the shoreline adjustment behind a segmented breakwater that is permeable and 

 overtopped. 



d. Segmented Breakwaters . A segmented breakwater offers a very func- 

 tional solution for a long section of shoreline that requires wave transmis- 

 sion to prevent tombolo formation. The structure can be built nearshore in an 

 economical water depth because it permits a constant proportion of wave energy 

 into the protected area. Also, the diffracted waves have the same period as 

 the incident waves. Segmented breakwaters can be designed to allow the beach 

 in their lee to accrete enough sediment to provide an erodible buffer during 

 storms and still maintain the natural longshore transport rate during normal 

 wave conditions. 



The amount of energy reaching the lee of the structure is controlled by 

 the width of the gaps between the breakwaters and the wave diffraction through 

 these gaps. The gaps should be at least two wavelengths wide, and the length 

 of each structure segment should be less than the distance offshore. Provid- 

 ing fewer gaps of greater width will cause the shoreline to respond with 

 spaced bulges and embayments with an enlarged relief (the seaward distance 

 from the more shoreward point of the embayment to the tip of the cuspate 

 spit), which does not provide uniform storm protection along the project. 

 If this is not acceptable, increasing the number of gaps and shortening the 

 length of each segment will promote features of less relief, providing more 

 uniform protection. Segmented offshore breakwaters are illustrated in Figures 

 5-30, 5-32, and 5-33. Figure 5-33 illustrates the use of offshore breakwaters 

 in conjunction with a beach fill. 



e. P ositioning with Respect to Breaker Zone . Placing the breakwater 

 landward of the normal breaker zone will advance the shoreline and may cause 

 tombolo formation (see Fig. 5-32). If positioned well shoreward of the 

 breaker zone, a large percentage of the total longshore transport will pass 

 seaward of the structure and the effect on the adjacent shoreline will be less 

 severe. This method is not recommended for coasts with steep beach slopes and 

 narrow surf zones because the area shoreward of the breakwater will tend to 

 fill completely, turning the breakwater into a seawall. 



f. Structure Orient ation. The orientation of the breakwater with respect 

 to both the predominant wave direction and the original shoreline can have a 

 marked effect on the size and shape of the resulting cuspate spit or tombolo. 

 A change in structure orientation modifies the diffraction pattern at the 

 shoreline, and subsequently, the shore response. An approximation of the 

 shape of the shore response when waves are normally incident to the shoreline 

 can be determined by using the procedures discussed in Chapter 2, Section IV 

 to determine the diffracted wave crest configuration. For waves that are 

 extremely oblique to the shoreline, it is recommended that the breakwater be 

 oriented parallel to the incoming wave crests. This will provide protection 

 to a longer section of shoreline for a given structure length; however, it 

 will probably increase the amount of construction material required for the 

 structure since one end of the breakwater will be in water deeper than if it 

 were oriented parallel to the bottom contours. 



6. Other Considerations. 



Apart from shore response, there are several other factors which affect 



5-71 



