Wind is the main agent forming sand dunes. There is an exchange of sediments between beaches and dunes and this is part of a natural process that maintains both morphological stability and ecological diversity. Once exposed, sand is vulnerable to aerodynamic processes.
Wide beaches are formed in the summer and narrower beaches during the winter. Storms erode beaches and transport sand out of the system or to other beaches. Bonding, both by moisture and chemical precipitates, may cause surface adhesions, raising thresholds and reducing erosion. Sometimes salt forms a whitish crust on the sand surface, also bonding sand grains.
Sand grains come in a wide variety of shapes, colors, and densities, depending on their origin and on how long they have been rolling in water currents and wind. Silicate sand and calcium carbonate sand (formed by fractured shells and skeletons) are the more common components of coastal dunes. Sand texture, as well as shape and density, affect transport. Smaller particles are easier to move than larger ones. Sediment size is measured on the Wentworth scale. It is harder for angular grains to become airborne but they may move further once they have. Denser grains are harder to move and often accumulate as lag deposits on the upper beach.
Almost all wind-blown sand travels quite close to the ground, through a mechanism called saltation. Individual grains move in a series of continuous leaps. Once airborne, a grain describes a curve path, and lands hitting the ground at a low angle, but with sufficient force to rebound into the air again. It hits other sand grains that become airborne and do the same thing. In a short time, there is a considerable amount of sand in the air. Under most circumstances, deposition takes place within a short distance although sometimes sand may be transported long distances alongshore where the wind blows parallel to the coast. Deposition is favored by obstacles such as driftwood, clumps of vegetation, boulders, and plastic objects which perturb air flow and create a shelter zone. Small dunes are formed with their tails - called trailing ridges - stretching downwind.
Changes in wind strength and direction cause rapid resedimentation. Often a dune's surface changes by the hour, creating complex stochastic patterns. Over time, these processes create recognizable dune bedforms such as ripples, sand waves, and barchans.
Most coastal dunes form in the presence of vegetation. An important determinant of dune form is the drag imposed by the vegetation on the air flow. Dunes can be classified according to the percentage vegetation cover. At one extreme are dunes that have been stabilized by their vegetation cover (fixed, shore-parallel ridges and parabolic dunes) and at the other are the free wind forms (barchan or sand wave dunes, transverse dunes). Transitional forms are typified by a fragmented topography (hummock dunes).
There is a strong interaction between vegetation and dune form, and there are several patterns of incipient dunes. Plant form modifies sand deposition, forming a leading edge (as in the case of Ammophila arenaria), a trailing edge (Spinifex hirsutus), or intermittent deposition in clumped vegetation. Perennial grasses such as Agropyron junceiforme and Elymus arenarius as well as tropical long-branched creepers (Canavalia rosea and Ipomoea pes-caprae) grow laterally and vertically and are able to raise a dune a meter or two high.
Sand dunes act as a buffer to extreme winds and waves and they also shelter landward communities. They replenish the depleted beach and near-shore during and after storms, and are important in the retention of freshwater tables against salt intrusion. They filter rain water and are also important habitats for plants and animals. People have always appreciated their beauty and recreational value.
R. W. Carter wrote that ''Of all the coastal systems, sand dunes have suffered the greatest degree of human pressure.'' Many have been irreversibly altered by human activities such as tourist developments, golf courses, and urban growth.
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