Laserfiche WebLink
<br />60 m reach extending upstream from the cable. <br /> <br />Determination of D50 Particle Sizes <br /> <br />The Dso, or median bed particle size, was determined by performing pebble <br /> <br />counts on representative exposed gravel deposits within the study reach. 6 Pebble counts <br /> <br />were performed on 3 exposed or slightly submerged cobble bars during April 1997. For <br /> <br />each pebble count, the intermediate axis of approximately 100 random particles was <br /> <br />measured and recorded. Pebble counts were conducted on a straight line which crossed <br /> <br />the cobble bar perpendicular to the direction of flow. <br /> <br />Calculation of Average Boundary and Critical Shear Stresses <br /> <br />The stage elevations and discharges necessary to inundate the three distinct <br /> <br />geomorphic surfaces identified adjacent to the channel were used to calculate the average <br /> <br />boundary shear stress at 12 of the 14 established cross-sections. We then compared the <br /> <br />average shear stresses applied by these discharges, with the critical shear stress necessary <br /> <br />to entrain the Dso particle sizes from pebble count locations in the reach. Discharges <br /> <br />where boundary shear stresses exceed the critical shear stress were considered sufficient <br /> <br />to mobilize sedimeht. <br /> <br />We used the Duboys equation (Duboys, 1879) to calculate the average shear <br /> <br />stresses applied to the boundaries of the channel. Average boundary shear stress is <br /> <br />calculated as: <br /> <br />to = gRS (3) <br />where to is the average boundary shear stress in N/m2, g is the specific weight of water at <br />16 <br />