9568

• UCREFRP Catalog Number: 9568
• Author: Resource Consultants Inc.
• Title: Sediment Transport Studies of the Little Snake, Yampa, and Green River Systems.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

Based on the incipient motion literature review it is concluded that the Parker type empirical
relationship (eg. Parker et al. 1982, Andrews 1983, and Diplas 1987) best describes the critical
dimensionless shear stress for armor layer stability in gravel bed systems. Results from this type of
relationship are consistent with field and laboratory observations and theoretical derivations, specifically
that a range of particle sizes can be put in motion at a given discharge. Further, the estimated values of
dimensionless critical shear stress resulting from the Parker type formulations are within the range
reported by various investigators.

Parameter values determined for this type of relationship by various investigators do not vary
dramatically. Therefore, the equation utilized may not be critical; however, considering that Andrews'
results were based on channels in the Rocky Mountain region it seems most appropriate to utilize his
parameter values for analysis of the Yampa River.

The results from this empirically based procedure for predicting critical dimensionless shear stress
may be compared with the analytically derived procedure described by Wiberg and Smith (1987), providing
the physical conditions are within the range of assumptions used in derivation. The only recommended
modification in application of these procedures is to check the magnitude of the particle Reynolds number.
If it exceeds 100,000, then the results should be carefully evaluated based on the results of Wang and Shen,
as discussed above.

If deposition of silt-size particles (wash load) is independent of velocity, one would expect that a
gravel bed channel with significant wash load will contain fine material at depth in the gravel bed regardless
of flow velocity maintained. Given that the wash load of the Yampa River is about 35 percent of the
sediment load, it can be concluded that occurrence of fine sediment at depth in the cobble spawning
reaches is a natural condition of the channel.

Based on available research it is reasonable to conclude that flushing of fine sediments from the
Yampa River gravel bed reaches without disrupting the surface layer can occur up to a depth equal to the
median cobble particle diameter. However, the available literature does not provide a proven method for
defining the critical dimensionless shear stress for flushing to occur. The empirical observations by
Milhous in Oak Creek, and the empirical relations by Everts and those by Fisher, Sill and Clark are the
only alternatives available. None of these approaches are expected to provide defmitive results to Yampa
Canyon conditions, illustrating the need for additional research (see Sections 7.3 and 7.4 .. proposals were
submitted to the Recovery Implementation Program for Fiscal Years 1991 and 1992 to conduct flume
studies to address this issue).

In summary, the following comments and conclusions can be made based on a comprehensive
review of incipient motion and flushing flow literature, and application of these concepts to the Yampa
River:

1. When considering incipient motion conditions, it is more appropriate to use the concept of
no motion of practical significance, rather than zero motion, and recognize that there is
always some movement occurring on the channel bed.

2 In a gravel bed system with a coarse bed layer two distinct incipient motion conditions
exist: the incipient motion of the bed layer itself, resulting in disruption of the bed
surface, and the incipient motion of the small particles deposited in the coarse bed, near
the surface, that can occur without disruption of the bed surface (i.e. flushing).

3. In a gravel bed channel the effects of hiding and protrusion can significantly influence
incipient motion conditions. As a result of these effects the following conditions may exist:

a given particle size may be entrained at different shear stresses (i.e., a singular
relation between particle size and shear stress, as given by the Shields curve, may

2-6

I
I
I
I
I
I
I
I

I
I
I
I
I
I
I
I
I
I
I