of the second degree (quadratic) effects that may be present. 



LONGSHORE- CURRENT VELOCITY 



Longshore- cirrrent velocity typifies a variable measured in this 

 study that depends rather obviously upon most of the other measured 

 variables. We may expect from theoretical and laboratory studies (cf. 

 Brebner and Kamphuis^ 19^3^ for example) that the following variables 

 will be significant ones in their influence on longshore- current velocity: 

 beach slope, wave period, wave height, and angle of wave approach. Other 

 measured variables that are not customarily considered in laboratory 

 studies may be expected to exert lesser, but possibly significant, effects 

 upon longshore- current velocity. Among these might be local wind veloci- 

 ties in onshore or offshore directions, which would affect the form of 

 the incoming swells, and water density, which would affect fluid drag and 

 the quantity cf sediment transported. Tidal currents opposed to or in 

 the same direction as the longshore currents might also be of signifi- 

 cance. 



Of the greatest importance in the evaluation of the influence of 

 the several variables considered as causes, on the one considered an 

 effect^ is the assessment of the simultaneous influence of each of the 

 various possible combinations of the causal variables. The method used 

 in investigation of this simultaneous influence was described in the 

 previous section. Here we take longshore- current velocity as a dependent 

 variable. 



Results of Analysis 



The results of the first stage of the regression analysis are 

 shown in table 2. This table shows the extent to which 13 environmental 

 factors (independent variables, or Xs) reduce the sum of squares of 

 longshore-current velocity when the factors are taken one at a time . The 

 total io SS reduction by all 13 Xs taken simultaneously is 69.97. ("The 

 number of longshore- current measurements used was 53.) An initial in- 

 ference is that some 70?^ of the interaction of longshore- current velocity 

 with the environment is "explained" by these 13 measured environmental 

 variables. While it is possible that certain important variables may 

 have been omitted from the analysis, it is also likely that a relatively 

 high "noise-level" may be present, due in part to local fluctuations in 

 the phenomena studied, a_s well as to errors of measurement. Mean long- 

 shore- c-ur rent velocity, V, for example, shows a great variation about 

 the true mean velocity because the point of actual observation relative 

 to the upstream and downstream rip-current boundaries was never know. 

 Thus the value used for V actually ranged from zero velocity, up through 

 the true mean velocity, and beyond to the maximum expectable one. 



Again, noise may have been introduced into the analysis owing to 

 the way that the independent variables were measured through time, relative 

 to the dependent variable. Wave height, for example, could have been 

 meas-ured at the instant that the longshore- current measurement was taken 

 or up to h hours before its measurement. In view of these difficulties 



