When does bedload transport begin in steep boulder-bed streams?

Authors

  • Mario Aristide Lenzi,

    Corresponding author
    1. Department of Land and Agroforest Environments, University of Padova, Agripolis, Viale dell'Università 16, 35020 Legnaro (Padova), Italy
    • Department of Land and Agro-Forest Environments, University of Padova, Agripolis, Viale dell'Università 16, 35020 Legnaro (PD), Italy.
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  • Luca Mao,

    1. Department of Land and Agroforest Environments, University of Padova, Agripolis, Viale dell'Università 16, 35020 Legnaro (Padova), Italy
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  • Francesco Comiti

    1. Department of Land and Agroforest Environments, University of Padova, Agripolis, Viale dell'Università 16, 35020 Legnaro (Padova), Italy
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Abstract

The paper presents the results of field measurements of critical conditions for bedload motion conducted in the Rio Cordon, a steep boulder-bed stream in the Italian Alps, under conditions of high Reynolds numbers and low relative submergence poorly explored before. Two methods have been used to determine threshold of motion: the displacement of marked clasts and the flow competence approach, which uses the largest grain size diameter transported by each flood event.

The high values of equation image confirm the great relevance of non-bedload effective shear stresses in step–pool streams given by the additional form drag associated with this morphology. Relative submergence effects on the dimensionless critical shear stress equation image have been quantified by considering the relative submergence ratio Rh/D84, and the major effect of relative size on the mobility of each particle in steep, widely graded bed mixtures has been evaluated. Finally, the dimensionless critical unit discharge equation image has also been adopted in the regression equations as the critical hydraulic parameter, because it may represent an easier parameter to use than the critical shear stress for steep, rough mountain rivers. Copyright © 2006 John Wiley & Sons, Ltd.

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