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Schematic of coastal parabolic dunes. U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12 km.
As barchan dunes migrate, smaller dunes outpace larger dunes, catching-up the rear of the larger dune and eventually appear to punch through the large dune to appear on the other side. The process appears superficially similar to waves of light, sound, or water that pass directly through each other, but the detailed mechanism is very different.
In those systems, other kinds of dunes may be closest to the water. [1] [5] Landscape of Northern California parabolic dunes. A foredune ecosystem begins with the first dune ridge directly behind an active beach. The ridge of a foredune can range in height from a few meters to tens of meters tall.
Sufficient vegetation has grown on the valley floor and there is little sand blowing into the main dune field from the valley; however, small parabolic dunes continue to originate in the sand sheet and migrate across grasslands, joining the main dunefield. Some of these migrating dunes become covered by grasses and shrubs and stop migrating.
For example, there are ergs or fields of linear dunes, of crescentic dunes, of star dunes, and of parabolic dunes, and these dune arrays tend to have consistent orientations and sizes. [10] [11] By nature, ergs are very active. Smaller dunes form and migrate along the flanks of the larger dunes and sand ridges.
The Egyptian “chevrons” are active, wind-generated dunes, but the “chevrons” in the Bahamas are inactive and have been variously interpreted. [3] The most common interpretation of large, chevron-shaped bed forms is that they are a form of parabolic dune, and that most examples are generated by wind action. [4]
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Guadalupe-Nipomo Dunes is the largest remaining dune system south of San Francisco and the second largest in the U.S. state of California. [1] It encompasses an 18-mile (29 km) stretch of coastline on the Central Coast of California and extends from southern San Luis Obispo County to northern Santa Barbara County.