Many coastal areas have well-developed sand dunes in the backshore area, some of which reach heights of several tens or even hundreds of feet (tens of meters). The presence or absence of dunes, and their shape and height, is mostly controlled by the amount of sediment supply available, although wind strength and type and distribution of vegetation also play significant roles. Dunes are fragile ecosystems that can easily be changed by disturbing the vegetation or beach dynamics, yet their importance is paramount to protecting inland areas from storm waves and surges, tsunami, and other hazards from the ocean.
Most coastal dunes are of the linear type, known as foredunes, which form elongate ridges parallel to the beach just landward of the foreshore. In some cases numerous foredunes are present, with the ones closest to land being the oldest and the younger ones forming progressively seaward of these older dunes.
sand dunes in the backbeach area are built by the windblown accumulation of sand derived from the foreshore area. The sands may grow far into the backshore environment, in some cases extending miles inland if not obstructed by vegetation, cliffs, or constructions such as buildings or seawalls. Vegetation is extremely effective at stabilizing mobile sand, and many examples of sand being trapped by plants are visible on beaches of the world.
Dunes are built by the slow accumulation of sand moved by wind, but may be rapidly eroded by storm surges and wave attack when the sea surface is elevated on storm surges. A single storm can remove years of dune growth in a few hours, transporting the dune sand offshore or along shore. Examples of this process were all too clear from Hurricanes Katrina in Louisiana and mississippi in 2005 and ike along the Texas coast in 2008. Likewise, tsunamis can remove entire dune fields in a single devastating event, as seen in many places during the indian ocean tsunami of December 2004. Rising sea levels pose a huge threat to many existing coastal dune fields, since a rise in sea level of one foot (0.3 m) on flat terrain can be equated with a 100-foot (305 m) landward migration of the shoreline, and the removal of the dune field from one location to an area farther inland, or to its complete elimination.
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