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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Incipient motion analysis requires evaluating the average boundary shear stress acting on the channel bed.
A common boundary shear stress equation is

T=yRS

(5.3)

where: R is hydraulic radius and
S is slope

However, this relationship requires knowledge of the local energy slope, which is difficult to obtain
accurately. An alternate approach, based on velocity profile equations, is

L = P V2 .

[5.75Iog( 12.27(y/ K ,))]2

(5.4)

y is flow depth
P is density of water
V is mean velocity and
Ks is roughness height.

which is based on the more commonly available velocity and flow depth. This equation was selected for use.

where:

All of the variables in equation 5.4 are readily obtainable from the field data with the exception of
the representative height of the bed roughness,~. Bray (1982) reviewed factors affecting resistance to flow
in gravel bed rivers. He showed that for gravel and cobble bed streams, ~ is approximately equal to 3.5
times the dS4 of the surface bed material. This value was adopted for use in the incipient motion
computations for this study.

Incipient motion conditions in the Yampa Canyon were evaluated at two locations: the point bar
located on the right bank at Mathers hole (rm 17.5) and the left bank channel at the spawning bar (rm 16.5).
Data necessary for the analyses were developed from the National Park Service (NPS) project report
(O'Brien, 1984) and the supporting data notebooks. Note that significantly more data was available for
Mathers Hole than at the spawning bar for developing hydraulic geometry relationships necessary in the
calculation (see Section 5.4.2).

The objective of the analysis was to derme the discharge at which various particle sizes on a cobble
bar would be at incipient motion. The following steps summarize the analysis approach:

1. A range of subsurface dSO typical of the cobble bars in Yampa Canyon was dermed based
on data at Mathers Hole.

2. The critical dimensionless shear stress was defined for particle sizes of 10, 20, 30 n. 150 mm
based on the Andrews type relationship (Equation 5.2).

3. For each particle size the boundary shear stress at incipient motion was defined based on
Equation 5.1.

4. The discharge necessary to produce this boundary shear stress was evaluated from
Equation 5.4 and hydraulic geometry relationships derived for the cobble bar.

5-15