breakwater in a large tidal range is shown in Figure 5-30. The mean tidal 

 range is 2.9 meters (9.5 feet) and there is a limited sediment supply. At low 

 tide (Fig. 5-30a) a double tombolo has formed because the structure is long, 

 close to shore, and has narrow breakwater gaps. At high tide (Fig. 5-30b) the 

 combined structure length is only about twice as long as the distance from the 

 original shoreline. A "high water tombolo" has not formed as might be 

 expected for this configuration due to the combination of the large tidal 

 range and the limited sediment supply. 



f. Natura l Beach Slope . The natural beach slope can play a major role in 

 the positioning and configuration of offshore breakwaters. If the profile is 

 gently sloping and the structure is to be placed outside the surf zone, the 

 breakwater may have to be lengthened in order to be an effective sediment 

 trap. A gently sloping beach with a large tidal range makes an optimum struc- 

 ture placement extremely difficult because such a large section of the profile 

 is active over the tidal cycle. 



g. Sediment Supply. If there is an insufficient supply of sediment, the 

 expected shoreline adjustment in the form of a cuspate spit will not fully 

 develop. Offshore transport will continue to erode and flatten the beach 

 profile in the lee of the structure, resulting in a different equilibrium 

 condition than expected. In locations where there is a seasonal variation in 

 sediment supply, it is possible that cuspate spits may accrete and recede 

 accordingly. 



h. Sediment Size . The sediment grain-size distribution on a beach 

 affects the shape and growth of a cuspate spit by affecting the slope of the 

 equilibrium beach profile and the sediment transport rate. Coarser sediments 

 have steeper profiles which cause more diffraction than finer grain-sized 

 sediments. The finer grained beaches will respond more rapidly to changing 

 wave conditions and are more likely to form tombolos. Graded materials may 

 settle differently between the shore and the breakwater. 



5. Design Consideration s. 



The main design considerations for an offshore breakwater center around 

 the resulting shoreline adjustment. In some cases it is desirable to ensure 

 a tombolo connection, but more often this connection should be avoided. The 

 formation of a tombolo is usually prevented by allowing sufficient energy to 

 pass into the protected region, using one or more of the techniques discussed 

 below. 



a. Breakwater Length Versus Di stance Offshore. Tombolo formation can 

 usually be prevented if the structure length, £ , is less than the distance 

 offshore, X ; i.e. , 



£ < X (5-20) 



This configuration usually permits the intersection of the diffracted wave 

 crests well before the undistorted waves adjacent to the structure reach the 

 shoreline. If the predominant wave direction is nearly shore normal, an 

 approximate location of the bulge apex is found at the point of the inter- 

 section of the two wave crests as the waves reach the shoreline, as shown in 

 Figure 5-31. When the structure length becomes greater than the distance 



5-67 



