Lecture 3
Sediment transport
Processes of transport
(And a few examples)
Transport in fluids
• By settling: Stokes Law
Transport in fluids
• By settling: Stokes Law
2
U = r gDr/18m
U = settling velocity
r = radius of particle
g = gravitational acceleration
Dr = difference in density
m = viscosity
Transport in fluids
• By settling: Stokes Law
2
U = r gDr/18m
What will fall faster?
Boulders or sand grains?
Transport in fluids
• By settling: Stokes Law
2
U = r gDr/18m
What will fall faster?
Heavier grains or lighter grains?
Transport in fluids
• By settling: Stokes Law
2
U = r gDr/18m
What will fall faster?
Grains in clear water or grains in turbid water?
Transport in fluids
• By settling: Stokes Law
2
U = r gDr/18m
What will fall faster?
Clasts on Earth or clasts on Mars?
The effect
of
turbulence
Multiple
grains also
slow
settling,
because
they
increase
the
effective
viscosity
Transport in fluids
• By settling: Stokes Law
• By rolling, jumping and suspension
Whether or not movement occurs is a function of ...
…density, mass, shape, size, flow velocity, and turbulence.
Consider, for example, the influence of flow velocity
Shear of the fluid near
the bed produces lift.
If the lift forces are
greater than the
gravitational forces on
the grain, the grain
moves off the bed.
Fine sand
~ 1 cm/sec
Coarse sand
Medium pebbles
~ 2 cm/sec
~ 8 cm/sec
This only applies to silt-sized and larger particles.
Why?
Rolling, saltation and suspension.
Bedload, suspended load.
Transport environments
• Gravity-dominated
– Rock falls, taluses
Transport environments
• Gravity-dominated
– Rock falls, taluses
• Water-dominated
– Rivers, lakes, oceans, estuaries
Transport environments
• Gravity-dominated
– Rock falls, taluses
• Water-dominated
• Ice-dominated
Transport environments
• Gravity-dominated
– Rock falls, taluses
• Water-dominated
• Ice-dominated
• Atmosphere-dominated
An example from the
Mono Craters
Pyroclastic fall deposits
An example from the
Mono Craters
Near-source (proximal)
With Stokes Law in mind,
how might one explain these
types of grading?
An example from the
Mono Craters
Far from source (distal)
Pyroclastic surge deposits