9605

• UCREFRP Catalog Number: 9605
• Author: Schmidt, J. C. and e. al.
• Title: Flow Recommendations for the White River, Utah-Colorado Draft Report.
• USFW Year: 2002.
• USFW - Doc Type: Logan, UT.
• Copyright Material: NO

1997. For each pebble count, the intermediate axis of approximately 100 random

particles was measured and recorded. Pebble counts were conducted on a straight line

which crossed the cobble bar perpendicular to the direction of flow.

Calculation of Average Boundary and Critical Shear Stresses

The stage elevations and discharges necessary to inundate the three distinct

geomorphic surfaces identified adjacent to the channel were used to calculate the average

boundary shear stress at 12 of the 14 established cross sections. We then compared the

average shear stresses applied by these discharges, with the critical shear stress necessary

to entrain the Dso particle sizes from pebble count locations in the reach. Discharges

where boundary shear stresses exceeded the critical shear stress were considered

sufficient to mobilize sediment.

We used the Duboys equation to calculate the average shear stresses applied to

the boundaries of the channel. Average boundary shear stress is calculated as:

to = gRS
where to is the average boundary shear stress in N/m2, g is the specific weight of water at

100 C (9797 kg/m2s2), R is the hydraulic radius ofthe channel at the indicated stage, and

S is the water surface slope at the indicated stage. When channel width is much greater

than depth, mean depth is a good approximation of hydraulic radius. All of the cross

sections measured in the study reach fit this criteria, and we approximated the hydraulic

radius with mean depth, calculated as the cross-sectional area divided by the top-width of

the channel. The slope of the water surface at the desired stages was not directly

measured because we did not observe high discharges during our field work. We used a
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