1.0 



0.8 



0.6 



CD EXPERIMENTAL MEASUREMENTS (ORVIOSON (I97l) 



O 0.2 



THEORETICAL 



PREDICTION 



0.1 0.2 0.3 0.4 

 Ratio of Breakwater Width-to-Wavelength, 



W/L 



0.5 



Figure 10. Comparison of linear theoretical model by Adee, Richey, 

 and Christensen, and experimental data by Davidson 

 (1971) for a proposed floating breakwater (after Adee, 

 Richey, and Christensen, 1976). 



To analytically simplify the problem, a breakwater with a simple form was 

 considered, and some assumptions common in naval hydrodynamics were adopted. 

 The breakwater model (Fig. 11) consisted of a vertical thin plate with the 

 upper part above the water surface and lower part extending to a depth, D, 

 beneath the surface. The breakwater mass per unit width is m, and the plate 

 may float freely or be anchored at depth, b, to cables which are represented 

 by linear springs having a spring constant, k, per unit width. The problem 

 is two-dimensional and the water depth is assumed infinite. Monochromatic 

 waves with frequency, w, approach from the left with crests parallel to the 

 breakwater. Part of the wave energy is reflected by the plate and part is 

 transmitted beneath it. The waves set the breakwater into periodic motion 

 which, in turn, generates outgoing waves both upstream and downstream. Using 

 the assumption of irrotational flow and considering linear waves, there exists 

 a velocity potential, <|>, satisfying the Laplace equation and the free- 

 surface boundary condition. The boundary conditions on the plate are the 

 plate velocities. The resulting hydrodynamical equations are solved analyt- 

 ically by Stiassnie (1980) , using complex variable techniques. The results 

 are presented in three groups that define the performance of the floating 

 breakwater. 



(1) Influence of the Mass Parameter . The effect of the mass of the 

 structure on performance was determined by setting k = 0, varying the param- 

 eter, W/L, and calculating the coefficient of transmission as a function of 

 relative structure draft, D/L. W is the width of the breakwater. The 

 results are presented in Figure 12 which shows the coefficient of transmission 

 as a function of D/L for selected values of W/L; also shown is the curve 

 representing a fixed plate. For the case W/L = 0.01, the transmission coeffi- 

 cient is almost the same as that for a weightless breakwater of zero width. 



41 



