Reflection). Transmission of wave energy over or through a breakwater should 

 be minimized to prevent damaging waves and resonance within a harbor. 



I 



Most information about wave transmission, reflection, and energy 

 dissipation for breakwaters is obtained from physical model studies because 

 these investigations are easy and relatively inexpensive to perform. Only 

 recently, however, have tests been conducted with random waves (for example, 

 Seelig, 1980a) rather than monochromatic waves, which are typical of natural 

 conditions. One of the purposes of this section is to compare monochromatic 

 and irregular wave transmission. Figure 7-36 summarizes som^ of the many 

 types of structures and the range of relative depths, '^g/s'^ » 

 model tests have been performed. 



for which 



Some characteristics and considerations to keep in mind when designing 

 breakwaters are shown in Table 7-3. 



b. Submerged Breakwaters . Submerged breakwaters may have certain 

 attributes as outlined in Table 7-3. However, the major drawback of a 

 submerged breakwater is that a significant amount of wave transmission occurs 

 with the transmission coefficient 



^T H. 



1 



greater than 0.4 for most cases, where H. and H 

 transmitted wave heights. 



(7-15) 

 are the incident and 



One of the advantages of submerged breakwaters is that for a given 

 breakwater freeboard 



F = h-d. 



(7-16) 



water depth, and wave period, the size of the transmission coefficient 

 decreases as the incident wave increases. This indicates that the breakwater 

 is more effective interfering with larger waves, so a submerged breakwater can 

 be used to trigger breaking of high waves. Figure 7-37 shows selected 

 transmission coefficients and transmitted wave heights for a smooth 

 impermeable submerged breakwater with a water depth-to-structure height ratio 

 dg/h =1.07 . 



Figure 7-38 gives design curves for vertical thin and wide breakwaters 

 (after Goda, 1969). 



c. Wave Transmission by Overtopping . A subaerial (crest elevation above 

 water level with positive F ) will experience transmission by overtopping any 

 time the runup is larger than freeboard (F/R < 1.0) (Cross and Sollitt, 1971), 

 where R is the runup that would occur if the structure were high enough so 

 that no overtopping occurred. Seelig (1980a) modified the approach of Cross 

 and Sollitt (1971) to show that the transmission by overtopping coefficient 

 can be estimated from 



K^Q = C(1.0 - F/R) 



(7-17) 



7-62 



