Longitudinal dune

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Longitudinal dunes (also Longitudinaldünen , linear dunes , stroke dunes or Seifdünen ) are elongated, parallel to the prevailing wind direction sand back. The areas in which there are longitudinal dunes often have only a small amount of sand and have a rather rough subsoil. Longitudinal dunes reach heights of up to 200 meters and in some cases extend over more than 20 kilometers.

etymology

The terms longitudinal , longitudinal , linear - and Strichdüne all relate to the elongated dunes form. The Arabic سيف Saif or Seif (spoken like English safe ) means sword and refers to longitudinal dunes with a curved floor plan in analogy to the Arabic sword shape.

description

The Big Red Longitudinal Dune in the Simpson Desert , Australia
Longitudinal dunes in the Kalahari , Namibia

Longitudinal dunes are characterized by their length, which can often be more than 20 kilometers. In the Simpson Desert in Australia , lengths of up to 300 kilometers can be achieved. Longitudinal dunes are straight aeolian forms of transport, the individual ridges of which are arranged parallel and at regular intervals. The distances between individual longitudinal dune ridges can vary between 400 and 3000 meters, in extreme cases they can even grow to 6000 meters.

Individual combs can also converge downwind like a tuning fork, in the shape of an inverted Y, and new combs grow halfway between two dunes.

Longitudinal dunes are able to migrate downwind, for example the Mauritanian dunes advanced 45 meters / year against the West African shelf edge during the last ice age.

In the Namib , large longitudinal dunes reach heights of almost 200 meters. Their distance to height ratio (d / h) is between 15: 1 and 20: 1. However, with smaller dunes in other deserts, this ratio is much higher (50: 1 to 200: 1).

Many longitudinal dunes consist of a moderately sloping substructure, which is often stabilized by vegetation. The upper, steeper ridge area, however, is plantless and in motion. Him to slip relationships develop ( English slip faces ) whose orientation is dependent on the prevailing wind direction.

Longitudinal dunes are generally symmetrical and triangular in elevation. However, this basic shape can become increasingly asymmetrical over the course of a year, with a convex windward side with a clear leeward side developing.

  • Simply constructed longitudinal dunes consist of two basic types:
  • Composite longitudinal dunes (engl. Compound dunes ) consist of two to four Seif-like ridges which rest a wide base. They are characteristic of the southern Namib .

Line dunes or silk dunes (from the Arabic Silk سلك meaning thread, string, wire ) are a special form of composite longitudinal dunes, which are created by the lateral merging of soap dunes in the direction of the prevailing wind. They are relatively low dunes a short distance apart with a sinusoidal shape of the ridge line. They occur in the Al Jafurah in eastern Saudi Arabia and are explained by two different wind systems - the shamal , a north-westerly wind that is replaced by easterly winds in spring.

  • Complex longitudinal dunes have a single, curved ridge, which is interrupted by star-dune-like summit regions. Their flanks are also superimposed by barchanoid transversal structures. These large, 50 to 150 meter high structures are to be placed next to the Draa . They run parallel to each other at a distance of 1 to 2 kilometers and occur in the Namib and Rub al-Chali of Arabia .

There are also complex longitudinal dunes that are 1 to 2 kilometers wide and are overlaid by barchanoid shapes in their ridges. They occur in the eastern Namib, in parts of the Wahiba Sands in Oman and in the Akchar sand sea of Mauritania .

Internal structure

Internally, longitudinal dunes are built up from complex, large-scale oblique stratification bodies whose angles of incidence have two clearly separated maxima. The individual inclined layering sets are separated from one another by discordances ( bounding surfaces ). The opposite, at a relatively high angle (by 33 degrees, sometimes even up to 36 degrees) and mostly parallel to the ridge, are created by avalanche-like, seasonally alternating sand sliding on the two slide slopes.

Much shallower to horizontally dipping layers are found primarily on the flanks and at the foot of the longitudinal dunes. They were not caused by landslides ( encroachment deposit ), but were blown ( accretion deposit ). They can be relatively coarse-grained and have parallel or oblique stratification. These are deposits that have emerged from Zibar or small transverse dunes or ballistic ripples.

A rather complicated structure Inter have the so-called Walrücken (Engl. Whalebacks ). These are composed dune shapes with platform-like elevations, which were left by several, partly overlapping longitudinal dunes.

Emergence

There is still disagreement about an explanation of the formation process of longitudinal dunes. The development models developed so far can be divided into four subject areas:

  • Directional dependence
  • Residual form
  • Modification of existing dune shapes
  • Taylor-Görtler vortex

Directional dependence

The common interpretation puts the course of the longitudinal dunes parallel to the prevailing wind direction. Their parallel, straight-line arrangement is associated with screw vortices ( English roller vortices ), which erode the sand from the intermediate dune area and deposit it again in the dune ridges. However, due to various inconsistencies, this theory is no longer shared by all authors.

