Table 5 



Representative Seawall 

 Crest Elevations for 

 Overtopping Study of 

 1991 Beach Profiles 



Profile No. 



Seawall 

 Elevation 

 ft, mlw 



la 



19.8 



lb 



20.7 



2 



21.3 



3 



20.6 



4 



20.3 



5 



20.4 



an elevation of +19.4 ft, then an exten- 

 sion was added to continue the slope to 

 an elevation of +20.3 ft, again providing 

 a freeboard of 0.4 ft. 



The wave generator in the 18-in. 

 flume was unable to reproduce wave con- 

 ditions at Profile 2 at a 1:30 scale. 

 Rather than change to a smaller scale. 

 Profile 2 was reproduced at a 1:30 scale 

 in the 3-ft flume. Similar to the 

 18-in. flume, the existing 1:20 slope in 

 the 3-ft flume was matched to a portion 

 of the surveyed profile, and the steeper 

 profile shoreward of the 1 :20 slope was 

 constructed of sheet metal. Surveyed 

 and idealized profiles for Profile 2 are 

 illustrated in Figure 12. 



Results 



Overtopping rate per linear foot of prototype seawall for each profile 

 and each hour of the storm that had measurable overtopping are shown in 

 Table 6. Physical model tests were not conducted on Profile 5 at hr 31, or 

 Profiles 3 and 4 at hr 42. Volumes listed in Table 6 for these tests were ob- 

 tained by multiple regression analysis using the other results listed in 

 Table 6. Regression analysis is discussed below. 



Storm conditions for the SPN were considerably worse than during the 

 1978 storm, with greater water depths and wave heights and longer wave 

 periods. Overtopping rates, however, were considerably less, attesting to 

 the incidental effectiveness of the 1991 beach fill. Overtopping rates mea- 

 sured in the wave flume for Profile 3 were surprisingly low, but incident 

 wave heights for Profile 3 were lower than for the other profiles. NED 

 confirmed that in the prototype, overtopping rates at Profile 3 appeared 

 lower than at the other profiles during the October 30, 1991, storm, and 

 the general trends observed in the wave flume agreed with observations of 

 the prototype. The model did not test erosion of the 1991 beachfill during 

 the SPN storm. Therefore, higher overtopping rates could be experienced 

 as the beach erodes during the storm. Tests in Task C+ below show re- 

 sults if the beach should erode to 1978 contours. 



24 



Chapter 3 Research Tasks A, B, and C 



