IV. COMPARISON OF THEORY WITH FIELD DATA FOR FRIDAY HARBOR BREAKWATER 



Although the Friday Harbor breakwater has a very complex geometry 

 and does not respond as a rigid body to the incident wave excitation, it 

 is important to draw some comparisons between the theoretical prediction 

 of performance and the field measurements. In seeking a "typical event" 

 from the enormous quantity of data gathered, the goal was to find a case 

 where the wind was reasonably close to being on the beam of the short 

 leg of the breakwater. 



The one striking item which emerges from the data is the similarity 

 of all the transmission coefficients examined. These curves seem iden- 

 tical no matter what the wind direction. This was not expected because 

 there were barges tied to the breakwater along the entire long leg, while 

 there were none along the shorter leg. A further investigation of the 

 reasons for the similarity is certainly warranted. 



The record selected for comparison with the theory was FH 7-8. 

 Figure G-3 in Appendix G shows the incident and transmitted wave spectra 

 and transmission coefficient. This record is also listed in the statis- 

 tical summaries of Appendix F. The spectral analysis using a high-pass 

 filter was performed as described in Section III. 



A comparison of the theoretically predicted and measured trans- 

 mission coefficient is shown in Figure 28. So long as the calculated 

 hydrodynamic damping is doubled in the theoretical analysis, the results 

 are quite good. As described in Section II, the peak in the transmission 

 curve at a frequency of 0.95 hertz probably results from the "irregular 

 frequency" phenomenon which occurs in this mathematical formulation. 



Comparisons of sway, heave, and roll acceleration predictions with 

 measurements are shown in Figures 29, 30, and 31, respectively. Here, 

 the acceleration response has been nondimensionalized by multiplying by 

 the beam or beam squared, as appropriate, and dividing by the accelera- 

 tion of gravity times the incident wave amplitude. 



In the case of sway acceleration, the theory overpredicts the values 

 throughout the entire frequency range. The peak at 0.5 hertz appears in 

 the correct location, but the measured values would need to be doubled 

 to bring the curves into better agreement. 



For heave acceleration the curves appear to be in closer agreement, 

 at least above the frequency of 0.4 hertz. Below 0.4 hertz there seems 

 to be little correlation. 



Roll acceleration seems to show the worst agreement of all. Here 

 again, the predicted accelerations are considerably higher than the mea- 

 sured values. 



There are several possible explanations for the discrepancy between 

 predicted and measured accelerations. In the field, even if the wind 



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