In the meantime, there is increasing evidence that longitudinal dunes are formed under the influence of two main wind directions, the resultant of which coincides with their longitudinal extent. This approach is supported by correlations between dune type and wind regime, investigations of their internal structure and detailed process studies.

The formation of meandering, pearl-like soap dunes is also explained by means of two main wind directions intersecting at an acute angle. It is believed that this dune shape emerges from one of the arms of sickle dunes.

Overall, however, it is becoming apparent that longitudinal dunes are not a primary form of dune, but that they developed from other types of dunes when they migrated to areas with a different wind pattern.

An alternative consideration to the wind result model sees the decisive factor for the formation of longitudinal dunes in wind currents running diagonally to the dune ridge. On the leeward side of the dune, the overflow creates a helicoidal, lateral deflection of the current (secondary current), so that on the leeward side sand is transported parallel to the course of the dune. With a longitudinal dune running in north direction, this effect is achieved by all winds that flow in from west to south to east and thus cover a sector of around 180 degrees. However, the sand transport is obviously maximized with a persistent angle of incidence of wind of 20 to 30 degrees in relation to the alignment of the longitudinal dune. Winds that are steeper than 30 degrees are no longer optimal for the growth in length, as they tend to deposit the sand directly on the leeward slide slopes and hardly shift it laterally. The sand thus remains in place and causes the dune to grow in height. The extreme case are winds that sweep vertically (at 90 degrees) over the dune body. There are then reversing dunes that grow vertically. On the leeward side, secondary flow cells form, which shift the sand towards the center of the dune, which ultimately results in star dunes .

Residual form

This very old explanatory model regards longitudinal dunes as residual forms created by the wind. It is believed that strong wind erosion carved out long furrows from massive alluvial deposits and left the only thin layer of sand in the dunes. Although some observations seem to confirm this theory, the bulk of the evidence speaks for an active, structural character of the longitudinal dunes.

Modification of existing dune shapes

Overall, it appears that longitudinal dunes are not a primary dune form, but that they developed from other dune types when they migrated to areas with different wind patterns.

The formation of meandering, pearl-like soap dunes is explained by means of two main wind directions intersecting at an acute angle. It is believed that this dune shape emerges from one of the arms of sickle dunes. Examples of this type can be found in Sinai.

Taylor-Görtler vortex

The parallel, straight-line arrangement of longitudinal dunes is also associated with counter-rotating Taylor-Görtler screw vortices ( Taylor-Görtler vortices or roller vortices ), which erode the sand from the intermediate dune area and deposit it again in the dune ridges. Analogies are linear rows of clouds ( cloud streets ) in the atmosphere (of a comparable scale) as well as the straight rows of snow and sand ( sand streamers - small-scale) that are often observed and blown by the wind over smooth, immovable surfaces (e.g. B. Ice).

The diameter of the screw vortices is determined by the thickness of the atmospheric boundary layer, which is around 1 kilometer in the area of ​​the trade winds. The Y-shaped, pairwise convergence of longitudinal dune rows can be explained very well with the lifting of a screw vortex from the earth's surface.

However, due to various inconsistencies, the Taylor-Görtler theory is no longer shared by all authors.

Influence of the wind speed

The wind speed parameter is highlighted by Glennie. All other things being equal, longitudinal dunes develop in a sandy desert at higher wind speeds than, for example, sickle dunes. If the wind speed increases, the higher the longitudinal dunes and the greater the distances between the individual ridges. According to Glennie, significantly more longitudinal dunes were created during the Pleistocene glaciations due to the increased wind speeds. The current, weaker wind regime can no longer maintain longitudinal dunes. Hence the observed overprinting by barchanoids and other forms.

Movement and age

As already explained above, longitudinal dunes not only move downwind, but they also shift sideways (especially complex longitudinal dunes). For complex longitudinal dunes in the Namib, Bristow et al. (2007) give a lateral displacement rate of 0.1 meters / year. With a width of 600 meters, the reconstitution time of the dunes is 6000 years. This is in good agreement with measured OSL ages of 5730 ± 360 years.

Occurrence

Lancaster estimates that around 50 percent of all dunes are made up of longitudinal dunes. In parts of the Kalahari, the Simpson Desert and the Strzelecki Desert, their proportion even rises to 85 to 90 percent and in the Sahara to 72 percent, whereas longitudinal dunes in Alashan and Gran Desierto of Mexico only make up 1 to 2 percent. Longitudinal dunes are the dominant dunes in deserts of the southern hemisphere as well as in the southern and western Sahara.

