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Geog375Gmphlgy > Aeolian Processes and Landforms > Flashcards

Aeolian Processes and Landforms Flashcards

1
Q

Bagnold

A
  • 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
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2
Q

What is required in any one area for wind to be an effective geomorphic agent?

A
  • 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
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3
Q

Supply-limiting factors

A
  • Quantity, Size, Availability (e.g. dryness) of fine sediment
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4
Q

Transport-limiting factors

A
  • Vegetation cover, Lag deposits, Topographic effects, Sand fences
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5
Q

What environments can Aeolian action be a geomorphic agent?

A
  • 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
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6
Q

2 main types of eolian sediment

A
  • Silt dominated, loess

- Sand dominated, sand plains/dune fields

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7
Q

True or false: Aeolian sediments are extremely well sorted, and well organized packages?

A
  • True
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8
Q

Global loess deposits

A
  • Often associated w/ ice sheets that produce large quantities of source material (rock flour)
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9
Q

Loess

A
  • Homogeneous, very well-sorted, silt-dominated sediment

- Deposited from suspension

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10
Q

Yukon Loess

A
  • 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
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11
Q

Loess stratigraphy provides excellent stratigraphic records b/c:

A
  • 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)
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12
Q

Where are the best loess stratigraphy records in Canada?

A
  • Beringia, Unglaciated parts of Yukon and Alaska

- Prominent researchers Duane Froese, John Westgate, G. Zazula

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13
Q

Why does loess readily form and become mobile after glaciers retreat?

A
  • No vegetation on exposed sediments to hold it in place

- Leads to loess

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14
Q

Aeolian Sed Transport (Graph of V in cm/sec vs. Grain diameter in mm)

A
  • 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)
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15
Q

What does sediment transport depend on?

A
  • Frequency, magnitude, and duration of the wind
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16
Q

On the aeolian sediment transport graph, why is the velocity so high to entrain small grains on the fluid curve?

A
  • Small grains (< 0.1mm) exhibit cohesion and low surface roughness (like hjulstrom curve)
  • Fluid in seds exhibits more cohesion in small grains?
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17
Q

Wind shear

A
  • Wind is viscous (Newtonian) fluid (like water)
  • Exerts shear stress on the bed
  • Which imparts frictional drag on the flow
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18
Q

Boundary layer

A
  • Zone of flow affected by frictional resistance

- Thickens w/ increased turbulence

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19
Q

Drag and the boundary layer theory

A
  • 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
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20
Q

Boundary layer and velocity profile: The profile response is a function of?

A
  • Fluid speed, u
  • Fluid density, rho
  • Fluid viscosity, mu
  • Surface roughness
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21
Q

Boundary layer and wind shear: Flow imparts what on the surface? Via?

A

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
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22
Q

Shape of velocity profile describes?

A
  • Shape of profile describes momentum transfer and surface sheer stress exerted by air flow and sediment transport
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23
Q

Shear stress drives?

A
  • Sediment transport, but can’t be measured directly
  • Therefore shear velocity, mu* is used
  • Derived from slope of velocity profile
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24
Q

Shear velocity and shear stress at the bed can be described by what eqn?

A

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

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25
Q

What does Boundary layer theory and wind shear mean?

A
  • 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
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26
Q

Determining shear velocity using the law of the wall

A
  • 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
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27
Q

Prantl-von Karman eqn: Law of the Wall

A
  • 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)
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28
Q

Bagnolds simplified eqn for determining shear velocity

A
  • 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)
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29
Q

How are the values for the Law of the Wall and Bagnolds eqn derived?

A
  • 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
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30
Q

Conventional instrumentation

A
  • Cup anemometry
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31
Q

New instrumentation

A
  • Ultrasonic anemometry

- Needs good amount of sensors in set up to produce decent velocity profile

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32
Q

Limitations to boundary layer theory and wind shear

A
  • 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
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33
Q

How does sediment transport alter the velocity profile response?

A

Sed transport alters profile response by extracting momentum and producing kinks in profile

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34
Q

Forces at work on a dry sand grain

A
  • Fluid force, FF, sum of vertical lift (FL) and horizontal drag (FD) forces
  • Resisting force, Fg, due to gravity
  • G
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35
Q

Forces at work on a dry sand grain, when do grains move?

A
  • 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
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36
Q

How is the U*t threshold estimated?

A
  • 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
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37
Q

Fluid threshold

A
  • Velocity the fluid must exceed to initiate transport for any given grain size
  • ie to overcome inertia and inter-particle friction
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38
Q

Impact threshold

A
  • 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
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39
Q

Aeolian sed transport: Thresholds

A
  • 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
40
Q

Aeolian transport eqns, qs: Bagnold 1936

A

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>
41
Q

Aeolian transport eqns, qs: Kawamura 1951

A

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
42
Q

Aeolian transport eqns, qs: White 1979

A

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

43
Q

Eqn’s for calculating sediment flux, qs

A
  • 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
44
Q

D, is a reference grain of what size?

A

0.25 mm

45
Q

qs can vary by?

