. sediment with a settling velocity less than found in the prototype, 
results of experiments by other investigators using prototype sediments 
resulted in the same exponential relationship. In addition, the bed 
roughness used by other investigators varied; therefore, this conclusion 
does not appear to be limited to the single-bed roughness used in this 
investigation. 
b. Using the above conclusion and the O'Brien's (1933) equation for 
continuity of sediment exchange results in a sediment exchange coefficient 
which is independent of elevation above the bed. Behavior of the rate of 
sediment concentration decay with elevation, which is related to the sedi- 
ment exchange coefficient by equation (13), was found to be a function of 
the flow velocity causing the suspension and the settling velocity of the 
sediment. A linear relationship between the flow velocity and the sedi- 
ment concentration decay rate was found for the constant sediment-settling 
velocity used in the majority of the experiments. This relationship is 
shown in Figure 6. The relationship between flow velocity and the sedi- 
ment concentration decay rate for other sediments studied in this investi- 
gation is shown in Figure 15. The limited data indicate a possible linear 
relationship for the different sediment-settling velocities. There is not 
enough data to determine the relationship between the concentration decay 
rate and settling velocity for a constant flow velocity. Only qualita- 
tive conclusions can be obtained from the eight concentration distribution 
Measurements shown in Figure 15. For a constant bed roughness and flow 
velocity, a higher sediment-settling velocity results in a higher sedi- 
ment concentration decay rate. The limited data consistently indicate 
that the concentration decay rate is not proportional to the settling 
velocity to the first power; i.e., they are not directly proportional. 
This and equation (13) imply that the settling velocity is an important 
variable influencing the sediment exchange coefficient. Therefore, in 
oscillating flows the sediment exchange coefficient cannot be accurately 
approximated by the momentum exchange coefficient as is commonly done 
in unidirectional flow analysis. No experiments were conducted in this 
investigation to determine how the above relationships would change with 
a change in bed roughness. 
The conclusions obtained from measurements of the turbulent velocity 
fluctuations are: 
a. The distribution of turbulent velocity fluctuations at a constant 
elevation in an oscillating flow was found to be approximately normal with 
a mean of zero. This relationship was determined from distribution analy- 
ses of measurements made at two elevations above the bed, both of which 
were above the boundary layer described in Section II. Results of these 
analyses are shown in Figure 22. 
b. The relationship between the root--mean-square turbulent velocity 
fluctuation and elevation above the bed was found to be exponential. This 
conclusion is based on measurements of distributions made for four differ- 
ent flow velocities, all using an amplitude of oscillation of 0.925 foot 
and covering approximately the same range of prototype flow velocities as 
Ue 
