Rounding and reduction in size and weight of coarse particles occur over a significantly shorter distance in a stream as compared to the “distance” of travel required to produce a similar result in a tumbling barrel or flume. This suggests that either the known processes of wear are more effective in streams than in the laboratory or that other processes are at work.
Investigations of riprap failure by hydraulic engineers provide data on the forces acting on coarse particles and particle behavior prior to downstream movement. The experiments demonstrate that lift forces acting on a submerged cobble are large and vary significantly with time. When this effect of lift is combined with drag forces, a particle vibrates in place without downstream movement. With increased velocity, the particle lifts off the stream bed and saltates downstream to a new position of temporary stability. During high-velocity flow, abrasion of cobbles in place and the chipping and fracturing during saltation can be significant factors causing rounding and size reduction of coarse sediments in rivers.