DETERMINATION OF SWELL 



Waves Advancing through Regions of Calm 



Plate VI is used to obtain the changes in 

 wave height and period, and the travel time 

 from the time the vi^aves leave the generating 

 area until they reach a point near the coast in 

 deep water 1 at the forecast locality, having 

 traveled through a region where the component 

 of the wind parallel to the direction of progress 

 of the swell does not exceed 10 knots. The 

 region through which the swell travels after it 

 leaves the generating area is known as the dis- 

 tance of decay. 



Entering plate VI with the distance of decay 

 and the period at the end of the fetch, one can 

 read off the travel time in hours, the wave pe- 

 riod in seconds at the end of the decay distance, 

 and the reduction factor to be applied to the 

 wave height at the end of the fetch. From the 

 inset in plate VI are found the wave speed 

 and wave length corresponding to any given 

 wave period. 



Example: To find Ho, To, and to. 

 Given Hp = 18 feet 



Ti? := 9 seconds 

 D = 600 nautical miles. 



From plate VI, to = 33 hours 



To = 12.1 seconds 

 Ho/Hp = 0.46 

 Ho = (Ho/Hp) (Hp) = (18) (0.46) = 8.3 feet. 



Effect of Following or Opposing Winds 



When, instead of traveling through an area 

 of calm, the waves are subjected to a wind that 



' Deep water is defined as water of depth equal to or 

 greater than one-half the length of the wave. 



Hf 



Tf 



is either following or opposing the motion of 

 the waves, their height and period are modified 

 as indicated in section I, page 12. This change 

 is made by substituting for the actual decay dis- 

 tance an effective decay distance which is then 

 used for the subsequent calculations. For calm 

 air the effective decay distance is equal to 

 the actual decay distance; for the following 

 winds the effective decay distance is shorter; 

 and for opposing winds the effective decay dis- 

 tance is longer. Thus, the effective decay dis- 

 tance is related to the movement of the waves 

 relative to the air. If De represents the ef- 

 fective decay distance and U' is the speed of 

 the following or opposing wind, then 



_ De/D = (C- U')/C (II.2) 



The direction of wave motion is positive and 

 the sign of U' is determined relative to it; fol- 

 lowing winds have a positive sign and opposing 

 winds have a minus sign. Use of equation 

 (II.2) involves a question as to the value which 

 should be used for C, but the value of Co (wave 

 speed at the end of the secondary wind area) 

 computed for a decay distance D without re- 

 gard to the following or opposing winds is suf- 

 ficiently accurate for this purpose. Equation 

 (II. 2) can then be used in the form 



De = D (1 - V/Co) (II.3) 



When a secondary wind is present the travel 

 time is computed according to the following 

 formula : 



toe = D/1.52Toe (II.4) 



However, in actual practice the following or 

 opposing winds seldom blow over the entire de- 

 cay distance. (See fig. II.3.) Therefore, the 





' SECONDARY WjND-x'i.V.;:' 



LOCALITY FOR 

 WHICH FORECAST 

 IS DESIRED 



Figure II.3. — General case of wind situation causing the growth and decay of waves. 



20 



