Fluid flow,
sediment transport
Modes of sediment transport
• Fluid transport
• Eolian (air)
• Fluvial (water)
• Glacial (ice)
• Dry mass wasting
• Gravity driven; non-fluid assisted
• Rockfalls (talus falls)
• Rockslides (avalanches)
• Wet mass wasting
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(sediment gravity flows)
Gravity driven; fluid assisted
Grain flows
Mud flows
Debris flows
Some slumps
Fluid flows vary in scale by orders of magnitude
• Characterized by depth, slope, timescale, fluid properties
e.g. Surface runoff
Riverine flows
Glaciers
Dust storms
• Dimensional scaling provides a means of comparing flows with
differing parameters to test for dynamic similarity
• Key measures for sediment transport: Reynolds and Froude
numbers
Laminar vs.
Turbulent flow
Transition
depends on:
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Flow velocity
Flow depth
Fluid viscosity
Fluid density
Characterized by the dimension-less Reynolds number
Reynolds number:
Re = Ud/
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Relates fluid inertial forces to viscous forces
Numerator measures fluid mass flux
Denominator measures fluid resistance to deformation
Laminar flow: Re < 2000
Turbulent flow: Re > 2000
Subcritical vs.
supercritical flow
Transition depends on:
• Flow velocity
• Wave velocity
• Characterized by the dimensionless Froude number
Froude number:
Fr = U/√(gd)
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Relates mean flow velocity to gravitational forces on flow
Numerator measures flow velocity
Denominator measures wave velocity
Subcritical (streaming, tranquil) flow: Fr < 1; waves propagate
upstream
• Supercritical (shooting, rapid) flow: Fr > 1; waves break
upstream
Why should a sedimentologist care?
• Turbulent flow can transport clasts more easily than laminar
flow
• Froude number transitions (hydraulic jumps) are associated with
breaking waves and thus erosion
• Specific bedforms are associated with different flow regimes
Bedforms and associated flow regimes
Riverine sediment transport
• Bed load - Coarse
• Saltation - Intermediate
• Suspended load - fine
• Wash load - finest
Forces acting on clasts
• Net Fluid Force
FFluid = FLift + FDrag
• Movement when FFluid > FGravity
• Lift = (Bernoulli) force leads to saltation and/or suspension
• Drag dominates in bed load transport.
Drag Coefficient, Cd
Hjulstrom Plot and Shield’s Diagram
Sediment erosion, transport, and
deposition are a function of
• Flow velocity
• Grain size
The Shields Diagram
 Accounts for the influence of more variables on initiation of
sediment transport.
 X axis is a measure of turbulence felt by the particle.
 Y axis is a measure of the critical boundary stress needed to
move the particle
What goes up must come down...
• Clasts follow one of two similar relationships when settling
• Impact law (turbulent wake)
particle Re > 0.5;
dg > 0.08 mm
(near silt/v.f. sand boundary)
high concentration
• Stokes settling (laminar wake)
particle Re < 0.5;
dg < 0.08 mm
low concentration
Impact Law:
ws = [4/3(1/Cd)((s-/)gd]1/2
• Predicts terminal velocity of larger clasts that generate turbulent
wakes
• Achieved when force of gravity is balanced by fluid drag on the
clasts following an initial period of acceleration
• Assumes spherical clast shape
• Applies for higher clast concentrations than stokes settling
Stokes Law:
ws = 1/18(1/)(s-)gd2
• Special case of the Impact law
• Predicts terminal velocity of smaller clasts that generate laminar
wakes
• Assumes spherical clasts
• Low particle concentration
• Low particle Reynolds number
• Obtained from the Impact law by noting that Cd ≈ 24/Re
Dry MassWasting
Examples:
• Rockfalls
(talus falls)
• Rockslides
(landslides; avalanches)
Eolian transport of dust from
the Sahara to the Atlantic Ocean.
Classes of wet mass wasting
Grain flows
• Begin when angle of repose is exceeded
• Lubricated by air
• Propelled by grain-grain impacts
Mud and debris flows
• Occur in steep landscapes
• Fines only (mud)
• Large/huge
clasts (debris)
Inverse grading in debris flow deposits
Turbidity Currents
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Sediment water mixtures
Lacustrine or Marine
Graded bedding
Fining upward sequence