drift. A cuspate spit is formed which will continue to grow until either the 

 longshore transport rate past the structure is reestablished or a tombolo is 

 formed. Depending on where the offshore breakwater is positioned relative to 

 the littoral zone, the formation of a tombolo can act as a complete littoral 

 barrier which can trap all the littoral drift until it is filled to capacity, 

 at which time sand will be shunted around the seaward side of the structure, 

 restoring the longshore transport rate. During this process severe erosion of 

 the downdrift beach would be expected. The cuspate spit that results from 

 oblique wave attack can be expected to be asymmetric with its shape dependent 

 on the structure length, the distance offshore, and the nearshore wave condi- 

 tions. Figure 5-27 illustrates the formation of asymmetric cuspate spits. 



A major concern in designing an offshore breakwater for shore protection 

 is determining if the resulting shore adjustment should be connected to the 

 structure. There are advantages and disadvantages for each shoreline config- 

 uration, and the designer is usually confronted with many aspects to consider 

 before making a choice between tombolos and cuspate spits. While both shore- 

 line adjustments affect the adjacent shoreline, cuspate spits are usually 

 preferred over tombolos. When a tombolo forms, large quantities of sediment 

 can be impounded, resulting in extensive erosion downdrift of the structure. 

 A cuspate spit formation will often allow the majority of littoral drift to 

 pass and thus have a lesser effect on the downdrift beach. During seasonal 

 changes in wave direction, a cuspate spit is more likely to allow the littoral 

 drift to pass landward of the offshore breakwater. Therefore, there is less 

 chance of the material being retarded by passage to the seaward of the struc- 

 ture where parts of the littoral drift may be lost permanently. Cuspate spits 

 and tombolos do not provide uniform erosion protection along an entire proj- 

 ect, and legal problems could arise if the protected region is not owned by a 

 Federal, State, or local government. Depending on the project, more uniform 

 protection may be needed. 



The formation of a tombolo increases the length of beach available for 

 recreation use and greatly facilitates the monitoring and maintenance of 

 the structure, but beach users may be inclined to use the structure or swim 

 immediately adjacent to it which could be dangerous. 



4. Siting Considerations . 



The most important parameters governing the shore response to offshore 

 breakwaters are those that affect diffraction. Wavelength, wave height, wave 

 direction, and the breakwater gap all affect the resulting diffraction pat- 

 tern. The shore responds by alining itself with the patterns of the wave 

 crests. The response rate is governed by the amount of wave energy available 

 to transport sediment. Other important parameters are the local tidal range, 

 the natural beach slope, the supply of sediment, and the sediment grain size. 

 Background information on the protective features and the functional limita- 

 tions of offshore breakwaters is discussed by Toyoshima (1972) and Lesnik 

 (1979). 



a. Waveleng th. In general, the amount of wave energy transferred into 

 the lee of a breakwater increases with increasing wavelength. According to 

 linear diffraction theory, the wavelength does not affect the pattern created 

 by the wave crest. However, wavelength does affect the amplitude of the 

 diffracted wave at a particular location. Longer waves will provide more 



5-64 



