6 Summary and Conclusions 



Report Summary 



From prototype experiences, detached breakwaters have proven to be a 

 viable method of shoreline stabilization in the United States. Breakwaters can 

 be designed to retard erosion of an existing beach, promote sedimentation at 

 the lee of the structure to form a new beach, retain placed beach fill material 

 and reduce renourishment intervals, and/or maintain a wide beach for damage 

 reduction and recreation. Low-crested breakwaters can also be combined with 

 dredge material disposal and marsh grass plantings to establish wetlands and 

 control erosion along estuarine shorelines. Most recent prototype applications 

 of detached breakwaters have been along sediment-starved shorelines with low 

 to moderate wave energy such as in the Chesapeake Bay, Great Lakes, and 

 some areas of the Gulf Coast. 



This report summarizes and presents design knowledge for both the 

 functional and structural design of detached breakwaters for shoreline 

 stabilization. Functional design of breakwaters in the United States relies on a 

 significant amount of engineering judgement, data from a few existing 

 breakwater projects for comparison, and an understanding of basic coastal 

 processes. The design process is an iterative one. Design guidance used to 

 predict beach response to breakwaters is also presented in Dally and Pope 

 (1986), Pope and Dean (1986), Rosati (1990), and Engineer Manual 1110-2- 

 1617. Guidance on the use of low-crested rubble-mound breakwaters for 

 wetland development purposes is limited, and has been mostly based on 

 experience from a few prototype sites. Ongoing research at WES under the 

 Wetlands Research Program is further investigating and evaluating the use of 

 breakwaters for these purposes. 



Functional design techniques and evaluation tools for detached breakwaters 

 can be classified into three categories: empirical relationships, physical and 

 numerical models, and prototype assessment. A three-phase design process is 

 suggested using these tools. First, a desktop study should be conducted 

 employing various empirical relationships to relate proposed structural and site 

 parameters to shoreline response and identify design alternatives. Second, a 

 physical or numerical model study can be used to evaluate beach response to 

 the breakwater project, and to assess and refine alternatives. Finally, if 



Chapter 6 Summary and Conclusions 



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