9605 - Laserfiche WebLink

Home Browse Search

high scarp, which drops to the lower flood plain. Natural levees may also be present. Average Boundary And Critical Shear Stress: Average boundary shear stress in the detailed study site was calculated for discharges of3,700 and 5,700 cfs, which are the discharges which just inundate the flood plain and low terrace, respectively, at twelve of the measured cross-sections. At 3,700 cfs, the average shear stress exerted on the bed ranges from 5.5 to 21.6 N/m2 (Table A=3) At 160 m3/s (5700 ft3/S), average boundary shear stresses range from 13.1 to 27.3 N/m2. For these high discharges, we assumed that the slope of the water's surface was 0.0014 at each cross section. Thus, mean depth (i.e., hydraulic radius, R) is the only variable that is different in the calculations, and shear stresses are highest where mean depth was greatest. The critical shear stress necessary to entrain the median particle size was calculated for the six pebble counts made at three gravel bars in the channel within the study reach (Table A-4). Median particle size (D50) ranged from 20 to 90 mm. Particle sizes were coarsest in a small riffle located at the extreme downstream end of the reach, and were finest near the center of the reach (Figure A-15) Average boundary shear stress generated by the bankfull discharge of 3,700 cfs was sufficient to mobilize sediment similar to the sizes of particles found on three of the six representative pebble count locations in the study site when assuming t* 50 = 0.034 (Figure A-16). The mobilized sites are located on a cobble bar in the riffle near cross sections 5 and 6, and had D50 values of 20, 24, and 32 mm. The location with the D50 of 32 mm was predicted to 21