Dune

A dune is a collection of sand that is blown in and deposited by the wind. Dune formation requires the presence of sand, and the absence of water or a closed vegetation. Therefore dunes are formed preferably in dry ( arid ) climates, but can also occur in humid regions, where the fastened vegetation has been eliminated. If the sand rather evenly to blows in the form of a blanket, it is called neutral by shifting sand.

• 3.1.2.1 parabolic dune / Paraboldüne / Bogendüne
• 3.1.2.2 Leedünen

• 3.2.1 Kreuzdüne / Sterndüne

Dune formation

The formation of dunes can be explained by the Helmholtz 's law: " currents, two media of different density past each other, the result is a wavy boundary surface. " This is flow energetically favorable at the generally prevailing winds as a planar boundary surface ( Cw). A distinction is large molds, the actual dunes, and small forms, the ripples. During dunes can reach considerable dimensions and thicknesses, ripples usually have a height of only a few centimeters, with an extension in the decimeter range.

Dune areas

The dune areas of the world are divided according to the general climates or for special cases:

• Dunes of the tropical and subtropical deserts and semi-deserts
• Dunes humid climates
• Dunes of the cold deserts and tundras
• Beach or coastal dunes and active floodplains

Dune types

In geomorphology, a distinction between different types of dunes, each depending on different wind directions and speeds ( aeolian dynamics), the density of the vegetation cover and the sand supply.

Despite all regular shapes are irregular dunes one of the most common types of dunes. The shape of the above-mentioned regular dunes in most cases is not perfect, but always somewhat modified. The causes are many and varied. They lie in a partially chaotic flow behavior of the wind, even momentarily fluctuating wind direction and the other in irregularities of the ground, vegetation and the moisture of the sand.

In the case of submarine formed ripple marks with a height from one meter ( mega ripples, large ripples ), one speaks of subaqueous dunes.

Primary dune

Apart from the parabolic dunes, which are linked to the presence of vegetation, one summarizes the above dune forms together to the free dunes. In contrast, the sand deposits are at obstacles, which are called bound dunes ( parabolic dunes, Leedünen, echoing dunes, sand ramps). They are often confused with brown dunes and white dunes.

Free dunes ( dunes )

Sicheldüne ( barchan )

The Sicheldüne, also referred to as Barchan, is the most common form of dry climates dunes. Its origin goes back to the vortex formation between the dunes crests. On the windward side of the surface of the dune increases with low gradient (about 15 ° ) and falls off to leeward steep ( about 30 °). At the two sides, the sickle ends, the height of the crest of the dune drops. The sickle ends move faster than the main ridge of the dune, causing the characteristic shape. Reason for the faster migration of the side arms, the lower mass of sand that has to be circulated by the wind. Crescent dunes caused by constant winds from a main wind direction and " migrate" to leeward. In Barchanfeldern no or only an extremely patchy vegetation is present, that impedes the movement of the sand.

Querdüne / Transversaldüne / Reihendüne

The transverse dunes, Transversaldünen or series dunes are elongated ridges which are arranged transversely to the prevailing wind direction. They are formed in areas where abundant sand is present and a lack of vegetation. Important in their formation is the presence of a dominant wind direction, so you can find cross dunes often on sandy coasts.

Längsdüne / Longitudinaldüne / Lineardüne / Strichdüne

The longitudinal dunes and linear dunes are elongate parallel to the prevailing wind direction running sand ridge. The areas in which there are longitudinal dunes, often have only a small sand delivery and more likely to have a rough surface. These dunes reach heights up to 100 meters and extend partly over many kilometers. Frequently found are those miles of sandy mounds in the Western Sahara, due to the strong, constant running in one direction trade winds. They often go back on parabolic dunes whose main ridge was breached due to the persistent winds.

Draa dune

The Draa dunes are huge or mega dunes with stable ridges. Unlike the other types of dunes they come only in the large sandy areas of the world before ( in the Erg of the Sahara ). The Draa is ancient forms from the Pleistocene. They are also referred to as inland dunes.

Bound dunes

Parabolic dune / Paraboldüne / Bogendüne

The Paraboldüne, also Bogendüne, the Sicheldüne is similar but narrower and has in plan an opposite curvature. It is partly covered with vegetation. The convex side of leeward. The vegetation of nearly free central part of the dune moves faster than the legs that can not follow the main ridge, because they are determined by the dense vegetation there ( heather, crowberry ). Parabolic dunes occur in regions with significant precipitation ( semiarid to humid ), allowing the development of a loose vegetation cover.

