Sand dunes are collections of grains of sand, forming hills, ridges or mounds, that accumulate on the shoreline by the effects of wind and sea waves. Sand dunes may vary in form and size, depending on the type of sand and the wind´s direction, forming as small as ant mounds or small hills some meters tall. They usually form parallel to the shoreline. Sand dunes help protect the beach against storms and strong sea waves. Sand dunes can form unique ecosystems, hosting plant and animal life. Although sand dunes are mostly associated with coastal areas, the largest expansions are found in dry regions inland where ancient sea beds existed in the past.
What are sand dunes?
Sand dunes are mounds of asymmetrical accumulations of sand particles. They´re characterized by a gentle slope in the windward side of the dune and a steep slope on the downwind side known as the slip face. Once formed, sand dunes migrate by erosion and deposition. Sand dunes are found along the shoreline on the land side of the beach; however, they´re also found in deserts, lakes and rivers. For sand dunes to form there needs to be a supply of sand particles, a means by which these sand particles can be transported and a fix obstacle where these particles can accumulate.
How sand dunes are formed?
Sand dunes form when the wind carries small particles of sand that are deposited against an obstacle, such as a shrub, rock, tree or any other kind of obstruction. As small particles of sand are deposited around the obstacle, the dune grows in size, forming rows that grow perpendicular to the wind´s direction. The wind will continue to erode the sand particles that were deposited on the windward side of the dune and deposit them on the leeward side of the dune. Sand dunes migrate when erosion by the wind forces the sand particles to move by saltation on the upwind slope. These particles are then deposited on the slip face, creating repose angles of 30-34º.
Parts of a sand dune
All sand dunes possess a windward (gentle) slope, a crest, a slip face and leeward slope. The gentle portion of the dune is perpendicular to the direction of the prevailing winds. The blowing winds deposits heavier sand particles on the windward slope of the dune; these particles move up the gentle slope by saltation, which refers to short leaps of sand that are dislodged from the nearby surface. accumulating on the dune`s crest. Eventually, the lighter sand particles reach their maximum height and begin to fall down the slip face of the dune. Over time, the dune will migrate (move), maintaining the same shape.
Types of sand dunes
Even though sand dunes are formed by a similar process and sand substances, they may vary significantly in form. Crescent dunes, which are some of the most common types of dunes, are typically wider than they´re long. The slip face, in crescent sand dunes, forms on the concave portion of the dune. They form where the prevailing winds blow in one direction. These types of dunes usually move very fast in deserts. Linear dunes are typically very straight dunes that can reach up to 150 km (94 miles) in length. Some linear dunes may combine to form y-shaped compound sand dunes.
Star dunes are pyramid-like types of dunes. They possess slip faces on three or more sand arms that radiate forming a central sand mound. Star dunes usually develop in region where the wind blows from distinct directions. Dome dunes, which are the less common types of dunes, are oval-shaped or circular mounds of sand. Parabolic dunes are u-shaped sand dunes where the u-shaped depression faces the direction of the wind. They´re usually formed where there is abundant vegetation and a constant one direction wind. Reversing dunes, which are variations of other dunes, occur where the direction of the wind continuously changes direction.
Sand dunes provide the habitat for varied plant and animal species. Sand dunes continually go through a process of erosion and accretion by the effects of the wind and ocean waves. Structures built close to the beach may prevent the natural cycle of erosion and accretion, leading to complete erosion of the beach sand dunes, which may eventually leave the beach more vulnerable to damage by storm surges. According to geography-site.co.uk, Sand dunes are vulnerable to erosion by natural processes and human activity; therefore, it´s very usual to see walking paths and fenced sand dune areas to prevent erosion by human action.
