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What Are Sand Dunes In Geography

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Sand Dunes In Death Valley

How are Sand Dunes formed on a coast? – Labelled diagram and explanation

Several dune fields are located in Death Valley in eastern California. These dunes are also placed in a large depression however, this basin is much deeper and is surrounded by giant mountain ranges that exceed 6,000 feet in elevation.

Large amounts of sediment are eroded and washed down from the mountains during rare and intense rainfall events, which are then deposited in the vast basin below. For example, in the case of the Mesquite dunes, the eroding Grapevine Mountains to the north provide an ample source of sand, which is blown into position by the prevailing westerly wind.

The sand builds up against the mountains, thus forming into an incredibly large dune field. Although the largest of the dunes reaches a height of 185 feet above the surrounding basin, it is spread out over a massive area totalling 5,000 square miles.

Where Are The Tallest Sand Dunes In The United States

The tallest dunes in the United States are located in the Great Sand Dunes National Park in Colorado, many of which peak over 600 feet above the surrounding valley floor. Star Dune is the tallest, the crest of which reaches a height of 750 feet.

With an area of some 30 square miles, the entire dune field is rather expansive. The history behind the formation of these dunes is a great example of how both wind and water processes work together to produce a vast dune field.

The dunes have built up on the western side of the Sangre de Cristos Mountains, and are located in the San Luis Valley, a large basin flanked by two mountain ranges. The basin is filled with sedimentary deposits which have been washed into the basin during intense rainfall and have since dried out. The prevailing westerly winds then blow the particles eastward, depositing them against the slopes of the Sangre de Cristos range.

Impacts Of Humans On Sand Dunes And Coastal Environments In Wales

Along the coast of south Wales, large scale developments such as the growth of towns and cities, expansion of transport networks and purpose built tourist facilities have put great pressure on sand dunes. However, more recently, our demand for a greener non-renewable energy source has added a further potential problem.

At Swansea, a large tidal lagoon has been proposed, which, if given the go-ahead, will create clean energy for the city using the power of the tide to generate electricity.

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Largest Active Sand Dunes In The Arctic

The largest active sand dunes in the Arctic are the Great Kobuk Sand Dunes in Alaska. These dunes rise out of the forest along the southern bank of the Kobuk River and are a relic of the last Ice Age. Over thousands of years, vegetation has slowly reclaimed all but 16,000 acres of the original 200,000 sand dunes that were formed.

Formation Of Sand Dunes

Pin by Topo Gigio on Deserts, Salt Flats, Sand Dunes

Many depositional features consist of sand and shingle and are therefore loose and unstable and easily eroded and transported.

Sand dunes are made of wind blown sand and are stabilised by a process called Plant Succession. Long low waves which roll onto the beach. A strong swash deposits beach material which forms a long, flat beach. These type of wave usually form when there is a short fetch

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Depositional Landforms Of Wind

Some geomorphologically remarkable Depositional Landforms caused by wind are sand dunes, loess etc. Wind-blown sediments are deposited as a result of a significant reduction in wind speed and obstruction caused by bushes, forests, marshes and swamps, lakes, large river walls, and so on. Sand is deposited on both the windward and leeward sides of fixed obstacles. This article explains the depositional landforms of the wind such as barchan, seif, parabolic, transverse dunes, longitudinal dunes, etc which is an important part of the geomorphology syllabus for the UPSC IAS Exam.

  • A dune is a landform created by the movement of sand by wind or water.
  • It’s usually shaped like a mound, ridge, or hill.
  • Sand dune formation is best in dry, hot deserts.
  • Sand dune forms include Barchans, Seifs, and others, depending on the shape of the dune.
  • They might be active or live dunes that are constantly moving, or they can be inert permanent dunes that are anchored in vegetation.

Established: September 13 2004 Size: 149137 Acres

Visitors to the Great Sand Dunes experience an undeniable sense of wonder, just as happens in so many of our most spectacular national parks. In contrast to the sudden shock of walking to the rim of the Grand Canyon, though, or topping a rise to view Crater Lake, the emotions evoked by this otherworldly landscape arrive in slow motion.

The dunes appear in the distance as you approach, but at first seem dwarfed by their backdrop, the 13,000-foot peaks of the Sangre de Cristo Mountains. Not until you’re nearly at their border does their vast scale become apparent: dunes up to 750 feet tall, extending for mile after milean ocean of sand hills of breathtaking magnitude. That’s just how the explorer Zebulon Pike described them in 1807: “Their appearance was exactly that of a sea in a storm , not the least sign of vegetation existing thereon.”

Winds that often top 40 miles an hour continually reshape the crests of the tall dunes, and smaller dunes may “migrate” several feet in a week. The dunes show a remarkable permanence of form, though, which geologists attribute to opposing winds. Prevailing southwesterly winds blow the dune mass northeasterly toward the mountains, and occasional but powerful northeasterlies blow the dunes back toward the southwest. This ‘back-and-forth’ action of the wind piles the dunes vertically, and contributes to the stability of the dunefield.

