Depending on grain size, river sediments fall into two main groups: wash load (grains that never fall out of suspension) and bed load (sediment that rests on the riverbed). Bed load sediment, similarly, can be either resting on the riverbed or temporarily in suspension, at which point it is known as suspended load.
Whether sand dunes that carpet the beds of some rivers are made primarily from bed load, which migrates by bouncing along the bottom, or suspended load, which moves by getting picked up and carried downstream, affects the shape and size of the dune. In turn, the size, shape, formation, erosion, and migration of riverbed sand dunes influence the river’s speed, the amount of turbulent mixing, and the migration of the river channel.
Measuring the role of suspended and bed load sediments in dune migration has traditionally been limited by the lack of a way to measure the concentrations of both types of sediment simultaneously. A new experimental apparatus known as an acoustic concentration and velocity profiler (ACVP) solves this problem.
Using an ACVP, which combines acoustic backscatter and acoustic Doppler sensors, Naqshband et al. studied the roles of different types of river sediments on dune formation and migration. With this instrument, the researchers found that bouncing bed load outpaces suspended load as the dominant mechanism for sand dune migration on the river’s bottom. (Journal of Geophysical Research: Earth Surface, doi:10.1002/2013JF003043, 2014)
—Colin Schultz, Writer