equipment moving with the flume. All other variables in these experiments 
were the same. The amplitude of oscillation was 0.925 foot. There is no 
statistically significant difference in the results, indicating uniform 
suspension along the flume. None of the measurements for large amplitudes 
(greater than 0.925 foot) (Table 1) were made with the optical equipment 
moving with the flume. The reason for this is at large amplitudes the 
optical equipment would move into an area of the fluid where secondary 
circulations due to the asymmetric roughness elements may exist and there- 
fore not give a representative concentration. 
The remaining 29 of the 65 experiments were made using amplitudes of 
05255), 08465) and On6i7) foots | Mabile) 2 as) a\stabullation o£ the data jvand 
Figure 8 shows the results of the 65 experiments. As shown in Figure 8, 
there is a great deal of scatter in the data for small amplitudes; there- 
fore, only qualitative conclusions have been made. In general, the 
smaller the amplitude the smaller the slope of the concentration distri- 
bution curve. 
Experiments, movies, and photos were used to determine why the small 
amplitudes do not obey the flow velocity relationship of the larger ampli- 
tudes given in equation (11). Based primarily on visual observation, the 
following explanation is hypothesized. For small amplitudes, the distance 
of travel of the flume bottom during a half cycle is not great enough for 
the boundary layer to fully develop during each stroke and to become tur- 
bulent. Only at the end of the half cycle when the acceleration forces 
cause separation is sediment thrown into suspension. Separation only 
occurs at the downstream face of the artificial dunes. The observed sus- 
pension pattern when a 0.235-foot amplitude of oscillation was used con- 
sisted of plumes of suspension separated by areas of zero concentration. 
These plumes were accentuated because, on the return, the half-cycle 
separation at the downstream face occurred such that the succeeding burst 
of sediment was thrown into approximately the same region of fluid as in 
the first half cycle; i.e., the amplitude of motion was about equal to a 
multiple of the wavelength of the dune shape. When the amplitude was 
increased to 0.465 foot, a more fully developed boundary layer was 
attained. In this case, separation occurred over a somewhat longer 
distance of travel but still less than the wavelength of the dune shape 
because less deceleration force was required. The suspension pattern 
was the same as with the 0.235-foot amplitude but much less distinct; 
the plumes were wider and overlapping. Finally, with the 0.693-foot 
amplitude, separation occurred over a distance equal to or greater than 
the wavelength of the dunes, and a uniform longitudinal concentration 
was attained. 
Special experiments were conducted to verify the above hypothesis. 
The vertical distribution of concentration at various horizontal loca- 
tions in the fluid was measured to determine if the concentration distri- 
bution varied. If the above hypothesis is correct, the concentration 
distribution should vary along the horizontal and in a regular manner 
determined by the shape of the bed dunes. In addition, if suspension 
30 
