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<br />Proceedings of the Seventh Federal Interagency Sedimentation Conference. March 25 (0 29. 2001. Reno. Nevada <br /> <br />. <br /> <br />WHAT REGULATES SUSPENDED-SEDIMENT TRANSPORT IN A GIVEN SETIING? <br />GRAIN SIZE OF BED SEDIMENT OR FLOW? <br /> <br />D. M. Rubin, Geologist, U.S. Geological Survey, Santa Cruz, California, and <br />D. J. Topping, Hydrologist, U.S. Geological Survey, Reston, Virginia <br /> <br />INTRODUCTION <br /> <br />Back2round and Durnose It might be argued that the key question to ask when beginning an investigation of a <br />natural sediment-transporting flow is whether transpon is limited mainly by flow strength or sediment supply, The <br />answer to this question detennines whether research should focus on the relation between flow strength and sediment <br />transport, the rate at which sediment of different grain sizes is supplied to the flow, or both. For a given setting, this <br />is the key question that must be answered prior to either designing a sediment-transport measurement program or <br />constructing a sediment-transport model or budget. This is also the key question to answer when calculating total <br />maximum daily loads (TMDLs) for sediment. <br /> <br />Rubin and Topping (in press) developed a technique to evaluate the importance of changes in flow strength relative <br />to changes in sediment supply in regulating the rate of sediment transport If that technique detennines that changes <br />in suspended-sediment transport in a river (or on a continental shelf) are regulated mainly by flow, then a measure of <br />the flow strength (e,g,. the boundary sbear stress, shear velocity, or discharge ofwater) may be an adequate predictor <br />of sediment transport, In contrast, if changes in sediment transport are regulated mainly by changes in bed,sediment <br />grain size, then measurements of sediment input wi11 be a more accurate predictor of sediment transport than any <br />measure of flow strength. <br /> <br />Definitions <br /> <br />. <br /> <br />Flow-regulated transport and bed-sediment-regulated transport In a system where flow-induced changes in <br />transport are large relative to bed-sediment grain-size-induced changes in transport, transport is defined to be flow~ <br />regulated. At the other extreme, where changes in bed-sediment grain size are the dominant factor regulating <br />sediment transport, transport is defined to be grain-size-regulated. <br /> <br />Suspended sediment, suspended bed material, and wash load Suspended sediment includes two kinds of load: <br />suspended bed material and wash load. In this paper, the tenn suspended sediment is applied to suspended bed <br />material (thus excluding wash load). Suspended bed material includes those grain sizes that occur in substantial <br />amounts in the bed. whereas wash load is finer than the bed sediment {Einstein and Chien, 1953). A.nother <br />approach---compatible with (Einstein and Chien, 1953)--might be to base definitions on the concentration gradient; <br />wash load would include t hose sizes having a concentration that remains constant with height above the bed. <br /> <br />APPROACH <br /> <br />Flow-rel!ulated tranSDort Laboratory flumes that recirculate both sediment and water are ideal for studying no\\'- <br />regulated transport, because grain size on the bed (Db) remains nearly constant. Under such conditions, increases in <br />shear velocity (u.) from one experiment to another cause increases 111 concentration (C), grain size of suspended <br />sediment (DJ), and sediment transport (q). D, and C increase because stronger flows are able to suspend coarser <br />sediment and more sediment, and q increases for two reasons: concentrations are higher and more water is <br />discharged. Because C and D, increase with shear velocity, they are positively correlated (Fig. la). <br /> <br />Bed~sediment l!rain~size-rel!ulated transport Grain.size-regulated transport can be studied by comparing dala <br />collected with differing bed-sediment grain sizes ror a narrow range or U.. Under such conditions, coarsening or the <br />bed sediment causes concentration to decrease, while causing the median diameter of the suspended sediment 10 <br />increase. As a result of these opposite responses to changes in Dh. C is inversely related to D, where transport is <br />grain.size regulated (Fig, I b), <br /> <br />. <br /> <br />I, t99 <br />