E8 Sediment Transport

Topic

Sediment transport is the movement of solid particles—sand, gravel, silt, and clay—driven by water or wind; this process shapes the majority of the Earth's landforms across both human and geological timescales.

In rivers, sediment moves in two distinct ways: as bed load, consisting of particles that roll or bounce along the riverbed itself, and as suspended load, consisting of particles kept suspended in the water by flow turbulence. In both cases, sediment movement begins only when the shear stress exerted by the flow on the bed exceeds a critical threshold—known as the critical Shields stress—which depends on both the size and density of the grains forming the bed.

The Shields criterion precisely establishes that the onset of motion occurs when a dimensionless number specific to each grain exceeds a critical value of just 0.047 for non-cohesive grains submerged in water.

Bed-load transport follows empirical relationships, such as that of Meyer-Peter and Müller, whereby the sediment flux moving along the bed scales with the excess shear stress—that is, the amount by which the stress exceeds the aforementioned critical Shields stress.

Suspended load, by contrast, follows the Rouse profile: its concentration decreases with height above the bed according to a power law. It is the Rouse number—the ratio of the grain's settling velocity to the product of the von Kármán constant and the flow's friction velocity—that determines the rate at which this concentration decreases with height.

Aeolian sand transport is, in many respects, analogous to this fluvial transport: saltation is the dominant mechanism, with grains launched into the air by the impact of other grains, tracing ballistic trajectories before falling back down; The mobilization threshold for dry sand—of the same type previously described in the context of wind erosion—is reached with wind speeds of five to six meters per second, measured at a height of one meter above the ground.

Dunes are the depositional pattern formed by this mobilized sand, ranging in length from barely one meter to a thousand meters; the transport capacity of this entire system scales between the cube and the fourth power of the flow's friction velocity, meaning that high-energy extreme events—however infrequent—ultimately dominate the long-term sediment budget of any river or dune field. Thus, from the solid particles with which this document began to the extreme events with which it concludes—and encompassing the Shields critical stress, the Rouse profile, and the aeolian mobilization threshold—sediment transport reveals that the landscape we see today is, at its core, the accumulated imprint of countless grains that once overcame their own threshold of motion.

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