dune

Formation

A universally precise distinction does not exist between ripples, dunes, and draas, which are all deposits of the same type of materials. Dunes are generally defined as greater than 7 cm tall and may have ripples, while ripples are deposits that are less than 3 cm tall. A draa is a very large aeolian landform, with a length of several kilometres and a height of tens to hundreds of meters, and which may have superimposed dunes.

Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. The upwind/upstream/upcurrent side of the dune is called the stoss side; the downflow side is called the lee side. Sand is pushed (creep) or bounces (saltation) up the stoss side, and slides down the lee side. A side of a dune that the sand has slid down is called a slip face (or slipface).

The Bagnold formula gives the speed at which particles can be transported.

Aeolian dunes

Aeolian dune shapes

Five basic dune types are recognized: crescentic, linear, star, dome, and parabolic. Dune areas may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types).

Barchan dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on the concave sides of the dunes. These dunes form under winds that blow consistently from one direction (unimodal

winds). They form separate crescents when the sand supply is comparatively small. When the sand supply is greater, they may merge into barchanoid ridges, and then transverse dunes (see below).

Some types of crescentic dunes move more quickly over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in China's Ningxia Province, and similar speeds have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's Taklamakan Desert.

Abundant barchan dunes may merge into barchanoid ridges, which then grade into linear (or slightly sinuous) transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to the ridge crest.

Seif dunes are linear (or slightly sinuous) dunes with two slip faces. The dunes lie generally parallel to each other The two slip faces make them sharp-crested. They are called seif dunes after the Arabic word for "sword". They may be more than 160 kilometres (100 miles) long, and thus easily visible in satellite images (see illustrations).

Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along the resultant direction of sand movement (hence the name "longitudinal"). Some linear dunes merge to form Y-shaped compound dunes.

Formation is debated. Ralph Bagnold, in The Physics of Blown Sand and Desert Dunes, suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by vortices in a unidirectional wind. In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes.

Dune complexity

All these dune shapes may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types). Simple dunes are basic forms with the minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed. Complex dunes are combinations of two or more dune types. A crescentic dune with a star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed.

Dune movement

The sand mass of dunes can move either windward or leeward, depending on if the wind is making contact with the dune from below or above its apogee. If wind hits from above, the sand particles move leeward; the leeward flux of sand is greater than the windward flux. Conversely, if sand hits from below, sand particles move windward. Further, if the wind is carrying sand particles when it hits the dune, the dune's sand particles will saltate more than if the wind had hit the dune without carrying sand particles.

Coastal dunes

Coastal dunes form when wet sand is deposited along the coast and dries out and is blown along the beach. Dunes form where the beach is wide enough to allow for the accumulation of wind-blown sand, and where prevailing onshore winds tend to blow sand inland. The three key ingredients for coastal dune formation are a large sand supply, winds to move said sand supply, and a place for the sand supply to accumulate. Obstacles—for example, vegetation, pebbles and so on—tend to slow down the wind and lead to the deposition of sand grains. These small "incipient dunes" or "shadow dunes" tend to grow in the vertical direction if the obstacle slowing the wind can also grow vertically (i.e., vegetation). Coastal dunes expand laterally as a result of lateral growth of coastal plants via seed or rhizome. Models of coastal dunes suggest that their final equilibrium height is related to the distance between the water line and where vegetation can grow. Coastal dunes can be classified by where they develop, or begin to take shape. Dunes are commonly grouped into either the Primary Dune Group or the Secondary Dune Group. Primary dunes gain most of their sand from the beach itself, while secondary dunes gain their sand from the primary dune. Along the Florida Panhandle, most dunes are considered to be foredunes or hummocks. Different locations around the globe have dune formations unique to their given coastal profile.

