Deserts

Sediment transport, dunes, dust, etc.

How does wind speed vary with altitude?

Wind speed increases exponentially with altitude. This is due to reduced friction higher up (as the wind isn't interacting with the landscape). 

Define saltation:

Where small grains are bounced along a surface by the wind. The length the grain travels is called "hop length", and this is generally 1m. 

Give the equation for shear velocity:

shear velocity = the square root of (shear stress / air density)

In what way is transport rate related to shear velocity?

Sand transport rate is proportional to the square of shear velocity.

Define resistance:

A substrate’s resistance to transport. Depending on factors such as moisture levels, roughness, vegetation type. 0.03-0.6% moisture content can double threshold speeds in sand.

Define sediment availability:

• The susceptibility of surface grains to transport by wind

• A perfectly susceptible supply would be spherical, well sorted, dry, and loose

• Pebbles will reduce sediment availability as they slow the wind down

Define aeolian deflation:

The movement of already loose particles by the wind 

Define aeolian abrasion:

The bombardment of surfaces by already moving particles 

Describe how wind flows over a sand dune:

• Wind speed is highest at the crest of the dune

• After the crest, it rapidly decelerates and becomes turbulent, leading to deposition

Define ballistic impact:

The collision of already moving saltated sediment and stationary surface grains. It can cause movement of particles below their threshold velocity.

Describe how a dune moves:

• Sand accumulates at the avalanche face, until it exceeds the angle of repose and falls down the lee slope

• Rates of 10-20m / year are typical

• Smaller dunes move faster because there’s less material

Describe the form of a barchan dune:

Crescent shaped individual dunes in areas of low sediment supply and one dominant wind direction

Describe the form of a barchanoid dune:

Clusters/stripes of interconnecting dunes with shapes reminiscent of barchans. Typically in areas with higher sediment supply

Describe the form of a transverse dune:

A further extension of barchanoids, where layers of dunes perpendicular to wind flow form with lots of sediment supply. They may have some curves, but aren't distinguishably like barchans

Describe the form of a star dune:

Form in areas of many wind directions, creating multiple slip faces.

Describe the form of a linear dune:

Form where two wind directions occur, creating long thin dunes roughly along the direction of the two winds. They can reach 200m tall and are common in desert interiors

Describe the form of a falling/climbing dune:

Where dunes form around other structures. 

Describe the formation of a dune:

• A small object or rough surface will disrupt the wind flow

• Hence sediment will be deposited, further disrupting the wind flow and creating a ridge (1-5m wide)

• Therefore the dune develops, getting taller and moving forward

Define equivalent sediment thickness:

Used to describe the amount of sediment in a dunefield. E.g. a barchan dune will have a lower sediment thickness than a transverse dune in the same area.

Give the equation for wind direction variability:

wind direction variability = resultant drift potential / sum of all drift potential

Describe some examples of Fryberger’s classifications:

• <0.3 is complex and obtuse bimodal e.g. star dunes

• 0.3-0.8 is obtuse bimodal - acute bimodal e.g. linear dunes

• >0.8 is wide to narrow unimodal e.g. barchan dunes

Define simple dunes:

Individual dunes, spatially separated e.g. crescentic dunes of the Skeleton Coast, Namibia

Define compound dunes:

Two or more dunes of the same type that are superimposed e.g. the linear dunefields of Mauritania, where the original pattern is 15-25ka old, and the smaller pattern is 10-13ka old.

Define complex dunes:

Two or more dunes of different types that are superimposed e.g. transverse and star dunes in Saudi Arabia.

Describe dune field self-organisation:

• If the wind regime is constant, a simple pattern emerges and the dunes get bigger and more spaced apart 

• If the wind regime is variable, the dune field will reorganise. The smaller dunes will show the current regime, the older dunes may reflect past conditions 

• The reorientation will only occur at the crest termination, and then travel along the length of the crest 

Describe defect migration:

Where defects move faster than ordinary dunes. E.g. a broken transverse dune will move through the ordinary ones faster, transferring the break along.

