+50 r— 



E 

 u 



-50 •— 



20 JULY 78 



17.1 34.1 51.2 68.3 85.3 102.4 119.4 136.5 

 TIME (min.) 



Figure 7. Time history of averaged (256 seconds) longshore current from five 

 closely spaced current meters, Scripps Beach, La Jolla, California, 

 July 1978 (from Guza and Thornton, 1978) . 



The latest results of the NSTS longshore current measurements are also 

 being analyzed by Guza and Thornton. Some early results of their November 

 1978 measurements at Torrey Pines, California, were reported in Guza and 

 Thornton (1980a). Again, 17.1-minute averages were used to obtain mean 

 values. The spatial and temporal averaging scales necessary to obtain mean 

 longshore currents will be discussed in a future paper by Guza and Thornton. 



As indicated in the above discussion, longshore current space variations 

 can extend for considerable distance along the coast (nearshore circulation 

 cells), normal to the coast (surf zone widths), and vertically (water depths), 

 as might be expected. Time scales for velocity variation extend from frac- 

 tions of incident wave periods in seconds to minutes and beyond, depending 

 upon the coastal phenomenon of interest. The very definition of longshore 

 current as a time-averaged velocity is now subject to review since a univer- 

 sally accepted averaging interval is not known. This result may not have 

 been expected, and it follows a general trend in science and especially in 

 physical oceanography. As new instruments are used with better resolution, 

 new phenomena are discovered. Also, the shear volume of the numbers collec- 

 ted (millions for the NSTS project) make analysis time consuming. The 

 variability of field observations in time and space has made the use of 

 controlled laboratory experiments attractive to researchers, as discussed in 

 the next section. 



33 



