protection over relatively short project distances, a single impermeable 



detached breakwater is recommended with a structure length as long as the 



project beach. If the depth of construction implied by the calculated 



distance offshore (see relationships below) is too great, Dally and Pope 



(1986) recommend moving the structure shoreward to a feasible depth and 



increasing structure transmissibility. Longer project shorelines should be 



protected with a relatively impermeable segmented system for nonuniform 



shoreline advance. 



a. The following structure -distance ratios (and gap distance, for 

 segmented systems) are recommended for tombolo formation: 



— ^ = 1.5 to 2 single breakwater (10) 



— - = 1.5, L <, Lg i L^ segmented breakwater (H) 



where L is the wavelength at the structure and Lg is the gap 

 distance between adjacent breakwater segments. 



b. The structure -distance ratio is reduced for salient formation: 



— 5 =0.5 to 0.67 single and segmented breakwaters (12) 



For uniform protection of a site, a structure located well outside 

 the normal surf zone is recommended. Either a highly permeable, 

 partially submerged structure (60-percent transmissibility is 

 recommended) or an impermeable, frequently segmented structure will 

 allow the required degree of wave energy to enter the shadow zone of 

 the structure for uniform shoreline advance. The recommended 

 structure-distance ratio for a segmented system is as follows: 



— 5 < 0.125 segmented breakwaters (13) 



Toyoshima (1972. 1974) 



23. Toyoshima recommends breakwater design guidance based on experience 

 gained through observation of prototype performance of 86 single and segmented 



14 



