Initial experiments were conducted in a flat‐bottomed flume with pebbles (median diameter slightly below 20 mm), with a view to test the validity of the methods which permit differentiation between shear stress due to grain resistance (τ″) and that due to bedform resistance (τ″). It is clearly confirmed that τ′ represents the total shear stress (τ). The method of Laursen (1958) is appropriate for evaluation of grain shear stress in the case of pebbly beds. Indeed the ratio τ′ is close to 1, except when the dimensionless flow depth (d/D50) is less than 4. A close relationship exists between the grain shear stress and the shear calculated from friction velocities. A second series of experiments was then carried out in the same flume, after creating differences in bedform (alternating riffles and pools). The ratio τ′/τ produced by Laursen's method represents on average 0.40. The relationship between these grain shear stresses and the shear stresses calculated from shear velocities is relatively good here as well, and is similar in presentation to that established in the case of flat bed. Finally, as confirmation of the adequacy of Laursen's method under the same initial conditions of bottom slope and discharge, the grain shear stress for an undulating bed is very close to the total shear stress of a flat bed.
Earth Surface Processes and Landforms – Wiley
Published: Sep 1, 1989
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