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<br />T <br /> <br />. <br /> <br />-.. <br /> <br />Enew Losses in Steep Tributary Streams at Flows Near <br />The Threshold of Bedload Entrainment <br /> <br />Matthew D. O'Connorl <br /> <br />Abstract <br /> <br />Mountain tributary streams with cbannel gradients greater than 0.05 <br />have extremely rougb beds with complex morpbology. Flow patterns sbaped <br />by channel morphology include abundant eddys, jumps, and small faUs. <br />Cobbles, boulders, and woody debris typically protrude through mucb of th, <br />vertical flow profile at many locations in the channel. Bank heigbl is bijh <br />relative to channel width, These couditions are expected to cause substantial <br />energy losses. <br /> <br />Energy losses can be estimated by partitioning the total shear slress" <br /> <br />I /I <br />"0"'''0+'1'0 <br /> <br />wbere T. is tbe depth-slope product formulation of tbe total shear stress. ': <br />is the portion of 'To expended on grains on tbe stream bed, and 1'0' is tbe <br />portion of 1'0 expended on bed forms and banks. If To' can be measured, thtn <br />To' can be calculated as a difference. <br /> <br />The tbeoretical thresbold of bedload entrainment, T:, was determined <br />from field observations of lhe median particle diameter and a value of critic. <br />Shields stress appropriate for a beterogeneous gravel bed, Flow deptb at 01 <br />near the threshold of entrainment was observed; bed slope was then used ,.' <br />calculate tbe total bed sbear stress according to the depth-slope formulation <br />of To' Finally, 'To' was calculated by subtraction. <br /> <br />The required field observations of stream stage, local cbann< <br />geometry, grain size distribution of sediment, and bedload tracelS were made <br /> <br />lCoUegc of Forest Resources, University of Washington,. Seattle, WA 9819S. <br /> <br />778 <br /> <br />-a- <br /> <br />. <br /> <br />ENERGY LOSSES-STREAMS <br /> <br />. <br /> <br />779 <br /> <br />in lh~ee stream cbannels. As an independent cbeck on th . <br />deSCribed above, near-bed flow velocity was d ' e tecbmque <br />. . measure )0 ODe chaonel to <br />nleasure Tc accordmg to a tecbnique wbicb scales the friction he 'ghl' th <br />l.aw of the Wall to d84 of the bed grain size distribution. Near-m:d v m , e <br />measurements can tben be used to calculate the sbear I 'ty fro thelOClty <br />of the Wall. ve OC1 m e Law <br /> <br />The resulting values of T .. were near 20 <br />estimated to be near 80 per~nt of Th perce I n! of TV) hence To' was <br />. _ , T., e ve oc.ty-based method of <br />eSllmatmg T. agreed reasonably well with the partJ'l1' , t h ' <br />onmg ec Dlque. <br /> <br />~ <br />