for small amplitudes is a function of location, the shape of the concen- 
tration distribution curve for a particular vertical may not be the same 
flume velocity, U,, but with a large amplitude. The following is a 
description of the experiments. 
The flume was adjusted to a 0.465-foot amplitude and a 3.18-second 
period was selected. A special brace with attached scale was construc- 
ted to hold the optical equipment stationary in space, and therefore 
stationary relative to the fluid. This brace allowed relocation of the 
optical equipment to any desired new location along its arc while main- 
taining the freedom to change the elevation of the optical equipment 
relative to the bed of the flume. An initial location of the optical 
equipment was chosen and a vertical concentration distribution measured. 
The optical equipment was then relocated to a new position and another 
concentration distribution was measured. This procedure was repeated 
until five concentration distribution curves were obtained. Each was at 
a different horizontal location relative to the position, at a fixed 
phase, of the flume bed, but with identical flow conditions of the flume. 
The horizontal range of the five distribution curves was about equal to 
one wavelength of the bottom roughness shape. The results of these ex- 
periments (Fig. 9) indicate that the distribution of concentration is not 
uniform along the flume bed for the 0.465-foot amplitude studied. Figure 
9 also indicates that the rate of sediment concentration decay is less in 
the areas between the dunes and greatest at the crest of the dunes. This 
implies that the plumes of suspension are above the trough of the bed 
shape, where it would be expected if the sediment is thrown upward and 
downstream from the downstream face of the dunes. 
Figures 10 to 14 are the concentration distribution curves measured 
for the locations indicated in Figure 9. Although not conclusive, these 
data indicate that the vertical distribution of sediment in the plumes 
does not conform to the exponential distribution of equation (9). The 
relationships shown in Figures 11 to 14 display a slight, but statisti- 
cally significant curvature and were obtained at locations in the plume; 
whereas, Figure 10 shows no curvature and was obtained at a location 
between plumes. This could be explained by the existence of periodic 
stationary eddies, one located on each side of the plume axis. The eddies 
would sweep sediment into the base of the plume axis giving a relatively 
high concentration and carry sediment out of the top of the plume, causing 
the concentration to be lower than would be predicted in a randomly tur- 
bulent flow field. 
The small amplitudes studied represented rare prototype conditions ; 
therefore, all subsequent experiments were limited to amplitudes greater 
than 0.617 foot. 
4. Experiments Using Sediments of Different Settling Velocities. 
Two different sediments of the same black plastic were selected 
and investigated to determine the effect of settling velocity on the 
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