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about the weather in remote areas. A second major difficulty arises when the 

 wave direction is needed at a site that is not exposed to the open sea. The di- 

 rection of the waves in deep water inay be known but the calculation of the wave 

 direction at the site may be difficult, if not impossible. 



Instruments to sense the direction of sub-surface wave-generated cur- 

 rents have been designed and built as wave direction indicators (Scripps Institu- 

 tion of Oceanography, June 1950) (Isaacs, 1948), but every attempt to analyze 

 the records obtained from the instruments has been unsuccessful. Extensive 

 theoretical studies and field tests have been made of one of these devices, the 

 Rayleigh disk, which indicated that the instrument was unsatisfactory as a wave 

 direction recorder (Beach Erosion Board, 1950). The difficulty encountered 

 when using the Rayleigh disk to indicate wave direction is not necessarily due to 

 the instrument; but rather the difficulty arises when sub-surface currents are 

 used to indicate wave direction. 



Sub-surface Current Recorders - Basically, the Rayleigh disk operates by 

 sensing the direction of the horizontal component of the orbital currents. The 

 Rayleigh disk, a round disk free to rotate about one of its diameters on a verti- 

 cal axis, senses the current direction by aligning itself perpendicular to the cur- 

 rent. When acted upon by the orbital currents of an ideal wave system the disk 

 will align itself perpendicular to the plane of the orbit. The disk will remain in 

 one position throughout the wave cycle even though the horizontal component of 

 the orbital current reverses since it is stable regardless of which surface of the 

 disk points upstream. Under laboratory conditions with a single uniform wave 

 system the Rayleigh disk will indicate the wave direction satisfactorily. 



If a steady state current flows in addition to the wave-system orbital cur- 

 rents, the disk will seek a positionperpendicular to the vector sum of the instan- 

 taneous horizontal connponents of the two currents. The disk, therefore, will 

 oscillate between the direction of the vector sum of steady state current and the 

 horizontal velocity of the orbital current at the crest of the wave, and the direc- 

 tion of the vector sum of the steady state current and the horizontal velocity of 

 the orbital current at the trough of the wave. The limits of the oscillation will 

 depend upon the relative velocity and direction of the steady state current and 

 the orbital current. Neither the direction of the steady state current nor the 

 direction of the horizontal component of the orbital currents can be determined 

 from the oscillations of the disk without additional information. 



The interference pattern produced by two wave systems with different 

 wave directions is that of a short-crested wave system (Fuchs, 1951). The 

 waves cannot be treated as being two-dimensional with the crests extending in- 

 definitely in the third direction and the sub-surface orbital currents do not nec- 

 essarily lie in a plane perpendicular to the bottom. Instead, the surface waves 

 must be described perpendicular to the wave direction as well as in the wave 

 direction, and the orbital currents must be described by a three-dimensional 

 system. In general, the orbital currents will have components in two planes: 

 (1) a vertical plane parallel to the direction of the short-crested wave propaga- 

 tion and (2) a plane parallel to the bottom. Directly below the center of the 

 crest of a short-crested wave, the orbit will lie within the vertical plane but at 

 the ends of the crest the orbit will lie in a plane parallel to the bottom. The 

 components of the orbital current in the vertical plane tend to align the disk with 

 the wave direction but the components of the orbital current in the horizontal 

 plane tend to cause oscillations or spinning of the disk. A record obtained from 

 a Rayleigh disk acted upon by a short-crested wave system, or by two or more 

 wave systenns with different wave directions, generally cannot be analyzed for 

 wave direction. 



