During experiments on sediment transport and resistance to flow with a uniform 0.33-mm sand, data were recorded on the movement of individual rocks having intermediate diameters from about 0.1 to 0.5 foot. The experiments were conducted in a flume 2 feet wide by 60 feet long and for most runs, depth was held constant at 0.5 foot.
The experiments showed that rocks on the sand bed moved downstream consistently only if the flow was in the upper regime—that is, only if the bed forms were plane bed, standing waves, or antidunes. The rocks moved at velocities that were approximately half the average velocity of the water. On all bed forms in the lower flow regime (ripples, ripples superimposed upon dunes, and dunes), the rocks moved upstream and down into the bed. That is, the rocks moved into a scour pocket that formed at the upstream side of the rock. The movement upstream and down into the bed is limited by and approximately equal to the distance below the original rock position of the minimum bed elevation plus approximately half the rock diameter.
The data indicate that cross-bedded sand deposits formed by the ripple or dune phases of transport would contain few, if any, pebbles or cobbles. Because the flow, in at least the downstream reaches, of most rivers is in the lower regime, the upstream movement and scour into the bed demonstrated in these experiments is an important factor in the sorting process.