HORIZONTAL CURRENTS 



The dominant horizontal current 

 in a shallow sea is produced by the short- 

 period, high-speed orbital motion of 

 surface waves. These currents are mod- 

 ified by the prevailing coastal, geostro- 

 phic, tidal, and wind-driven currents. 



An important low- speed current 

 pattern is produced by the orbital motion 

 of long-period internal waves. The re- 

 sulting horizontal current patterns are 

 detected and measured at the tower with 

 standard oceanographic current meters 

 and tracking floats which suspend vaned 

 drogues at various depths. 



One type of water flow is caused 

 by the daytime coastal winds which pro- 

 duce a shoreward- moving surface cur- 

 rent. As a result, the subsurface water 

 flows offshore during the day and a re- 

 verse flow takes place at night. 



Horizontal currents are also de- 

 termined by measuring the movement 

 of float-suspended vaned drogues 

 (above). The drogues are positioned 

 at predetermined depths, usually 

 above and below the thermocline. 

 The surface float is tracked by 

 transit and camera from the tower's 

 upper deck. Cameras mounted on 

 kites have also been used to photo- 

 graph the sea surface. 



Horizontal currents are measured with several types 

 of current meters. This one from BuShips charts 

 current direction and speed. It is being lowered from 

 the tower boom on a single support wire. For direc- 

 tional orientation, other types of meters utilize twin 

 guide wires extending from the ocean floor to the top 

 of the instrument house. The greatest problem, yet 

 unresolved, is the inability to separate short period, 

 rapid, irregular wave motions from slow coastal drift. 



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