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use in calibrating a one dimensional hydrodynamic model for computing roughness coefficients, <br />velocities and boundary shear stresses for various flow levels (discussed below). <br />Samples of the bed sediment were taken at a number of locations within the study reach. <br />The bed surface (armor layer) was sampled with point counts of 100 or 200 particles following <br />the method described byWolman [1954]. Particles were sampled randomly within specific areas <br />of the channel, and measured at 1/2-phi intervals using a metal template (gravelometer). A <br />separate sample of the subsurface sediment (substrate) was obtained in order to determine the <br />bulk size distribution of the bed material. A total of 135 kg of sediment was collected in this <br />sample, with the largest rock weighing 10 kg, or 7% of the total sample weight. The coarse <br />fraction (>32 mm) of the subsurface sample was sieved in the field and the fine fraction (<32 <br />mm) was sieved in the laboratory, again at 1/2-phi intervals. A graphical plot of the grain size <br />distribution of this sample (Fig. 6) indicates that the substrate has a median grain size, D50,, of 30 <br />mm, and 17% is finer than sand (2 mm). The size distribution of this sample is very similar to <br />two other samples collected previously in the 15-mile reach (Pitlick et al., 1999). <br />100 <br /> <br /> Subsurface Sediment ° <br /> <br />80 <br /> -0 RM184 <br />m <br />S 60 -¦- RM 177 <br /> <br /> -?-- RM 176 <br />d <br />4 <br />0> <br />0 <br />CL <br />2 <br />0- <br /> <br />0 <br />0.01 0.1 1 10 100 1000 <br />Grain Size (mm) <br />Figure 6. Grain size distributions of subsurface sediment at 3 locations in the 15-mile reach. <br />15