Their occurrences in detail:

Individual evidence

  1. ^ A b c N. Lancaster: Dune Morphology and Dynamics . Ed .: AD Abrahams, AJ Parsons. Chapman & Hall, London 1994, ISBN 0-412-44480-1 .
  2. ^ A b R. L. Folk: Longitudinal dunes of the northwestern edge of the Simpson Desert, Northern Territory, Australia. 1: Geomorphology and grain size relationships . In: Sedimentology . tape 16 , 1971, p. 5-54 .
  3. ↑ M. Sarnthein, E. Walger: The aeolian sand stream from the W-Sahara to the Atlantic coast . In: Geologische Rundschau . tape 63 , 1974, pp. 1065-1087 .
  4. ^ RJ Wasson, R. Hyde,: A test of granulometric control of desert dune geometry . In: Earth Surface Processes and Landforms . tape 8 , 1983, p. 301-312 .
  5. ↑ H. Tsoar: profile analysis of sand dunes and Their steady state significance . In: Geografiska Annaler . 67A, 1985, pp. 47-59 .
  6. ↑ H. Tsoar: Dynamic processes acting on a longitudinal (seif) dune . In: Sedimentology . tape 30 , 1983, pp. 567-578 .
  7. ^ N. Lancaster: Controls of Dune Morphology in the Namib Sand Sea . In: ME Brookfield, TS Ahlbrandt (Ed.): Developments in Sedimentology . Volume 38. Elsevier, 1983, ISSN  0070-4571 , pp. 261-289 .
  8. ↑ G. Kocurek et al .: Dune and dunefield development stages on Padre Island, Texas: effects of lee airflow and sand saturation levels and implications for interdune deposition . In: Journal of Sedimentary Petrology . tape 62 , 1992, pp. 622-635 .
  9. ↑ H. Tsoar: Internal structure and surface geometry of longitudinal (seif) dunes . In: Journal of Sedimentary Petrology . tape 52 , 1982, pp. 823-831 .
  10. ^ RA Bagnold: The physics of blown sand and desert dunes . Methuen, London 1954.
  11. ↑ H. Tsoar: Linear dunes - forms and formation . In: Progress in Physical Geography . tape 13 , 1989, pp. 507-528 .
  12. ↑ IG Wilson: Aeolian bedforms - their development and origins . In: Sedimentology . tape 19 , 1972, p. 173-210 .
  13. ↑ a b c S. R. Hanna: The formation of longitudinal sand dunes by large helical eddies in the atmosphere . In: Journal of Applied Meteorology . tape 8 , 1969, p. 874-883 .
  14. ↑ SG Fryberger: Dune forms and wind regimes . In: ED McKee (Ed.): A study of global sand seas, United States Geological Survey Professional Paper . Paper 1052, 1979, pp. 137-140 .
  15. ^ E. McKee: Sedimentary structures in dunes of the Namib Desert, South West Africa, Geological Society of America Special Publication . Paper 188, 1982.
  16. ^ I. Livingstone: Monitoring surface change on a Namib linear dune . In: Earth Surface Processes and Landforms . tape 14 , 1989, pp. 317-332 .
  17. ^ A b H. Wopfner, CR Twidale: Formation and age of desert dunes in the Lake Eyre depocenters in central Australia . In: Geologische Rundschau . tape 77 , 1988, pp. 815-834 .
  18. ↑ a b H. Tsoar: Desert dunes morphology and dynamics, El Arish (northern Sinai) . In: Journal of Geomorphology Supplement Volume . tape 20 , 1974, p. 41-61 .
  19. ^ RC Sprigg: Stranded and submerged sea-beach systems of southeast Australia and the Aeolian desert cycle . In: Sedimentary Geology . tape 22 , 1979, pp. 53-96 .
  20. ^ KW Glennie: Desert Sedimentary Environments . In: Developments in Sedimentology . tape 14 . Elsevier, Amsterdam 1970, p. 222 .
  21. ^ CS Bristow, GAT Duller, N. Lancaster: Age and dynamics of linear dunes in the Namib desert . In: Geology . tape 35 , 2007, p. 555-558 .
  22. ^ N. Lancaster: Linear dunes . In: Progress in Physical Geography . tape 6 , 1982, pp. 476-504 .
  23. ^ WM Jordan: Prevalence of sand-dune types in the Sahara desert . In: Ann. GSA & Assoc. Soc. Joint. Meet. Miami Beach, Program 1964, pp. 104-105 .