I[I. Sediment Transport
.......
61
Several investigators have observed that the constant of Van Karman falls below
the value 0.4 (clear flow) in the case of a heavy sediment-laden flow over a rigid,
flat bed. It has also been observed that the flow velocities in a layer close to the
bed are reduced, while in the remaining part of the flow there are larger flow
velocities. Apparently, the mixing is reduced by the presence of a large amount of
sediment particles.
According to Einstein [1950], and Chien [1954], who determined the amount of
energy needed to keep the particles in suspension, the constant of Von Karman is
a function of the depth-averaged concentration, the particle fall velocity and the
bed-shear velocity.
Although Ippen [1971] supposed that the constant of Van Karman is primarily a
function of some concentration near the bed, an investigation of Einstein and
Abdel-Aal [1972] showed only a weak correlation between the near-bed
concentration and the constant of Von Karman.
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Fig. 5 :
(a) 0-factor
(b) Concentration profile for experiment ofEinstein-Ning Chien (1950,
1954)
Coleman (1980] questioned the influence of the sediment particles on the
constant of yon K~trm~n. He re-analyzed the original data of Einstein-Chien and
Vanoni-Brooks and concluded that they used an erroneous method to determine
the constant of Von Karman.
These contradictions allow to question whether the concept of an overall constant
of yon Karman for the entire velocity is correct for a heavy sediment-laden flow.