(A) HAMMER SHOCK 



(B) COMPRESSION 

 SHOCK 



Figure 6-2. Compression- and hammer-shock pressure curves 

 [after Lundgren, 1969) . 



c. Composite Breakwaters and Jetties . For the composite type of 

 breakwater or jetty where the rubble base is of considerable height rela- 

 tive to the depth of water (Fig. 6-3), the rubble base absorbs a larger 

 part of the incident wave energy than the vertical wall absorbs, when 

 situated only on a filter blanket, and a larger percent of the waves can 

 break on the vertical -wall part of the structure. However, the same types 

 of pressure curves can be formed as for the simple vertical -wall type. 

 Therefore, for a composite structure, the model should be designed in 



the same manner as for the vertical wall, except that the rubble-mound 

 base part would be designed according to Froude's law, with scale-effect 

 corrections for the viscous shear in a manner similar to that for sta- 

 bility tests of rubble-mound breakwaters (see Sec. VI, 2, a and 3,c). 



d. Seawalls . Since seawalls may be either rubble mound, vertical 

 wall, or composite structures (or constructed similar to vertical-wall 

 structures except that their seaside face is in the form of steps or is 

 concaved seaward) , the forms of the attacking waves can be any of those 

 discussed previously (clapotis, ventilated shock, etc.). Thus, for the 

 composite breakwater or jetty the model design procedures would be the 

 same as those discussed in Section VI, 2 and VI, 3. 



e. Floating Breakwaters . The methods by which floating breakwaters 

 reduce incident wave heights to provide a measure of protection to the 

 harbor area are reflection, forced instability, out-of-phase damping, 

 destruction of orbital motion, and viscous damping (Bulson, 1964). Except 

 for rare occasions (e.g., the formation of portable harbors for military 

 use) , the use of floating breakwaters of any type is not very practical 

 unless the wave environment consists of short -period waves of moderate 

 height and water depths that are deep relative to wavelength. For such 



a wave environment, a practical type of breakwater structure is one that 



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