Table 5. Twin-log floating breakwater tests No. 

 (after Jackson, 1964). 



Center- to- center 

 log spacing 

 (ft) 





B 



=• 45° 







Wave period 

 (s) 



d/L 



H i 

 (ft) 



H t 

 (ft) 



V H i 





d - 10 



ft; y/d - 



.24 







4.0 



2.0 



0.49 



2.0 



0.6 



0.30 



5.7 



1.7 



0.69 



0.9 



0.2 



0.22 



5.7 



2.0 



0.49 



2.0 



0.7 



0.35 



5.7 



2.5 



0.32 



2.5 



1.6 



0.64 



8.2 



2.0 



0.49 



2.0 



0.6 



0.30 





d - 20 



ft; y/d = 



.12 







4.0 



2.0 " 



0.98 



2.0 



0.7 



0.35 



5.7 



1.7 



1.35 



0.9 



0.2 



0.22 



5.7 



2.0 



0.98 



2*0 



0.7 



0.35 



5.7 



2.5 



0.63 



2.6 



1.6 



0.62 



8.2 



1.7 



1.35 



0.9 



0.2 



0.22 



8.2 



2.0 



0.98 



2.0 



0.7 



0.35 



8.2 



2.5 



0.63 



2.6 



1.6 



0.62 



with the plates on the channel-side log, did not appreciably improve the wave 

 attenuation characteristics of the structure. However, the addition of the 

 barrier plates to the 3-f oot-diameter logs increased the forces on the 

 restraining piles. For the conditions tested, the flotation depth and wave 

 period were more critical than water depth. This occurred because increasing 

 the water depth from 10 to 20 feet for the wave periods tested did not 

 increase the wavelengths under investigation. 



2. Twin-Cylinder Floating Breakwater . 



A variation of the twin-log floating breakwater uses the concept of hollow 

 cylinders filled or partially filled with water for mass variability effects. 

 The twin-cylinder floating breakwater (Fig. 102) consists essentially of two 

 circular cylinders rigidly connected at intervals by structural supports. 

 This design, based on a concept of LaSalle Hydraulic Laboratories, Canada, 

 has been investigated experimentally in the laboratory by Ofuya (1968). The 

 depths of submergence of the cylinders and the stability of the structure 

 depend on the amount of fluid in the cylinders. This design attempts to 

 attenuate wave energy mainly through forced instability of the incident waves 

 and to produce wave breaking and turbulence in the gap between the cylinders. 

 The optimum depth of submergence, a /a , was determined by Ofuya (1968). 



The wave damping performance of the twin-cylinder breakwater is discussed 

 in terms of its transmission coefficient, C t , as functions of the incident 

 wave steepness, H-/L, and relative water depth, L/d. The stability and 

 natural period of oscillation of the twin-cylinder float in still water were 

 determined experimentally. The breakwater remained stable over a wide range 

 of values of a. /a when the lower cylinder was filled with water and the 

 upper cylinder was empty. Its natural periods of rolling were found to depend 



153 



