6 r- 



5 _ 



penetration of 

 structure below 

 Stillwater level 

 wave amplitude 



0.2 



0.4 



0.6 



0.8 



1.0 



F /F 

 offset rigid wall 



Figure 190. Effect of relative penetration, h/a, on ratio of force 

 on offset breakwater, F r r to force on vertical 

 rigid wall, ^ x \ a\<\ wall' ^ or °^^ set floating breakwater 

 configuration (after Sethness and Moore, 1973). 



A two-dimensional experimental investigation of a breakwater constructed 

 of 20-gage aluminum sheeting and styrofoam was conducted in a wave basin 25 

 feet long, 6 feet wide, and 2 feet deep. The structure was a series of L- 

 shaped pieces connected so that the offset between the forward and rear faces, 

 and the span between the side faces, could be adjusted. An offset distance of 

 6 inches was used as the primary setting for the 6-inch-high model. In still 

 water, 4.5 inches of the model was submerged with 1.5 inches above the water 

 surface. The rectangular units were joined to extend the model to 70 inches 

 long, the width of the wave basin. Steel weights anchored the model and were 

 attached to allow the breakwater to pivot freely. 



Sethness and Moore (1973) conducted tests with the offset breakwater 

 penetrating one-half, one-third, and one-fourth of the water depth. Trans- 

 mission coefficient data for the tests are presented in Figure 191(a); the 

 transmission coefficient, CL , approaches the same value when the relative 

 wavelength, L/2D , approaches the optimum value of 1.00. Here L is the 

 design wavelength and D is the breakwater offset distance. The angle of 

 incidence was investigated by placing the axis of the model at angles of 30° 

 and 45° to the incident wave fronts. These data are shown in Figure 191(b), 

 where the deviation from direct wave incidence is apparent. For all wave- 

 lengths, an increase in transmission coefficient occurred as a result of 

 indirect incidence, with the greatest increase occurring at the optimum wave- 

 length. The best location for attaching mooring cables was determined to be 

 the top part of the structure (Fig. 191, c). A final evaluation determined the 



250 



