beach 

 planform. The local breaking wave height and angle are mainly controlled by 

 the detached breakwater. The shoreline that would result if there were no 

 seawall is indicated by a dashed line. 



79. Figure 9b shows the result of waves arriving almost normal to the 

 shoreline for a period of 84 hr. Convergence of waves behind the detached 

 breakwater causes a bulge, or salient, to form. The wave direction then 

 changes, Figure 9c, and waves arrive obliquely from the right for an elapsed 

 time of 180 hr. This results in a loss of sand on the beach next to the 

 jetty. The seawall prevents the shoreline from eroding farther landward imme- 

 diately next to the jetty; the price paid is that more sand is removed from 

 along the front of the seawall. Finally, as shown in Figure 9d, the wave di- 

 rection changes again and waves arrive obliquely from the left. The wave 

 shadow zone behind the detached breakwater also shifts and the potential re- 

 gion for erosion moves to the middle of the seawall. Sand returns next to the 

 jetty, and an eroded sector forms at the middle of the seawall. 



80. Although differences in shoreline positions with and without the 

 seawall are moderate in this example, by altering the input wave conditions 

 (e.g., by increasing the difference in breaking wave angle between applied 

 wave conditions) a much greater disparity in resultant shorelines can be 

 generated. 



Example 2: Pocket Beach 



81. The initial shoreline configuration for this example is shown in 

 Figure 10a. A curved pocket beach approximately 2 km long is bounded by two 

 long headlands which contain the littoral transport. A curved seawall is lo- 

 cated 4 m landward of the initial shoreline. 



82. Waves first arrive obliquely from the right side of the figure for 

 126 hr to produce the planform shown in Figure 10b. As a result, beach mate- 

 rial moves toward the left headland. The seawall has protected the area on 

 the right side of the beach, as seen by the shoreline change that would have 



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