tend to compartmentalize the shore and the long-shore current system. Sedi- 

 ment moving alongshore is forced into deeper water in order to move around the 

 structure ends, thereby increasing offshore losses. Frequently, the presence 

 of a groin field will displace the nearshore bar system seaward. If detached 

 breakwaters are designed properly, sediment will continue to move alongshore 

 at a reduced rate behind the structures. The degree of reduction for given 

 transport conditions is a function of the design. Sediment may be trapped 

 temporarily and then removed when conditions change. Breakwaters have not 

 been observed to increase offshore losses of sediment (unless tombolos form) 

 and, in fact, are capable of decreasing the offshore transport rate. There- 

 fore, unlike groins, breakwaters (a) seldom promote offshore sediment losses, 

 (b) do reduce the potential for longshore movement, and (c) can allow regional 

 littoral transport patterns to continue. 



8. There are several disadvantages to the use of detached breakwaters. 

 They are expensive to construct, often involving the use of marine-based 

 equipment. Also, available design experience and guidance is limited. The 

 parameters which control the complex interaction of sediments and structures 

 are poorly understood, setting the stage for potential judgmental errors. 

 Probably the greatest disadvantage is a perceived one. The scarcity of func- 

 tioning examples especially for segmented detached breakwaters in the United 

 States reduces the public and even the technical confidence level. People are 

 reluctant to support a project if they cannot see a similar plan that is 

 working. 



9. Of primary interest to the coastal engineer when designing a 

 detached breakwater are the equilibrium size and shape of the salient and its 

 stability. Accurate prediction of the eventual shoreline configuration is 

 beyond the present state of knowledge; however, the conditions necessary for 

 equilibrium can be deduced. If the length of a structure is great enough in 

 relation to its distance offshore, the salient may connect to the structure 

 forming a tombolo. If the incoming breaking waves are normally incident to 

 the original shoreline and there is no predominant longshore transport direc- 

 tion, the diffracted waves will transport sand from the shore adjacent to the 

 structure into the structure's shadow. This process will continue until the 

 shoreline is so aligned that the waves break parallel to the shoreline and the 

 longshore transport again becomes zero along the entire coast. Figure 3 is an 

 example of such a case, where the tombolo 's equilibrium condition is such that 



