BOTTOM SLOPE AND PAETICLE SIZE IE THE 



SHOALING-WAVE ZOKE 



Mean Slope in the Shoaling-Wave Zone (Sg) 



Also studied was the interaction of 12 environmental variahles with 

 a portion of the beach slope roughly 250 feet heyond the breaker zone_^ In 

 the case of measurement of the mean slope in the shoaling-wave zone (Sg), 

 or average mean grain size on the slope /^O^z'^sT) ^^^ measurements of the 

 independent variables were nearly always made exactly k, Q, 12, l6, or 20 

 hours prior to measurement of the dependent variable. This exact lag- 

 time convention was considered more realistic than the U-hour-long one 

 used for lower-foreshore measixrements, because the measurement of the de- 

 pendent variable was not tied' to a dynamic condition, such as obtained at 

 the time of low tide for lower- foreshore measurements. 



Variables measured . — Eecent laboratory work on the equilibrium char- 

 acteristics of sand beaches beyond the breaker zone ( cf . Eagleson, Glenn, 

 and Dracup, 19^3 ) commonly utilizes measurement or determination of the 

 following variables, where one is concerned with the mechanics of slope 

 alteration: bottom slope itself at the point in question, particle size 

 on the slope, wave period, wave length, wave height, water depth, specific 

 gravity of the sand particles and the fluid, and kinematic viscosity of 

 the fluid. The variables described in the paragraphs below were measured 

 in this study in an effort to approximate the variables found useful in 

 laboratory studies. The subset of data used earlier in the description of 

 the regression method was taken from the fourth lag period of this section 

 of the study. 



The segment of the shoaling- wave- zone slope selected for study here 

 was one which was reasonably flat and yet which exhibited a relatively 

 large range of slope values dirring the periods of observation. As seen 

 in figure 3, the slope was determined from soundings at four stations at 

 the 15th Street pier, each of which was 25 feet apart. The slope was 

 determined by plotting the soimding values for these four, and for several 

 surrounding stations, on cross-section paper and then drawing an estimated 

 mean slope through the four stations by eye. Still -water depth (h) at 

 the midpoint of this mean slope (fig. 3) was determined for each lag period 

 from precision tide data or direct measurement by sounding line. Mean grain 

 size, Mg, determined as explained in appendix A, was obtained for each of 

 the four stations used in slope determination. The four values were aver- 

 aged to give {M^)s values for the regression analyses. Wave parameters 

 were again estimated from wave-spectrum-analyzer records furnished by 

 the Coastal Engineering Research Center. Specific gravity of the particles 

 was assumed constant (2.65, or that of common quartz). Rather than deter- 

 mine kinematic viscosity, we instead determined water density, from pre- 

 cision temperature and salinity measurements (see appendix A for details). 



In addition to the above variables that are similar or identical to 



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