SUMMARY OF STATE-OF-THE-ART 



1 . Physical Description . 



A review of more than 60 years of observations and measurements of 

 longshore current, nearshore circulations and rip currents worldwide 

 reveals that longshore currents are primarily caused by oblique wave inci- 

 dence and breaking at the coast which creates an excess longshore momentum 

 flux to drive the current. In nature, the longshore current varies across 

 the surf zone, along the coast, with depth and in time at any location. 

 Typical values are less than 1 meter per second with maximum 3.5 meters 

 per second or more. The use of continuous recording current meters at 

 fixed points arrayed across the surf zone and down the coast in recent experi- 

 ments has permitted researchers a look at far more details of the current 

 structure. Literally millions of instantaneous values of these currents 

 are now available from the two NSTS experiments and most have yet to be 

 fully analyzed. The point velocity measurement in the longshore direction 

 continuously varies in time due to the unsteadiness of nature and because 

 many forces are at work together. Wind waves, surface winds, tides, and 

 turbulent eddies from many sources all contribute to the signals recorded. 

 The influence of surface winds has yet to be examined in detail. By 

 definition, the longshore current is a time-averaged current. But, as of 

 today, an appropriate temporal averaging time is not known. This fact plus 

 the ability of these meters to record storm- induced currents has resulted 

 in even larger longshore current values being reported. 



Far more is known about horizontal circulations around vertical axes 

 and resulting rip currents than is known about the vertical velocity 

 distributions and circulations about horizontal axes in the surf zone. 

 Many plausible mechanisms have been advanced for triggering rips to form 

 and resulting neashore circulation cells to develop. Whatever the origi- 

 nal mechanism, strong bottom currents in rips can create troughs in 

 offshore bars to fix rip positions. The local bathymetry then controls 

 the future flow patterns observed. Beach slope, profile, and sediment 

 characteristics all play important roles in coastal hydrodynamics. A 

 complete understanding of steep (reflective) and flat (dissipative) 

 beaches has yet to be achieved. The existence and role of secondary 

 currents has yet to be firmly established. 



New instruments are being studied to measure surf zone currents. 

 Even though problems exist with dynamic calibrations, the EM current meter 

 remains the most proven and rugged type available for multiphase (liquids, 

 sediment, air bubbles) flows in the surf zone, and the new slope array 

 device should prove to be a valuable tool to examine the theory of long- 

 shore currents in both the field and the laboratory. 



Because of the unsteadiness of nature and all the uncontrollable 

 forces present simultaneously, controlled laboratory experiments have 

 been conducted. Problems with the boundaries are being overcome so that 

 infinite beach theories can be examined at the scale of the laboratory 



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