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

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Lyons (1989), however, concluded that the Green River channel below Flaming Gorge had reached
a new equilibrium (no net aggradation or degradation) and no more significant changes in channel geometry
should occur. The reach immediately below the dam (rm 237 to 310) was found to have reached a new
equilibrium by 1974, while the reach further downstream (rm 94 to 121) did not reach equilibrium until
1981. In contrast to the narrowing that had occurred after closure, leading to new equilibrium conditions,
the high runoff during the 1980's resulted in an increase of channel width. The conclusion was that the
Green River channel has adjusted to the changed sediment regime created by Flaming Gorge, but that the
potential for future channel changes exists in response to other changes in water and sediment discharge
conditions. (See Chapter 6.0 for an update of these conclusions.)

2.4 Incipient Motion Literature Review

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The defmition of incipient motion is based on the critical or threshold condition where the
hydrodynamic forces acting on a grain of sediment have reached a value that, if increased even slightly, will
move the grain. Some of the concepts and ideas presented in the literature on incipient motion are now
generally well accepted and documented; others are relatively new and untested. For example, some
concepts that have been well established by the literature since Shields first presented his famous paper
(see Appendix A) include knowledge of the importance of turbulence in incipient motion. Hydrodynamic
forces are not constant for a given set of flow conditions, but rather can vary dramatically. This turbulent
nature can result in some particle motion under almost any flow condition if observations are carried out
long enough. Therefore, when defining 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 bed.

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The significance of nonuniform bed material and protrusion, as opposed to the uniform, plane bed
material utilized by Shields, in defining incipient motion conditions has also been well demonstrated.
Protruding particles will be entrained by lower shear stresses than non-protruding particles, and smaller
particles located between larger particles may be sheltered, or hidden, requiring a large shear stress relative
to their small size. However, hiding is not as significant as originally thought, for example as given by
Einstein (1950) in his bed load function, because of the additional turbulence generated downstream of the
large particles that provide the sheltering. In any case, it is important to recognize, particularly in a gravel
bed, that a given particle may be entrained at different shear stresses depending on its position in and the
composition of the bed.

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The concepts of protrusion and hiding in a gravel bed suggest that at a given shear stress, many
different size particles may be at incipient motion. That is, protrusion can result in incipient motion of
large particles of the armor layer at small shear stresses (relative to particle size), while hiding can prevent
incipient motion of the smaller particles until relatively high shear stresses occur. The net effect is that the
range of shear stresses necessary for incipient motion of a broad range of particles is compressed in a
gravel bed channel. On this basis some researchers have assumed a concept of equal mobility for all grain
sizes. It is apparent that a singular relation between particle size and shear stress, as given by the Shields
curve, may not adequately explain incipient motion in gravel bed channels. The work of Parker et al.
(1982), Andrews (1983), and Diplas (1987), resulted in similar type equations for quantifying critical
dimensionless shear stress in gravel-cobble channels.

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One concept that is relatively new in the literature, but is based on well established principles of
fluid mechanics, is that there may be a significant increase in the critical dimensionless shear stress at very
large particle Reynolds numbers. This effect, described by Wang and Shen (1985), was observed in data
representing very large particles in a flood channel in China. The effect was also recognized by Wiberg and
Smith (1987) and accounted for in their analytical formulation of critical dimensionless shear stress. From
fluid mechanics this effect can be explained by the reduction in drag coefficient at high Reynolds numbers
(based on flow velocity). Wang and Shen suggested that this effect would occur at particle Reynolds

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