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How Are Sand Dunes Formed

The conditions required for sand dune formation are:

  • a large supply of sand
  • a large flat beach
  • time for the sand to dry, so an extensive tidal range is needed
  • an onshore wind for sand to be transported to the back of the beach
  • an obstacle for the dune to form against, e.g. pebble or driftwood

The image below shows Harlech beach, North Wales. Its sizeable tidal range supports the development of sand dunes. The tidal range is the difference between the high tide and the following low tide.

A beach with a large tidal range

How Does Wind Transport Sand

Singing Sand Dunes | National Geographic

Aeolian Transport is the first process of coastal dune formation and involves the movement and weathering of sand particles behind and along the shoreline. Aeolian transportation is when the wind transports sediment. Wind transports sand in 3 ways. These are:

Aeolian transportation

1% of the movement of sand is by suspension when sand is picked up and carried within the wind. 95% of sand movement results from saltation when grains of sand bounce along the beach as they are picked up and dropped by the wind. Finally, 4% of transportation is by creep when sand grains collide and push each other along.

The video below shows a combination of these processes of transportation.

As the wind blows up the beach, it will transport material. Larger pieces of sediment will rest against an obstacle forming a ridge, while smaller particles will settle on the other side. On the side facing the wind, the material begins to reach a crest. This is because the pile of material becomes steep and unstable and begins to collapse. When this happens, smaller particles fall down the other side. Once there is a stable angle , the sand stops slipping, and the cycle repeats. As the sand becomes an obstacle, more dunes may form in front of it the stronger the wind, the higher the dunes.

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Beaches And Associated Features: Berms Runnels And Cusps

Beaches are dynamic environments which for the buffer between land and sea. They have 3 main components, the nearshore , the foreshore and the backshore . The backshore typically features the material deposited by storm waves.

Wave zones and beach morphology

The gradient of the beach tends to change during the year. Beaches are typically steeper in summer. This is because constructive waves are more common in summer, but destructive waves are more common in winter.

The strong swash of a constructive wave deposits the largest material at the top of the beach. As the upper beach builds up, the backwash becomes even weaker because a greater proportion of the water drains away by percolation, rather than running down the beach. The weak swash of a destructive wave deposits material at the base of the beach. It cannot advance further up the beach because it is destroyed by the backwash from the previous breaking wave. Ridges and runnels form parallel to the shoreline in the foreshore zone. Ridges are areas of the foreshore that are raised above the adjacent shore which dips into a runnel. The runnels are disrupted by channels that help to drain the water down the beach.

Ridges and Runnels

If you were to look at the cross-section it would appear similar to that of hills and valleys but at a much smaller scale.

Beach profile containing ridges and runnels

Ridge and runnels on Harlech beach

The images below show ridges and runnels along the beach at Harlech, North Wales.

Where Are Sand Dunes Located In The United States

Within the United States, dune fields are quite rare, and are most commonly found in southwestern desert areas of the country. These include the Great Sand Dunes in southern Colorado, and the Mesquite Flat Sand Dunes located in Death Valley, eastern California, both of which are considered to be some of the U.S.s largest and most striking dune fields.

Many other dune fields are also situated in parts of Nevada, Arizona, Utah, New Mexico and Texas, areas which receive little precipitation, and are often subject to long dry seasons. Some of the USAs prairie-type territory is also home to dunes, such as the vast Nebraska Sandhills.

Sand Dunes are also found along coastal areas of the United States, in particular along the coast of Oregon between Barview and Florence, parts of the Great Lakes, and scattered along sections of the east coast.

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How Do Sand Dunes Change As You Move Inland

How do sand dunes change with distance from the beach?

Dunes become taller further inland. Embryo dunes are only a few metres high whereas mature dunes are up to 15m high. This is because marram grass and other vegetation colonise the sand dune and hold it together with long roots, stopping the migration of the dune. Dunes closer to the beach are more yellow, whereas further away, they are grey due to humous and bacteria from plants and animals being added. A trough separates each dune , called a slack. They are formed by the removal of sediment from the sheltered lee side of the dune and the windward side of the next dune. Slacks can be eroded so much that they reach the water table resulting in the formation of salty dunes. The video below illustrates how vegetation in a dune ecosystem changes as you move inland .

The 360° aerial image below shows the sand dunes at Morfaa Harlech nature reserve in North Wales.

The image below shows vegetation succession on dunes at Harlech, North Wales.

Sand dune vegetation succession

Geography Igcse: Dune Formation

Sand dunes of Monument Valley. National Geographic

GEOGRAPHY IGCSE: DUNE FORMATION MURIWAI BEACH NEW ZEALAND. It contains: dune formation, examples of eroded dunes and dune vegetation. Case study: Muriwai Beach, New Zealand.

