The length of the breakwater parallel to shore should be sufficient to provide the desired
protection. This will vary with the structure's distance from shore. The width depends on the design
wavelength at the site. This can be determined by timing eleven successive wave crests as they pass a
stationary point (when large waves are present). Divide this time by 10 to obtain the wave period, T. The
breakwater width should be 2.5 x T2 (e.g., if the wave period is 5 seconds, the width should be 2.5 x 5 x 5
= 62.5 feet). The depth of penetration in the water (draft) will determine the structure's effectiveness. In
general, this should be greater than one-half the wave height. Two-layer structures or the use of truck or
tractor tires will help achieve greater draft.
The air trapped within the top of vertical tires provides sufficient flotation in most cases. In quiet
conditions, the air is eventually dissolved by the surrounding water, but wave action will replenish the air
supply. 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 will
eventually sink the structure unless it is periodically scraped off. Sand can also collect in the tires and
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.
Stainless and galvanized steel cable, polypropylene, nylon, Poly-D, and Kevlar rope, galvanized
and raw steel chain, and rubber conveyor belt edging have all been used as fastening materials. Of these,
the conveyor belt edging has proven most satisfactory. The others failed because of corrosion in
seawater, 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 in a wide range of widths and
thicknesses.
Floating breakwaters must be securely anchored to prevent displacement. Danforth, screw
anchors and large concrete blocks have been used with mixed results. They are satisfactory for seasonal
use in mild waves, but they tend to creep over long periods in soft bottoms and are not always desirable
for permanent installations. In these cases, driven piling is usually the best means of stable anchorage
over long periods. Pile driving" of course, adds considerably to total installation costs.
Floating breakwaters can also be constructed using other materials. For instance, bundles of logs
can be chained together or other barriers can be fabricated from treated timber. Modules of lightweight
concrete filled with flotation foam have also been successful. The proportioning and design factors
presented for rubber tire breakwaters also apply to these.
Fixed Breakwaters
The most important feature of a fixed breakwater is its height, which determines how much wave
energy is dissipated. In building a fixed breakwater, some settlement should be anticipated in the
structure's design height. The amount depends on the type of soil, the structure's weight, and type of
foundation. Moderate, but uniform, settlement may not necessarily adversely affect performance, but if
one portion of the breakwater sinks significantly below the others, there will be increased wave
transmission over the low section.
Longard Tubes. The same advantages and disadvantages mentioned for a Longard tube bulkhead
apply. An added disadvantage is that the protective epoxy coating cannot be applied to a wet tube so that
damages are more likely. Therefore, a tube should not be used at a location where it will be exposed to
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