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<br />...-x--.. Above LCR gage O.715U5p sana <br />. Grand CyIl gogo P-61 SU$l) sand <br />._~...... Grand CyIl gage o.n susp sand <br />~ 122-Mile eddy 0-74 SU5P sand <br />. Nabooal Cyn gag& P-6' ~$P sano <br />--0--- Grand CyIl goge sand, sIN & clay on bed <br /> <br />~ :;.u__'_~..,;. <br />..-............ . ..... <br />~ ........ ..rl <br />......... <br />x. .......... ~ <br />~ <br /> <br /> 1400 <br /> B <br /> 1200 <br />~ 1000 <br />M <br />E <br />'" <br />e> 800 <br />'" <br />.c: <br />u <br />'" <br />C5 600 <br /> <br /> <br />400 <br /> <br />200 <br /> <br />-1 <br /> <br />2 <br /> <br />3 <br /> <br />4 <br /> <br />Day of Flood <br /> <br /> TOPPING ET AL. 79 <br />0,50 <br />0.45 <br />0.40 <br />0.35 E <br /> E <br />0.30 0 <br />"' <br /> 0 <br />0,25 <br />0.20 <br />0,15 <br />0.10 <br />+1 +2 +3 <br /> <br /> <br /> <br />5 <br /> <br />6 <br /> <br />7 <br /> <br />Figure 3 (continued) <br /> <br />progressively higher velocities (and are deposited first in <br />eddies), the concentration of the finest grain sizes will <br />change the fastest during mainstem "000 evenls, <br />Furthennore, as the concentration of lhe finer grain sizes in <br />suspension decreases below the concentration of these sizes <br />that can be supported by the grain-size distribution of the <br />bed sediment, a mass transfer of these sizes from the bed to <br />the suspended sediment will occur (i,e" the finest sizes will <br />be winnowed from the bed), and the bed will coarsen, <br />The physical coupling between the coarsening of the bed <br />and the suspended sediment in the channel can be explained <br />in the following manner. For a sandy bed. lhe overall <br />equilibrium concentration of suspended sediment near the <br />bed scales approximately with lhe shear stress in excess of <br />the critical value for initiation of motion for the median size <br />comprising the bed, whereas the equilibrium concentration <br />of each size fraction scales with the fraction of the bed <br />composed oflhat size class [e,g" Smilh and McLean. 1977; <br />McLean, 1992; Topping, 1997], Thus, lhe near-bed lotal <br />concenlration of suspended sediment will decrease as the <br />median grain size of (he hed increases. but, as observed in <br />hOlh the suspended sediment and in the deposits of lhe 1996 <br />controlled flood. lhe concentration of the coarser fractions <br />will increase as a resulL of higher representation in the hed <br /> <br />material. The mathematical representation of this hypothe- <br />sized physical balance is: <br /> <br />(cm),. = As im cb Y [('sf - ''')/'e,] <br /> <br />(I) <br /> <br />where (cm),. = the near-bed. lime-averaged concentration <br />of suspended sediment in size-class m, As = the fractional <br />area of the bed that is covered by fine sediment (i,e" sand, <br />silt, and clay), im = the volume fraction of sediment size- <br />class m in thaI ponion of the bed covered by fine sediment, <br />cb;;:: 0.65 is the volumetric concentration of fine sediment in <br />that portion of the bed covered by fine sediment, Y= 0,0045 <br />[P. Wiber;g. University of Virginia. pers, comm. 1989), 'sf is <br />the skin-friction component of (he boundary shear stress, <br />and 'e' is the critical shear stress of the median size (D50) of <br />the fine sediment on the bed, Toppillg [1997) showed lhat <br />equation 1, used in combination with Ihe suspended- <br />sediment theory of Smilh alld McLean [1977), is a good <br />predictor of bOlh the measured depth-integrated concen- <br />tration and grain-size distribution of suspended sediment in <br />the "ume experiments of Kelllledl' [1961] and Guy el ai, <br />[1966], and is also a good prediclor of both the measured <br />near-bed concenlration and grain-size distribution of <br />suspended !:.edimenl in the Rio Puereo data of Nordin <br />[ 1963), <br />