28 



7. DISCUSSION 



Previous to any discussion of the results obtained in tliis study 

 certain remarks should be made concerning their limitations. 



Primarily, it should be noted that the generated wave requires a 

 certain distance of travel after its generation to reach a stable form. 

 It was noted during the course of the experiments that the measured 

 values of wave velocity, wave length, and wave height, changed with 

 time and position in the tank. For example, an average decrease in 

 wave height of 0.01 1 foot was noted over a space of 12 feet immediately 

 preceding the center observation window of the wave tank ; while over 

 the succeeding 13 feet the average decrease was only of the order of 

 0.001 foot. Further observations indicated that at a distance of 2 

 feet from the plunger, the shape of the particle orbits was essentially 

 as indicated by theory, but that the forward travel of the water 

 particles was small and practically constant from siu-face to bottom. 



In the generation of the wave it seems reasonable to assume that 

 since the water is not displaced according to the laws governing the 

 particle motions then a certain number of cycles, or oscillations, of 

 the water surface will be required after displacement of the water by 

 the generating plunger, before the water particles take on the more or 

 less stable movement indicated by both theory and observation. The 

 variation of wave height should be a sufficient criterion as to the 

 attainment of a stable form by the generated wave. Observations of 

 many waves indicate that in the present wave tank and with the 

 present method of generation, a distance equivalent to about 6 to 

 10 wave lengths is required for the assumption of a stable form by the 

 wave. These experiments were by no means definitive and the 

 results cited must be regarded as merely indicative of a condition, 

 rather than as quantitative values accurately defining the condition. 



The distance from the wave generator to the section of the wave 

 tank selected for making the observations for this study is about 24 

 feet. It follows from the above discussion that insofar as this effect 

 is concerned the most accurate results are those for waves whose 

 lengths were in the neighborhood of 4 feet or less. 



It is believed that the magnitude of the error resulting from observa- 

 tion of the wave characteristics before a stable form is attained is, in 

 general, small. However, for the determination of mass transport, 

 which is a secondary effect, the error introduced by this condition may 

 be appreciable. The quantitative determinations of mass transport 

 reported herein therefore may not be the true magnitudes of the 

 effect. 



The ratio y has been adopted in wave experimentation as a criterion 



to differentiate deep- and shallow-water waves. The reasoning lead- 