Coastal sand dunes can provide privacy and/or habitats to support local flora and fauna. Animals such as sand snakes, lizards, and rodents can live in coastal sand dunes, along with insects of all types. Often the vegetation of sand dunes is discussed without acknowledging the importance that coastal dunes have for animals. Further, some animals, such as foxes and feral pigs can use coastal dunes as hunting grounds to find food. Birds are also known to utilize coastal dunes as nesting grounds. All these species find the coastal environment of the sand dune vital to their species' survival.

Over the course of time coastal dunes may be impacted by tropical cyclones or other intense storm activity, dependent on their location. Recent work has suggested that coastal dunes tend to evolve toward a high or low morphology depending on the growth rate of dunes relative to storm frequency. During a storm event, dunes play a significant role in minimizing wave energy as it moves onshore. As a result, coastal dunes, especially those in the foredune area affected by a storm surge, will retreat or erode. To counteract the damage from tropical activity on coastal dunes, short term post-storm efforts can be made by individual agencies through fencing to help with sand accumulation.

Gypsum dunes

In deserts where large amounts of limestone mountains surround a closed basin, such as at White Sands National Park in south-central New Mexico, occasional storm runoff transports dissolved limestone and gypsum into a low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as selenite. The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes. These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into the sea.

Nabkha dunes

A nabkha, or coppice dune, is a small dune anchored by vegetation. They usually indicate desertification or soil erosion, and serve as nesting and burrow sites for animals.

Subaqueous dunes

Subaqueous (underwater) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural channels such as rivers and estuaries, and also form in engineered canals and pipelines. Dunes move downstream as the upstream slope is eroded and the sediment deposited on the downstream or lee slope in typical bedform construction. In the case of subaqueous barchan dunes, sediment is lost by their extremities, known as horns.

These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in wavelength and height. The shape of a dune gives information about its formation environment. For instance, rivers produce asymmetrical ripples, with the steeper slip face facing downstream. Ripple marks preserved in sedimentary strata in the geological record can be used to determine the direction of current flow, and thus an indication of the source of the sediments.

Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river flooding.

Lithified dunes

A lithified (consolidated) sand dune is a type of sandstone that is formed when a marine or aeolian sand dune becomes compacted and hardened. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the colour of the rock. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in Zion National Park in the western United States.

A slang term, used in the southwest US, for consolidated and hardened sand dunes is "slickrock", a name that was introduced by pioneers of the Old West because their steel-rimmed wagon wheels could not gain traction on the rock.

Desertification

Sand dunes can have a negative impact on humans when they encroach on human habitats. Sand dunes move via a few different means, all of them helped along by wind. One way that dunes can move is by saltation, where sand particles skip along the ground like a bouncing ball. When these skipping particles land, they may knock into other particles and cause them to move as well, in a process known as creep. With slightly stronger winds, particles collide in mid-air, causing sheet flows. In a major dust storm, dunes may move tens of metres through such sheet flows. Also as in the case of snow, sand avalanches, falling down the slipface of the dunes—that face away from the winds—also move the dunes forward.

Sand threatens buildings and crops in Africa, the Middle East, and China. Drenching sand dunes with oil stops their migration, but this approach uses a valuable resource and is quite destructive to the dunes' animal habitats. Sand fences might also slow their movement to a crawl, but geologists are still analysing results for the optimum fence designs. Preventing sand dunes from overwhelming towns, villages, and agricultural areas has become a priority for the United Nations Environment Programme. Planting dunes with vegetation also helps to stabilise them.

Conservation

Dune habitats provide niches for highly specialized plants and animals, including numerous rare species and some endangered species. Due to widespread human population expansion, dunes face destruction through land development and recreational usages, as well as alteration to prevent the encroachment of sand onto inhabited areas. Some countries, notably the United States, Australia, Canada, New Zealand, the United Kingdom, Netherlands, and Sri Lanka have developed significant programs of dune protection through the use of sand dune stabilization. In the U.K., a Biodiversity Action Plan has been developed to assess dunes loss and to prevent future dunes destruction.

