horizontal lines represent the wave height and 

 period as functions of the distance from the 

 coast at intervals from 5 to 40 hours after the 

 onset of the 40-knot wind. 



The fetch shown in figure 1.8 can be limited 

 either by the presence of a coast line or by 

 the characteristics of a wind system over 

 the open ocean. It will be seen from the figure 

 that the wave heights produced by any given 

 wind speed can be limited either by the length 

 of the fetch or the duration time of the wind. 

 Plate III has been devised to determine 

 whether the fetch or the duration time is the 

 limiting factor. Its use will be explained later 

 in the section dealing with practical applica- 

 tions. 



Plates IV and V show wave heights and pe- 

 riods as functions of duration time and wind 

 speed, provided that the fetch is not limiting. 



When using plates III to V it should be borne 

 in mind that the curves are constructed on the 

 assumption that a constant wind suddenly be- 

 gins to blow over an undisturbed water sur- 

 face. If the wind speed changes gradually, an 

 average velocity has to be introduced according 

 to rules which are discussed when dealing with 

 the practical applications. Also, allowances 

 must be made for waves that are present when 

 the wind starts blowing. 



Some other characteristics of the growing 

 waves are shown in figure 1.9. In the upper 

 curve the wave steepness as expressed by the 



I- 

 z 



UJ 



o 

 q: 

 ui 



0. 



Il-i 



en 

 (/) 

 u 



z 



Q. 

 LU 

 UJ 

 H 

 (/) 



UJ 



> 



< 



1.0 



0.8 



S 0.6 



< 



UJ 



> 



> 0.4 



0.2 



100 



200 



300 



400 



500 



600 



FETCH, F, IN NAUTICAL MILES 



100 



200 



300 



40O 



500 



600 



FETCH, F, IN NAUTICAL MILES 



Figure 1.9. — Wave steepness (upper graph), expressed by the ratio B./L as a function of distance from coast 

 line at 10, 20, and 30 hours after a wind of 40 knots started to blow over an undisturbed water surface, and 

 corresponding representation of wave age (lower graph) expressed by the ratio of wave velocity to wind 

 velocity, C/TJ. 



10 



