100 



20 



> -i 



,-a- 



>..-' 



J«: 



_^fii 



->-- 



rr^or-" 



jfcn ^-4— -^T * 



. — -°— 



• — .-.a 3 ^ 1 ^' 



i ! , ML. . 



Wove 

 Characteristic! 



Number o( Rov»» 



Li 



% 



1 



2 



3 



4 



4.50' 



0.777 







* 



B 



e 



600' 



0.583 



9 



A 



B 



9 



8.00' 



0437 







A 



a 



a 



11.00* 



0318 



• 



A 



■ 



• 



0.04 



0.05 



Incident Wave Steepness, H./L. 



Figure 135. Effect of incident wave steepness, H./L^, and relative 

 breakwater width, L T /L^, on power loss, P D , for open- 

 tube floating breakwater (after Ippen and Bourodimos, 1964). 



primarily through interference with the wave-induced velocity pattern and 

 through turbulence generated by motion around and through the tube arrays. 

 Energy dissipation rates for some arrays reached 80 percent. No wave breaking 

 occurs with this type breakwater, which is in contrast to conventional struc- 

 tures. The forces on the breakwater (determined from limited tests) were 

 generally only a small percentage (2 to 5 percent) of the total forces exerted 

 on a vertical wall with complete wave reflection. The forces were essentially 

 the same in both directions, which indicates negligible drift by wave action 

 for floating breakwaters of this type and similar mooring. The results were 

 achieved with arrays extending over only a limited part of the depth, from 20 

 to 35 percent; however, the length of the tubes had to be about one-half to 

 one wavelength. Wave steepness, H./L., significantly affected the results, 

 with increasing wave steepness causing a decrease in reflection coefficients, 

 C r , and transmission coefficients, C t , and an increase in power losses, 

 P D , for a given array and wavelength. 



IX. PNEUMATIC AND HYDRAULIC BREAKWATERS 



Most floating breakwaters can be classified in general as passive systems; 

 i.e. , the devices provide no energy to attenuate waves. The incident wave 

 energy is either reflected or dissipated by the system. The most effective 

 natural mechanism of wave energy dissipation is the phenomenon of wave 

 breaking. Active wave attenuation systems produce and inject kinetic energy 



196 