Examples

Africa

Alexandria Coastal Dunefields, in the Eastern Cape, South Africa

Witsand Nature Reserve in the Kalahari Desert, South Africa

The white dunes of De Hoop Nature Reserve, South Africa

The dunes of the Suguta Valley, a desert part of the Great Rift Valley in northwestern Kenya

The dunes of the Danakil Depression, northeastern Ethiopia toward the border with Eritrea

The dunes of Sossusvlei in the greater Namib-Naukluft National Park, Namibia

Chad Basin National Park in northern Nigeria

The coastal dunes of Iona National Park in the southwesternmost part of Angola

Khawa dunes in the Kgalagadi Transfrontier Park, the southwesternmost part of Botswana

La Dune Rose in the city of Gao in northern Mali near the Niger River

Erg Aoukar in southeastern Mauritania extending into Mali

Erg Chech in southwestern Algeria and northern Mali

Erg Chebbi and Erg Chigaga in southern Morocco

Grand Erg Oriental in northeastern Algeria and southern Tunisia

Grand Erg Occidental in western Algeria

The Idehan Ubari and the Idehan Murzuq in southwestern Libya

Asia

Sunset view dunes in the Alankuda village on Kalpitiya peninsula in Sri Lanka.

The dunes in the Thar Desert in India and Pakistan

Tottori Sand Dunes, Tottori Prefecture, Japan

Chara Sands, Zabaykalsky Krai, Russia

Rig-e Jenn in the Central Desert of Iran.

Rig-e Lut in the Southeast of Iran.

The Ilocos Norte Sand Dunes in the Philippines, particularly Paoay Sand Dunes.

Moreeb Dune in Liwa Oasis, United Arab Emirates, used as an arena for drag motor sports and Sandboarding.

Gumuk Pasir Parangkusumo near Parangtritis beach in Yogyakarta, Indonesia.

Lautan Pasir, a volcanic dune in Bromo Tengger Semeru National Park, Indonesia.

Mui Ne, Vietnam.

Sharqiya Sands, Oman

Teri, red dune complex in southern India

The dunes of the Taklamakan Desert in southwest Xinjiang in Northwest China

Tukulans of the Central Yakutian Lowland, Russia

Europe

The Dunes of Dyuni, near Pomorie, Bulgaria, vast area of sand dunes in the Burgas Province

The Dune of Pilat, not far from Bordeaux, France, is the largest known sand dune in Europe

The Dunes of Piscinas, in the south west of Sardinia island.

Sands of Forvie within the Ythan Estuary complex, Aberdeenshire, Scotland.

Oxwich Dunes, near Swansea, is on the Gower Peninsula in Wales.

Winterton Dunes – Norfolk, England

Słowiński National Park, Poland

Siedlec Desert, Poland

Starczynów Desert, now mostly forested dunes, Poland

Sand dunes of Lemnos, Lesbos Prefecture, Greece

Akrotiri Sand Dune, Lemesos, Cyprus

Råbjerg mile, Northern Jutland, Denmark

Thy National Park, North Denmark Region, Denmark

Dunes of Corrubedo, Spain

Cresmina Dune, Portugal

Northern Littoral Natural Park, Portugal

Dune of Salir, Portugal

São Jacinto Dunes Natural Reserve, Portugal

Rëra e Hedhur in Shëngjin, Albania

North America

Victoria Island Sand Dunes,160 km North West of Cambridge Bay, Nunavut, Canada. Approximately 600 square kilometres, the largest in Canada, third largest in North America and the largest in the Arctic. There are two lakes with direct access to the Dunes from float planes.

The Athabasca Sand Dunes, located in the Athabasca Sand Dunes Provincial Park, Saskatchewan.

Carcross Desert near Carcross, Yukon

Alaska:

Nogahabara Sand Dunes in western Alaska.

Great Kobuk Sand Dunes, Kobuk Valley National Park, Alaska

California:

The Cadiz Dunes in the Mojave Trails National Monument in California.

