t 60 



TRANSMITTED 

 ENERGY 



0.8 1.0 1.2 



RELATIVE CREST HEIGHT, h /d. 



Figure 55. Distribution of wave energy in the vicinity of a reef breakwater (Ahrens 1987) 



A conventional rubble-mound structure is normally composed of a bedding 

 layer and a core of quarrystone covered by one or more layers of larger stone 

 and an exterior layer of large quarrystone or concrete armor units. Figure 46 

 shows a typical rubble-mound section for high wave energy environments 

 where moderate overtopping conditions are expected. The traditional multi- 

 layer design may not be required or constructable for projects located in lower 

 wave energy environments or shallow water. Geometry places some serious 

 constraints in shallow water, where it is difficult to include all theproper 

 layers, proper thickness, proper stone weight, etc. when the structure is only 

 4 ft high. Reef breakwaters have recently become more widely used as beach 

 stabilization structures. This type of breakwater is little more than a 

 homogeneous pile of stones placed on a bedding or filter layer. Figures 56 

 and 57 show cross sections of existing reef breakwater projects. 



Developing a breakwater cross section consists of determining the required 

 crest elevation, crest width, structure slope, armor requirements, and bedding 

 layer requirements to provide the desired stability and functional performance 

 characteristics under anticipated design wave and water level conditions. 

 General design guidance used to develop the cross section of a conventional 

 rubble-mound breakwater can be found in Chapter 7 of the Shore Protection 

 Manual and in Chapter 4 of EM 1 1 10-2-2904, Design of Breakwaters and 



Chapter 4 Structural Design Guidance 



95 



