The selection of linear scales for vertical -wall stability models is 

 based partly on practical considerations, such as size and capability of 

 available wave flume and wave-generating facilities. Another factor in 

 scale selection is the size, sensitivity, response characteristics, and 

 range of pressures over which- available pressure transducers will operate 

 with sufficient accuracy for purposes of the model study. IVhen pressure 

 transducers are used, the intensity of pressure must be measured simul- 

 taneously at different locations over the structure face so that the 

 maximum total force can be calculated. Another method is the use of 

 strain gages to measure the overturning moment and the vertical and hori- 

 zontal force components, from which the stability of the structure in 

 overturning and sliding can be calculated (Leendertse, 1962). Several 

 research studies have been conducted which determined the magnitude of 

 breaking wave pressures and impulses on vertical -wall surfaces (Bagnold, 

 1939; Mitsuyasu, 1966; Garcia, 1968; Kamel, 1968b). A few model studies 

 have also been conducted concerning the measurement of wave forces due to 

 breaking waves (Lundgren, 1962; Jackson, 1966; Nagai, 1968). The linear 

 scales of these models ranged from about 1:20 to 1:50. 



(3) Floating Breakwaters . The selection of linear scales for 

 floating breakwater studies is, at times, based on the structure size, 

 water depth, and wave dimensions in the prototype, versus the size of 

 available wave flume and wave generator capability. However, prototype 

 floating breakwaters are usually impractical except for relatively deep 

 water and short wavelengths fii^ this respect the floating breakwater is 

 similar to the pneumatic and hydraulic breakwaters) , and the structures 

 are not unusually large in cross section. Thus, if the structures were 

 freely floating, the models could be of considerable size, relative to 

 their prototype. However, the structures must be moored, in most in- 

 stances with elastic cables, and the selected model scale may also be 

 based on the similarity requirements of the mooring-line assembly. Most 

 of the model studies conducted have used linear scales between 1:6 and 

 1:27 (Jackson, 1964; Davidson, 1971a). 



(4) Pneumatic Breakwaters . A patent for a device to protect 

 coastal areas and structures from water waves by a controlled flow of 

 compressed air from a submerged pipeline was obtained by Philip Brasher 

 in 1907 (U.S. Patent No. 843926); the device was installed at Crotch 

 Island, Maine, in 1908 (Laurie, 1952). Other installations were made 

 in Maine and Massachusetts in 1908, 1911, and 1912, and in El Segundo, 

 California, in 1915 (Scientific American, 1916). These installations 

 were credited with various degrees of success, but the estimates of wave 

 height reductions were mostly subjective. Full-scale tests have been 

 made in England (Evans, 1955; Heath, 1959; Bulson, 1961), Japan (Kurihara, 

 1955, 1956), Russia (Bogolepoff, 1937; Teplov, 1958; Radionov, 1960), and 

 the United States (Sherk, 1960); experimental and theoretical investiga- 

 tions have been conducted in several countries (Schiff, 1943; Carr, 1950; 

 Wetzel, 1955; Taylor, 1955; Bulson, 1968) in which the effectiveness of 

 pneumatic breakwaters was studied in wave fliomes using water depths of 

 less than 1 foot to about 35 feet. However, a comprehensive series of 

 controlled tests to determine the quantity of compressed air required to 



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