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

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 tau(ci)* 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 tau(ci)* have been quantified by considering the relative submergence ratio R-h/D-84, 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 q(ci)* 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 (C) 2006 John Wiley & Sons, Ltd.