EOLIAN PROCESSES AND LANDFORMS

• aeolus: Greek god of the wind
• aeolians: a Greek tribe
• aeolian harp: produces music as air passes over the strings
• wind is an important geomorphic agent in arid environments and in other smaller areas where fine sediments are exposed to wind, i.e. where surface cover is lacking: beaches, floodplains, deserts, soil disturbed by agriculture
• otherwise wind is not an important geomorphic agent due to its low density relative to rock: 1/2000 as opposed to 1/1.6 for water/rock
• given the buoyant force of water, little energy is required to keep sediment suspended, whereas in air only the finest sediments (dust) remain in suspension

Controls on wind erosion

1. wind velocity
   • E = V³rho, where E is erosivity , V is velocity and rho is air density
   • thus the erosivity of wind is an exponential function of wind velocity, i.e. if the wind velocity doubles, the wind is 8X more erodible or, if it triples, the wind is 27 times more erodible
   • that is why we observe massive wind erosion (dust) with a significant increase in wind speed

   
   

2. surface cover
   • an extremely important factor since there is no wind erosion on a vegetated surface
   • wind velocity decreases exponentially near the ground and is theoretically zero on a natural (i.e. rough) surface; thus erosivity (V³ is dramatically reduced)
   • on a windy day, put your nose next to the ground and you will discover there is no wind; small birds and insects take advantage of this on windy days
   • the zone of little or no wind is called the laminar sublayer (or the boundary layer), the rougher the surface (e.g. taller the vegetation) the deeper the layer of laminar air flow (i.e. no turbulence to entrain and suspend sediment)
   • thus there is no wind in the interior of a closed forest

   
   

3. grain size
   • threshold erosional velocity is related to the square root of particle size
   • thus when the threshold velocities for various particle size plot as a straight line when the particle size axis is on a square root scale
   • the threshold velocities are slightly lower for sand when impact among grains (saltation and creep) is taken into account
   • the fluid threshold velocities (wind shear) plot as two straight lines that slope down to converge at a minimum threshold velocity for coarse silt and fine sand (i.e. these are the most easily eroded grains)
   • with smaller particle sizes grains tend to cohere when wet and resist erosion
   • larger grains resist erosion by virtue of their greater size (mass)

Erosional processes and landforms

1. deflation: entrainment of loose sediment
   • deflation hollow (blowout)
     • shallow depression produced by deflation
   • often originates from the destruction of vegetation (e.g. fire, recreational use of dunes)
   • depth is limited by lag gravel or the water table, since wet sands resists deflation and favours the establishment of plants

   
   

2. abrasion (sand blasting)
   • impact of entrained sand grains against rock surfaces and other grains
   • yardangs
     • wind abraded ridges oriented with the prevailing winds and separated by abraded chutes that conduct windblown sand
   • ventifacts
     • stones faceted (planed) by abrasion
     • with changing direction of dominant winds, different facets merge along sharp ridges to transform rounded stones to angular ventifacts

Eolian sediment transport

1. suspension
   • air suspends particles less than 0.2 mm in diameter
   • this dust is carried 1000s m upward and 1000 kms downwind, held in suspension by turbulent eddies

   
   

2. saltation
   • transport of sand grains in long (1 m or more) low (within 1-2 m of the ground) trajectories as momentum is passed from grain to grain
   • grains are momentarily suspended but too heavy to remain in suspension
   • most of the transport of dune sand
   • at high wind speeds saltation is more or less continuous and appears as a fuzzy layer next to the ground

   
   

