information available on the condition of the beach immediately prior to this 

 storm. 



23. A review of existing literature on seawalls and their effect on 

 beaches has recently been conducted at CERC (Kraus, in press). Several labo- 

 ratory and field studies have suggested a rule-of-thumb estimate of the depth 

 of scour in front of a seawall equal to the height of the incident wave which 

 could be supported by the water depth (approximately 2 ft in this case) . The 

 physical model tests were set up with eroded beach elevation at +1.0 ft NGVD 

 during the Phase I tests to allow for testing of the stability of the stone 

 berm under a severely eroded beach condition and kept at that elevation for 

 the Phase II tests. 



24. A total of 110 tests was made for the Phase I template with four 

 water levels of 6.0, 7.0, 8.0, and 9.5 above NGVD for the two design storms 

 discussed in Part III of this report. The Phase I results were higher than 

 those predicted in the Phase I GDM study (i.e., 0.125). In an attempt to 

 reduce the overtopping rates, two modifications to the Phase I testing plan 

 were considered: modifying the stone toe berm and modifying the seawall. The 

 stone toe berm was proposed to control overtopping by controlling scour during 

 overtopping events. The width of the berm (three stones wide) was determined 

 in Phase I testing to be stable. A wider toe would be more conservative but 

 felt to be unnecessary and would have little influence on overtopping unless 

 the width was increased substantially. Modifications to the seawall included 

 adding a 0.75-ft wider lip at the top of the curved parapet. The addition of 

 this lip required a slight redesign of the curved parapet and steps. 



25. The Phase I seawall was constructed at a scale of 1:13 which, for 



the wave tank used, allowed only 60 to 70 percent of the maximum H to be 



mo 



produced at the wave board. Although the tank wave gages suggested that 



shallow-water wave heights were not significantly increasing with increasing 



deepwater wave heights, it could not be determined whether the shallow-water 



wave height had reached its maximum. Thus, to assure the design events could 



be reproduced, the Phase II tests were conducted at a 1:19 scale which allowed 



100 percent of the design H spectra to be produced. 

 ° mo 



26. The Phase II seawall template was constructed at the smaller scale 

 and installed in the same tank for testing. Figure 6 shows the modified sea- 

 wall design and test facility cross section. A foreshore slope of 1 on 16 

 and offshore slope of 1 on 100 were used in the model tests. A total of 155 



18 



