IV. AN APPROXIMATE METHOD FOR THE PREDICTION OF REFLECTION 

 AND TRANSMISSION COEFFICIENTS OF TRAPEZOIDAL, MULTILAYERED BREAKWATERS 



1 . Description of the Approximate Approach . 



In Section II an explicit solution for the transmission and reflection 

 coefficients of homogeneous rectangular crib-style breakwaters was 

 developed. In Section III a semiempirical procedure for the prediction 

 of reflection coefficients of rough impermeable plane sloped structures 

 was developed. When viewing the interaction of incident waves with a 

 trapezoidal, multilayered breakwater as a problem of energy dissipation 

 the problem treated in Section II may be regarded as an idealized analysis 

 accounting for the internal dissipation of energy within the structure, 

 Eq j^j^^ , whereas Section III may be regarded as an idealized analysis 

 of'the energy dissipation on the seaward face of the breakwater, i.e., 

 the external energy dissipation, E^^g^^. This section presents a 

 synthesis of the results obtained in Sections II and III into an 

 approximate procedure for the prediction of wave reflection from and 

 transmission through trapezoidal, multilayered breakwaters. 



The basic assumptions of this approximate procedure are those 

 inherent in the analyses and procedures developed in Sections II and III: 



(a) Relatively long normally incident waves which may be considered 

 adequately described by linear long wave theory. 



[b) Incident waves do not break on the seaward slope of the 

 breakwater, so that the external energy dissipation may be 

 considered mainly due to bottom frictional effects. 



(cj The cover layer on the seaward slope of the breakwater consists 



of natural stones, so that the empirical relationships for the 



wave friction factor developed in Section III.3.C may be 

 considered valid. 



With these assumptions stated, the following procedure is suggested 

 as being physically realistic although approximate in nature. 



For most trapezoidal, multilayered breakwaters, the stone size in 

 the layer under the cover layer of the seaward slope is small relative 

 to the stone size of the cover layer. As a first approximation the 

 structure may therefore be regarded as resembling an impermeable rough 

 slope. Thus, with the incident wave characteristics and the stone 

 size, djj, of the cover layer as well as the seaward slope of the 

 trapezoidal breakwater, tanB^, specified, the procedure developed in 

 Section III may be used to approximately account for the energy 

 dissipation on the seaward slope, i.e., the external energy dissipation 

 may be estimated. This energy dissipation approximately accounts for 



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