"Significant" Waves 



Because of the irregular appearance of the 

 sea surface, it is necessary to describe the 

 waves that are present by means of some statis- 

 tical term. This term should give emphasis 

 to the higher waves because they are opera- 

 tionally more important than the lower ones, 

 although the actual number of lower (and 

 shorter) waves may be greater. For this rea- 

 son it is not advisable to state the mean wave 

 height for say a V2- or 1-hour period of obser- 

 vation, but rather to use the average height of 

 the highest one-third of all observed waves. 

 This average is used herein and is called the 

 "significant" wave height. This measure, as 

 well as the mean, is not an exact measure be- 

 cause it depends upon the extent to which small 

 waves have been recorded. If every ripple is 

 counted as a wave, both the mean height and 

 the average height of the one-third highest 

 waves are reduced. In practice all waves less 

 than one foot are eliminated from considera- 

 tion. Tests indicate that the average height of 

 the one-third highest waves is less dependent 

 upon the scope of the observations than the 

 mean height and is, therefore, a more consistent 

 measure. Furthermore, a casual observer tends 



to pay more attention to the higher waves and 

 reports a wave height which lies closer to the 

 significant wave height than to the mean. 



Table 1.1 shows the wave height characteris- 

 tics of this measure. The significant wave 

 height has been given a relative value of 1.00. 

 Therefore, if the significant wave height is 

 known, the height of the maximum wave, the 

 average height of the highest 10 percent, and 

 the average height of the entire wave train can 

 be computed. 



Table I.l. — Comparison of wave-height characteristics 



Wave Relative Height 



Significant 1.00 



Average .64 



Highest 10 percent 1.29 



Highest 1.87 



By using this table it is seen, for example, 

 that if a wave train has a significant wave 

 height of 10 feet, the highest wave is 18.7 feet, 

 the average of the highest 10 percent is 12.9 

 feet, and the mean wave height is 6.4 feet. 



This table represents preliminary results and 

 should be used only as a guide until a larger 

 number of records of sea and swell have been 

 analyzed statistically. 



EMPIRICAL KNOWLEDGE OF WIND WAVES AND SWELL 



Measurements of Waves and Swell 



Wind waves are defined as waves which are 

 growing in height under the influence of the 

 wind. 



Swell consists of wind-generated waves which 

 have advanced into regions of weaker winds 

 or calms and are decreasing in height. 



So far the discussion of surface waves has 

 dealt mainly with waves which appear as rhyth- 

 mic and regular deformations of the surface. 

 Because of interference, the formation of short- 

 crested waves, and the breaking of waves, there 

 is little regularity in the appearance of the sea 

 surface, particularly when a strong wind blows. 

 Although individual waves can be recognized 

 and their heights, periods, lengths, and veloci- 

 ties measured, such measurements are extreme- 

 ly difficult and comparatively inaccurate. The 

 lengths of most waves and the heights of low 

 waves are likely to be underestimated, while 

 the heights of large waves are generally over- 



estimated. Wave heights above 55 feet are 

 extremely rare yet the literature contains many i 

 reports of waves exceeding 80 feet in height. I 

 Such errors are probably due to the complexity | 

 of the sea surface and the movement of the | 

 ships from which measurements are made. 



Reliable measurements of ^vave height H are 

 so difficult that in general the reported values 

 represent crude estimates. The height of a 1. 

 large wave is estimated as the eye height of 

 the observer above the water line when the j 

 ship is on an even keel in the trough of the 1 

 wave, provided that the observer sees the crest 

 of the wave coincide with the horizon. The ' 

 height of a small wave is estimated directly by 

 using the dimensions of the ship for compari- 

 son. On board a small ship the height of the 

 waves which are more than twice as long as 

 the ship can be recorded by a microbarograph. 



The ivave period T can be measured by re- i 

 cording the time interval between successive ' 



