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

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.