Aeolian Processes and Landforms Flashcards
Bagnold
- Military, British soldiers of the long range desert group, WWII
- Developed ‘Physics of blown sand and desert dunes’ 1941
- Studied how sand moved so vehicles could be driven on dunes
What is required in any one area for wind to be an effective geomorphic agent?
- Competent winds strong and steady (>6m/s or 22km/hr)
- Abundant sand-sized or finer seds
- Low moisture (<5 percent)
- Sparse vegetation and few wind breaks
Supply-limiting factors
- Quantity, Size, Availability (e.g. dryness) of fine sediment
Transport-limiting factors
- Vegetation cover, Lag deposits, Topographic effects, Sand fences
What environments can Aeolian action be a geomorphic agent?
- Not limited to desert
- Found in fluvial, glacial, and coastal sediments
- Beaches, outwash valleys (delta’s and glacial, braided streams, exposed topsoil (erosion, agriculture), recently burned areas, mine tailings
- Also agricultural (Dirty ‘30’s blew away fertile top soil due to not enough vegetation cover)
- Also cold, dry, low vegetation polar areas
2 main types of eolian sediment
- Silt dominated, loess
- Sand dominated, sand plains/dune fields
True or false: Aeolian sediments are extremely well sorted, and well organized packages?
- True
Global loess deposits
- Often associated w/ ice sheets that produce large quantities of source material (rock flour)
Loess
- Homogeneous, very well-sorted, silt-dominated sediment
- Deposited from suspension
Yukon Loess
- Primarily generated during glacials/deglacials when bare seds exposed (e.g. large braided glaciofluvial floodplains)
- Transported/deposited large distances, primarily in unglaciated terrain
- Loess in Dawson Range originated in vast braid-plains of White and Donjek rivers
- Loess thickness decreases northward b/c southerly winds off the St. Elias ice sheets dominated
Loess stratigraphy provides excellent stratigraphic records b/c:
- Largely depositional, erosional unconformities uncommon
- Incorporates paleo-envr material (fossils, paleosols, early human sites), fossils frozen in Berringia provide recoverable DNA
- Numerous chronostratigraphic markers (Volcanic ash, paleomag)
Where are the best loess stratigraphy records in Canada?
- Beringia, Unglaciated parts of Yukon and Alaska
- Prominent researchers Duane Froese, John Westgate, G. Zazula
Why does loess readily form and become mobile after glaciers retreat?
- No vegetation on exposed sediments to hold it in place
- Leads to loess
Aeolian Sed Transport (Graph of V in cm/sec vs. Grain diameter in mm)
- 2 curves on graph, fluid and impact
- Fluid shows higher velocities needed to entrain stationary small grains, once moving they impact stationary grains and entrain those on the impact curve
- Wind velocity must exceed resisting forces (grain size, except for small grains where cohesion and low surface roughness dominate like Hjulstrom curve)
- Winds move sand-sized grains and finer but also gravels at higher velocities
- Once moving, grains impact the stationary grains on the bed surface
- Entrains impacted grains at lower velocities (Impact threshold curve)
What does sediment transport depend on?
- Frequency, magnitude, and duration of the wind
On the aeolian sediment transport graph, why is the velocity so high to entrain small grains on the fluid curve?
- Small grains (< 0.1mm) exhibit cohesion and low surface roughness (like hjulstrom curve)
- Fluid in seds exhibits more cohesion in small grains?
Wind shear
- Wind is viscous (Newtonian) fluid (like water)
- Exerts shear stress on the bed
- Which imparts frictional drag on the flow
Boundary layer
- Zone of flow affected by frictional resistance
- Thickens w/ increased turbulence
Drag and the boundary layer theory
- Drag decreases w/ height above bed (where u=0)
- Until drag=0 which is free-stream velocity (Umax or Uinfinity)
- Drag affects shape of velocity profile
Boundary layer and velocity profile: The profile response is a function of?
- Fluid speed, u
- Fluid density, rho
- Fluid viscosity, mu
- Surface roughness
Boundary layer and wind shear: Flow imparts what on the surface? Via?
Imparts shear stress on sediment particles via momentum transfer
- Newton’s law of viscosity which states that shear stress is proportional to velocity gradient w/ height above the surface (stress is approx. du/dy, derivative of fluid speed over height)
- Shape of profile describes momentum transfer and surface sheer stress exerted by air flow and sediment transport
Shape of velocity profile describes?
- Shape of profile describes momentum transfer and surface sheer stress exerted by air flow and sediment transport
Shear stress drives?
- Sediment transport, but can’t be measured directly
- Therefore shear velocity, mu* is used
- Derived from slope of velocity profile
Shear velocity and shear stress at the bed can be described by what eqn?
Shear stress at the bed, t0 = [rho (density) x mu* (shear velocity)]^2
- Shear velocity also proportional to sediment flux a the surface, qs (qs proportional to mu*^3