For this example point P and those lines of equal K' situated nearest P 

 are shown on a schematic overlay (Fig. 2-41). This overlay is based on 

 Figure 2-36 since the angle of wave approach is 135° . It should be noted 

 that Figure 2-41, being a schematic rather than a true representation of the 

 overlay, is not drawn to the hydrographic chart scale calculated in the 

 problem. From the figure it is seen that K' at point P is approximately 

 0.086. Thus the diffracted wave height at this point is 



H = K'H^ = (0.086)(3) = 0.258 m, say 0.26 m (0.853 ft) 



The above calculation indicates that a wave undergoes a substantial height 

 reduction in the area considered. 



OVERLAY 

 { Fig. 2 - 35 ) 



X and y are measured in units 

 of wavelength. 

 (These units vary depending 

 on the wovelength and the 

 chart scale.) 



180' 



*^x 



Wave crests 



Direction of wove opprooch 



Figure 2-41. Schematic representation of wave diffraction overlay • 



*************************************** 



b. Waves Passing a Gap of Width Less Than Five Wavelengths at Normal 

 Incidence. The solution of this problem is more complex than that for a sin- 



gle breakwater, and it is not possible to construct a single diagram for all 

 conditions. A separate diagram must be drawn for each ratio of gap width to 

 wavelength B/L. The diagram for a B/L ratio of 2 is shown in Figure 2-42 

 which also illustrates its use. Figures 2-43 to 2-52 (Johnson, 1952) show 

 lines of equal diffraction coefficient for B/L ratios of 0.50, 1.00, 1.41 

 1.64, 1.78, 2.00, 2.50, 2.95, 3.82, and 5.00. A sufficient number of diagrams 

 have been included to represent most gap widths encountered in practice. In 

 all but Figure 2-48 (B/L = 2.00), the wave crest lines have been omitted. 

 Wave crest lines are usually of use only for illustrative purposes. They are, 

 however, required for an accurate estimate of the combined effects of refrac- 

 tion and diffraction. In such cases, wave crests may be approximated with 

 sufficient accuracy by circular arcs. For a single breakwater, the arcs will 

 be centered on the breakwater toe. That part of the wave crest extending into 

 unprotected water beyond the K' =0.5 line may be approximated by a straight 

 line. For a breakwater gap, crests that are more than eight wavelengths 



2-92 



