2. www.geo.utep.edu/ pub/ hurtado/2412

3. Aeolian Sediment Transport
Air moving as wind is always turbulent flow.
There will always be a thin boundary layer at the surface where the velocity of the flow is zero.
Above boundary layer, velocity increases (velocity gradient).

4. Aeolian Sediment Transport
Movement of particles in a fluid is determined by…
Mass of particles
Density of fluid
Friction and interparticle cohesion
Drag, lift, impact forces

5. Aeolian Sediment Transport
Lift force – can cause particles to “jump” into airflow similar to the Bernoulli model for airfoil lift.

6. Aeolian Sediment Transport
Drag force – results from difference in pressure on windward and leeward sides of a grain which can cause a particle to roll or slide. Surface creep.
Can initiate motion at lower wind velocity than necessary for lift and can move particles too large to be moved by lift.

7. Aeolian Sediment Transport
Turbulence – implies that airflow is not uniform, but is characterized by bursts of higher velocity flow, producing variations in pressure that can create lift and drag forces.
Drag and drag forces vary with the square of fluid velocity  increase velocity by factor of 2 and forces increase by factor of 22 = 4

8. Aeolian Sediment Transport
Factors determining whether sediment will move:
Density
Shape
Cohesion
There is a direct relationship between size and fluid threshold velocity, for dry particles of a given shape and density…

9. Grains larger than 1 mm will be rarely moved by drag alone
Grains smaller than 0.6 mm will require more fluid velocity as grainsize decreases due to reduced surface roughness and turbulence, interparticle cohesion, size of particles with respect to boundary layer.

11. Aeolian Sediment Transport
In air, boundary layer on top of a flat granular surface is ca. 1/30 grain diameter
Above boundary layer, velocity increases proportional to logarithm of height.
“Drag velocity”, V* = sqrt(t/r)
When V* (velocity gradient) exceeds V*t (threshold velocity for lift or drag), particle will be entrained into turbulent flow.

13. Aeolian Sediment Transport
Threshold velocity (V*t) is proportional to square root of grain diameter for grains larger than 0.1 mm.
Grains smaller than 0.06 mm (silt and clay) are transported in supsension.
Entrainment of 0.06 mm and smaller grains is due to impact by larger grains.

14. Aeolian Sediment Transport
Can draw a Shields curve showing the threshold for velocity for erosion vs. deposition.
Note that there is a minimum threshold velocity-grainsize. For grainsizes larger and smaller, there needs to be increasingly faster flow to move particles.

17. Aeolian Sediment Transport
For sand particles, there is an impact threshold that is less than the fluid threshold.
This is caused by impact of sand grains already in motion striking particles at rest.
Impact of small grains on large grains can cause motion at 80% of the fluid threshold velocity.
More important for larger grainsizes.
Once motion has been initiated, it can be sustained by ballistic impact even if wind has died down below fluid threshold level.

19. Aeolian Sediment Transport
Saltation: sand grains transported by wind do so in short, asymmetic trajectories no more than 1-2 m above the ground (usually cm), and up to 1-2 m in length.
Distance traveled depends on gravity and drag, which depends on particle size, wind speed, and surface characteristics.
Falling grain impacts at degree angle, hits other grains on granular surface, and entrains many more grains into air flow  curtain of sand gets swept forward.
Some portion of the sand will remain on the ground, but will migrate by surface creep (fine bedload).

21. Aeolian Sediment Transport
Increased wind speed will impart a greater forward velocity to saltating grains  rapid increase in the amount of sand in transport.
Rate of sand transport by saltation depends on third power of wind speed.
Sand can be best moved at highest wind speed. But very strong winds are infrequent. So most volume of sand moved at intermediate wind speeds.

24. Aeolian Sediment Transport
Creep and saltation and suspension lead to very effective sorting mechanism…
Creep only moves grains a few millimeters per impact and is effective for grainsize ca. 6x saltating fraction. (25%)
Saltation moves grains 1-2 m or more. (75%)
Finest fractions are moved and sequestered by suspension.
Some material isn’t moved at all and is left behind.

25. Aeolian Sediment Transport
Wind passing over terrain with different surface roughness will either deposit or erode.
Smooth surface  high surface velocity  transport and erosion.
Rough surface  lower surface velocity due to surface drag  deposition.

26. Aeolian Sediment Transport
Hammada – bare rock in desert environments.
Reg (gobi) – gravel or stony pediments.
Erg – sandy or dusty surfaces (dune or loess)

30. www.geo.utep.edu/ pub/ hurtado/2412