|Table of contents|
2 Coastal dunes
3 References & Links
5 See Also
Mesquite Flat dunes in Death Valley
Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In all such cases the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds.
The most common dune form on Earth (and on Mars) is the crescentic. Crescent-shaped mounds generally are wider than long. The slipface is on the dune's concave side. These dunes form under winds that blow from one direction, and they also are known as barchans, or transverse dunes. Some types of crescentic dunes move faster over desert surfaces than any other type of dune. A group of dunes moved more than 100 meters per year between 1954 and 1959 in China's Ningxia Province; similar rates have been recorded in the Western Desert of Egypt. The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than 3 kilometers, are in China's Taklimakan Desert
Coastal dunesDunes form on coasts where the backshore can support and onshore winds encourage the accumulation of sand blown inland from off a beach. Any part of the upper beach, once dry, can lose sand to the wind, especially if the sand is fine, and dune formation proceeds in the direction towards which the predominant wind direction is blowing.
Succession on coastal dunes
succession occurs. The conditions on an embryo dune (is this a made up word?) are harsh, with salt spray from the sea carried on strong winds. The dune is well drained and often dry. Rotting sea weed brought in by storm waves adds enough nutrients to allow pioneer species to colonise the dune. These pioneer species are marram grass, sea wort grass and other sea grasses in England. These plants are well adapted to the harsh conditions of the fore dune, typically having deep roots which reach the water table, rhibozomes which produce nitrogen compounds, and protected stoma, reducing transpiration. The deep roots also bind the sand together, and the dune grows into a fore dune as more sand is blown over the grasses. The grasses add nitrogen to the soil, meaning other, less hardy plants can then colonise the dunes. Typically these are heathers and gorses. These too are adapted to the low soil water content and have small, prickley leaves which reduce transpiration. Heathers add humus to the soil, but have a pH of lower than 7, so make the soil slightly acidic. Heathers are usually replaced by coniferous trees which can tollerate the low pH. Coniferous forests and heathland are common climax communities for sand dune systems.
Young dunes are called yellow dunes, dunes which have high humus content are called grey dunes. Leeching occurs on the dunes, washing humus into the slacks, and the slacks may be much more developed then the exposed tops of the dunes.