pipes lashed together, or an assembly of auto tires, rubber spheres, 

 or other material. Some of these structures require built-in flotation 

 chambers which may be filled with polystyrene. Model structures are at 

 times fabricated from the same materials used for the prototype, but 

 most model materials are different from the prototype. However, in all 

 cases the outside geometry and total weight of the prototype structure 

 must be reproduced from the length and weight scales, and the moments of 

 inertia about the different axes must be reproduced properly. Thus, any 

 variations between model and prototype values caused by the use of model 

 material different from the corresponding prototype material must be con- 

 sidered in the calculations involving geometric, kinematic, and dynamic 

 reproduction of the prototype structure in the model. Figure 6-14 shows 

 cross sections of a floating breakwater proposed for a prototype instal- 

 lation at Oak Harbor, Washington, together with the corresponding cross 

 section used in the model (Davidson, 1971a). 



PROTOTYPE STRUCTURE 



I 'lO-SCALS MODEL 



Figure 6-14. Model repTroduction of a proposed floating breakwater 

 (after Davidson, 1971a). 



d. Measurement of Waves and Wave Forces . 



(1) Measurement of Wave Characteristics . The wave height and 

 wave period are usually required measurements in model studies concerning 

 the stability of coastal structures. With a known wave period and depth 

 of water, it is more convenient and usually more accurate to determine 

 wavelength using the equation 



X - ST^ , u 27rd 

 A = ^ — tanh — ^— 

 lit X 



(6-38) 



which is based on the linear theory for progressive waves. Although wave 

 heights may be measured approximately by eye with a graduated scale, they 

 are usually determined with electronic wave gage apparatus and recorded 

 on an oscillograph or magnetic tape. The wave period may be determined 

 by a stopwatch, or by a photoelectric counting device if more accuracy is 

 required. If the waveform is also desired in some model studies, it can 



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