Laserfiche WebLink
<br />78 GRAIN-SIZE EVOLUTION DURING FLOODS <br /> "T;-'r ,-- <br /> 0,16 <br /> X Above LeR 0.77 sand <br /> 1200 + fobove LeR [)..11 S~\ &. cla'j 0,14 <br /> . GranO'CynP-61 sand <br /> Il 0 Gralld Cyn P.61 sill & clay <br /> . Grand Cyn 0.77 sand 0.12 <br /> 1000 0 Grand Cyn 0.77 sill & clay ~ <br /> ~ A 122-Mile eddy 0.74 sand <br /> M /', 122-1,4118 eddy 0.74 Slit" clay 0.10 !C.. <br /> .S- P c <br /> . National Cyn p.eT sand ,Q <br /> Q) 800 0 Nat1Ol'la1CynP.61 sill & clay ~ <br /> E' 0,08 <br /> '" i E <br /> .J:: Q) <br /> 0 0 <br /> ,!Q nIt c <br /> 0 600 0,06 0 <br /> u <br /> 2 x 0.04 <br /> 400 <br /> 0 0.02 <br /> + 000 Oot>Q, 80 ~ <br /> 200 0.00 <br /> -1 1 2 3 4 5 7 +1 +2 +3 <br /> Day of Flood <br /> <br />Figure 3. (A) Hydrograph of {he 1996 controlled flood (as measured at the Grand Canyon gage) and spalially averaged. <br />depth-integraled sand and sill & clay concentrations measured at the Grand Canyon gage, I 22-mile eddy site. above <br />LCR gage. and !he National Canyon gage. Error bars are one slandard deviation. The travel-lime oflhe flood bel ween (he <br />various main-channel and eddy siles has been removed in (his figure such lhat lhe beginning of day one al each sile <br />corresponds [0 lhe lime of lhe first arrival of the Rood wave al each sileo Mean values and error bars for the 122-mile <br />eddy da.ta shown in this plOl, and in Figure 4b, supercede Ihose in Rubi,l and others (1998}. The above LCR gage and <br />Nalional Canyon gage dala are from KonieCl.ki and others [1997]. (8) Hydrograph of [he 1996 controlled flood (as <br />measured at the Grand Canyon gage)~ the spatially averaged, depth-integrated median grain sizes of suspended sand <br />measured at the Grand Canyon gage, 122-mile eddy sile, above LCR gage, and lhe Narional Canyon gage; and the <br />spatially averaged, median grain sizes of the sand, ~ilt, and clay on the bed measured at the Grand Canyon gage. The <br />samples collected with the D-77 samplers are finer lhan those collected wilh lhe P-61 samplers for the reason described <br />in (he text. <br /> <br />smooth trends through time, This reduced spatial and <br />temporal variahility suggests thai tracking the temporal <br />evolution of (he concentration and grain-size dislribulion of <br />sediment in suspension could be an effective 1001 for <br />monitoring the stale of the sediment supply in Marble and <br />Grand Canyons, <br /> <br />5.3. Mechanics of Suspended.Sedimef/t COllcentration <br />Decrease and Coarsening <br /> <br />Rub;" el ai, II998J interpreted the decrease In <br />suspended-sand concentration and the associated coars- <br />ening of the suspended sand during the 1996 conlrolled <br />flood to result from winnowing of the finesl sizes of sand <br />from the channel bed and preferential deposition of these <br /> <br />finest sizes of sand in eddies. In their interpretation, as the <br />finest sizes of sand were winnowed from the bed, the bed <br />coarsened, which caused lhe median size of sediment <br />supplied to deposits in the eddies 10 coarsen, On the reach <br />scale in any river, because of their lower settling velocilies, <br />the finer sizes of suspended sediment travel downstream at <br />progressively higher velocities than the coarser grain sizes. <br />Winnowing or the nnest grain sizes rrom the bed occurs <br />during mains tern floods in J system like the Colorado River <br />in Marble and Grand Canyons hecause of both Ihe upstream <br />supply limitation of sediment and the mismatch 1n the <br />liming of maximum tflbul;.lry sediment supply and <br />mainstcm sediment transport. B~cause there is a limited <br />upstream supply of sedllnent during each mainstem flood <br />event, and the finer grain Silt'S travel downstream al <br />