Discussion of Research 

 Taslcs A, B, and C 



44 



A major uncertainty in the physical model tests was the wave spectrum 

 being tested. Wave information furnished for the storm profile consisted 

 of a monochromatic wave height and wave period, obtained by refracting 

 and diffracting a representative wave (peak period and significant wave 

 height) of a wave spectrum. This representative wave, after shoaling to 

 the approximate distance offshore modeled by the wave generator in the 

 wave flume, then was used as the peak period and zeroth moment wave 

 height to reproduce a new irregular wave spectrum. This would be accu- 

 rate if the entire spectrum shoaled to the same extent as the representative 

 wave. In reality, each frequency in the incident spectrum will shoal differ- 

 ently, and an entirely new spectrum will exist after shoaling. Although we 

 have the capability of dividing the incident spectrum into a number of 

 bandwidths, transforming each bandwidth individually through numerical 

 models REF/DIF and SBEACH, and then reassembling the transformed 

 spectrum from the individual bandwidths, the procedure is time-consuming, 

 not economically feasible, and other uncertainties in the prototype and 

 physical model do not justify attempting such a level of precision. This 

 uncertainty applied to Tasks A, B, C, and C+, and the net effect on over- 

 topping rate caused by this approximation of the wave spectrum is unknown. 



With the 1978 profiles (Tasks A, C, and C+), there was considerable 

 freeboard between the beach and seawall crest. Waves striking the sea- 

 wall were forced into a vertical sheet of water and spray, frequently ex- 

 ceeding the height of the seawall. Because the motion was nearly vertical, 

 much of this water fell back on the seaward side of the seawall in the 

 flume, but wind effects may cause more of the water to overtop the sea- 

 wall in the prototype. 



Wind effects on overtopping rates in Task B are expected to be mini- 

 mal. Wind has two effects on seawall overtopping rates: modification of 

 wave runup on the beach, and blowing spray over the seawall. Modifica- 

 tion of the wave runup has been calculated to have only a minor effect on 



Chapter 4 Discussion of Research Tasks A, B, and C 