Define a nebkha:

A dune with a discrete plant at its core

Define a foredune:

A dune with continuous vegetation on the backshore

Define a blowout:

An erosional hollow with a depositional lobe attached

Define a parabolic dune:

Like the inverse of a barchan. Vegetation is unlikely to form on the crest, but form on the arms because it's less disruptive to their growth. They have trailing arms as the plants grow on the edge, stabilising it, and then only the front of the dune continues to move.

Describe the vegetated dunes in Israel:

• In Israel, there are many coastal sand dunes, occupying 460km²  

• Most were formed within the last 1000 years, during which human activity exterminated natural vegetation 

• They were mainly barchan and transverse dunes along the Ashdod coast 

• Vegetation cover increased by 13% between 1944 and 1990 

• Ammophila arenaria is a hardy plant that often pioneers dune vegetation cover 

Describe stabilisation:

Sometimes, when plants grow on barchan dunes, they can transform into parabolic dunes. 

Define the Lancaster Index:

Can describe how mobile a dune is likely to be based on windiness, potential evapotranspiration and precipitation.

M = (Windiness x PET) / Precipitation

Describe how climate change may affect dunes:

• Various climate modelling engines such as Hadcm2 were used to model the impacts of the predicted temperature increase in the next ~100 years in southern Africa

• It predicted that by just 2039, significantly increased dune mobility will occur 

• This is largely due to vegetation damage destabilising the dune 

Define the gross bedform-normal rule:

• Dunes align themselves so that sand transport across the crest is maximised

• Gross transport is defined as transport in which the backward amount is added to, rather than subtracted from, the forward amount) 

• Hence both angular variability and the ratio of sand-carrying capacity affect the dune alignment 

Describe the importance of dust:

• Dust travels globally – the path length of a dust particle is 5x that of sediment carried by a river 

• They aren't constrained to flood plains 

• 2m+ are directly affected by the effects of dust 

• Dryland areas cover 40% of the Earth's land surface 

• Drylands also cover other planets e.g. Mars, so research on Earth can be applied there 

Describe the origins of dust:

• Continental interiors e.g. the Gobi desert 

• Rain shadows, where moisture levels are low (e.g. Death Valley, east of the Sierra Nevada mountains and Pacific Coast Range) 

• Coastal and polar deserts 

Define moisture balance and categories of environment based on it:

• Moisture balance = monthly precipitation / PET

• Hyper-arid = <0.05

• Aird = 0.05-0.2

• Semi-arid = 0.2-0.5

• Dry-subhumid = 0.5-0.65

Define a hamada:

Large, rocky plateaus where the sand has been eroded, leaving bare rocks and pebbles. Also referred to as desert pavements.

Define an erg:

Areas covered with sand dunes.

Define and erg:

Areas of flat, hardened sand.

Define an ephemeral lake:

• They may not contain any water for several years 

• If they are vegetated, they are often economically important – e.g. some lakes in the Chihuahuan desert are covered in Panicum obtusum 

• In lakes where salts accumulate, water loss is primarily from evaporation, but in lakes with clays or silty clays, it is largely from infiltration 

Describe the 1930s Dust Bowl:

• Droughts lasting 100 months in some places caused severe crop damage (at one point only 50% of crops were harvested)

• Inappropriate farming techniques (e.g. dust mulching) worsened the damage

• 43% of a 6.5m ha area was seriously damaged

Give some examples of major dust source areas:

• Gobi desert

• Taklamakan desert

• Bodele depression

• Aral sea

Give some impacts of dust on health:

• Dust itself can be aerosol particulates that impact respiratory health and aggravate prior conditions such as asthma 

• Dust can be formed of toxic materials, for example in the SW US, it contained arsenic and lead 

• In the Aral Sea Region, there is the highest incidence of childhood pneumonia due to pesticides in the dust 

Describe some impacts of dust on the atmosphere:

• Affects air temperatures as it absorbs and scatters solar radiation 

• Affects cloud formation 

• Affects atmospheric sulphur dioxide levels 

• Influences marine productivity as carbon dioxide levels change 

• It has large spatial scales, so can impact people well beyond the original environment 

Describe the biocrust and impacts of its destruction:

• Biocrust is the uppermost layer of the soil, comprising of the soil and a variety of organisms such as mosses, algae, lichens, and bacteria 

• Biocrusts currently reduce atmospheric dust emissions by 60% 

• About 25% of dryland soil surfaces are covered by biocrusts