4. Draw the orthogonals and from the spacing 

 of the orthogonals determine K^ at a selected 

 depth contour. 



5. Determine the average wave period of the 

 major wave train by {a) measuring from the 

 photographs the time interval between breakers 

 or between the instants at which crests pass 

 identifiable points such as rocks or small patches 

 of foam or (6) measuring the wave length at points 

 where the depth is known and compute the period 

 (or obtain it from available graphs). 



Except for 56, this procedure may be followed 

 even when the hydrography is unknown. 



This aerial method of preparing a refraction 

 diagram has the practical advantage that it deals 

 with real waves, which vary in period and direc- 

 tion, and it truly represents the effect of local 

 irregularities in the bottom. It has a number of 

 disadvantages among which may be mentioned: 



1. Obtaining photographs representing a range 

 of periods and directions, and possibly low and 

 high tide, will require considerable flying time and, 

 almost invariably, a long period of waiting for 

 the desired wave conditions to occur in good 

 photographic weather. 



2. The swells, which dominate the breaker zone, 

 are frequently obscured at a relatively short dis- 

 tance from shore by small steep waves. It is 

 usually the long period waves which are of interest. 



If the refractive effect occurs in a limited area, 

 as in a small bay or near a harbor entrance, direct 

 use of aerial photographs as outlined here is 

 feasible. If the refractive effect takes place gradu- 

 ally over large areas, the aerial photographs 

 become too time consuming because of the problem 

 of ground control. A judicious combination of the 

 graphical method with measurements of aerial 

 photographs will yield the most reliable results at 

 reasonable cost. 



DETERMINATION OF REFRACTION COEFFI- 

 CIENTS FROM AERIAL PHOTOGRAPHS 



In the preceding section, the method outlined 

 utilized the aerial photographs only for the pur- 

 pose of determining crest positions. The refrac- 

 tion coefficients, Ka, were then obtained by draw- 

 ing orthogonals and measuring the spacing between 

 them just as in the completely graphical method. 

 Accurately timed aerial photographs, preferably 

 corrected verticals, may be used to go a step 

 further and permit measurement of the refraction 

 coefficients directly. The steps in the procedure 

 are as follows: 



1. Secure accurately timed aerial photographs 

 of the shore line and offshore area. 



2. Determine offshore direction directly from 

 photographs if photography shows waves in deep 

 water or indirectly from the period, depth, and 

 angle at the offshore edge of the photographs. 



3. Determine the depth by the wave velocity 

 method ("Underwater Depth Determination," 

 U. S. Navy Photographic Interpretation Center 

 Report 46) "if the available charts do not show 

 hydrography in and near the line of breakers. 



4. Measure the photographs to determine the 

 depth in which the waves break and compute 

 breaker height from ^6=1.3 d„. Repeat for as 

 many waves at each shore-line point as photog- 

 raphy permits. 



5. Determine period (a) from interval between 

 breakers or (6) from water depth and wave 

 length outside breaker line. 



6. Compute L„, from T (Lo=5.12T^), and 

 dt/Lo and obtain HtlH„' from plate I, Breakers 

 and Surf, H. O. 234. 



7. From the value of flj obtained in step 5 and 

 Hi/ Ha' compute Ho', the wave height which 

 would have been required to produce the observed 

 breaker had the original crest been parallel to the 

 shore and the shoreline straight. 



8. If it is assumed that the breakers ob- 

 served were generated by a train of waves of 

 uniform period and height, the relative refraction 

 coefficient is obtained by taking the ratio of H„' 

 at each point along the shore to its value at a 

 single reference point, say, the point where H„' is 

 maximum. 



9. To obtain an absolute value of Ka for each 

 point, it is necessary to find at least one reliable 

 value by (a) computing the refraction coefficient 

 for a point along a straight stretch to which 

 figure 4 applies or (6) obtaining an independent 

 measure of Ho from an offshore recorder, from 

 photographic measurements of breakers on an 

 adjacent straight beach, or other means. Divide 

 Ho' from step 8 by Ho from step 9 to obtain the 

 absolute value of Ka. 



10. The experimental values of K^ in step 9 

 apply to the varying depths in which the waves 

 broke. Usually, no further analysis is necessary, 

 but it should be remembered that Ka should be 

 corrected back to a common depth, if the difference 

 between the standard depth and the depth of 

 breaking changes K^. 



In addition to the disadvantages, previously 



17 



