Fluvial Sediment Transport

Fluvial sediment transport is the movement of sediment by flowing water in rivers, the process that erodes, carries, and deposits material and shapes channels and floodplains.

Definition

Fluvial sediment transport is the entrainment, conveyance, and deposition of mineral and organic particles by river flow, occurring as bedload moving along the bed and suspended load carried within the flow.

Scope

This topic covers the entrainment and transport of sediment as bedload and suspended load, the threshold conditions for particle motion, sediment-rating relationships, and the link between sediment transport and channel form. It treats the sediment dynamics of rivers; the hydraulics of the water itself is covered in a separate topic.

Core questions

• Under what conditions does flowing water entrain sediment?
• How do bedload and suspended load differ, and how are they estimated?
• How does sediment transport relate to discharge?
• How does sediment transport shape river channels and floodplains?

Key concepts

• Bedload and suspended load
• Shear stress and threshold of motion
• Shields criterion
• Sediment rating curve
• Erosion, transport, deposition
• Channel adjustment

Key theories

Threshold of motion (Shields criterion)

Shields related the onset of sediment motion to a dimensionless shear stress, providing the standard criterion for whether a given flow will entrain particles of a given size.

Sediment transport and channel form

The interaction of water and sediment transport governs erosion, deposition, and the adjustment of channel geometry, the basis of fluvial geomorphology linking flow, sediment, and form.

Mechanisms

Flowing water exerts a shear stress on the bed; when it exceeds a threshold that increases with particle size, grains begin to move, rolling and sliding as bedload or, for finer particles and stronger flows, lifted into suspension. The total load thus depends jointly on flow strength and on the size and supply of available sediment, and where transport capacity falls, sediment is deposited.

Clinical relevance

Sediment transport controls reservoir sedimentation, channel and bank stability, the design of bridges and intakes against scour, the morphology and habitat of rivers, and the delivery of sediment and associated contaminants to estuaries and coasts.

History

Quantitative sediment transport began with Shields's 1936 threshold criterion and parallel bedload formulas; the 1964 synthesis by Leopold, Wolman, and Miller integrated sediment transport with channel form to establish modern fluvial geomorphology.

Key figures

• Luna B. Leopold
• M. Gordon Wolman
• Albert Shields

Seminal works

• leopold1964
• shields1936
• dingman2015

Frequently asked questions

What is the difference between bedload and suspended load?

Bedload is the coarser sediment that rolls, slides, or bounces along the bed, while suspended load is the finer sediment held aloft within the turbulent flow; together they make up the total sediment load a river carries.

Why do most rivers carry the most sediment during floods?

Sediment transport rises steeply with flow strength, so the large shear stresses during high flows entrain far more material; a large share of a river's long-term sediment load is moved during relatively infrequent high-flow events.