1 Introduction 



With increased use and development of the coastal zone, beach erosion in 

 some areas may become serious enough to warrant the use of protective 

 coastal structures. Based on prototype experience, detached breakwaters can 

 be a viable method of shoreline stabilization and protection in the United 

 States. Breakwaters can be designed to retard erosion of an existing beach, 

 promote natural sedimentation to form a new beach, increase the longevity of 

 a beach fill, and maintain a wide beach for storm damage reduction and recre- 

 ation. The combination of low-crested breakwaters and planted marsh grasses 

 is increasingly being used to establish wetlands and control erosion along 

 estuarine shorelines. 



General Description 



Detached breakwaters are generally shore-parallel structures that reduce the 

 amount of wave energy reaching the protected area by dissipating, reflecting, 

 or diffracting incoming waves. The structures dissipate wave energy similar to 

 a natural offshore bar, reef, or nearshore island. The reduction of wave 

 action promotes sediment deposition shoreward of the structure. Littoral 

 material is deposited and sediment retained in the sheltered area behind the 

 breakwater. The sediment will typically appear as a bulge in the beach 

 planform termed a salient, or a tombolo if the resulting shoreline extends out 

 to the structure (Figure 1). 



Breakwaters can be constructed as a single structure or in series. A single 

 structure is used to protect a localized project area, whereas a multiple seg- 

 ment system is designed to protect an extended length of shoreline. A seg- 

 mented system consists of two or more structures separated by gaps with 

 specified design widths. 



Unlike shore-perpendicular structures, such as groins, which may impound 

 sediment, properly designed breakwaters can allow continued movement of 

 longshore transport through the project area, thus reducing adverse impacts on 

 downdrift beaches. Effects on adjacent shorelines are further minimized when 

 beach fill is included in the project. Some disadvantages associated with 



Chapter 1 Introduction 



