Dunes and Dune Erosion
Dunes and dune erosion
Dunes are small hills, mounds or ridges of wind-blown soil material, usually sand, that are formed in both coastal and inland areas. The formation of coastal or inland dunes requires a source of loose sandy material and dry periods during which the sand can be picked up and transported by the wind. Dunes exist independently of any fixed surface feature and can move or drift from one location to another over time. They are the result of natural erosion processes and are natural features of the landscape in many coastal areas and deserts, yet they also can be symptoms of land degradation. Inland dunes are either an expression of aridity or can be indicators of desertification—the result of long-term land degradation in dryland areas.
Coastal dunes are the result of marine erosion in which sand is deposited on the shore by wave action. During low tide, the beach sand dries and is dislodged and transported by the wind, usually over relatively short distances. Depending on the local topography and direction of the prevailing winds, a variety of shapes and forms can develop—from sand ridges to parabolic mounds. The upper few centimeters of coastal dunes generally contain chlorides from salt spray and wind-blown salt. As a result, attempts to stabilize coastal dunes with vegetation are often limited to salt-tolerant plants.
The occurrence of beaches and dunes together have important implications for coastal areas. A beach absorbs the energy of waves and acts as a buffer between the sea and the dunes behind it. Low lying coastlines are best defended against high tides by consolidated sand dunes. In such cases, maintaining a wide, high beach that is backed by stable dunes is desirable.
Engineering structures along coastal areas and the mouths of rivers can affect the formation and erosion of beaches and coastal dunes. In some instances it is desirable to build and widen beaches to protect coastal areas. This can require the construction of structures that trap littoral drift, rock mounds to check wave action, and sea walls that protect areas behind the beach from heavy wave action. Where serious erosion has occurred, artificial replacement of beach sands may be necessary. Such methods are expensive and require considerable engineering effort and the use of heavy equipment.
The weathering of rocks, mainly sandstone, is the origin of material for inland dunes. However, whether or not sand dunes form, depends on the vegetative cover condition and use of the land. In contrast to coastal dunes, that are often considered to be beneficial to coastal areas, inland dunes can be indicators of land degradation where the protective cover of vegetation has been removed as a result of inappropriate cultivation, overgrazing , construction activities, and so forth. When vegetative cover is absent, soil is highly susceptible to both water and wind erosion. The two work together in drylands to create sources of soil that can be picked up and transported either downwind or downstream. The flow of water moves and exposes sand grains and supplies fresh material that results in deposits of sand in flood plains and ephemeral drainage systems. Before dunes can develop in such areas, there must be long dry periods between periodic or episodic sediment-laden flows of water. Wind erosion occurs where such sand deposits from water erosion are exposed to the energy of wind, or in areas that are devoid of vegetative cover.
Where sand is the principle size soil particle and where high wind velocities are common, sand particles are moved by a process called saltation and creep. Sand dunes form under such conditions and are shaped by wind patterns over the landscape. Complex patterns can be formed—the result of interactions of wind, sand, the ground surface topography, and any vegetation or other physical barriers that exist. These patterns can be sword like ridges, called longitudinal dunes, crescentic accumulations or barchans, turret-shaped mounds, shallow sheets of sand, or large seas of transverse dunes. The typical pattern is one of a gradual long slope on the windward side of the dune, dropping off sharply on the leeward side.
Exposed sand dunes can move up to 11 yd (10 m) annually in the direction of the prevailing wind. Such dunes encroach upon areas, covering farmlands, pasture lands, irrigation canals, urban areas, railroads and highways. Blowing sand can mechanically injure and kill vegetation in its path and can eventually bury croplands or rangelands . If left unchecked, the drifting sand will expand and lead to serious economic and environmental losses.
Worldwide, dryland areas are those most susceptible to wind erosion. For example, 22% of Africa north of the Equator is severely affected by wind erosion as is over 35% of the land area in the Near East. As a result, inland dunes represent a significant landscape component in many desert regions. For example, dunes represent 28%, 26%, and 38% of the landscape of the Saharan Desert, Arabian Desert, and Australia , respectively (Heathcote 1983). In 1980, Walls estimated that 1.3 billion hectares of land were covered by sand dunes globally. Although dunes can be symptoms of land use problems, in some areas they are part of a natural dryland landscape that are considered to be features of beauty and interest. Sand dune have become popular recreational areas in parts of the United States, including the Great Sand Dune National Monument in southern Colorado with its 229-yd (210-m) high dunes that cover a 158-mi2 (254.4-km2) area, and the Indiana Dunes State Park along the shore of Lake Michigan.
When dune formation and encroachment represent significant environmental and economic problems, sand dune stabilization and control should be undertaken. Dune stabilization may initially require one or more of the following: applications of water, oil, bitumens emulsions, or chemical stabilizers to improve the cohesiveness of surface sands; the reshaping of the landscape such as construction of fore-dunes that are upwind of the dunes, and armoring of the surface using techniques such as hydroseeding, jute mats, mulching and asphalt; and constructing fences to reduce wind velocity near the ground surface. Although sand dune stabilization is the necessary first step in controlling this process, the establishment of a vegetative cover is a necessary condition to achieve long-term control of sand dune formation and erosion. Furthermore, stabilization and revegetation must be followed with appropriate land management that deals with the causes of dune formation in the first place. Where dune erosion has not progressed to a seriously degraded state, dunes can become reclaimed through natural regeneration simply by protecting the area against livestock grazing, all-terrain vehicles, and foot traffic.
Vegetation stabilizes dunes by decreasing wind speed near the ground and by increasing the cohesiveness of sandy material by the addition of organic colloids and the binding action of roots. Plants trap the finer wind-blown soil particles, which helps improve soil texture , and they also improve the microclimate of the site, reducing soil surface temperatures. Upwind barriers or windbreak plantings of vegetation, often trees or other woody perennials, can be effective in improving the success of revegetating sand dunes. They reduce wind velocities, help prevent exposure of plant roots from the drifting sand, and protect plantings from the abrasive action of blowing sand. Areas that are susceptible to sand dune encroachment can likewise be protected by using fences or windbreak plantings that reduce wind velocities near the ground surface. Because of the severity of sand dune environments, it can be difficult to find plant species that can be established and survive. In addition, any plantings must be protected against exploitation, for example, from grazing or fuelwood harvesting.
The expansion of sand dunes resulting from desertification not only represent environmental problems, but they also represent serious losses of productive land and a financial hardship for farmers and others who depend upon the land for their livelihood. Such problems are particularly acute in many of the poorer dryland countries of the world and deserve the attention of governments, international agencies, and nongovernmental organizations who need to direct their efforts toward the causes of soil erosion and dune formation.
[Kenneth N. Brooks ]
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Food and Agricultural Organization (FAO) of the United Nations. Sand Dune Stabilization, Shelterbelts, and Afforestation in Dry Zones. Rome: FAO Conservation Guide 10, 1985.