desired, the next tasks are to determine to what extent the incident wave 

 energy must be reduced or modified to stimulate formation of the desired 

 shoreline, and the best possible means for accomplishing the required changes. 

 Average seasonal shoreline variations give an indication of how the shore 

 responds to different wave climates. Comparing the local winter and summer 

 wave climates and observing how much the beach progrades during the summer 

 months may provide insight into the degree of incident wave modification re- 

 quired to obtain the desired shoreline. From this comparison,, an assumption 

 can be made regarding the degree of energy reduction required to advance the 

 shoreline the desired distance and the resultant sinuosity of the salients. 

 Techniques available for reducing incident wave energy and controlling shore- 

 line response are separated into two categories, those used for assuring tom- 

 bolo development and those used for developing salients only. 

 Assuring tombolo development 



41. Tombolos usually develop if the two diffracted wave crests behind a 

 structure do not intersect before the undisturbed portions of the wave reach 

 the shoreline. Figure 17 shows a highly idealized case and the resulting dif- 

 fraction pattern. This single detached breakwater has a length £ and off- 

 shore distance x , and is parallel to the shoreline. Table 1 summarizes 

 these and other parameters for various United States detached breakwater proj- 

 ects. The incident waves are assumed to be shore-normal and to obey linear 

 wave theory, and the water landward of the structure is assumed to be uniform 

 in depth. Diffracted waves will have a circular planform and the arcs will 

 not intersect before the rest of the wave reaches the shore if the single 

 breakwater length or segment length for a segmented breakwater is greater than 

 two times the distance offshore, that is: 



I > 2x for a single detached breakwater 



I > 2x for a segmented detached breakwater 



(where i is the individual segment 

 length) 



For oblique waves, the above condition also ensures no intersection of the 

 arcs. However, the longshore current generated on the updrift side may be 

 enough to penetrate the shadow zone and keep the tombolo from connecting. 

 This is a general relationship and there are instances of tombolo connection 

 where the single breakwater or segment length is equal to or less than 



37 



