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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 <br />