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
What does Boundary layer theory and wind shear mean?
- Shear stress occurs on all surfaces and is subject to fluid flow and tends to develop or deepen w/ distance downstream
- Boundary layer extends from bed to elevation where wind speed is 99 percent of the free-stream velocity
- Velocity profile can be used to estimate shear stress, shear velocity, and sediment flux at the surface
Determining shear velocity using the law of the wall
- Log scale to transform height, then linear regression
- Z = indep variable
- mu* = slope
- Zo = length of viscous sub-layer, height at which wind vel=0
- Law of the Wall provides estimate of shear velocity ant any height above the bed based on time-averaged velocity profile
Prantl-von Karman eqn: Law of the Wall
- Avg flow velocity, muz = mu* x ln (height above bed, z/roughness length, Zo)
- Zo is the height at which wind vel=0, reflects local roughness elements (sand grains, rocks, veg, which all increase Zo)
Bagnolds simplified eqn for determining shear velocity
- Avg flow velocity, muz = 5.75 x mu* x log (height above bed, z/roughness length, Zo)
- Zo is the height at which wind vel=0, reflects local roughness elements (sand grains, rocks, veg, which all increase Zo)
How are the values for the Law of the Wall and Bagnolds eqn derived?
- Measure velocity profile above surface from > or = to 3 log-spaced instruments
- Apply Prandtl-von Karman eqn
- Regress ln z against u and recall that velocity is dependent variable
- Thus avg flow vel at z = a (ln z) plus b
- To find Zo, recall that u=0 at Zo: 0 = a (ln Zo) plus b, then ln Zo = -b/a, then Zo= e^(-b/a)
- mu, determine slope of constant stress region, Slope = muk or mu* = 0.4 x slope (rise/run)
- Shear stress = density x mu*
- Sediment flux = mu*^3
Conventional instrumentation
- Cup anemometry
New instrumentation
- Ultrasonic anemometry
- Needs good amount of sensors in set up to produce decent velocity profile
Limitations to boundary layer theory and wind shear
- Assumes steady uniform flow (time-averaged approach)
- Log-linear portion of profile (constant-stress region) is subjective, region typically exists in lower 10-15 percent of boundary layer
- Assumes constant surface roughness, complicated by changes in roughness or bedforms, veg, etc.
- Sed transport alters profile response by extracting momentum and producing kinks in profile
How does sediment transport alter the velocity profile response?
Sed transport alters profile response by extracting momentum and producing kinks in profile
Forces at work on a dry sand grain
- Fluid force, FF, sum of vertical lift (FL) and horizontal drag (FD) forces
- Resisting force, Fg, due to gravity
- G
Forces at work on a dry sand grain, when do grains move?
- Grains move when shear velocity (Ut) exceeds the threshold U
- i.e. when U* > U*t
- Pivot angle, alpha, is btwn Fluid force and ground
- As pivot angle decreases, less Fluid Force FF is required to move the grain and U*t decreases
How is the U*t threshold estimated?
- Using Bagnolds formulation, 1936
- U*t = A x sq. root [g x D x ((density sediment x density fluid)/density fluid)]
- Where A is constant, 0.1 and D is particle diameter
Fluid threshold
- Velocity the fluid must exceed to initiate transport for any given grain size
- ie to overcome inertia and inter-particle friction
Impact threshold
- Shear velocity required to maintain grains in transport after impact has begun
- Bagnold showed in wind tunnels that impact threshold is approx. 80 percent less
- This reduced U*t is due to positive feedback caused by impacting grains that eject sediment into the airstream
Aeolian sed transport: Thresholds
- Higher wind velocities are needed to get grains moving, upper fluid threshold curve
- Grains moving, sediment transport occurs at lower velocities, lower impact threshold curve
- This reduced U*t is progressively more significant as grain-size increases b/c larger grains transfer more momentum to the surface
Aeolian transport eqns, qs: Bagnold 1936
qs = C x [(d/D)^0.5] / (fluid density/g x U*^3)
- Where C = empirical constant 1.5 for fine, well sorted sand, 1.8 for naturally graded sand, 2.8 for coarse poorly sorted sand
- Where d = grain diameter, D = standard sand diameter, 0.25mm, g = gravitational acceleration
- U* = shear velocity
- Limitations: contains no threshold term and predicts flux at U*<u></u>
Aeolian transport eqns, qs: Kawamura 1951
qs = C x fluid density/g x (U* x Ut)(U x U*T)^2
- Where C = empirical constant 2.78
- g = gravitational acceleration
- U* = shear velocity
- Limitations: Incorporates a threshold term U*t but still relies on empirical constant
Aeolian transport eqns, qs: White 1979
qs = 2.61 x fluid density/g x U^3 x (1 - Ut/U) / (1 plus Ut/U*)
- Most advanced for dry sediment on flat surfaces
Eqn’s for calculating sediment flux, qs
- Bagnold, 1936 but has no threshold term and relies on empirical coeffiecients
- Kawamura, 1951, has threshold term but still relies on empirical constant
- White, 1979, most advanced for dry sed on flat surfaces
- Other eons incorporate moisture, slope, adhesion, organics
- Direct measurements use sed traps and wind speed profiles to collect empirical data for comparison w/ models
D, is a reference grain of what size?
