15 Chapter Aeolian Processes and Arid Landscapes 11-15-2007 Frank Hopf
Outline
Work of wind erosion, transport and deposition A look at resulting landforms beach, desert and loess Arid lands discussion The perils of overpopulating arid lands The Colorado River example Desertification and climate change
Work of wind erosion, transport and deposition
Wind is the geomorphic agent that takes over once the water, ice or waves stop. Where all is moist, the wind has a hard time moving things, partially because of added cohesion and largely because vegetation protects the soil or sand from the wind But in dry areas and beaches AEOLUS can be dominant .
Work of wind erosion, transport and deposition (2)
Not unlike water erosion (removing) transport (moving) and deposition (leaving) is mostly determined by fluid velocity and particle size. Aeolian forces add complication of elevation change and moisture content. A particle (sand grain) will remain in place as long as it's threshold velocity is not exceeded. If the threshold velocity of an indivdual grain is exceeded, then the wind will begin to pick up and move those individual grains by creep, saltation, or suspension. When the air speed drops below threshold for a particular grain, the wind will deposit the grain at that point. For non-cohesive materials (sand, cobbles, gravel), the threshold velocity for movement increases with grain size, i.e. finer materials (sand) require lower wind velocities to move.
Chart of Bagnold's work 1941 showing the increasing amount of a certain sand grain size as the wind velocity Increases above the threshold
Unfortunately it is difficult to measure the wind speed and other formulas have been proposed as shown on the next page
Models of relationship of shear velocity and sand transport
Note the log scale use on this chart for q, the amount of sediment transported
CEM 2002
Shear velocity (m/sec)
R2D2 And CP30 doing sand transport vs wind velocity experiments on some beach on Titan, the largest moon of Saturn (just kidding)
Aeolian Erosional Landforms
Yardang - vertifact
http://marlimillerphoto.com/ wind.html Ventifact Any rock abraded by the wind found on deserts of earth, Mars, and Titan, moon of Saturn.
Martian ventifact, picture by Martian rover
Aeolian Erosional Landforms
Yardang elongated ridges carved by wind, abrasion on leading edge, may be 150 meters high
Refurbed yardang?
Yardang from Meadow TX http://www.answers.com/topic/yardang
Deflation
Erosion of large surface wind, by creating a hole or blowout Can leave lag deposits (pebbles and gravel) called desert pavement
http://geology.wr.usgs.gov/parks/ deva/galmisc.html
h t Australian Blowout burying all in its way t p : / / h u m a n i t i e s . c q u . e d u
http://humanities.cqu.edu.au/geographyGEOG11023/week_6.htm
Aeolian Erosion (cont.)
Depression Qattara Depression Egypt 2 130 m below S.L. 18,000 km
Blowouts often caused by damage to vegetation at beach
Gulf War damage to Saudi and Kuwait desert pavement, now deflation and downwind dust storms Deserts and beach dunes are very fragile environments
From Process Geomorphology Ritter et al. 2002
Methods of Aeolian Transport Creep sliding of larger particles on surface Saltation bounding or hopping of particles along surface Suspension only the smaller particles lifted high in the air and keep there by air turbulence. Can circle globe
Depositional Landforms
Smallest Sand ripples picture below from text, picture is only one meter wide
Dunes
Dunes = wind (or water) sculpted piles of sand Lots of dunes = Erg or sand sea (only about 10% of world's desert surfaces) Active and inactive Sand Hills of Nebraska and much of Permian Basin stabilized or inactive ergs. Grand Erg Oriental in Sahara, sand is 1200 m thick Rub'al Khali in Saudi Arabia is an ergs Mars has ergs.
Sand Movement across a dune or ripple
http://www.indiana.edu/~g103/G103/Week8/week8.html
Dune Types - Basics
Transverse (wind moving sand) Longitudinal (moving w...