3. creep (traction)
   • movement of coarse sand and pebbles (up to 6x larger than saltating grains) as they slide and roll impacting one another and transferring momentum
   • usually does not occur with velocities less than 4.5 m/sec

note similarities and differences with transport by running water

• larger grains are suspended in water than in air
• saltation accounts for most sand transport in air, but is much less common in water because sand grains tend to remain in suspension in turbulent water
• larger particles (gravel) move as bedload (traction) in water
• air has no dissolved load

sorting

• the finest fraction is removed from the eolian landscape as dust and accumulates elsewhere as loess
• saltating grains out distance the traction load, leaving a lag of creeping and non-transported grains
• with exponential increase in sand transport with wind velocity, energy is quickly diverted from erosion to transport dissipating much of the wind energy
• thus wind velocity increases over barren rock surfaces, where sediment transport and the friction among saltating grains and with the stationary sand is not a factor
• sand is transported until friction over a rough surface (sand or vegetation) or an obstruction causes a decrease in wind velocity and deposition
• therefore eolian landscapes are characterized by a mosaic of 1) windswept and sandblasted surfaces, 2) stony lag deposits, 3) sand sheets or dune fields, and 4) loess sheets
• unlike other geomorphic processes wind does not result in the lowering of the landscape (denudation) towards an ultimate base level, rather sediment is usually just moved within a closed system in the direction of prevailing winds, unless it gets exported (e.g. transferred into a river)

Depositional landforms

1. ripples
   • small sand waves with a wavelength of about 1 m, i.e. the typical path length of saltating grains
   • they are ephemeral and mobile, i.e. move, disappear and reform during wind storms
   • common the windward slopes of sand dunes

   
   

2. dunes
   • classic eolian landform
   • stable or advancing landform of windblown sand
   • originates as a mound of free sand from a sandy surficial deposit (e.g. beach, weathering sandstone) or from a blowout
   • as the mound grows it develops the dune asymmetry characterized by a gentle windward slope and a leeward slip face at the angle of repose for sand
   • same longitudinal shape as a ripple but several orders of magnitude difference in size, and thus dunes are much less mobile and more persistent
   • dunes migrate downwind as sand saltates up the windward face (i.e. ripples migrate), accumulates where the wind dies just over the crest, and then flows (mass wasting) over the slip face

classification of depositional landforms

1. barchan dune
   • classic desert dune
   • crescentic in plan view, horns (cusps) project downwind and thus the head faces into the wind and the slip face is concave downwind
   • isolated, freely migrate across desert plains maintaining their form

   
   

2. parabolic dune
   • associated with vegetation, so form in subhumid and semiarid environments (rather than arid) where vegetation is nearby (e.g. beaches, grasslands - sw sask)
   • originate as a blowout, dune forms as the head of the dune at the downwind edge of the blowout develops the dune asymmetry and advances beyond the horns
   • stability of the sides and horns used to be attributed to vegetation but recent research (including P. David and S. Wolfe in Saskatchewan) suggest that water is a more important factor, so the stability of parts of a parabolic dune and the presence of vegetation are both related to water
   • eventually deflation lowers the blowout to the water table or to an underlying stratum lacking sand (e.g. bedrock or stony clay till) and the dune becomes impoverished

   
   

3. transverse dune
   • linear, cuspate and forms perpendicular to the wind, with large sand supply and low winds
   • with stronger winds they evolve into barchans
   • usually occur on beaches, floodplain alluvium or erodible sandy bedrock rather than in dry deserts

   
   

4. longitudinal dune
   • large (kms in length, ~ one km wide) linear forms parallel to the strong persistent winds
   • form in dry subtropical deserts with irregular sand supply
   • separated by lag gravel
   • whaleback: a ridge of coarse sand left in the path of a migrating longitudinal dune

   
   

5. erg
   • "sand seas", vast sand sheets (Lawrence of Arabia stuff)
   • 1/4 - 1/3 of the area of true deserts
   • the largest sandy deserts overlie poorly consolidated sandy bedrock

   
   

6. fixed sand sheets
   • undulating sandy hills in subhumid environments (e.g. large parts of the Great Sand Hills of Saskatchewan and the Sand Hills of Nebraska)
   • deflation hollows interspersed with subdued stable parabolic dunes

   
   

7. sand shadow
   • accumulation of sand on either side of a fixed obstacle (e.g. shrub or tuft of grass)

   
   

8. sand drift
   • accumulation of sand in the lee of a gap between obstacles or in the still air at the bas of an escarpment

[ Course Outline | Next Topic ]