Flashcards
Aeolian processes
Cards in this deck (23)
- 1Aeolus/ windGreek god- god of the winds
- 2The are oaf the world with the most wind power...1- Terre de forgo
2- South greenland
3-UK
4- North west USa
5- Central North Africa
6- central AUS - 3Air flow patterns 1Globally, air circulation is thermally-driven
Rising warm air (equator)= low pressure- (e.g. equator- low pressure area)
Sinking cool air (sub-tropics)= high pressure
Air moves high to low (Hadley cells) Pressure gradient force. - 4Air flow patterns 2:Air parcels moving from high to low pressure at a large scale are deflected - Coriolis force
Reflects wind right in northern hemisphere & left in southern hemisphere.
Trade winds - 5Air flow patterns 3:Momentum that winds gets from earth's rotation is greatest near the equator.
Momentum is conserved & making up for this lack of momentum in the poles there is higher wind speeds (compositing for the lack of momentum). - 6Air flow patterns 4:Global spatial variation of wind speeds:
e.g. Oceans, continents.
Rough surfaces- mountains, trees.
Regional/ local variations.
Mid-lats- 30-60degrees N/S- westerly flow contains large scale eddies (e.g. low pressure systs.). - 7Near surface boundary layer:Atmospheric boundary layer: 1-2km high
Near-surface boundary layer: 10m/s
Wind speed changes with height (velocity profile).
Laminar flow- (individ. horizontal layer- no mixing- smooth surface needed.)
Turbulent flow (eddies lots of flow- mixing momentum (mass x velocity). - 8Reynolds number (Re)Re= intertail forces/ viscous forces.
Intertial forces- what are encouraging the air to deform).
Viscous forces- how thick- ability to resist deformation.
Dimensionless ratio- used to distinguish b/w laminar and turbulent flow. - 9Reynolds ne 2:Laminar flow Re: < (less than) 500
Transitional flow: 500 < Re < (over) 2000
Turbulent flow: > (eql. to) 2000
Airflow is nearly always turbulent because air has low viscosity & boundary layer depths are high.
High no. at top of equations & low no. at bottom (viscosity)= Re no. usually high. - 10Measuring turbulent boundary airflow:Anemometer- things that spina round-
Can easily derisive a velocity profile form wind speed (u) at heights (z). - 11Wind speed velocity profile:See graphs:
Linear axes- v. low to ground wind speed dis retired due to friction- then there is efficient mixing of air near the bed causing a rapid increase (0-2m): then 2- 10m there is less of an increase.
If plotted on Log-scale: straight line.
- Flat & unvegitated: Known as the 'LAW OF THE WALL' - 12Law of the wall1) Aerodynamic roughness length: Zo
- v. thin layer of Zero-veolocity flow exists above the surface
e.g tiny sand bed Zo= d/30 (d=grain diameter)- 0.0007m
Rougher surfaces, greater Zo
2) When L of W stands: Prandtl- von Karman - describes the velocity profile/ distribution of u with height above surface. - 13Shear velocity1Is the no. estimates the ability of the wind to do work.
Drag of airflow exerts drag... creating shear stress.
Ability to do work.. can't calc. shear steps directly.. so there we calc. shear velocity (slope of velocity profile).
Proportional to y/x vel. prof. - 14Shear velocity 2Faster wind speeds at a particular height results in a steeper velocity profile- greater drag therefore ^ shear velocity.
U*- ability to move sediment- erosivity.
CRITISMS: assumes unvegitated and wind speed Zero near ground level (Zo).
Vegetation incorporates 'd' term- raising Zo- modify the equation (z-d) value. - 15Sand/ quartzSedimentary rocks make up 80% total land surface, comprising
20%- carbonates
15% sandstone
5% evaporites
60% clay minerals and quartz.Quartz dominates aeolian bedforms and dust deposits because:
a)quartz is highly abundant
b)quartz is chemically try stable, low solubility below pH 10- chemically inert. - 16Sediment size:Sediment size:
Sand= 63- 2000(micrometer) diameter particles:
Silt & clay= <63 diameter
(often referred to as ‘dust’ in aeolian processes).
1(micrometer) = 1/1000mm - 17Particle entrainment:Gravity
Cohesion (moisture etc.)
Force- wind (^ and Low pressure- causing a vertical effect- (lift)
Surface drag (friction)
Form drag (due to pressure differentials).
Tipping point- threshold: (critical shear velocity) - 18U*cCircle shear velocity- point at which a prcile will start to move
- threshold of entrainment
It is a result of lots of different things: Particle size, turbulence, gravy, atmospheric density.) - 19Bagnold (1941)U*c= critical shear velocity of a grain-
Ps + density of the grain
P= density of art
g= gravitational acceleration
D= diameter of the grain
A= coefficient of flow - 20Three modes of Aeolian particle transportCreep- rolling(>500 microm)-25% movement.
Saltation- Leap (70-500microm)- dislodges particles- they move up & down wind- with the energy (50-60% energy lost hitting bed).
Heathcote (1983)- 90% sand carried in lowest 50cm. Wiggs (1992)- 35% sand transport in lowest 2.5cm.
Suspension- smaller held in airstream. - 21ThresholdsFluid threshold: For a still sand bed, fluid critical threshold must be exceeded to start grain movement.
Impact threshold: Saltation occurring therefore the is a lower threshold needed for movement (lower critical shear velocity- Anderson & Haff- (20% less). - 22Bagnolds (1941) sand transport equationEst. the man of sand in a period of time... or the rate (flux) of sand transport (q).
q- quantity of sediment transported
C= coefficient of sediment sorting.
dp= mean grain size of pop.
D= mean grain size of standard pop
g= gravity
u*= shear velocity - 23Key controls on sand transport ratesSand texture- x=3 but value ^ e.g. x=4.1 (glass spheres) x= 2.76 (crushed quartzite)
Moisture- ^ moisture=^ cohesion ^shear stress.
Vegetation- 20% to reduce sand transport.
slope (distribution matters too)- accelerate around vegetation.
Slope- Downward slope less fluid shear velocity needed- upward slope more needed.
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- Earth surface processes and landforms Flashcards on Aeolian processes, created by Lucy Harris on 13/06/2018.
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- Lucy Harris
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