DETACHED BREAKWATERS FOR SHORE PROTECTION 



PART I: INTRODUCTION 



1. Detached breakwaters protect the shore by modifying wave action, 

 thereby promoting sediment deposition shoreward of the structure and resulting 

 in the development of a beach salient. The concept of detached breakwaters 

 combines the wave energy dissipation of a natural shore-parallel sandbar or 

 reef with the wave diffraction effects of a nearshore island. Detached break- 

 waters locally reduce incident wave energy and alter wave direction to create 

 a "shadow zone" where sediment transported alongshore or placed as beach-fill 

 is retained. This sediment will typically appear as a bulge in the beach 

 planform. If this depositional feature or salient becomes connected to the 

 structure, it is called a tombolo. This report examines the conditions which 

 cause different degrees of sediment buildup, the philosophy and history behind 

 the use of detached breakwaters for shore protection, and the advantages and 

 disadvantages of detached breakwaters. 



2. Detached breakwaters can be constructed as a single structure for 

 very localized shore protection or as a segmented breakwater to protect a 

 longer section of beach. A detached breakwater is not connected to shore by 

 any type of sand-retaining structure, and is usually built approximately par- 

 allel to shore or to the predominant wave train. A segmented detached break- 

 water consists of two or more relatively short breakwater segments separated 

 by gaps. Both the segment and gap length are usually regular. Figure 1 

 illustrates the general characteristics of detached single and segmented 

 breakwaters. 



3. Segmented detached breakwaters have many advantages over other, more 

 conventional forms of shore protection. Unlike groins, segmented breakwaters 

 can be designed to provide substantial protection without becoming a complete 

 barrier to littoral transport, nor promoting offshore losses. Unlike revet- 

 ments, bulkheads, and seawalls, they aid in the retention of the beach. If 

 the breakwater system is properly sited and designed, and beach-fill is in- 

 cluded as an item of construction, the impact to neighboring shores should be 

 minimal. The main disadvantages of segmented detached breakwaters are that 

 they are more expensive to construct than land-based structures and there are 



