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<br />Proceedings of the Seventh Federal/nteragency Sedimentation Conference. March 15 to 19.100/. Reno. Nevada
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<br />Ouantifvine: the relative imDortance of ~rain-size rel!ulation and flow ret!ulation of susDended-sediment
<br />transoort If transport in all flows were regulated purely by changes in flow or by changes in bed sediment, then the
<br />sign of ACIAD! would be a definitive distinguishing characteristic (positive for flow-regulated transport and negative
<br />for grain-size-regulated transport). Because all intennediate conditions are possible, however, definitive evaluation
<br />is more complicated. Rubin and Topping (in press) followed the approach discussed below.
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<br />Most models ofsuspended-sediment transport express transport as a function ofsom e combination of now properties
<br />(such as U', water slope, and water depth) and bed-sediment grain-size properties (such as median diameter Db and
<br />standard deviation), The simplest approach to quantifying the relative importance of a single change in both flow
<br />and a change in bed-sediment texture is to evaluate their individual impacts on the transport rate. for such a change,
<br />this measure a. can be defined as
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<br />a = ~q<jlow"grain-size,) (q<jlow"grain-size,ll
<br />log[q<jlow"grain-size,) / q(jlow"grain-size,)]
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<br />(I)
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<br />where q gives the sedimenHransport rate as a function of both flow and bed-sediment grain size; subscripts refer to
<br />conditions at two times. The numerator quantifies the extent to which a change in transport rate is influenced by the
<br />change in bed.sediment grain size (holding flow constant), while the denominator quantifies the effect of the change
<br />in flow atone. a is a dimensionless number that describes how much of a change in transport is caused by a change in
<br />bed sediment relative to a change in flow. Where sediment transport is regulated primarily by changes in bed-
<br />sediment grain size, I ex 1 >> I; where transport is regulated primarily _by changes in flow, 10.1 << I; and where
<br />transport is regulated equally by changes in flow and bed sedim ent, I a I = I,
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<br />To evaluate the functional relations expressed in eq (1), Rubin and Topping (in press) used the following approach:
<br />(I) A numerical model based on McLean (1992) was used to calculate concentration of suspended sediment at SOO
<br />logarithmically spaced elevations above the bed, for l29 size classes of bed sediment binned in 1/16 $ lncrements.
<br />The algorithm was used to predict mean concentration and median grain diameter for more than 1000 combinations
<br />of flow variables. including II median grain diameters (0.03 to 1.2 mm), 20 values of u. (from below threshold of
<br />transport to upper plane,bed regime), 3 depths (10, 100, and 1000 em), and both narrow and wide log,normal bed,
<br />sediment grain-size distributions. The computations were repeated for a more complex algorithm that included
<br />development of dunes.
<br />(2) Concentrati on and grain size of suspended sediment were a veraged through the water column.
<br />(3) The computed results were then approximated by equations expressing the dependent variables (C and Ds) as
<br />power functions of the independent variables (u. and Db).
<br />(4) The equations derived in step (3) were rearranged, so that the independent variables u. and Dh were expressed in
<br />terms of the more easily observed dependent variables C and DJ. This sequence of steps led to
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<br />logt!.C
<br />+J
<br />logt;D.
<br />logt;C
<br />--K
<br />logt;D,
<br />where the values of J and K for various models listed in step (I) are given in Table I.
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<br />a=L:J
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<br />(2)
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<br />Table. I Values of J, K, L, and M in equations (2 and 5), determined by fitting power la ws to computational results.
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<br />Model J K l M
<br />Without dunes; sigma Dhi=O,55 3,5 ,2,S O,IS 1.0
<br />Without dunes; si~ma ohi=I,4 3.5 ,1,5 0.4 0,5
<br />With dunes; sigm a Dhi-O.5S 5,0 ,3,0 0,2 0,7
<br />With dunes; sigma ohi= 1.4 3,5 ,1.5 OJ 0,5
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