PART IV: ESTIMATING WAVE TRANSMISSION 

 Wave Transmission by Diffraction 



69. Waves are transmitted by rubble-mound breakwaters in three ways: 

 around, over, and through. The first way plainly refers to diffraction of 

 incident waves around the heads of breakwaters at the entrance channel or 

 through other gaps in the structure. Wave transmission by diffraction, the 

 most substantial of the three modes, can be limited by careful orientation of 

 the breakwaters. Diffracted waves combine with waves transmitted over and 

 through a breakwater within the area influenced by diffraction. All three 

 modes must be addressed in this area. Methods to define the limits of pene- 

 tration of diffracted waves, including estimates involving directional irregu- 

 lar incident waves, are presented in the SPM (1984) (see also Goda, Takayama, 

 and Suzuki 1978). 



70. Many projects, such as boat harbors and ports where the breakwater 

 is relatively extensive and the principal physical feature providing wave pro- 

 tection, can deal with optimization of the breakwater in plan as a separate 

 measure. This optimization can precede the optimization of the breakwater 

 cross section and include layout of all the other major features associated 

 with the proposed coastal development. Procedures for systematic optimization 

 of breakwater lengths and orientation with respect to wave penetration by dif- 

 fraction are discussed by Groeneveld et al. (1983) and in EM 1110-2-1615 

 (Headquarters, Department of the Army 1984). 



Wave Transmission by Overtopping 



71. Rubble-mound breakwaters are designed usually with the intention 

 that waves do not overtop the structure except in the most extreme incident 

 wave conditions. Traditionally this has been a matter of estimating runup on 

 the seaward face for an extreme wave height and period combination and setting 

 the crest just above the maximum runup. This method can provide a crude ap- 

 proximation of crest elevation for concept formulation, but it should not be 

 carried further into the design process. More precise techniques for esti- 

 mating wave transmission by overtopping were devised by Cross and Sollitt 

 (1971) and later refined by Seelig (1980b). 



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