As sand accumulates on a beach area, several forces work together to move deposited sediments inland, where they form dunes. Waves continue to push sand up onto the shore, and winds carry dry sand away from the water. Eventually, piles of this sand become contained and fortified by various grasses, shrubs and other coastal vegetation. These piles of sand are known as coastal sand dunes.
Sand dunes are very important to coastal ecosystems; they may need a beach's sand in order to form, but a beach needs a dune's reservoir of sand to replenish itself after a storm. Coastal areas are protected from erosion because sand dunes absorb the effects of storms and surges. Sand dunes also provide a foundation for ecosystems made up of a wide variety of coastal life. Sea turtles nest in these dunes, rodents carve out extensive burrows, and vegetation and insects of all kinds have adapted to live in a dune's sandy environment. Coastal sand dunes can also be particularly effective at protecting the inland-facing parts of barrier islands, which are often vulnerable to storms and other natural disasters [source: NOAA].
Dunes themselves, especially those of barrier islands, can also be damaged or destroyed by storms. The U.S. Geological Survey's storm impact scale for barrier islands defines specific storm damage on a scale of 1 to 4. Impact 1 indicates that wave erosion occurred only in beach areas without lasting damage. Dunes can replenish beaches with their stores of sand. Impact 2 indicates that waves eroded the dunes to an extent that the damage could be permanent. At Impact 3, waves reach heights greater than that of the dune. They destroy the dune itself and create overwash by moving the sand and other sediments toward land. At Impact 4, the barrier island experiences permanent changes. The storm surge engulfs the island, destroying the dunes and pushing all barrier island sediments toward land [source: Sallenger].
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