Figure 32. Two-dimensional model arrangement of catamaran- 

 type (twin pontoons) floating breakwater, Oak 

 Harbor, Washington (after Davidson, 1971). 



pounds per foot of structure width perpendicular to the direction of wave 

 travel). When the water level was lowered to the 10-foot depth, most of the 

 anchor chains lay on the flume bottom; thus the initial tension on the anchors 

 was reduced to zero and the draft of the floating module was decreased to 

 approximately 3.8 feet. Figure 33 shows the chain mooring system at the 29.5- 

 foot water depth. 



Transmitted wave heights at the two depths were measured at a distance of 

 one and one-half wavelengths from the structure (Fig. 34). These data indi- 

 cate that the transmitted wave height varies more with wave period than with 

 change in water depth. For an allowable transmitted wave height of 0.5 foot, 

 proposed modules, using a chain mooring system, would not be adequate for 

 incident waves greater than approximately 2.0 feet in height and approximately 

 2.5 seconds in period. During wave attack, the module oscillated about its 

 longitudinal centerline and at the same time rocked with the waves. Over- 

 topping of the module began with lesser wave heights at the 29.5-foot depth 

 because the initial tension in the chain restraints limited the upward motion 

 of the module at this depth more than at the 10-foot depth. During the trans- 

 mission tests, the module was not observed to be in resonance with any of the 

 wave periods tested. Also, an analysis of the wave data did not indicate that 

 the breakwater module would create larger transmitted wave heights due to 

 resonance. 



b. Transmission Tests with Pile Mooring System . Davidson (1971) also 

 performed tests at the 10- and 29.5-foot water depths to determine the effec- 

 tiveness of the proposed catamaran-type breakwater with a pile mooring system 

 (Fig. 35). The flotation depth at both water depths was 5.0 feet. The 

 results of the transmission tests with a pile mooring system are shown in 

 Figure 36. These data indicate that, with the exception of the 3.0-second 

 wave period, the transmitted wave height again varied more with wave period 

 than with change in water depth. The 3.0-second wave period at both depths 

 caused the breakwater module to rock in such a fashion that larger transmitted 

 wave heights were produced than had been anticipated, resulting from resonant 

 action of the system. For a maximum incident wave height of 2.0 feet and an 

 allowable transmitted wave height of 0.5 foot, a breakwater constructed of the 

 catamaran-type modules would be inadequate for wave periods greater than 2.5 

 seconds. 



65 



