26 



these methods will vary with each breakwater project site. If possible, 

 shoreline morphology, such as a natural headland or creek, should be used to 

 terminate the breakwater project and minimize impacts on adjacent shorelines. 



Wave climate 



Structural effects on wave environment. Breakwaters reduce wave 

 energy at the shoreline by protecting the shoreline from direct wave attack and 

 transforming the incoming waves. Wave energy is dissipated on and reflected 

 from the structure, or diffracted around the breakwater's ends causing the 

 waves to spread laterally. Some wave energy can reach the breakwater's lee 

 by transmission through the structure, regeneration in the lee by overtopping 

 waves, or diffraction around the structure's ends. As most detached 

 breakwater projects are constructed in shallow water, incident wave energy is 

 often controlled by local water depth and variability in nearshore bathymetry. 

 Average wave conditions, as opposed to extreme or storm wave conditions, 

 generally control the characteristic condition of the shoreline. 



Wave diffraction. Shoreline response to detached breakwaters is 

 primarily controlled by wave diffraction. The diffraction pattern and wave 

 heights in the breakwater's lee are determined by wave height, length, and 

 angle, cross-sectional design, and for segmented structures, the gap-to-wave 

 length ratio. The resulting shoreline alignment is generally parallel to the 

 diffracted wave crests. 



If incident breaking wave crests are parallel to the initial shoreline (a 

 condition of no longshore transport), the waves diffracted into the 

 breakwater's shadow zone will transport sediment from the edges of this 

 region into the shadow zone (Fulford 1985). This process will continue until 

 the beach planform is parallel to the diffracted wave crests and zero longshore 

 transport again results (Figure 20). For oblique incident waves, the longshore 

 transport rate in the breakwater's lee will initially decrease, resulting in 

 sediment deposition (Figure 21). A bulge in the shoreline will develop and 

 continue to grow until a new equilibrium longshore transport rate is restored 

 or a tombolo results. 



Wave height. The magnitude of local diffracted wave heights is generally 

 determined by their distance from the breakwater's ends, or by their location 

 relative to the gaps in a segmented system (EM 1 1 10-2-1617). Wave height 

 affects the pattern of diffracted wave crests, and therefore affects the resulting 

 beach planform. For shallow water of constant depth, linear wave theory 

 predicts the circular pattern of diffracted wave crests shown in Figure 22a. 

 However, for very shallow water where wave amplitude affects wave celerity 

 C, the celerity decreases along the diffracted wave crests in relation to the 

 decrease in wave height. Figure 22b shows the distorted diffraction pattern, a 

 series of arcs of decreasing radius, which results. The latter situation usually 

 results in tombolo formation if the undiffracted portion of the wave near the 



Chapter 2 Functional Design Guidance 



