106 



integrity is defined as the structure's ability to survive an extreme storm 

 without substantial damage and the rate it sustains damage from storms more 

 extreme than the design event. Additional guidance on design optimization is 

 provided in EM 1 1 10-2-2904. 



Since a project constructed by the USACE can no longer be justified based 

 on recreational benefits alone, the benefit-cost ratio for a breakwater project 

 has become even more difficult to defend. However, with increased use of 

 beach nourishment as a method of shoreline erosion control, use of shoreline 

 stabilization structures such as detached breakwaters can be justified to protect 

 and retain the fill. Detached breakwaters can substantially increase the 

 amount of time for which beach nourishment remains on the beach. Jf the 

 savings realized by reducing the time required between renourishments 

 exceeds the cost of the structures, their construction can be justified and the 

 beach fill design optimized. For example, if the renourishment period of a 

 beach fill project is increased from 3 to 6 years, the amortized savings 

 accruing from less frequent nourishment is then available to build stabilization 

 structures (EM 1 1 10-2-1617). During the plan formulation stage of a beach 

 fill project, alternative plans should be evaluated to determine if the use of 

 breakwaters and the reduction of nourishment requirements is more cost- 

 effective than the beach fill project without structures. 



With new emphasis on wetlands and environmental concerns in the 

 USACE, the creation and restoration of wetland areas can also justify the use 

 of detached breakwaters. Although limited prototype applications exist at this 

 time, ongoing research is being conducted at WES to further develop this 

 concept. The use of relatively inexpensive low-crested breakwaters appears to 

 be a viable method to protect and stabilize a wetland area. This application 

 may also be justified on the basis of beneficial uses of dredged material 

 related to a dredging operation. Dredged material can be placed behind a 

 breakwater and planted with marsh grass to establish wetland areas, creating 

 project benefits. The cost of this type of nearshore placement may be 

 considerably less than other dredged material disposal options, which will also 

 increase the benefit-cost ratio. 



Several structural design alternatives are available to optimize the cross 

 section, and, subsequently, the costs of a breakwater project. Smith (1986) 

 presents some of the structural parameters which should be considered when 

 determining the structural design of the breakwater. Depending on the length 

 of shoreline to be protected, construction of multiple segments reduces 

 construction costs since less material is required than for a single continuous 

 breakwater. Low-crested reef-type breakwaters can also be more cost- 

 effective since less material is required to develop the crest elevation. 

 Additionally, low-crested and reef-type breakwaters use a homogeneous stone 

 size instead of the traditional multilayer construction, and therefore require 

 less accuracy in stone placement during construction. 



Chapter 5 Other Design Issues 



