depth. The procedure was repeated until readings had been made for 11 posi- 

 tions over the calibration range of the rods . The voltage readings were con- 

 verted to vertical distances from the horizontal datum, SWL, and compared with 

 the known physical position of the rod at each reading. The voltage readings 

 of the rods were accepted if the maximum deviation for each of the 11 posi- 

 tions was less than 0.01 ft. The maximum deviation was usually less than 

 0.01 ft, and deviations greater than this were typically caused by residue on 

 the rods. In these cases, the rod was cleaned, and the procedure was repeated 

 for the individual rods that exceeded the tolerance . 



171. The SWL was measured by a point gage, graduated to 0.001 ft, lo- 

 cated in the horizontal section of the tank. The water depth was checked 

 before conducting each test. Measurements of water depth at gages and at the 

 bar were made with a rule that was accurate to 0.05 ft. The same rule was 

 used for measuring seaward and shoreward faces during construction of the 

 bars. All measurements during video analysis were made by scaling the 2-in. 

 grid as it appeared on the monitor to 1/4 in. , which gave an error of ±1/8 in. 



172. Monochromatic waves were analyzed and averaged for 15 waves from 

 the wave record, and measurements from videotapes were averaged for 10 waves. 

 Although the waves were monochromatic (constant period) and regular (constant 

 height) , it was important to average the measured quantities to reduce fluc- 

 tuations caused by reflections and currents present in the tank. It was also 

 critical to limit the number of waves averaged to those that were not re- 

 flected from the wave board. 



173. To illustrate the importance of averaging values, breaker depth 

 index was plotted as a function of H /L for five tests, as shown in Fig- 

 ure 79. The average value of 7 b is marked by the symbol, and the range be- 

 tween maximum and minimum values of 7 b computed from the 10 measured values 

 of each test is shown by the vertical line at each wave steepness. It is seen 

 that differences of 15 percent in individual values occurred despite care to 

 eliminate wave reflection and seiching. The considerable scatter in the data, 

 often much more than 15 percent, shown in many figures of this report is at- 

 tributable in part to the difficulty in defining the given quantity, such as 

 breaking of collapsing waves, and not to direct measurement. The process of 

 averaging 10 values as done in the present study helped to reduce scatter due 

 to individual wave variation and thus presented a more realistic picture of 

 the variability in the developed relationships. 



142 



