L = d/(0.1083) 



L = 10/(0.1083) = 92.34 m (303 ft) 



The harbor entrance is, therefore, 300/92.3 = 3.25 wavelengths wide; 

 interpolation is required between Figures 7-63 and 7-64 which are for gap- 

 widths of 2 and 4 wavelengths, respectively. From Figure 7-63, using the 

 diagrams for Sj^^^^ = 75 and noting that 1000 meters equals 5.41 gap-widths 

 (since B/L = 2.0 and, therefore, B = 2(92.34) = 184.7 meters (606 

 feet) ), the diffraction coefficient 5.41 gap-widths behind the harbor 

 entrance along the center line is found to be 0.48. The period ratio is 

 approximately 1.0. Similarly, from Figure 7-64, the diffraction coefficient 

 2.71 gap-widths behind the gap is 0.72 and the period ratio is again 1.0. 

 Note that the gap width in Figure 7-64 corresponds to a width of 4 

 wavelengths, since B/L = 4.0 ; therefore, B = 4(92.34) = 369.4 meters 

 (1212 feet), and 1000 meters translates to 1000/(369.4) = 2.71 gap 

 widths . The auxiliary scales of y/L and x/L on the figures could also 

 have been used. Interpolating, 



B/L K' Period Ratio 



2.0 0.48 1.0 



3.25 0.63 1.0 



4.0 0.72 1.0 



The diffraction coefficient is, therefore, 0.63, and the significant wave 

 height is 



H = 0.63(3) = 1.89 m (6.2 ft) 



There is no change in the period of maximum energy density. 



For the point 1000 meters off the center line, calculate y/L = 1000/(92.34) 

 = 10.83 wavelengths , and x/L = 1000/(92.34) = 10.83 wavelengths . Using 

 the auxiliary scales in Figure 7-63, read K' = 0.11 and a period 

 ratio = 0.9 . From Figure 7-63, read K' = 0.15 and a period ratio = 

 0.8 . Interpolating, 



B/L K' Period Ratio 



2.0 0.11 0.9 



3.25 0.135 0.86 



4.0 0.15 0.8 



The significant, wave height is, therefore, 



H = 0.135(3) = 0.4 m (1.3 ft) 



and the period of maximum energy density is 



T = 0.86(10) = 8.6 s 

 P 



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7-99 



