Horizontal Wove Trap 



Vertical Wave Trap 



Short Choppy Wfl 



Long Period Wave 



Figure If 



Conceptual model of horizontal and vertical Wave Trap floating 

 membrane breakwater developed by U.S. Rubber Company. 



Figure 161. Preliminary research by the U.S. Rubber Company indicated that 

 the most successful membrane-type wave damping system consisted of a flexible 

 floating blanket, which rides on the top of the sea surface, attached by 

 numerous flexible cords to a second weighted blanket suspended beneath the sea 

 surface. The bottom sheet is valved for maximum resistance when rising and 

 minimum resistance when falling. In this arrangement, the rising and falling 

 of the wave tends to accelerate the large mass of water contained between the 

 two sheets, which effectively reduces the wave amplitude. The efficiency 

 of the system was good in the preliminary tests on waves of high steepness 

 because of the large vertical accelerations (vertical forces). Low steepness 

 required a long trap (in width) to achieve a substantial reduction in wave 

 height. U.S. Rubber Company has conducted research on variations of the basic 

 conceptual arrangement. 



In a prototype situation, the Wave Trap assembly would be anchored in the 

 ocean in relatively shallow water but beyond the breaker formation point. 

 Waves moving in from deeper water will pass through the first baffle sheet. 

 Because one of the remaining valve sheets will generally be positioned at 

 least one-half a wavelength away, the forces of the wave against the shoreward 

 baffle will be balanced by opposing forces acting against the seaward baffle 

 sheet. This effect tends to exert an accelerating effect on the mass of water 

 in the wave train without putting a large strain on the anchor lines (U.S. 

 Rubber Company, 1961). 



(1) Wave Attenuation Effectiveness . For Ripken's (1960a) exper- 

 imental investigation, the float sheet of the Wave Trap was composed of four 

 18-inch-wide strips lashed together to form one large, impermeable sheet. The 

 valve sheet was a coated, synthetic fabric with the edge stiffened and trans- 

 versely weighted with metal rods. Closely spaced holes (diameter, 1-5/8 

 inches), fitted with 2-3/4- by 3-inch coated fabric flaps to serve as check 

 valves, were arranged in a staggered pattern (Fig. 161) with about one-half 

 inch of fabric between holes. The valve flaps readily opened and closed 

 for upward and downward flow, respectively. The assembly was tested in 20- 

 foot lengths (breakwater width) and was exposed to various wavelengths and 

 wave steepnesses, with a 1:10 mooring line slope and a 4.5-foot water depth. 



220 



