7995

• UCREFRP Catalog Number: 7995
• Author: Mussetter, R. A. and M. D. Harvey.
• Title: Yampa River Endangered Fish Species Habitat Investigations.
• USFW Year: 1994.
• USFW - Doc Type: Ref. No. 93-166.01,
• Copyright Material: NO

Dc =

1"

(3.3)

1'. (y s -y )
c

Reported values for 't *c for the median particle size of the surface bed material range from 0.03
(Meyer-Peter and Muller 1948; Neill 1968) to 0.06 (Shields 1936). A value of 0.047 is commonly used
in engineering practice based on the Meyer-Peter. Muller bedload transport equation (Meyer-Peter.
Muller, 1948). More recent studies indicate that a value of 0.03 may be more reasonable for gravel and
cobble bed streams (Parker et at. 1982; Andrews 1983). For purposes of this analysis. incipient
conditions were computed using values of 't *c of 0.03 and 0.047 to address the range of conditions
associated with uncertainty in selection of single appropriate value. The bed shear stress due to grain
resistance ('t ') is a better descriptor of near-bed hydraulic energy in gravel-cobble bed streams than the
more commonly used total shear stress because it eliminates the effects of flow resistance due to
irregularities in the channel boundary. nonlinearity of the channel. variations in channel width, and other
factors that contribute to the total flow depth. but not the energy available to move individual particles
on the channel bed (Einstein 1950; Mussetter 1989). The grain shear stress ('t') is computed from the
following relation:

l' = yylS

(3.4)

where Y' is the portion of the total hydraulic depth associated with grain resistance (Einstein 1950)
and S is the energy slope at the cross section. The value of Y' is computed by iteratively solving the
semilogarithmic velocity profile equation:

V
v'
.

yl
= 5. 75 +6 . 251 og ( - )
Ks

(3.5)

where V is the mean velocity at the cross section. ~ is the characteristic roughness height of the bed.
and V; is the shear velocity due to grain resistance given by:

v.' = JgylS .

(3.6)

The characteristic roughness height of the bed (Ks) was assumed to be 3.5 084 (Hey 1979). where 084
is the particle size for which 84 percent of the bed material is smaller. The average 084 of the bar
surface sediments is about 123 mm (4.8 in.) (based on MH7 and MH8) and that of riffles or tertiary bars
in the study reach is approximately 146 mm (5.7 in.) (Figure 3.7). Mean velocities predicted by the
HEC-2 model vary from less than 0.5 fps in the pool area upstream of the primary bar (XS 14 - Figure

3.16 Resource Consultants & Engineers. Inc.