Leedünen

Leedünen are sand drifts on the lee side of an obstacle caused by the division of the streamlines of the air flow a slipstream. The fact that the vortex in the lee are weaker than in the main wind flow, leeward the sand grains settle out.

Secondary dunes

Secondary dunes form

• From other dunes ( Kreuzdüne / Sterndüne )
• On other dunes ( patch secondary dune / grid dunes)

They are immobile.

Kreuzdüne / Sterndüne

The cross or star dunes are particularly high dunes and are formed by seasonal overlapping wind directions in the presence of large amounts of sand. The star dunes can rise several meters above the surrounding 100 under certain circumstances. This dune type does not migrate, but the sand is shifted repeatedly.

Dunes in Central Europe

In Central Europe, except the coastal dunes have only the inland dunes formed in the waning ice age a significant spread.

• Coastal dunes are associated with flat coastal spatially and genetically. The sand dunes of this type will approach flown by prevailing onshore winds from the sandy beach, that originally came from the sea, building sand beach. Depending on the development of the vegetation in the hinterland of the beach can create different types of dunes, ranging from irregular shapes to parabolic dunes.

• The inland dunes were originally formed in the cold tundra during the fading Weichseleiszeit. A further development or reactivation in the post-glacial period was under the influence of man. Through intensive land use, z.T. Overexploitation, the vegetation cover was destroyed and reactivated the sand drift.

Dunes in the world ( examples)

One of the highest dunes in the world include those in the Namib Desert at Sossusvlei. You can reach well over 200 meters. In the desert of Alashan highest sand summit Biluthu reaches almost 520 meters above sea level. He will probably stabilized by moisture from the soil. At the same time they are the oldest dunes in the world. As the wind blows, only the top layer over the dune crests, it is assumed that the foundations of the giant dunes were last moved about 5000 years ago. Giant dunes are considered tourist attractions.

The Rub al Khali is the largest sand desert in the world. Here, sand dunes cover an area of over 500,000 square kilometers and reach the dune Tel Moreeb up to 300 meters in height.

The largest dune in Europe ( Dune du Pilat ) is currently about 110 m high and is located in France in Arcachon. Large dunes there were and are also at Pillkoppen (61 m) and Nida on the Curonian Spit, in the Slowinski National Park ( Lontzkedüne, 42 m) at Leba in Poland and Denmark, for example, the Råbjerg Mile.

Singing Dunes

Singing dunes are found in virtually all deserts of the world and beyond, even in areas with large beach dunes. Thereby generated as sliding on the sand dunes deep and loud buzzing sounds are described as singing, more than 10 km are partially audible and last up to 15 minutes.

The resulting sound is often compared to hum, moan, drums, thunder, fog horns or low-flying propeller aircraft. Due to this large bandwidth and long unexplained origin of the sounds the singers have found dunes input in legends and literature. Even Marco Polo reported from the Gobi desert by evil spirits that " sometimes the air with the sounds of various musical instruments crowded, but sometimes with drums or the noise of weapons ." Recent mentions can be found in such diverse texts such as Charles Darwin's Voyages of the Beagle (1889 ) and Frank Herbert's Dune (1984).

The sounds generated by the dunes can reach a volume of up to 100 dB and are mostly around 100 Hz; but there are also documents frequencies up to 770 Hz.

Although the phenomenon of singing sand dunes so has long been known and has been studied scientifically, the essential mechanisms of action have been studied only in recent years. Are generated by the noise from slipping sand avalanches, in which move the sand grains down. By this movement, the adjacent air is vibrated, so that a small pressure wave and thus the sound wave. Since the layer of sand Slipping up to 500 grain of sand layers, the individual tones add in synchronous motion of the sand grains to the loud humming sounds. It is still unclear as yet the mechanism by which the sand grains synchronize their movement.

Singing dunes are found in almost all the dry deserts of the world, inter alia,

• The Namib at Terrace Bay ( Skeleton Coast )
• The Sahara
• The Gobi
• The Arabian desert
• On Sand Mountain in Nevada
• And Hawaii.