Deserts and Winds

1. Deserts and Winds

2. Definition of Desert • A desert is an area with less than 25 cm (10 inches) of annual precipitation • aridity index = potential evaporation/precipitation greater than 4.0 • Deserts may be cold, temperate or hot. All major continents have one type of desert or the other.

3. Distribution and causes of dry lands • Dry regions cover 30 percent of Earth’s land surface • Two climatic types are commonly recognized • Desert or arid • Steppe or semiarid

4. about 1/5th of land World's Deserts

5. Atmospheric Convection and Subtropical Deserts Tropopause Barrier RAIN

6. Wind in the desert • Transportation of sediment by wind • Differs from that of running water in two ways • Wind is less capable of picking up and transporting coarse materials • Wind is not confined to channels and can spread sediment over large areas

7. Wind in the desert • Wind erosion • Wind is a relatively insignificant erosional agent with most erosion in a desert performed by intermittent running water • Mechanisms of wind erosion • Deflation • Lifting of loose material • Deflation produces blowouts (shallow depressions) and desert pavement (a surface of coarse pebbles and cobbles)

8. Desert pavement is formed from larger rocks and fragments left after deflation.

9. Desert Pavement

10. Wind in the desert • Transportation of sediment by wind • Mechanisms of transport • Bedload • Saltation – skipping and bouncing along the surface • About 20 to 25 percent of the sand transported in a sandstorm is moved this way • Suspended load

11. Wind in the desert • Wind erosion • Mechanisms of wind erosion • Abrasion • Produces ventifacts (stones with flat faces) and yardangs (wind sculpted ridges) • Limited in vertical extent

12. Yardang: A small wind-sculpted rock formation caused by abrasion. Figure 15.4

14. Wind in the desert • Wind deposits • Significant depositional landforms are created by wind in some regions • Two types of wind deposits • Dunes • Mounds or ridges of sand • Often asymmetrically shaped • Windward slope is gently inclined and the leeward slope is called the slip face

15. Formation of a desert blowout

16. Formation of sand dunes

17. Sand dunes in the western United States

20. Wind in the desert • Wind deposits • Two types of wind deposits • Dunes • Slow migration of dunes in the direction of wind movement • Several types of sand dunes including transverse, longitudinal, parabolic

21. 9. What are the three classes of dune forms? • We can simplify dune forms into three classes • 1–crescentic2- linear3- star dunes, and others. • Crescentic dunes are divided into four types: Barchan, Transverse, Parabolic, and Barchanoid Ridge. Barchan dunes are crescent shaped dunes with horns pointed downwind. Winds are constant with little directional variability. Limited sand availability. Transverse dunes are asymmetrical ridges which are transverse (perpendicular) to the wind direction. Surface has abundant sand supply. Parabolic dunes are generated by vegetation, open end faces upwind with U-shaped and arms shaped by the vegetation. Barchanoid ridge dunes, are wavy, symmetrical dune ridges aligned in right angels to the winds. Formed from coalesced barchans. (See next slides).

23. Barchan

26. WIND • a) barchan dune • b) parabolic dune • c) longnitudinal dune • d) transverse dune depression

27. Parabolic

28. Longnitudinal

29. Longitudinal Dunes

30. Transverse

32. 9. What are the three classes of dune forms? (continued) • The next class of dunes is called linear dunes. They are divided into two types: Longitudinal and Seif. Longitudinal dunes are long, ridge-shaped dunes that are aligned parallel to the wind direction and have two slipface. Average 100 meters high and 100 kilometers long and can reach to 400 meters high. Seif dunes (means sword in Arabic) a sharp-crested sand dune with curved edges, often several miles long. Runs in a series of parallel ridges; most common in the Sahara desert.

33. Longitudinal Dunes (left) and a Satellite photo of Seif Dunes in Saudi Arabia (Right).

34. 9. What are the three classes of dune forms? (continued) • The third class of dunes is called a Star dune (One type only). Star dunes are giant dunes; Pyramidal or star shaped. Slipsurfaces in multiple directions. Resulting from winds shifting in all directions. (See next slide).

35. Star Dunes

36. Star

37. 9. What are the three classes of dune forms? (continued) • The last class is named “other” for other types. There are two of them: Dome dunes and Reversing dunes. Dome dunes are circular or elliptical mounds with no slipface. Reversing dunes are asymmetrical ridges formed intermediately between star dunes and transverse dunes formations. Wind direction can alter their shapes between forms. (See next slide).

38. Dome and Reversing dunes.

39. Wind in the desert • Wind deposits • Two types of wind deposits • Loess • Blankets of windblown silt • Two primary sources are deserts and glacial outwash deposits • Extensive deposits occur in China and the central United States

40. 10. Another form of material deposits are loess deposits. How are loess materials generated? What form do they assume when deposited? • Pleistocene glaciers advanced and retreated in many parts of the world, leaving behind large glacial outwash deposits of fine-grained clays and silts (<0.06 mm). These materials were blown great distances by the wind and redeposited in unstratified, homogeneous deposits named loess. Loess deposits form some complex weathered badlands and some good agricultural land.

41. 2. Loess = silt and clay • Blown from glacial outwash plains, deserts, and floodplains to areas where they have been stabilized by vegetation • From rich top soils of the Ukraine to Germany, China and the northern plains of U.S.

42. Figure 15.24: Desertification Figure 15.24

43. Wind defects and Engineering solution • Wind Deffects: • Covering rail ways, roads,drainage canals, erosion of wooden pools,cover building by sand • Solutions: • Planting • Fencess • Bitumin coat