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

provided in Appendix C. The effective discharge at Jensen is also tabulated in Table 5.4 for reference as
determined by Andrews (1986).

5.4 Yamoa Canyon Incipient Motion

5.4.1 Definition. Incipient motion of particles resting on the bed of a channel is defined as the flow
condition at which the forces resisting the motion of a bed particle are equal to the fluid forces tending to
move the particle. Under ideal conditions, a slight increase in fluid forces will cause motion of the particles.
In terms of shear stress, this means that motion is possible when the shear stress acting on a particle, "t 0 ,
exceeds a critical value denoted as "t e' At incipient motion the critical and particle shear stress are equal.

It is important to note that the concept of incipient motion only defines the conditions at which
motion begins. Incipient motion does not imply that all the particles on the bed will move once the
threshold shear stress has been exceeded. Rather, as Neill (1968) states "At [incipient motion] occasional
movement of single grains may be obtained". In fact,. the beginning of motion is difficult to define because
the motion of particles is random. As Simons and Senturk (1977) state:

"When the stress over the bed is near its critical value it is possible to observe a few particles
moving on the channel bottom. The time history of the movement of a particle involves long
rest periods. In fact, it is difficult to conclude that particle motion has begun..

The ensuing analysis was performed to estimate the minimum discharge required to move particles
making up the coarse surface of the gravel and cobble bars of the lower Yampa canyon. It cannot however,
be assumed that the entire surface of the bars will be disturbed at the minimum critical discharge. Rather,
the analysis indicates the range of discharges which must be exceeded to substantially alter the bars. (See
Section 2.4 and Appendix A for more details.)

5.4.2 Procedure. The Shields type relationship provides the basic equation for evaluation of
incipient motion; however, review of the literature on incipient motion concluded that the Andrews (1983)
relationship should be utilized for quantifying the critical dimensionless shear. Additionally, the particle
Reynolds number should be checked to insure that the change in boundary layer described by Wang and
Shen (1985) had not occurred for the given flow conditions and particle size (see Section 2.6). Therefore,
the basic formulas recommended for incipient motion analysis are:

Le= L.(ys-y)d;

(5.1)

-0.872
't.= O.0834(d;/ds-o)

(5.2)

where:

"t e is the critical boundary shear stress
-r. is the critical dimensionless boundary shear stress
Y is the specific weight of water
d i is particle diameter
dSO is subsurface dsp
Y s is the specific weight of sediment

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