Bed loadmotion and grain sorting in ameandering stream

A three-dimensional moment analysis defines both particle mobility and the average orientation angle of moving bedload particles in meander bends. Accordingly, under identical bed shear stress and near-bed streamline orientation angle on a side slope, bedload particles of different sizes move in different directions. This sorting mechanism has been verified in the sharp meander bends of Fall River, Colorado. The extensive field data set includes near-bed sediment transport measurements by size fractions from 0.125 to 32 mm using a Helley-Smith sampler. The field measurements in two meander bends corroborate the theoretical model: particles finer than d50 preferentiallymove up the point bar and particles coarser than d50 move toward the thalweg. The measured deviation angle between the mean trajectory of 0.125 mm and 32 mm particles reaches 20 ˚ near the bend apex. RÉSUMÉ Une analyse tri-dimensionnelle des moments de force sur une particule définit la mobilité et l’angle de déplacement des particules en charriage dans une rivière à méandres. Sous une contraine de cisaillement donnée, les particules sur un plan incliné se déplacent dans différentes directions selon leu