Alternative Structure Configurations 



406. After the modeling system had been calibrated, verified, and 

 tested, it was possible to study alternative strategies for maintaining the 

 beach fill in place. Walker, Clark, and Pope (1980) also discuss alternatives 

 considered in arriving at the final choice of using detached breakwaters . 

 Obvious alternatives are to remove the (expensive) detached breakwaters and/or 

 groins in order to assess quantitatively the necessity for keeping them in 

 place. This type of information might be useful if another project is to be 

 constructed on a similar coast. An important limitation in this analysis is 

 the absence of the probable mitigating effect of the breakwaters on offshore 

 transport, which is not accounted for in GENESIS. 



407. Shoreline change over the verification period 9 October 1978 to 

 17 November 1979 for three alternative configurations was investigated: 



a. Existing groins without detached breakwaters. 



b. Existing breakwaters without the terminal groins. 



c. Extended groins without detached breakwaters. 



For case c, by trial and error the groins were extended to the length required 

 to give the same volume change for the site as the existing (design) condition 

 of detached breakwaters and shorter groins. Results of the simulations are 

 shown in Figure 46. For the case with only the groins of existing length, the 

 salients are absent, as was expected. More serious is the significant loss of 

 57,000 cu yd of fill, about half of the initial fill of 110,000 cu yd. 



408. To simulate the case of removing the two groins, 20 cells were 

 added on each side of the original calculation grid. The added shoreline/sea- 

 wall positions were read from aerial photographs except for the farthest few 

 cells, which were not covered by the photographs and were extrapolated by 

 hand. Thus, the model contained 89 cells for this particular simulation. 



The value of NN on Line A. 3 in the START file was set to 89, and the grid 

 cell numbers of the detached breakwaters on Line G.6 were incremented by 20. 

 As seen from Figure 46, the beach fill did very well on the updrift (west) 

 side. In fact, slight accretion may be observed here since the west groin had 

 been removed. Evidently this groin not only prevents sand from leaving the 

 enclosed beach, but also prevents it from entering. 



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