CHAPTER M 



CURRENT MEASUREMENTS 



M-1 General. — Probably more types of in- 

 struments are used for measui-intj currents than 

 for any other single oceanographic measure- 

 ment. Devices range from tlie simple drift bottle 

 to sophisticated electronic instruments. 



Types of current-measuring instruments may 

 be divided into four broad and general cate- 

 gories : Free-floating, fixed, tethered, and ship- 

 board. Those in the first categoi-y include dye 

 marks and floats or drogues that can be observed 

 from ship, shore, or aircraft. Those in the second 

 categoi-y include instruments that are attached 

 to piers, towers, or beacons, or placed on the 

 bottom of rivers, bays, estuaries, and other near- 

 slioi'e areas. Those of the third category include 

 buoys in either deep or shallow water, and those 

 of the fourth category include instruments that 

 can be operated when the ship is underway and/ 

 or anchored. Dye marks, drogues; Ekman, Eob- 

 erts, and Woods Hole (Richardson) current 

 meters; and the Geomagnetic Electrokineto- 

 graph (GEK) method of measuring currents 

 will be discussed in this chapter. 



M-2 Dye Marks. — Rhodamine-B dye is used 

 to determine current patterns in coastal waters. 

 This technique involves releasing quantities of 

 the dye at a given point and checking the 

 dispersion of the dye by means of visual observa- 

 tion, color photography, or fluorometric meas- 

 urement. In some applications, divers carry the 

 containers of dye to a predetermined depth and 

 release it, and in other projects, the dye may be 

 dumi^ed over the side of a vessel. 



M-3 Parachute Drogues. — The parachute- 

 drogue method of measuring current speed and 

 direction has become increasingly important at 

 the Xaval Oceanographic Office during recent 

 years. 



In making these observations, an impro- 

 vised array consisting of a parachute, a length 

 of wire rope, and a lighted, radar reflector 

 equipped buoy is launched fi'om a ship and 

 tracked. Since the parachute sinks to a prede- 

 termined depth, opens, and moves with the pre- 

 \-aiIing currents, tracking the surface buoy and 

 recording time and position results in a record 

 of current speed and direction. This method is 



Change 1—1970 



^•ery satisfactory for measuring surface and 

 shallow water current velocity, but because of 

 drag force and depth uncertainty, drogues are 

 less accurate for deeper observations. The tech- 

 nique of launching a series of drogues with par- 

 achutes at various depths is especially effective 

 where counter currents exist, or where topog- 

 raphy may have an influence on currents. The 

 path followed by the drogue will be that of the 

 general water mass, and internal waves or minor 

 current fluctuations generally will not be re- 

 flected ; however, by recording positions at more 

 frequent intervals rotary tidal currents and 

 changing current patterns can be detected. The 



RADAR REFLECTOR 



WATERPROOF LIGHT 



.GUY WIRE CLAMP 



WOODEN PLATE 6"»6" 



STYROFOAM BLOCK 



UPPER CHAIN OR WIRE 



COUNTERBALANCE 

 WEIGHT (APPROX, 25LSS.) 



MIDDLE CHAIN OR WIRE 

 WIRE ROPE OR PIANO WIRE 

 LOWER CHAIN OR WIRE 



Figure M-1. Parachute drogue array. 



M-1 