0.25 mm
qs can vary by?
- An order of magnitude for a given U*
- Ideal conditions never present = 1
- Steady uniform wind = 2
- Flat dry substrate = 3
- Transport is dominated by saltation in equilibrium w/ the wind
Modes of sediment transport and grain size
- Creep, >0.5mm
- Saltation and modified saltation, 0.7-0.5mm
- Reptation
- Suspension (long term <0.02mm) (short-term 0.02-0.07mm)
Creep
- Rolling and sliding
- Can be enhanced by saltation
- Very large grains can be jerked forwards short distances by descending grains
Saltation
- Short hops < 1m
- 90 percent of sed transport
- cm-scale hops near surface
- Modified or extended occurs when sand grains are caught in turbulent eddies
- Saltation causes reputation and can enhance creep (landing grains explodes other grains upwards)
Reptation
- Splash
- Particles displaced by saltating grain bombardment
Suspension
- Transport of fine grained silts and clay sized seds
Saltation and wind speed
- Wind speed increases above ground surface
- Windspeed 5m/sec to initiate sand movement
- Windspeed 4-5m/sec to continue saltation
Suspended sediment from the Sahara
- Can extend across the Atlantic all the way to North America
Describe Sediment source areas to deposition areas
- Source area = lag gravel
- Sand transportation from lag gravel to sand dunes
- Sand deposition from sand sheets to sand dunes
- Dust transportation from lag gravel through sand sheets, dunes and loess
- Dust deposition at Loess area
What are aeolian source areas often dominated by?
- Erosional landforms
- Deflation basins, desert pavements winnowed of finer sediments, ventifacts, yardangs
Deflation/lag
- Original gravel is dispersed
- Deflation removes fine grains
- Deflation develops lag gravel surface (desert pavement)
Ventifacts
- Often confused w/ artifacts and meteorites
- Larger rocks that have sand blasted faceted shapes based on where they faced the dominant wind direction for a length of time
- Subsequent rolling exposes other sides to the blasting and creates a new surface
Yardang
- Wind-abraded bedrock ridges
- Form when 1 wind direction dominates
- Several times longer than wide
- Stoss end is steep and higher
- Vary from cm high to several km long and 100’s m high
- Composed of softer rocks in areas w/ low veg and high saltation/strong abrasion
- Found on Mars as well
Bedforms
- Occur across wide spectrums of height and wavelength at the sam particle sizes
- Regularity in form and spacing, even at vastly diff scales
- Result from instabilities generated by the interaction of a fluid w/ an underlying layer of diff density
- Varying bedform wavelength from cm-km scale w/ wide variety of grain sizes, height scale from cm-km
Flow-form interactions
- Bedforms, dunes, are formed by fluid flow (wind)
- Dunes in-turn modify near-surface flow which modifies the dunes
- Complex feedback
- Fluid flow and bedform morphology
Flow-transport interactions
- Flow provides energy to transport sand
- But that in-turn reduces wind competency (sed extracts momentum from the wind)
- Fluid flow and sediment transport
Form-transport interactions
- Transport varies w/ moisture, topography, veg etc.
- But this variability affects dune forms
- Bedform morphology and sediment transport
What results from bedrooms protruding into the boundary layer?
- Streamlines compressed on stoss side
- Results in flow acceleration and erosion
What does a rapid change in relief at the crest result in?
- Causes flow separation from the surface and flow stagnation in the lee
How does steep stratification build on the slip face?
- Sand moves up stops side, over-steepens at the crest
- Grain flow failures occur on slip face, creates stratification
At what distance does the streamflow reattach to the surface downstream of a before?
- Flow reattaches at a distance 5-10 times the height of the bedform
Bedforms do not simply respond passively to the wind, but rather?
- Don’t respond passively
- Rather they induce secondary flow regimes that help maintain their form
Flow-form interactions: How does the dune mass migrate downwind?
- Flow re-circulates in the lee
- Casses low or even negative wind speeds
- Gravity settling of the grains in addition to grain flows (grain flows lead to forsets in record)
- Scour may occur in the lee due to turbulence and eddies, steepening the lee face
- Basically windward entrainment followed by downwind deposition due to gravitational settling and subsequent granular flow on slip-face
Ripples
- Smallest landform, asymmetric
- Wavelength 5-25cm, h 0.5-25cm
- Windspeed 0.01-0.13cm/s
- Scales directly w/ saltation and reptation
- Indicates near-surface flow and transport direction
- Crests are coarser w/ finer grains in sheltered lee zone
Ripple initiation and propagation
- Initiate due to bed irregularities that increase the angle of incidence of saltating grains
- Creep intensity increases on stoss side, grains transported up slope and accumulate at top
- Lee-side hollow forms behind first ripple and sequence propagates down wind
What limits ripple height?
