Boundary Layer

When a fluid flows over a solid surface such as a streambed, flow in the immediate vicinity of the boundary is retarded by the frictional resistance of the boundary. ^[1] This zone of retardation is called a boundary layer - the region of fluid flow next to the boundary across which the fluid velocity grades from that of the boundary, commonly zero, to that of the unaffected part of the flow above. ^[1]

Boundary layers may be comparatively thin or they may extend all the way to the free surface of a flow. ^[1] Flow within boundary layers may be laminar, turbulent, or may grade from laminar to turbulent. ^[1] The structure of the boundary layer - particularly the presence or absence of the viscous sublayer - is one of the most important factors controlling sediment transport, because it determines the local flow regime that grains on the bed actually experience.

The Viscous Sublayer

For smooth beds with turbulent flow in the main body of the fluid, there is a thin layer close to the bed boundary where molecular viscous forces dominate. ^[1] Molecular adhesion causes the fluid immediately at the boundary to remain stationary, and successive overlying layers slide relative to those beneath at a rate dependent upon fluid viscosity. ^[1] This layer is the viscous sublayer, or laminar sublayer. ^[1]

The viscous sublayer is technically not truly laminar - it is characterised by streaks of faster and slower moving fluid - but viscous forces dominate within it, and it behaves in a quasi-laminar way relative to the turbulent flow above. ^[1] The thickness of the viscous sublayer varies with the intensity of turbulence and with the bed roughness. On a smooth bed under moderate current velocities, the sublayer may be a fraction of a millimetre thick; it thins further as turbulence intensifies.

Most sediment transport takes place within boundary layers, and turbulent boundary flow is much more effective in eroding and transporting sediment than is laminar flow. ^[1] The presence or absence of a viscous sublayer may be an important factor in initiating grain movement - extremely small grains that lie entirely within the viscous sublayer may be difficult to move. ^[1]

Hydraulically Smooth vs. Hydraulically Rough Flow

The relationship between grain size and viscous sublayer thickness defines two contrasting flow conditions that have opposite effects on sediment erosion.

If sediment particles on a streambed are so small - mud to fine-sand size - that they lie within the viscous sublayer, near-bed flow is dominated by viscous forces and the flow is said to be hydraulically smooth. ^[1] Under hydraulically smooth conditions, even though the overlying flow is turbulent, grains on the bed are shielded from direct turbulent impingement by the sublayer. This partly explains why fine clay and silt particles can be surprisingly resistant to erosion: they are cohesive, and they lie in a zone of relatively calm, viscous flow.

If the grains are so large that they exceed the thickness of the viscous sublayer and thus protrude into the turbulent part of the flow, the flow is hydraulically rough. ^[1] Over a very rough or irregular bed such as coarse sand or gravel, the viscous sublayer is destroyed by these irregularities, which extend through the layer into the turbulent flow. ^[1]

The flow of fluid over a boundary is thus affected by the roughness of the boundary - obstacles on the bed generate eddies at the boundary, and the larger and more abundant the obstacles, the more turbulence is generated. ^[1] This locally generated turbulence near the bed increases bed shear stress, which increases the capacity of the flow to erode and entrain sediment - rougher beds, counterintuitively, promote more vigorous sediment transport than smooth beds at the same mean flow velocity.

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