Table 3 



Computation of refraction coefficients for Monterey Bay 

 [Kd values apply to the 5-fathom contour for waves of 14-second period from W. N. W.) 



1 Distance between orthogonals measured at crest 20. Refraction shoreward of this crest should be determined from larger scale chart. 



3 Coefficients apply to crest 20. 



3 Refraction seaward from crest 14 is based on distance between orthogonals H and J. 



depth, of period, and of the initial angle of the 

 wave crest. In the preceding examples, repre- 

 sentation of the results was simplified by report- 

 ing the coefficient of refraction at a constant 

 depth of 5 fathoms, thus eliminating one variable. 

 The question now arises as to the magnitude of 

 the difference in this coefficient if the wave breaks 

 in a lesser depth, say, 10 feet. For this purpose, 

 the 30-foot contour may be assumed as parallel 

 to the shore line and the effect worked out approxi- 

 mately from figure 4. Using the example of 

 Monterey Bay, with a period of 14 seconds and 

 waves from W. N. W., the refraction coefficient 

 between stations E and F is around 0.60 at 

 d/L„=0.03. On a straight shore line, from figure 

 4 this coefficient and depth would correspond to 

 ao=7l° and a=24°. At d/L,=Om, a=13 de- 

 grees so that the further change in a between 

 d/Lo=Om and (i/L„ = 0.01 would be about 11 

 degrees. The change in coefficient would bf 

 determined from 



^Vi 



K,o=-xi^ 



Km — -%/r^ 



Kio 



K,o 



6io~VCos 



«30 



«10 



= 0.97 



With the change in angle from 24 degrees at 

 d=30 feet to 13 degrees at d=10 feet, the value 

 of Ka at the 10-foot contour is only 97 percent of 

 the value at the 30-foot contour. Evidently, the 

 exact depth to which the refraction diagram is 

 carried does not greatly affect the value. As a 



working rule, to be modified if special circum- 

 stances so indicate, carry refraction diagrams 

 shoreward to at least d/Lo=O.OS. 



If the refraction diagram is to be drawn for the 

 purpose of determining the local angle between the 

 shore line and the the breaking crest, then it is 

 obviously necessary to continue the diagram to 

 the depth in which the wave breaks. This refine- 

 ment is unnecessary in determining wave heights 

 but is necessary in estimating the strength of the 

 littoral current which depends upon the angle 

 between the breaking crests and the shore line. 



CONSTRUCTION OF REFRACTION DIAGRAMS 

 FROM AERIAL PHOTOGRAPHS 



The graphical method of preparing refraction 

 diagrams may be replaced by a purely photo- 

 graphic method, using accurately timed aerial 

 photographs. Various components of the method 

 have been described in other reports and the 

 procedure will be summarized only briefly here. 

 Steps in the analysis are: 



1. Obtain aerial photographs of the shore line 

 and offshore area, preferably verticals, taken at 

 an accurately timed interval of approximately 3 

 seconds and with about 85 percent overlap. 



2. Check photographs for altitude and tilt and 

 determine ground scale. Enlargements corrected 

 for tilt are desirable. 



3. Trace crest of major wave train from selected 

 photographs in the set and transfer to overlay of 

 hydrographic chart. The set of photographs will 

 show different crest angles at the same position 

 and averaged curves should be drawn. 



776699 0^8 3 



15 