A
  • 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
46
Q

Modes of sediment transport and grain size

A
  • Creep, >0.5mm
  • Saltation and modified saltation, 0.7-0.5mm
  • Reptation
  • Suspension (long term <0.02mm) (short-term 0.02-0.07mm)
47
Q

Creep

A
  • Rolling and sliding
  • Can be enhanced by saltation
  • Very large grains can be jerked forwards short distances by descending grains
48
Q

Saltation

A
  • 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)
49
Q

Reptation

A
  • Splash

- Particles displaced by saltating grain bombardment

50
Q

Suspension

A
  • Transport of fine grained silts and clay sized seds
51
Q

Saltation and wind speed

A
  • Wind speed increases above ground surface
  • Windspeed 5m/sec to initiate sand movement
  • Windspeed 4-5m/sec to continue saltation
52
Q

Suspended sediment from the Sahara

A
  • Can extend across the Atlantic all the way to North America
53
Q

Describe Sediment source areas to deposition areas

A
  • 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
54
Q

What are aeolian source areas often dominated by?

A
  • Erosional landforms

- Deflation basins, desert pavements winnowed of finer sediments, ventifacts, yardangs

55
Q

Deflation/lag

A
  • Original gravel is dispersed
  • Deflation removes fine grains
  • Deflation develops lag gravel surface (desert pavement)
56
Q

Ventifacts

A
  • 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
57
Q

Yardang

A
  • 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
58
Q

Bedforms

A
  • 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
59
Q

Flow-form interactions

A
  • 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
60
Q

Flow-transport interactions

A
  • Flow provides energy to transport sand
  • But that in-turn reduces wind competency (sed extracts momentum from the wind)
  • Fluid flow and sediment transport
61
Q

Form-transport interactions

A
  • Transport varies w/ moisture, topography, veg etc.
  • But this variability affects dune forms
  • Bedform morphology and sediment transport
62
Q

What results from bedrooms protruding into the boundary layer?

A
  • Streamlines compressed on stoss side

- Results in flow acceleration and erosion

63
Q

What does a rapid change in relief at the crest result in?

A
  • Causes flow separation from the surface and flow stagnation in the lee
64
Q

How does steep stratification build on the slip face?

A
  • Sand moves up stops side, over-steepens at the crest

- Grain flow failures occur on slip face, creates stratification

65
Q

At what distance does the streamflow reattach to the surface downstream of a before?

A
  • Flow reattaches at a distance 5-10 times the height of the bedform
66
Q

Bedforms do not simply respond passively to the wind, but rather?

A
  • Don’t respond passively

- Rather they induce secondary flow regimes that help maintain their form

67
Q

Flow-form interactions: How does the dune mass migrate downwind?

A
  • 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
68
Q

Ripples

A
  • 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
69
Q

Ripple initiation and propagation

A
  • 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
70
Q

What limits ripple height?

A
  • Ripple height is limited by wind velocity (decreases upward)
  • Wavelength largely reflects reptation length
71
Q

Dunes

A
  • 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
72
Q

What are dune types distinguished by?

A
  • Size and shape
  • Orientation to dominant winds
  • Vegetation
  • Sediment supply
  • Most are depositional, some can be erosional (blowouts)
73
Q

How does vegetation influence dune type?

A
  • Free vs. anchored
  • Shadow dunes in lee of shrubs
  • Parabolic dunes anchored by vegetated arms
74
Q

Dune type classification triangle

A
  • 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
75
Q

What are the main ways to classify dunes?

A
  • Size and shape
  • Relation to wind directionality
  • Free or anchored
76
Q

Free dunes

A
  • Transverse (transverse, barchan, dome, reversing)
  • Linear (Linear, seif)
  • Star (star, network)
77
Q

Anchored dunes

A
  • Vegetated (parabolic, nebkha, blowout)

- Topographic (sand ramp, climbing, falling, lee)

78
Q

Incident wind angle and dune form

A
  • 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
79
Q

Resultant Drift Potential, RDP

A
  • 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)
80
Q

DP, RDP

A
  • 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)
81
Q

Sediment supply

A
  • 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
82
Q

Barchan dunes

A
  • Unimodal wind regime
  • 2 horns facing resultant drift direction
  • Usually on substrate w/ limited sand supply
  • Desert pavement
  • can coalesce into barchanoid ridges
83
Q

Transverse dunes

A
  • Unimodal wind regime
  • Less limited sand supply than barchan
  • Long linear-ish looking ridges perpendicular to dominant wind direction
84
Q

Longitudinal/linear dunes

A
  • 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
85
Q

Seif dunes

A
  • AKA Longitudinal dunes, Linear dunes
86
Q

Simpson desert, Belet, Oman, Fensal, and Kalahari dunes

A
  • Longitudinal (AKA linear or seif) dunes
87
Q

Parabolic dunes

A
  • Horns point upwind, opposite to barchan dunes
88
Q

Evolution of parabolic dune from a blowout

A
  • 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
89
Q

Where can relict parabolic dunes be found?

A
  • NE BC, on the mountains
  • Revealed by LiDaR
  • Could be locally reactivated
90
Q

Blowout

A
  • Erosional dune form

- Break in form, removal of vegetation?

91
Q

Star dunes

A
  • Multimodal wind regime (all or any direction)
  • Forms start shaped dune w/ various arms
  • Namib desert
92
Q

Coppice dunes

A
  • AKA shadow dunes, nebkah

- Dust/sand deposited in the lee of bushes or boulders

93
Q

Coastal foredunes

A
  • Shore-parallel vegetated dune ridges backing beaches
  • Sand removed from swash zone by wind
  • Buffer against storm surges and sea level rise
94
Q

How does log debris affect aeolian landforms on coasts?

A
  • Log debris can act as an accretion anchor and prevent sed from being removed
95
Q

What are economic applications of Aeolian landforms?

A
  • Silica sands like NWT, Great Slave Lake

- Iron sands like Peru

96
Q

Where can neat records of past aeolian landscapes be found?

A
  • Navajo sandstones in Arizona

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