CURRENT EFFECTS 



53 



another, independent of the motion of the wave forms across it. 

 Actually, however, this is the normal state of the sea, except for very 

 brief periods of time and over very small areas, water being the most 

 mobile of common substances next to air. Therefore, the effects that 

 currents may have on wave motion deserve consideration. The prob- 

 lem, reduced to its simplest terms, is one of the effects on waves of 

 currents that are either contrary or are following, because it is the 

 contrary or the following component of motion that comes into con- 

 sideration in the cases of currents that are flowing at an angle with 

 the run of the waves. 



Briefly, the effect of a contrary current is to decrease the lengths 

 and hence to increase the heights of waves, since the amount of energy 

 is unchanged, thus rendering them steeper. But it does not alter their 

 periods, because it decreases their velocities in the same proportion 

 that it decreases their lengths. The effects of following currents are 

 the reverse, i. e., they increase the lengths of waves and decrease their 

 heights so that the waves are rendered less steep, though again with- 

 out altering their periods. The degree to which a wave is altered by 

 a current depends on the ratio between the velocity of the wave while 

 in still water and the velocity of the current it encounters, the latter 

 being regarded as positive if it is in the same direction as the wave, 

 negative if it runs against the wave (table 18). 



Table 18. — Ratios between the heights, lengths, and steepness of waves in still water 

 and in currents of different relative velocities 



[Based on a theoretical study made at the Scripps Institution of Oceanography] 



The general rule is, the stronger the current, the greater its effects 

 upon the waves that may be advancing either with it or against it. 



According to table 18, if a wave that has a period of 4.2 seconds, 

 hence is 100 feet long in deep water and is travelling at the rate of 

 13.4 knots, encounters a current flowing in the opposite direction at 2 

 knots (ratio of current velocity to wave velocity of —0.15), the height 

 of the wave should theoretically be increased by a factor of 1.39 by 

 the time a steady state was reached, but its length would be decreased 

 to 67 feet. The alterations would be smaller in both these respects, 

 if the current were flowing more slowly, larger if it were flowing more 

 rapidly. If the wave in question ran into a following current of the 



