In the range of flow velocities where equation (23) is valid, the calcu- 
lated so is an approximation of a base velocity at an arbitrary eleva- 
tion, the crest of the bed dunes. To apply the experimental results to 
a real situation, the constants in equation (23) must be adjusted to give 
So values at the elevation of the top of the bedload layer. This eleva- 
tion depends on the grain diameter of the sediment being considered and 
on the bed geometry; therefore, no attempt was made to express Ss, at 
the bedload elevation. 
IV. THE SUSPENDED LOAD IN OSCILLATING FLOW 
1. Suspended-Load Theory in Unidirectional Flow. 
The suspended-load theory for unidirectional flow and the available 
field data to test the theory supply valuable insight to some problems 
which exist in determining the suspended load in oscillating flow. For 
this reason, Einstein's (1950) suspended-load theory and field data from 
the Missouri and Atchafalaya Rivers are presented. 
The suspension theory in unidirectional flow is based on an equilib- 
rium equation for mass flux across a unit horizontal area in the flow. 
Assume the unit horizontal area is at elevation Y. Across this area 
fluid is being exchanged by the vertical component of the random motion 
of fluid particles caused by turbulence. From continuity, the picture of 
fluid exchange can be simplified by assuming that through one-half of the 
unit area, fluid is moving upward with an average velocity of v; through 
the other half area the fluid is moving down with an average velocity -v. 
If the exchange occurs over an average distance of 1, it can be assumed 
that the downward-moving fluid originates, as an average, from an eleva- 
tion Y + 1/2 le while the upward-moving fluid originates from Y - 1/2 le. 
The important assumption is made that the fluid preserves, during its 
exchange), the propertiesvot ithe) fluid iat ats pointiof or1vein. | bi. the 
concentration of sediment at elevation Y is C and the sediment has 
a settling velocity of V,, the equilibrium equation for sediment flux 
is given by: 
[c - Fle (ac/ay)| (3) (ava) + 
[c ce (ac/ay)| (3) (-v-Vz) = 0. (24) 
This equation reduces to: 
CV, + sl, v (dc/d¥) = 0. (25) 
To solve this equation the term, 1/2 fv omust ibevevaluated? eathuaisi as 
normally done by equating this term to the corresponding term in a similar 
equation of momentum exchange; i.e., the sediment exchange coefficient is 
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