energy to the shadow zone, especially the obliquely arriving waves, which 

 tends to prevent tombolo formation. The amount of energy transferred into the 

 lee of the structure can be found using Figures 2-28 to 2-39 in Chapter 2 for 

 the appropriate position, water depth, wavelength, and wave direction. The 

 diffraction technique must be performed for both ends of the breakwater, with 

 the resultant energy quantities being summed. 



b. Br eakwater Gap Width . The ratio of the gap width, B , to the wave- 

 length, L , for segmented offshore breakwaters greatly affects the distribu- 

 tion of wave heights in the lee of the structures. Generally, increasing the 

 ratio B/L will increase the amount of energy reaching the shadow zones, 

 while the diffraction effects will decrease. Figures 2-42 to 2-52 in Chapter 

 2 can be used to estimate the diffraction patterns caused by breakwater gaps. 

 It is important to note that these diagrams do not contain refraction, shoal- 

 ing, or breaking effects. 



c. Wave Direc tion. The general shape of the shoreline behind an offshore 

 breakwater is highly dependent on the directional nature of the wave climate. 

 Very oblique waves produce a strong longshore current that may prevent tombolo 

 formation and restrict the size of the cuspate spit. The bulge in the shore- 

 line tends to aline itself with the predominant wave direction. This is 

 particularly evident for tombolos, which seem to "point" into the waves. How- 

 ever, if the predominant waves are oblique to the shoreline, the tombolo' s 

 apex will be shifted to the downdrift direction, its equilibrium position 

 becoming more dependent upon the strength of the longshore current and the 

 length of the structure. 



d. Wave Height . Besides its role in generating currents and entraining 

 sediments, wave height also affects the pattern of diffracted wave crests. 

 Linear diffraction theory assumes that the diffracted wave crests move at a 

 speed given by 



C =-y/gd" (5-18) 



where C is the wave celerity, g the acceleration of gravity, and d the 

 water depth. Assuming a constant water depth gives the circular diffracted 

 wave crests as shown in Figure 5-28. In this case all the wave crests move at 

 the same speed, even though the wave height has decreased along the crest 

 toward the breakwater. However, in very shallow water, studies have shown 

 that wave amplitude dispersion plays an important role in wave diffraction 

 (Weishar and Byrne, 1978). The wave celerity in very shallow water is more 

 accurately expressed as 



'4 



'g(d + H) (5-19) 



which is a function of wave height, H . With a constant water depth, the 

 wave celerity will decrease along the diffracted wave crest as the wave height 

 decreases. In other words, the farther along a diffracted wave crest into the 

 undisturbed region the more the wave height decreases, which in turn decreases 

 the speed of the wave crest. This action distorts the wave pattern from the 

 circular shape to an arc of decreasing radius as shown in Figure 5-29. In 

 situations where amplitude dispersion is important, tombolos are more likely 

 to form because the diffracted parts of the wave crests are less likely to 



5-65 



