effects of calm and secondary wind areas must 

 be calculated separately. 



The method of computing the decay of waves 

 for the general case is as follows : 



1. Measure distances D', D", and D. 



2. Using Tp and D', determine from plate 

 VI Hd; Tb; and to-, the height period, 

 and travel time, respectively, to the end 

 of Z»'. 



3. Calculate V, the velocity component of 

 the secondary wind parallel to the direc- 

 tion of wave movement. 



4. Determine To., (uncorrected for secondary 

 wind) and the corresponding Co- from 

 plate VI. 



5. Compute an effective decay distance from 

 equation (II.3) equivalent to D"-D'. 



6. Using Td' and De, determine Ho- and 

 Td" (corrected). 



7. Using equation (II.4), Td" (corrected), 

 and distance D" - D', compute the travel 

 time tn- through the secondary wind area. 



8. Using To,, (corrected) and distance D- 

 D", determine from plate VI Hoe/Hp, Toe, 

 and t(D-D")- 



9. Add to; to", and icD-D-j to find t-nr, the 

 travel time through the entire decay 

 distance. 



It is possible to combine some of these 

 steps by defining the effective decay as the total 

 distance, both calm and secondary wind areas 

 through which the waves must travel. Then 



De = D' + (D-D") + {D"-D') (l-U'/Co..) 

 = D-{D--D-) iUyCn-') (II.5) 



This formula will allow the determination of 

 Hoe and Toe directly, but the travel time must 

 still be computed by steps 1 through 9 above. 



Example: To find Hoe, Toe, and toe with fol- 

 lowing or opposing wind over decay area. (See 

 figure II.3.) 



AtD" 



For comparison purposes the results are 

 given as if there were no secondary wind. 



Ho = 1.8 feet 



To = 12.5 seconds 



to = 64 hours. 

 It will be noted that the effect of the following 

 wind is to make the swell arrive later but 

 higher and with a shorter period. An opposing 

 wind will cause just the opposite effects. 



The general case described here probably oc- 

 curs only infrequently in nature. More likely 

 are the special cases shown schematically in 

 figure II.4. 



Diminution of Swell 



Occasionally, precise determination of the 

 diminution of swell is necessary. It may be 

 necessary to state when the swell will first fall 

 below a certain height. This problem may be 

 solved by considering the relations between 

 wave height at any point in the fetch, the 

 length of the fetch, and the actual duration 

 time. In considering the growth of waves it 

 was assumed that a wind of constant velocity 

 began to blow over an undisturbed water sur- 

 face. If the fetch is unlimited, the constant 

 wind will cause at any time waves of constant 

 height and period in an area determined by the 

 duration of the wind only (see p. 8) . A mini- 

 mum fetch (F^in.) can be defined which is the 

 shortest possible fetch for the wind speed to 

 establish the highest significant wave possible 

 in the time during which it has been blowing. 

 This minimum fetch can be determined from 

 plate III by using the actual duration time and 

 the wind speed. In all parts of the fetch except 

 this minimum fetch the significant waves will 

 have the same height and period. Now, if we 



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