theories of that time. Thornton (1969) discusses these early efforts and 

 that of Zenkovitch (1960) as examples of the distribution of longshore cur- 

 rent across the surf zone. Although Thornton measured currents through the 

 surf zone, he did not include enough detail to verify his own theories in 

 this regard. 



About 1970, the radiation stress theory clearly demonstrated how the 

 time-averaged longshore current velocity must vary across the surf and beyond 

 the breakers. As indicated in Appendix A, some subsequent field measurement 

 programs have concentrated on obtaining data to validate the theory (Sasaki, 

 1975; Huntley and Bowen, 1974; Manohar, Mobarek, and Morcos, 1974; Wood, 1976;' 

 Meadows, 1977; Allender, et al., 1978; Guza and Thornton, 1978, 1980a, 1980b; 

 Kraus and Sasaki, 1979). The magnitudes of the currents were in the same 

 range as those previously discussed. Unfortunately, many of these early data 

 sets were obtained from visual observations. Wave angle observations at the 

 breaker lacked the needed precision. The general trend has been to record 

 even stronger longshore currents in recent experiments than the maximum value 

 of 2 meters per second previously cited (Guza, personal communication, 1980). 

 This is probably due to the use of contiuous recording current meters on 

 solid supports that were operational during storm events. In this regard, no 

 evidence was found of a continuous velocity reading in the surf zone over ex- 

 tended time periods (months or years) . 



b . Along the Coast . It was apparent from even the earliest observa- 

 tions in the late 1940 's that the mean current varied along the coast. This 

 is especially true on a scale of the circulation cell with rip currents pre- 

 sent or near coastal obstructions. Even on relatively straight beaches with 

 approximately parallel bottom contours, Inman and Quinn (1951) measured vari- 

 ability along the coast (measured by its standard deviation) that was equal 

 to or greater than the mean longshore current. Keeley and Bowen (1977) 

 measured mean currents over a 1-kilometer length of a Canadian beach and 

 found large-scale variation. Mechanisms and theories to account for the 

 variations will be discussed later. 



c. With Depth . The most difficult space variable to observe is over 

 the water depth. Dye released in the surf will rapidly mix over the depth 

 and give a depth- integrated and time-averaged current value. To be visible, 

 surface floats must move with surface currents. Drogues help to obtain the 

 depth-averaged current. A vertical array of fixed current meters is the only 

 practical way to measure the current distribution over the depth. 



Shepard and Inman (1950, 1951) made more than 1,600 observations of 

 surface and bottom current tendencies in the surf zone of southern 

 California beaches. Inside the breaker zone, their observations Indicated 

 a net seaward drift along the bottom and a net shoreward movement at the 

 surface. These top-to-bottom, onshore-to-offshore tendencies were not obser- 

 ved beyond the breakers. Galvin (1967) stated that the nonuniformity of 

 currents had not been studied quantitatively at that time except as described 

 above, and it is assumed he meant variations over the vertical. The vertical 

 uniformity of the horizontal currents is implied by many field researchers 

 (e.g., Thornton, 1969; Komar and Inman, 1970^; Horikawa and Sasaki, 1972) who 

 never mention it. 



KOMAR, P.D., and INMAN, D.L., "Longshore Sand Transport on Beaches," Journal 

 of Geophysical Research, Vol. 75, No. 30, 1970, pp. 5914-5927 (not in bibli- 

 ography) . 



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