a. Hinged parabolic beach 



ffi/ r // j ////>>/>//// J ' / ' // s s ' t S >///// s ' // ''/' r > ' 



b. Freely floating parabolic beach 



Figure 186. Parabolic beach floating breakwater 

 concept (after Ofuya, 1968). 



freely floating beach was moored by an anchor line. The performance of the 

 beaches was evaluated by their porosities, defined as the number of slat gaps 

 on the beach surface to the number of slats required to cover the entire 

 surface; e.g., a fully covered beach has zero porosity. The porosity varied 

 exponentially from the harbor to the seaward end. 



Ofuya (1968) conducted the experimental investigation in a wave channel 

 (two-dimensional), rectangular in the cross section (100 feet long by 2 feet 

 wide by 4 feet deep). The porosity effect of the hinged beach on wave 

 attenuation properties was determined for beach porosities ranging from to 

 60 percent. The breakwater was progressively more effective in wave damping 

 with decrease in porosity (except below 10 percent). Wave attenuation 

 resulted from several complex processes which occur both within the beach 

 structure and in the vicinity. Incident waves propagating toward the slope 

 were forced to steepen, become unstable, and sometimes break completely. The 

 oscillation of the hinged parabolic beach produced reverse currents through 

 the rectangular slat openings. Water jets produced an additional source of 

 energy dissipation. The slightly sloping surface of the slats and the reverse 

 currents, which were produced, induced partial reflection of the incident 

 waves. 



Ofuya (1968) tested the parabolic beach as a freely floating structure 

 primarily for energy dissipation, with wave energy reflection as secondary. 

 The floating parabolic beach differs from the floating pontoon, floating plas- 

 tic membrane, or floating water-filled plastic bag, due to the following 

 features: (a) the slightly sloping leading edge induces steepening of the 

 incident wave; (b) the porous internal structure dissipates wave energy by 

 turbulence and wave breaking; and (c) a sufficient length can be attained 

 for energy absorption to partially occur through repeated bending of the 

 structure. . 



246 



