100 



alternative products are designed to function similar to a breakwater. Some 

 have had limited success, and some have not. Some may be applicable in one 

 region and are not valid in another region. However, proponents of various 

 alternative schemes can make unsubstantiated claims of product success. An 

 engineering assessment of the product relative to a specific site is critical prior 

 to its purchase and use. 



Some of the structures were evaluated under the Shoreline Erosion Control 

 Demonstration Act, and their performance has been summarized by the Chief 

 of Engineers in his report to Congress (Dunham 1982). All field tests 

 conducted under this program were in sheltered waters and not on the exposed 

 ocean coast. Experience with beach stabilization systems on the open coast is 

 limited, with many cases being selectively reported according to their limited 

 success. 



Evaluations of alternative beach stabilization structures should be based on 

 their functional performance, economics relative to traditional types of 

 breakwater construction, aesthetics, and ability to be removed or modified if 

 they do not function as expected or become aesthetically unacceptable (EM 

 1 1 10-2-1617). The economics and aesthetics of alternative systems often 

 make such systems favorable; however, the lack of laboratory or prototype 

 experience with many alternative structures means limited data are available to 

 help determine the structure's stability and performance characteristics under 

 given design conditions. 



Stability of alternative structures is typically not as great a concern as the 

 performance characteristics provided by the structure. Such structures are 

 normally placed in low to moderate wave energy environments where wave 

 loadings are not very severe and structures can often be overdesigned to 

 provide greater stability for a relatively low increase in cost. However, the 

 uncertainty of performance characteristics and their resulting effect on the 

 expected shoreline planform is critical when evaluating alternative structures. 

 Wave transmission characteristics for any structure are critical in determining 

 resulting shoreline configuration as discussed in Chapter 2. Reflection 

 characteristics must be considered for potential scour and navigation problems. 

 Highly reflective near-vertical -faced structures such as sheet-pile breakwaters 

 should be avoided, since extensive toe protection will be required to avoid 

 scour. Also, such structures pose threats to navigation and nearby shorelines 

 due to increased wave activity. 



Conservation of energy principles can be applied to initially evaluate the 

 suitability of any structure in terms of functional performance. The basic 

 principle states that all incident wave energy can be accounted for by the 

 summation of energy transmitted, reflected, and dissipated within the 

 structure. For example, a high non-overtopped steel sheet-pile wall can 

 prevent the majority of incident wave energy from being transmitted. Since 

 energy dissipation of the structure is expected to be minimal, the majority of 

 incident energy will be reflected and may potentially cause scour. If the 

 objective is to provide low wave energy transmission and minimal reflective 



Chapter 4 Structural Design Guidance 



