Measurement of the Dependent Variable 



An attempt is made to understand the changes in the overall alti- 

 tude of the lower-foreshore surface along a 100-125 foot segment running 

 perpendicular to the shoreline (fig. 3). The dependent variable has the 

 dimensions of a length, the magnitude of which is believed sufficient to 

 adequately reflect the environmental processes producing the foreshore 

 changes. This length is the net thickness of material added to or sub- 

 tracted from the foreshore surface at five or six stations in a 24.5-hour 

 period; i.e., over two tidal cycles. 



The section of the "lower foreshore" referred to here includes the 

 zone of breaking waves somewhere in its lower half (fig. 4) at the time of 

 measurement of the altitude of the beach surface (time of low tide). The 

 remainder of the lower-foreshore surface studied here consists of the 

 region covered by the breaker and swash-backwash zones of the previous two 

 tidal cycles. Measurements of the altitude of the beach surface were made 

 at six stations at 15th Street (northern transect, fig. 1) and five stations 

 at the Camp Pendleton line (southern transect, fig. 1), at the low-tide 

 times designated by the letter "R" on figure 2. Thus, the measurements 

 cover 25 consecutive days in June and July. 



Precision of the sounding-line or leveling-rod measurement is esti- 

 mated at +0.10 foot. Altitudinal variations at the stations were summed 

 over the five or six stations ahd the net erosion or deposition recorded. 

 Unfortunately, the large sampling distance involved (25 feet) undoubtedly 

 missed minor irregularities in the profiles that could have been signifi- 

 cant. Figure 4 shows the range of changes in lower-foreshore altitudes at 

 the two transects during the 25 consecutive days (fig. 2) of measurements. 

 Clearly, the beaches were not anywhere near an equilibrium condition, such 

 as might be expected (cf. Strahler, 1964) during the late summer. Daily 

 values for the increment of deposition, Jf , averaged about 0.7 foot at 15th 

 Street; for erosion, Kf , averaged about 0.5 foot. Respective average values 

 at the Camp Pendleton line were 0.5 foot (Kf) and 0.4 foot (Jf). The values 

 compare with daily net fluctuations of 0.2-0.3 foot on Cape Cod beaches 

 (Zeigler and Tuttle, 1961). 



For this part of the study, the variations in the causal independent 

 variables were followed over six lag periods (two tidal cycles), as shown 

 below: 



32 



