period of about 2 seconds. Problems were encountered with the strapping mate- 

 rial which proved inadequate when subjected to continuous twisting action; 

 better protection was needed for the foam which provided buoyancy for the tire 

 structure. 



Based on Kowalski's (1974) experience, recommendations are made for a 

 more effective breakwater configuration to be constructed by considering the 

 actions of the different parts of the structure. The leading edge could be 

 made into a sloping beach so that incoming waves will break by running up the 

 incline. By arranging the foamed tires so that the resultant buoyancy is 

 negative along the first few rows of tires, the leading edge will stay sub- 

 merged, providing runup. The breakwater should dissipate as much of the wave 

 energy as possible by internal friction and interference with the structure. 

 This is accomplished by adjusting the flexibility of the strapping and by the 

 arrangement of the tires. The trailing edge of the structure should be so 

 designed as not to create new waves due to its motion. The breakwater should 

 end with a thin, scalloped edge to prevent continuous, long-crested waves. 

 Finally, the orbital motion of the wave can be suppressed by vertical tire 

 rows suspended beneath the breakwater. 



5. Theoretical Considerations . 



a. Estimate of Transmission Coefficient . Harms (1979b) analyzed a simple 

 model for estimating the transmission coefficient, C t , of a scrap-tire 

 floating breakwater by considering the power required to propel a tire of neg- 

 ligible mass at velocity, U(t), unidirectionally through a viscous fluid at 

 rest. The product of the drag force on the tire and the velocity of motion is 

 equivalent to the time rate of change of the kinetic energy of the surrounding 

 fluid. It was assumed that this basic relationship is still applicable when 

 the tire is fixed in place and U(t) represents the instantaneous velocity of 

 an external current, even when this flow is not only unsteady but also period- 

 ically reverses. In view of the complexity of the flow field within the 

 structure, which cannot be accounted for even by the most sophisticated wave 

 theories, it was deemed sufficient to use linear wave theory and deepwater 

 conditions for a first approximation. The drag-related dissipation was con- 

 sidered to vary only in proportion to H 2 (x), so that the total rate of 

 energy dissipation anywhere along the breakwater was proportional to the local 

 wave energy density, yH 2 (x)/8. The energy flux balance, integrated across the 

 beam of the breakwater, yielded 



H t 



— = exp 



H, 



f [-20i 



(52) 



3P(L/W) J 



where C^ and P are the dimensionless drag coefficient and structure poros- 

 ity, respectively. 



For each structure tested, Harms (1979b) determined the appropriate value 

 of C^ from equation (52) after measuring the transmitted wave height. The 

 value of Cj did not change significantly from one structure concept to 

 another; hence, the transmission performance was deduced to be governed pri- 

 marily by the porosity parameter, P, for the same value of H^/L and L/W. 

 In the principal region of interest (where the breakwater provides significant 

 wave protection, typically L/W < 2), the theoretical values were found to 



140 