- Ripple height is limited by wind velocity (decreases upward)
- Wavelength largely reflects reptation length
Dunes
- Large landform, varied forms
- Wavelength 3-500m, up to 100m or more high
- Indicate flow and transport direction
- Formed by primary aerodynamic instabilities
- Create complex flow-form-transport interactions
What are dune types distinguished by?
- Size and shape
- Orientation to dominant winds
- Vegetation
- Sediment supply
- Most are depositional, some can be erosional (blowouts)
How does vegetation influence dune type?
- Free vs. anchored
- Shadow dunes in lee of shrubs
- Parabolic dunes anchored by vegetated arms
Dune type classification triangle
- Sand vs. vegetation vs. wind
- Longitudinal dunes = Lots of wind, little sand, no-lots veg
- Crescent dunes = Middle sand, no veg, middle wind
- Transverse dunes = Little wind, more sand, no veg
- Parabolic dunes = Middle veg, more sand, little-lots wind
- No dunes = No-lots of wind/sand but always lots of veg
What are the main ways to classify dunes?
- Size and shape
- Relation to wind directionality
- Free or anchored
Free dunes
- Transverse (transverse, barchan, dome, reversing)
- Linear (Linear, seif)
- Star (star, network)
Anchored dunes
- Vegetated (parabolic, nebkha, blowout)
- Topographic (sand ramp, climbing, falling, lee)
Incident wind angle and dune form
- Wind regime and directional variability exerts control on morphology and maintenance of aeolian landforms
- Frequency, magnitude, and directional modality of competent winds shown to eat major control over dune form and morphodynamics
- 90-75 degrees = Transverse dunes
- 75-15 degrees = Oblique dunes
- 15-0 degrees = Longitudinal dunes
Resultant Drift Potential, RDP
- Used to classify dunes
- Dunes align normal to net transport vector (except linear, opposing oblique modes; star, multimodal)
- Describes net sand transport direction, for winds above threshold 6m/s (resultant vector down flow)
DP, RDP
- Drift Potential
- Resultant Drift Potential
- RDP/DP gives ratio
- High ratio = narrow unimodal (transverse dunes, lower ratio to barchan dunes)
- Mid ratio = Acute bimodal to obtuse bimodal (Linear dunes)
- Lowest ratio = Complex (star dunes)
Sediment supply
- Same wind can produce many dune forms
- But depends on sand supply
- e.g. both barchans and transverse are unidirectional wind, but barchans usually have limited sand supply
Barchan dunes
- Unimodal wind regime
- 2 horns facing resultant drift direction
- Usually on substrate w/ limited sand supply
- Desert pavement
- can coalesce into barchanoid ridges
Transverse dunes
- Unimodal wind regime
- Less limited sand supply than barchan
- Long linear-ish looking ridges perpendicular to dominant wind direction
Longitudinal/linear dunes
- Ridges parallel to wind direction, product of high wind velocities
- Straight or irregularly sinuous, elongate ridges
- Often wider and steeper upwind, gradually tapering
- AKA seif dunes
- Unvegetated areas, often on bedrock in warm deserts
- Bimodal wind regime
Seif dunes
- AKA Longitudinal dunes, Linear dunes
Simpson desert, Belet, Oman, Fensal, and Kalahari dunes
- Longitudinal (AKA linear or seif) dunes
Parabolic dunes
- Horns point upwind, opposite to barchan dunes
Evolution of parabolic dune from a blowout
- Foredune veg reduced by storm wave erosion (blowout, therefore no longer anchored by veg, more movement/erosion)
- Erosion continues, deflation basin expands, depositional lobe advances downwind, parabolic form develops
- Foredune reforms across parabolic dune throat, parabolic continues to advance downwind forming elongated trailing ridges
Where can relict parabolic dunes be found?
- NE BC, on the mountains
- Revealed by LiDaR
- Could be locally reactivated
Blowout
- Erosional dune form
- Break in form, removal of vegetation?
Star dunes
- Multimodal wind regime (all or any direction)
- Forms start shaped dune w/ various arms
- Namib desert
Coppice dunes
- AKA shadow dunes, nebkah
- Dust/sand deposited in the lee of bushes or boulders
Coastal foredunes
- Shore-parallel vegetated dune ridges backing beaches
- Sand removed from swash zone by wind
- Buffer against storm surges and sea level rise
How does log debris affect aeolian landforms on coasts?
- Log debris can act as an accretion anchor and prevent sed from being removed
What are economic applications of Aeolian landforms?
- Silica sands like NWT, Great Slave Lake
- Iron sands like Peru
Where can neat records of past aeolian landscapes be found?
- Navajo sandstones in Arizona