The Kelso Dunes in the Mojave Desert of California.

Eureka Valley Sand Dunes and Mesquite Flat Sand Dunes in Death Valley National Park, California.

Algodones Dunes near Brawley, California.

Guadalupe-Nipomo Dunes, on the central coast of California.

Cat Dune in the Cronese Mountains is a rare type of dune called a sand ramp.

Great Sand Dunes National Park, Colorado.

The Great Dune found in Cape Henlopen State Park in Lewes, Delaware.

Bruneau Dunes State Park – Owyhee Desert, Idaho

South America

Argentina:

Duna Federico Kirbus in Catamarca Province, Argentina

Villa Gesell in Buenos Aires Province, Argentina

Brazil:

Lençóis Maranhenses National Park in the state of Maranhão, Brazil

Ilha Comprida Environmental Protection Area in the state of São Paulo, Brazil

Joaquina Beach Dunes in Florianópolis, in the state of Santa Catarina, Brazil

Jericoacoara National Park, in the state of Ceará, Brazil

Genipabu in Natal, Brazil

Chile:

Cerro Medanoso in Atacama Region, Chile

Colún Beach, Valdivian Coastal Reserve in Chile

Dunas de Concón, Chile

Cerro Dragón (Chile), urban dune in Iquique, Chile

Peru:

Cerro Blanco in Nazca Province, Peru

Huacachina in Ica Region, Peru

Venezuela: Medanos de Coro National Park near the town of Coro, in Falcón State, Venezuela

Oceania

Simpson Desert sand dunes in the Northern Territory, Queensland and South Australia, Australia

Coorong National Park in South Australia

Lincoln National Park in South Australia

Coffin Bay National Park in South Australia

Fraser Island in Queensland, Australia

Cronulla sand dunes in New South Wales, Australia

Stockton Beach in New South Wales, Australia

Lancelin sand dunes in Western Australia, Australia

Te Paki sand dunes near Cape Reinga, New Zealand

Sand dune systems

(coastal dunes featuring succession)

Athabasca Sand Dunes Provincial Park, Alberta and Saskatchewan

Ashdod Sand Dune, Israel

Bamburgh Dunes, Northumberland, England

Bradley Beach, New Jersey

Circeo National Park, a Mediterranean dune area on the southwest coast of the Lazio region of Italy

Cronulla sand dunes, NSW, Australia

Crymlyn Burrows, Wales

Dawlish Warren, Devon, England

Fraser Island, Queensland Australia, largest sand island in the world

Indiana Dunes National Park, Indiana

Kenfig Burrows, Wales

Margam burrows, Wales

Murlough Sand Dunes, Newcastle, Co Down, Northern Ireland

Morfa Harlech sand dunes, Gwynedd, Wales

Newborough Warren, North Wales

Oregon Dunes National Recreation Area, near North Bend, Oregon

Penhale Sands, Cornwall, England

Sleeping Bear Dunes National Lakeshore, Michigan

Sandy Island Beach State Park, Richland, New York

Extraterrestrial dunes

Dunes can likely be found in any environment where there is a substantial atmosphere, winds, and dust to be blown. Dunes are common on Mars and in the equatorial regions of Titan.

Titan's dunes include large expanses with average lengths of about 20–30 km. The regions are not topographically confined, resembling sand seas. These dunes are interpreted to be longitudinal dunes whose crests are oriented parallel to the dominant wind direction, which generally indicates west-to-east wind flow. The sand is likely composed of hydrocarbon particles, possibly with some water ice mixed in.

Dunes are a popular theme in science fiction, featuring in depictions of dry Desert planets appearing as early as the 1956 film Forbidden Planet and Frank Herbert's 1965 novel Dune. The environment of the desert planet Arrakis (also known as Dune) in the Dune franchise Dune in turn inspired the Star Wars franchise, which includes prominent theme of dunes on fictional planets such as Tatooine, Geonosis, and Jakku.