The Red Rock Canyon study reach (fig. 5) was
<br />approximately 2,300 ft long and was surveyed in
<br />1994; selected cross sections were resurveyed in 1995.
<br />Ten channel cross sections were surveyed in the Red
<br />Rock Canyon reach and were used to calibrate the
<br />one-dimensional, water-surface profiles model.
<br />Hydraulic-geometry characteristics were calculated at
<br />6 of the 10 cross sections, and sediment-entrainment
<br />potential was calculated at 4 cross sections. Geomor-
<br />phic features surveyed included streambanks, alluvial
<br />bars, and the distal margin of a tributary debris fan.
<br />Sediment-Size Analysis
<br />Sediment-size characteristics, including the
<br />median particle size (dsp), were determined for a
<br />variety of surfaces deemed geomorphically distinct
<br />and (or) botanically significant using the Wolman
<br />(1954) method. These surfaces included the streambed
<br />and streambanks, the lateral and top surfaces of large
<br />alluvial bars, and the distal margin of a debris-flow
<br />deposit (tables 3 and 4). The streambed and low-flow
<br />channel of most cross sections in the study reaches are
<br />composed of material in the large cobble- and boulder-
<br />size range; however, streambanks and alluvial bars are
<br />composed of finer material in the gravel-, cobble-, and
<br />occasionally boulder-size range. Most of the initial
<br />sediment measurements were made in 1994; however,
<br />a few sites were first measured in 1995. Replicate
<br />measurements were made in 1995 at several sites
<br />initially measured in 1994 to determine the effect of
<br />the 1995 peak discharge (9,470 ft3/s) on sediment-size
<br />characteristics. Subsurface sediment size was not
<br />determined except for the left bank at cross section V4
<br />in the Warner Point study reach, where the ratio of
<br />surface dsp (47 mm) to subsurface dgp (20 mm) was
<br />2.35, within the range stated by Parker and others
<br />(1982) as typical for streams having gravel and cobble
<br />beds.
<br />One-Dimensional Streamflow Modeling
<br />The one-dimensional water-surface profiles
<br />model HEC-2 (Hydrologic Engineering Center,
<br />1990) was used to estimate water-surface elevations,
<br />flow depths, and hydraulic conditions through the
<br />study reaches for a range of discharges from 2,000 to
<br />20,000 ft3/s. The model used surveyed channel cross
<br />sections and was calibrated with observed water-
<br />surface elevations and high-water marks associated
<br />with discharges of 336, 614, 766, 1,024, 1,584, 3,450,
<br />5,170, 6,800, and 9,400 ft3/s in the Warner Point
<br />reach. HEC-RAS (Hydrologic Engineering Center,
<br />1997) was used to model the Red Rock Canyon reach.
<br />This model was calibrated using discharges of 400,
<br />1,000, 2,310, and 3,170 ft3/s. Discharges at the Warner
<br />Point and Red Rock Canyon reaches were assumed to
<br />be comparable to discharges recorded a few hours
<br />earlier at USGS gaging station 09128000 upstream.
<br />The hydraulic models were calibrated by
<br />varying the Manning's n roughness coefficient until the
<br />calculated water-surface elevations matched the
<br />surveyed water-surface elevations as closely as
<br />possible. Calculated water-surface elevations were
<br />within 0.2 to 0.5 ft of the surveyed elevations for most
<br />calibration discharges, having differences less than
<br />10 percent of the calculated flow depth for a specific
<br />discharge. Although the Manning's n roughness coeffi-
<br />cient was used as a calibration tool, reasonable values
<br />for each cross section were maintained based on esti-
<br />mated values from Barnes (1967) and Arcement and
<br />Schneider (1989).
<br />Computational errors were minimized, and the
<br />accuracy of the models was improved by inserting
<br />interpolated cross sections to balance velocity heads
<br />and water-surface elevations. Additionally, multiple
<br />friction loss equations were evaluated, and conveyance
<br />ratios were kept within tolerances where possible and
<br />were otherwise determined acceptable. Hydraulic
<br />output from the modeling runs included water-surface
<br />elevations and energy gradients, from which flow
<br />depths and boundary shear stresses at specific loca-
<br />tions on the cross sections were calculated. Tables 5
<br />and 6 summarize hydraulic-geometry characteristics
<br />in the study reaches for simulated discharges.
<br />Shear Stress Distribution
<br />Sediment entrainment in stream channels is
<br />partly a function of the boundary shear stress acting on
<br />sediment particles resting on or in the streambed or
<br />other inundated alluvial surfaces. Shear stress is
<br />proportional to the square of streamflow velocity and
<br />is most accurately determined by measurements of
<br />velocity vectors in downstream, lateral, and vertical
<br />directions. When velocity data are unavailable, mean
<br />shear stress in a channel cross section commonly is
<br />GEOMORPHIC AND SEDIMENTOLOGIC CHARACTERISTICS 13
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