velocities. He also used photos taken from a tethered balloon to record dye 

 trajectories. These innovative techniques produced the first measurements 

 showing velocity variations with depth in the rip currents, and with time at 

 any position in the surf zone. Figure 2 shows the strengths of the circula- 

 tions and rip currents reported as transport velocity (i.e., flow rate per 

 unit width) at Seagrove Beach, Florida (Sonu, 1972). 



These early field efforts concentrated on measuring mean values of the 

 currents in the nearshore zone and the wave heights, angles, etc. that 

 created them; however, it became evident that space and time variations of 

 the currents must also be measured. Mechanisms, cause-effect relations, and 

 theories stated by these researchers are discussed later in this report. 



2. Space Variations of Longshore Currents . 



a. Across Surf Zone . Ingle (1966) reported current measurements at 

 three to six points across the surf zone and beyond . A fluorescein dye patch 

 was timed and its travel distance noted. Although considerable scatter oc- 

 curred, it was noted that the largest longshore current developed midway be- 

 tween the swash zone and the breaker zone. Currents outside the breaker line 

 were small but in the same downcoast direction and never exceeded 0.3 meter 

 per second. The maximum velocity was 1.3 meters per second at the midsurf 

 position with values dropping rapidly on either side. 



Galvin (1967) briefly mentions this variation across the surf, but does 

 not mention it as a possible reason for differences between field data and the 



-<o -10 -ao 



Figure 2. Rip current and nearshore currents at Seagrove, Florida, July 1968 

 (from Sonu, 1972). 



26 



