3. Time Variations 



By definition, the longshore current is a time-averaged current. Early 

 field measurements with floats, dyes, drogues, etc., measured distance 

 traveled over finite-time increments to obtain a mean value over the time 

 interval employed. These are Lagrangian views of the velocity field. The 

 use of continuous recording velocity meter arrays during the 1970's has 

 permitted local Eulerican velocity fields and their time histories to be 

 constructed. As shown previously CFig. A) and in Figure 5 (Dette 

 and Fuhrboter, 1974), the longshore velocity components vary considerably 

 with time and within a wave period. Early researchers gave little thought 

 to the time interval for use in calculating the longshore current, but it 

 has now become the central issue,. The velocity recorded is due to the wave 

 orbital motions, local return flows, mass transport by waves, tidal grad- 

 ients, surface wind shears, effects of rip current shears, turbulent eddies, 

 and other factors as discussed in more detail_in a later section. Figure 5 

 also shows a mean longshore current value of V =0.97 meter per second 

 which is apparently the average over the 28- second record shown. Why was 

 this time interval employed? Use of a shorter time interval such as the 

 time for wave height repetitions (i.e., the wave period) would clearly give 

 longshore current values that change in time (Fig. 5). Thus, Dette and 

 Fuhrboter (1974) were perhaps the first to claim that longshore currents 

 could not be regarded as steady or quasi-steady flows. They showed fluc- 

 tuations in the range of ± 100 percent that occurred with periods from 1/4 

 to 1/9 of the wave period. A mean longshore current of 1.0 meter per 

 second commonly fluctuated from to ± 2 meters per second within shorter 

 time intervals. 



Somewhat similar results were reported by Wood and Meadows (1975) and 

 Meadows (1976, 1977). Using demeaned (averaging time about 15 minutes) and 

 band pass filtering techniques on the time series (see Fig. 4), Meadows 

 concluded that the mean longshore current velocity varied in time at any 

 point due to the horizontal wave particle velocity and a longer period com- 

 ponent due to nearshore zone-induced wave interactions. An analysis was 

 also made of the phase lag between the water surface time series and the 

 surface longshore current time series. The long-period component was 

 generally in-phase and the wave orbital component (short-period) was out- 

 of-phase. Meadows (1976, 1977) stated that these fluctuations were at 

 times more than 150 percent of the mean longshore current. Variations 

 occurred over time periods of 3 to 80 seconds, i.e., at periods about equal 

 to or greater than the wave height periods present (4.2 seconds). 



Although primarily interested in the three-dimensional structure of 

 the velocity field in the surf zone, Brenninkmeyer, James, and Wood (1977) 

 also commented on the time histories of their velocity measurements. They 

 found many strong oscillations within any one breaker period, the com- 

 ponents of which appeared to move together. Maximum velocities (different 

 values) were recorded in three component directions at the same time, 

 indicating a dominant velocity vector. Superimposed were smaller, higher 

 frequency oscillations with opposite phases. These were thought to show 

 the secondary water motions (vertices and helices) found within a bore. 



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