those recommended by the JMC method and Toyoshima's median-depth 

 system (Toyoshima 1972, 1974) was conducted. Both of these methods 

 resulted in segment lengths and gap distances smaller than the constructed 

 project, with structures positioned closer to shore than indicated by the 

 Diffraction Energy Method. 



Empirical methods used in U.S. design 



This section briefly describes four methods presented in Table 3 that have 

 commonly been used in the design of more recent U.S. breakwater projects. 

 These methods were not specifically evaluated with prototype data by Rosati 

 (1990), and were therefore excluded from the previous section. Two of the 

 methods, Pope and Dean (1986) and Ahrens and Cox (1990), are applied in 

 the case example presented in Appendix B. 



Dally and Pope (1986). Dally and Pope present several techniques for 

 controlling shoreline response to a single or segmented detached breakwater 

 project. They recommend the following limits for the structure length- 

 distance offshore ratio (and gap distance for segmented systems) based on the 

 type of beach planform desired and the length of beach to be protected. 



For tombolo development: 



— = 1.5 to 2 single breakwater ( n ) 



X 

 X 



= 1.5, L <, L <. L s segmented breakwater ( 12 ) 



46 



where L is the wavelength at the structure. 

 For salient formation: 



-i = 0.5 to 0.67 single and segmented breakwaters "*> 



For uniform protection over a long distance and an unconnected shoreline, a 

 structure located outside of the surf zone is recommended. Either a permeable 

 (60 percent), partially submerged structure or an impermeable, frequently 

 segmented structure will allow ample wave energy into the area. In order to 

 provide sufficient distance for the diffracted waves to reorient themselves via 

 refraction before reaching the shoreline, the recommended ratio for a 

 segmented system is: 



— < 0.125 segmented breakwaters ( 14 ) 



Chapter 2 Functional Design Guidance 