GEOGRAPHY IGCSE: DUNE FORMATION MURIWAI BEACH NEW ZEALAND. It contains: dune formation, examples of eroded dunes and dune vegetation. Case study: Muriwai Beach, New Zealand.

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The Establishment Of Plants

As sand dunes establish, plants start to form and grow in size along the sand dune. Primary vegetation gives stability to the sand dune which, in turn, traps sand. Wind is then deflected over the ridge and vegetation, creating shelter for larger plants to grow and establish.

The sand dunes at Merthyr Mawr are very unusual because they have formed against a carboniferous limestone plateau that runs from east to west along the coast, making the dunes at Merthyr Mawr the second highest in Europe. This reserve also contains examples of the successional stages of dune formation, from bare sand, embryonic dunes to stabilised dunes and slacks.

The site is managed by Natural Resources Wales , and it is designated as a site of special scientific interest because of its wealth of plants and insect life.

Many of our sand dune systems are covered by both UK and international legislation in order to protect the habitats and the species that depend upon them. Despite these legislations, there is an increasing recognition that further protection is required, otherwise many of the species will soon become extinct.

Geology Of Great Sand Dunes National Park

What are sand dunes?

Weathering and Erosion

Nothing has a greater impact on the landscape around us, especially in the Great Sand Dunes, than the processes of weathering and erosion. Typically seen as interchangeable words, these are actually two distinct processes in the world of geology. Weathering is the physical and/or chemical breakdown of surfaces due to wind, water, or ice. Erosion is the process of moving the broken down material from one place to another via wind, water, or gravity. At Great Sand Dunes National Park, erosion is responsible for recycling escaping sediments back into the main dune field and bringing in new sediments from the surrounding mountains. Alluvium, or sediments deposited by bodies of water, is the source most of the sand in the dunes.

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How Are Sand Dunes Created

Typically, both wind and water processes act together to create sand dunes, whether they are located in the desert or along the coast. The wind picks up sand particles, transports them over a distance, and deposits them in a depression, or against a large structure such as a hillslope or mountain range. This is particularly common in dryland zones, where a lack of surface moisture means that many sand particles can be easily lofted into the air by the wind.

Water processes work in a rather similar way, but involving surface flow, which transports sand particles from a source and deposits them in a depression or against a slope. On the coast, frequent strong winds and strong wave and tidal action work together to build dune fields.

Reason For Cresting Of Sand Dunes

Formation of Sand Dunes: Coastal Processes Part 5 of 6

In the process of wind moving sand particles up the sandpile, the pile becomes severely steep that it starts to collapse due to the impact of its own weight. This leads to mass movement of sand down the slip face. The collapsing of the sandpile will stop the moment the slip face attains the required angle of steepness. When the right angle is achieved, the dune becomes stable. The right angle required for the stability of a dune, better known by scientists as angle of repose, is normally approximately 30 to 34 degrees.

When enough sand has accumulated around the obstacle, the dune turns into an obstacle itself, and its growth continues. Sand dunes will grow into varying shapes and sizes depending on the direction and velocity of the wind. High-velocity winds are associated with taller sand dunes. Low-velocity winds, on the other hand, tend to spread the dunes out. If wind direction remains same over the years, the sand dunes will shape towards that direction. Any vegetation that comes up will prop up the dune, stabilizing it and will prevent any possibility of shifting.

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Transport Of Sand By Wind

Transport by wind takes place almost entirely within 0.5 m of the ground surface with nearly 90% of this movement within 2.5 cm of the surface . Once the grain is lifted it is transported downwind in a parabolic trajectory. The speed and distance that the grain will be transported is determined by grain size, shape and density in relationship to the drag component . For fine-grained sand the threshold wind velocity for sand is approximately 10 km/hr .

Once the threshold value is exceeded, sediment can be transported in one, or a combination of, four main ways: traction or creep, saltation, reptation and/or suspension .

  • Traction load: The traction load refers to particles that are too large and/or dense to be entrained by the lift component , or are just pushed along by other grains colliding with the surface grains, and are “rolled” along the surface due to the traction of the drag force .
  • Saltation: Saltation refers to the processes when the sand grains are lifted up into the air and suspended for a short distance before falling back to the surface with a parabolic trajectory. The height and time that the grains remain suspended is determined by grain size, shape, and density in relation to the lift and drag force, and the turbulence and velocity of the wind flow.
  • Small Scale Ecosystems Sand Dunes

    The coasts of both north and south Wales are well known for their wonderful sandy beaches, making them popular holiday destinations for visitors from the UK and many other parts of the world. Behind many of these beaches are sand dune systems which provide homes for diverse forms of wildlife including plants and insects, some so highly specialised that without these dune habitats, they might be driven to the point of extinction. In addition, these sand dunes act as a vital natural barrier to the power of the waves.

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