Bed load motion and grain sorting in a meandering 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 d(50) preferentially move up the point bar and particles coarser than d(50) move toward the thalweg. The measured deviation angle between the mean trajectory of 0.125 mm and 32 mm particles reaches 20degrees near the bend apex.