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

Downstream control for the model was established by setting the starting water-surface elevation
for the flow at the time of the survey (300 to 350 ds) to the measured elevation, and adjusting the
starting water-surface elevation and/or energy gradient for higher flows untl the computed water-surface
profile at the downstream end of the bend matched the measured high-water marks. The resulting water-
surface elevation at XS 1 varied from Elevation 87.7 at about 300 cfs to about 95.3 at 32,300 cfs (Figure
3.4). For the conditions indicated in figure 3.4, the flow regime at XS 1 Increases from subcritical at
low flows to near critical flow at 15,000 cfs and greater flows.

The Manning's -n- roughness values for the main channel were determined by first estimating
the value predicted by the Umerinos Equation, which is applicable for grain roughness in cobble and
gravel bed streams and is given by:

O.0926R1/6

n =

R
1.16+2.01oq(-)
D84

(3.1)

This relationship indicated that Manning's -n- should vary from about 0.04 for the depths associated
with flows less than about 500 ds to less than 0.035 for depth associated with flows of 20,000 to 30,000
cfs, based on the characteristic (Ow particle size in the study reach of about 120 mm. This range of
values underpredicted the water-surface elevations at low flows, and significantly overpredicted the
water-surface elevations at higher flows. Based on the amount of sand In the system, it Is probable that
sufficient sand is being transported at higher flows to reduce the characteristic roughness size in the
channel bed. After numerous iterations, a relationship between Manning's -n- and discharge was
developed which varied from a maximum value of 0.045 for discharges of 500 cfs and less to a minimum
value of 0.025 for flows of 15,000 ds and greater. Overbank Manning's -n- values used in the model
varied from 0.05 to 0.06 based on field observations of the amount of vegetation and other roughness
conditions at the site.

As shown on the site location map (F'lQure 1.2), the river makes a sharp turn to the right at the
downstream end of the primary bar. In addition, the bedrock outcrop on both sides of the channel at
XS 5 creates a significant flow constriction in the bend at high flows. The ratio of radius of curvature
to width has a minimum value of about. 1.2 at the sharp portion of the bend near XS 4, and averages
about 1.5. Energy loss through the bend reduces the local energy gradient at the bend entrance,
creating backwater conditions at higher discharges In the upstream channel. The enecw loss through
the bend increases with increasing discharge, causing the energy gradient at the bend entrance to
decrease with increasing discharge. The 1993 high-water marks verify this observation, showing a sharp
drop between XS 4 and XS 6 where the channel enters the bend and a very flat upstream profile,

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