COMPARISON OF OBSERVED WAVE DIRECTION 

 WITH A REFRACTION DIAGRAM 



by 

 Donald R, Forrest 



During the course of routine wave observations at Mission Bay, 

 San Diego, California, by the Field Research Group, there occurred 

 recently a comparatively rare combination of visibility and wave 

 condition which made possible a comparison of observed wave direction 

 with a refraction diagram. Observations have been made twice daily 

 for more than a year and the usual case is that the region on the 

 sea surface where ; swell can be clearly defined from the observation 

 point is limited. On 19 January 1951* however, visibility was ex- 

 ceptionally good and even though the swell was less than one foot 

 high it was possible to measure its direction of travel over a 

 relatively large area. 



Directions were measured by the transit-sighting bar method 

 described in Beach Erosion Board Bulletin, Volume 4 S Number 2, 1 April 

 1950. The 536-foot elevation of the instrument at the observing 

 point, U. S. Coast and Geodetic Survey triangulation station HI Jolla w , 

 permitted direction measurements to be made up to about five miles 

 distant. 



The most seaward of a group of fifteen observations was 

 plotted on a hydrographic chart and an orthogonal drawn seaward to 

 deep water. This depth was the 72 fathom contour for the 13-second 

 wave observed. The deep water azimuth thus established was 292°, 

 a change in direction of two degrees from that observed at 38 fathoms. 

 Other orthogonals were drawn, resulting in the accompanying re- 

 fraction diagram. The method of plotting orthogonals was that 

 developed by Johnson, O'Brien and Isaacs (Hydrographic Office 

 Publication No. 605). The remaining fourteen observations were then 

 plotted on the refraction diagram. These are shown as short heavy 

 lines , 



On the whole, agreement is good. However, directions observed 

 close inshore show a little less curvature than does the refraction 

 diagram, while those observed at the 15 to 20*fathom region tend to 

 indicate a little more curvature. The largest difference, about 

 ten degrees, seems to be that of the most northerly observation, 

 close to shore. The observing technique dictates that this particular 

 measurement should not be in error by more than about two degrees. 

 The reason for the difference probably lies in the fact that the 

 bottom in this area is shallow, very rocky and irregular. This por- 

 tion of the refraction diagram, based on rather sketchy hydrographic 

 information, is probably erroneous. 



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