The basic elements of design for floating tire breakwaters are listed below.
Length. The length parallel to shore should be sufficient to provide the desired protection and will
vary with the structure's distance from shore.
Width. The width should be chosen to yield a satisfactory decrease in the transmitted wave
height (the wave height behind the structure). No definite criteria would apply, but wave height
reductions of 30 percent may be an acceptable starting point for design. This would reduce the energy
reaching the protected shoreline to about 49 percent of that of the incident waves (0.7 x 0.7=0.49). If
later experience shows this to be an unsatisfactory or excessive level of protection, the breakwater can
be made wider or narrower by adding or removing modules, or its distance from shore or length can be
changed.
The design breakwater width is a function of the wavelength at the site. with a known water
depth and wave period, the wavelength can be found using either Figure 26, or Equation (3). Figure 61
gives the wave transmission coefficient, K as a function of the design wave height. The transmitted
wave height is determined by multiplying the incident wave height by K. For instance, if the local
wavelength, L, is 80 feet, and k breakwater width, W, of 40 feet is proposed, Wsh/L is 0.50, and K, is
0.90. If the incident wave height is 5 feet, the transmitted wave will be 4.5 feet. This wave will contain
0.9 x 0.9, or 81 percent of the energy of the incident wave. This may not be a satisfactory level of
protection in many cases.
Draft. Increased depth of penetration in the water column increases the effectiveness of floating
breakwaters. In general, the draft should be greater than one-half the design wave height.
Two-layer structures or the use of truck or tractor tires will achieve greater draft.
Flotation. The air trapped within the top of vertical tires provides sufficient flotation in most
cases. In quiet water, the air is eventually dissolved by the surrounding water and the structure sinks.
Wave action, however, replenishes the air supply, but care must be taken not to use tires with puncture
holes. More permanent flotation is possible with Styrofoam blocks or foam injected into the crowns of
the tires. In salt water, marine growth that is not periodically removed will eventually sink the structure.
Sand also collects in the tires and can sink them, but this can be prevented by drilling holes in the
bottoms of the tires. In that case, flotation aids such as Styrofoam blocks should be used.
Fastening Materials. Stainless and galvanized steel cable; polypropylene, nylon, Poly-D and
Kevlar rope; galvanized and raw steel chain; and rubber conveyor belt edging have been used for tying
tires together. Davis (1977) presented the results of tests using all of these, and found that conveyor belt
edging was the most satisfactory. The others failed because of either corrosion, abrasion by the tires,
fatigue, or deterioration from other factors. Steel cables sawing through the tires have caused some
devices to fail. Rubber belt edging, a scrap material derived from the manufacture of conveyor belts, is
available from several rubber companies and comes in a wide range of widths and thicknesses. For tire
breakwater construction, the belting should be at least 2 inches wide and 0.375 inches thick.
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