Sand Dunes: Structure and Types

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  • 0:01 What Are Sand Dunes?
  • 2:02 Types of Sand Dunes
  • 4:41 Lesson Summary
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Lesson Transcript
Instructor: Margaret Cunningham

Margaret has taught many Biology and Environmental Science courses and has Master's degrees in Environmental Science and Education.

Although most people think of sand dunes simply as piles of sand, they actually come in many different shapes and sizes. In this lesson, we'll discuss the unique features of barchan, transverse, parabolic, longitudinal and star sand dunes.

What Are Sand Dunes?

Imagine you are walking through a desert, and the wind begins to blow around you. When the wind settles, you see piles of sand. You might wonder what these piles are and how they got there.

Mounds of loose sand grains that are piled up by wind movement are referred to as sand dunes. There are many sand dunes located within the deserts of the Southwestern United States. You can also see sand dunes along beaches where sand blows inland and creates a dune that separates the water from the habitat. In the Sahara Desert of Africa, the sand dunes are so large they are referred to as sand seas. The sand dunes move with the wind and resemble a moving sea.

Most sand dunes are considered asymmetrical, meaning that if you cut down a central line, they are not identical on both sides. The side of the sand dune that faces the direction of the wind movement normally has a gentle slope, while the downwind side has a steeper slope. The downwind side of the dune is referred to as the slip face. Sand grains are blown up the gentle slope of the dune, and when they reach the top, they fall onto the slip face. When enough loose sand settles on the slip face, it can cause a small sand avalanche to occur as the sand slips down the dune.

Over time, entire sand dunes move as wind erodes the upwind side and moves it to the slip face. This results in the sand dune slowly moving in the downwind direction. On average, a dune can move around 10 to 15 meters per year. Although this may not seem like much, if you think about sand dunes along a beach, 10 to 15 meters can make quite a difference in the landscape.

Sand dunes stop moving when they become overgrown with vegetation. In beach areas where sand dunes can move towards houses, vegetation is often planted on the dunes to stop them from moving. The roots of the plants help stabilize the sand and prevent it from blowing in the wind.

Types of Sand Dunes

There are a variety of different types of sand dunes found around the world. The different types have certain characteristics based on the velocity and direction of wind, the sand supply and how vegetation covers the dune.

One type of sand dune is called a barchan. A barchan sand dune is a crescent-shaped dune, with tips that point in the downwind direction, and has a steep slip face. Individual barchan dunes are separate from each other and gradually move along the barren desert surface. This type of sand dune is created when sand is limited. If the sand supply increases, ridges will form if barchan sand dunes start to grow and connect to each other. In some cases, these ridges lose their crescent shape and become straight, forming a second type of sand dune. Transverse dunes are straight, long dunes that are orientated perpendicular to the wind direction and have steep slip faces.

The third type of sand dune shares the crescent shape with the barchan dune but is inverted. The parabolic dune is crescent-shaped, with vegetation-anchored tips that point in the upwind direction, and has a steep downwind slip face. This type of sand dune requires a large amount of sand and strong winds.

The fourth and largest type of sand dune is the longitudinal dune. Longitudinal dunes are large symmetrical ridges of sand that form parallel to the primary wind direction. These dunes are straight and regularly spaced, with barren ground in between each dune. Longitudinal dunes require strong winds from two directions in order to create such long and straight lines. There is a primary wind direction and also a cross-wind.

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