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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Barrier island | 1/4 | https://en.wikipedia.org/wiki/Barrier_island | reference | science, encyclopedia | 2026-05-05T07:34:21.177659+00:00 | kb-cron |
Barrier islands are a coastal landform, a type of dune system and sand island, where an area of sand off the coast has been formed by wave and tidal action parallel to the mainland coast. Barrier islands are commonly formed in long, narrow systems parallel to shorelines and are shaped by waves, tides, sediment movement, and sea-level change, which cause them to shift, grow, or erode over time. They protect coastlines by absorbing energy, and create areas of protected waters where wetlands may flourish. A barrier chain may extend for hundreds of kilometers, with islands periodically separated by tidal inlets. The longest barrier island in the world is Padre Island of Texas, United States, at 113 miles (182 km) long. Sometimes an important inlet may close permanently, transforming an island into a barrier peninsula, often including a barrier beach. Barrier islands are related to other coastal landforms such as barrier beaches and spits, which share similar physical features but differ primarily in their degree of attachment to the mainland and the presence of water on their landward side. Though many are long and narrow, the length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range, wave energy, sediment supply, sea-level trends, and basement controls. The amount of vegetation on the barrier has a large impact on the height and evolution of the island. Globally, there have been approximately 1,500 barrier islands identified, totaling over 15,000 kilometers in combined length. Barrier islands are most abundant in the northern hemisphere with North America accounting for over one-third of the barrier islands and Asia nearing one-quarter of the total. There are chains of barrier islands along approximately 13 to 15% of the world's coastlines. They display different settings, suggesting that they can form and be maintained in a variety of environments. Numerous theories have been proposed to explain their formation. A human-made offshore coastal engineering structure constructed parallel to the shore is called a breakwater. Its coastal morphodynamic effect is to dissipate and reduce the energy of the waves and currents striking the coast in the same way as a naturally occurring barrier island.
== Constituent parts == Barrier islands function as part of an interconnected coastal system where environments exchange both sediment and energy, where changes to one component can influence the entire barrier system.
Upper shoreface The shoreface is the part of the barrier where the ocean reaches the shore of the island. The barrier island body itself separates the shoreface from the backshore and lagoon/tidal flat area. Characteristics common to the upper shoreface are fine sands with mud and possibly silt. Further out into the ocean the sediment becomes finer. The effect of waves at this point is weak because of the depth. Bioturbation is common and many fossils can be found in upper shoreface deposits in the geologic record.
Middle shoreface The middle shoreface is located in the upper shoreface. The middle shoreface is strongly influenced by wave movement because of its depth. Closer to shore the sand is medium-grained, with shell pieces common. Since wave action is heavier, bioturbation is not likely.
Lower shoreface The lower shoreface is constantly affected by wave action. This results in development of herringbone sedimentary structures because of the constant differing flow of waves. The sand is coarser.
Foreshore The foreshore is the area on land between high and low tide. Like the upper shoreface, it is constantly affected by wave action. Cross-bedding and lamination are present and coarser sands are present because of the high energy present by the crashing of the waves. The sand is also very well sorted.
Backshore The backshore is always above the highest water level point. The berm is also found here which marks the boundary between the foreshore and backshore. Wind is the important factor here, not water. During strong storms high waves and wind can deliver and erode sediment from the backshore.
Dunes Coastal dunes, created by wind, are typical of a barrier island. They are located at the top of the backshore. The dunes will display characteristics of typical aeolian wind-blown dunes. The difference is that dunes on a barrier island typically contain coastal vegetation roots and marine bioturbation. They also help Barrier Islands grow.
Lagoon and tidal flats The lagoon and tidal flat area is located behind the dune and backshore area. Here the water is still, which allows fine silts, sands, and mud to settle out. Lagoons can become host to an anaerobic environment. This will allow high amounts of organic-rich mud to form. Vegetation is also common.
== Processes ==
=== Migration and overwash === Water levels may be higher than the island during storm events. This situation can lead to overwash, which brings sand from the front of the island to the top and/or landward side of the island. During these washover events, the sand that is being transported may end up on the landward side, beyond the dune, forming a washover fan. The overwash can continuously alter the surface of the barrier, requiring the ecosystem to adapt to periodic disturbances. This process leads to the evolution and migration of the barrier island.
==== Types of migration ==== Barrier island systems are dynamic and they may shift positions over time through three main types of migration. Lateral migration happens when sand is pushed along the coast, causing erosion at one end and accumulations at the other. This results in sideways movement along the shoreline. Prograding/regressive migration is when sand builds up on the ocean-facing side of the barrier island, leading to growth on the seaward side. Transgression migration occurs when the barrier island is being built up toward the mainland. This is commonly caused by the sea levels rising or the land sinking.