Variation in the reference Shields stress for bed load transport in gravel-bed streams and rivers

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Abstract

The present study examines variations in the reference shear stress for bed load transport (τr) using coupled measurements of flow and bed load transport in 45 gravel-bed streams and rivers. The study streams encompass a wide range in bank-full discharge (1-2600 m3/s), average channel gradient (0.0003-0.05), and median surface grain size (0.027-0.21 m). A bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate W* versus the reach-average dimensionless shear stress τ*. The reference dimensionless shear stress τr* was then estimated by selecting the value of τ* corresponding to a reference transport rate of W* = 0.002. The results indicate that the discharge corresponding to τ r* averages 67% of the bank-full discharge, with the variation independent of reach-scale morphologic and sediment properties. However, values of τr* increase systematically with average channel gradient, ranging from 0.025-0.035 at sites with slopes of 0.001-0.006 to values greater than 0.10 at sites with slopes greater than 0.02. A corresponding relation for the bank-full dimensionless shear stress τbf*, formulated with data from 159 sites in North America and England, mirrors the relation between τr* and channel gradient, suggesting that the bank-full channel geometry of gravel- and cobble-bedded streams is adjusted to a relatively constant excess shear stress, τbf* - τr*, across a wide range of slopes. Copyright 2005 by the American Geophysical Union.

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Mueller, E. R., Pitlick, J., & Nelson, J. M. (2005). Variation in the reference Shields stress for bed load transport in gravel-bed streams and rivers. Water Resources Research, 41(4), 1–10. https://doi.org/10.1029/2004WR003692

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