PROTOTYPE SCALE MOORING LOAD AND TRANSMISSION 

 TESTS FOR A FLOATING TIRE BREAKWATER 



by 



Michael L. Giles and Robert M. Sorensen 



1. INTRODUCTION 



Floating breakwaters of varying size, shape, and constituent material 

 have been in lise for several decades. However, during the past few years 

 there has been an increased interest in floating breakwaters, particularly 

 at sites exposed to short-period waves. They are being used for shore- 

 line erosion control, as combination breakwaters and docks for marinas, 

 and for the temporary protection of dredging and logging operations, 

 waterfront construction, and other coastal activities. 



One class of floating breakwaters currently being used at several 

 locations, particularly in sheltered waters, is the floating tire break- 

 water (FTB) . FTB's are constructed of scrap automobile or truck tires 

 made buoyant by plastic foam or some other material and connected to 

 form assemblages of modules with a variety of configurations (Candle and 

 Fischer, 1977). Scrap tires are available at no cost (except transport 

 costs to the site) and are extremely durable when placed in the marine 

 environment. Thus, a floating breakwater made of scrap tires can often 

 be constructed and installed at a low cost in comparison to breakwaters 

 constructed with many other materials. 



The most important information needed for an adequate functional 

 design of a floating breakwater is the wave energy transmission character- 

 istics, usually given in terms of the transmission coefficient (ratio of 

 transmitted to incident wave height) . The transmission coefficient 

 depends on the incident wave period, height and direction, the water 

 depth, the characteristics of the mooring system, and the characteristics 

 of the breakwater. Floating breakwaters prevent wave energy transmission 

 by reflection and by dissipation primarily through the generation of tur- 

 bulence in breaking and the interaction of water particle motion with 

 the breakwater structure. Also, the dynamic response of the breakwater 

 to wave motion and the consequent regeneration of waves in the seaward 

 direction helps to diminish wave energy transmission. 



An important aspect of the structural design of a floating breakwater 

 is the determination of mooring loads for the range of possible incident 

 wave conditions and water depths. Mooring loads must be evaluated to 

 determine the required mooring line strength and anchor requirements. 



There has been little field or laboratory research into the wave 

 transmission and mooring load characteristics of floating tire breakwaters 

 Kamel and Davidson (1968) conducted model tests, using 15. 3- centimeter- 

 diameter (6 inches) tires, for an FTB known as the "Wave-Maze" (Noble, 

 1969) . They evaluated wave transmission and mooring loads for a range 



