The concentration distribution in the vertical was then measured for 
the flow condition selected. Again, for each elevation, voltage records 
were made for the sediment-laden water, clear water, ambient light, and 
clear water plus the filter. If the optical equipment was held stationary 
in space, the records for clear water and clear water plus filter were 
made while the flume was oscillated at such a long period that no sediment 
was in suspension. The time interval of the record for sediment-laden 
water, clear water, and clear water plus filter was always an integer 
multiple of the flume oscillation period. This procedure automatically 
allowed any irregularities in the transmissibility of the flume windows 
to be compensated for when calculating the fraction of light passed. 
The output signal of the photoelectric cell, which was recorded on 
Magnetic tape at the rate of 58 samples per second, was not constant 
With time, due to instantaneous concentration fluctuations. Because the 
concentration is related to the logarithm of the output voltage, it was 
necessary to calculate the concentration for each sample and then average 
the concentrations over the period of record to determine the true mean 
concentration. 
The concentration measurements were usually started as near the bound- 
ary as possible, about 0.5 centimeter above the crest of the roughness 
elements. The flume was stopped, the elevation of the optical equipment 
was raised and recorded, and new measurements were taken. This procedure 
was followed until an elevation was reached at which the concentration 
was too low for the optical equipment to measure. The optical equipment 
was then lowered in a stepwise fashion until near the boundary to obtain 
concentration measurements at intermediate elevations. In this manner, 
7 to 15 concentration measurements were obtained to describe the concen- 
tration distribution for one flow condition. 
3. Results. 
Sixty-five concentration distribution curves were obtained of the 
form, 
C=C, exp(MyY) , (9) 
where 
M = slope of the curve (in feet 1) 
C, = concentration at the base elevation (in grams per liter) 
C = concentration of sediment (in grams per liter); average of 
the concentrations calculated from equation (8) for each 
sample of record during the period 
Y = elevation above the crest of the artificial roughness (in feet) 
23 
