Hydro-morphometric parameters controlling travel distance of pebbles and cobbles in three gravel bed streams

This study investigates the parameters controlling bedload distances in several gravel-bed rivers of a medium mountain (Morvan, France). Using PIT tags introduced into pebbles and cobbles, an examination of the relationship between several hydro-morphometric variables and travel distance was first undertaken with bivariate analysis. Bedload distances of our 6 study sites show important differences and this discrepancy cannot fully be understood considering only the stream power for the peak discharge, especially in multi-peak surveys such as ours. Stream impulse, a variable combining flow intensity and flow competence duration, has a stronger correlation with bedload distances. The analysis also indicates a varying influence of relative grain size, relative flow depth, bed slope and width/depth ratio. Among all these parameters, in order of importance, the relative grain size, the slope, and the stream impulse, emerge as the most significant explanatory variables from a multivariate analysis. The role of the relative grain size on bedload transport underlines the importance of grain size sorting and microtopography in plane-bed rivers. The influence of the slope is ambivalent: favoring bedload distances under certain circumstances and lowering them under others. The direction of the influence of the slope seems to depend on its combination with other morpho-sedimentary or hydraulical parameters. Finally, we propose a single equation for bedload distance prediction that predicts travel distance rather well.