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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Barrier island | 4/4 | https://en.wikipedia.org/wiki/Barrier_island | reference | science, encyclopedia | 2026-05-05T07:34:21.177659+00:00 | kb-cron |
=== Wave-dominated === Wave-dominated barrier islands are long, low, and narrow, and usually are bounded by unstable inlets at either end. The presence of longshore currents caused by waves approaching the island at an angle will carry sediment long, extending the island. Longshore currents, and the resultant extension, are usually in one direction, but in some circumstances the currents and extensions can occur towards both ends of the island (as occurs on Anclote Key, Three Rooker Bar, and Sand Key, on the Gulf Coast of Florida). Washover fans on the lagoon side of barriers, where storm surges have over-topped the island, are common, especially on younger barrier islands. Wave-dominated barriers are also susceptible to being breached by storms, creating new inlets. Such inlets may close as sediment is carried in them by longshore currents, but may become permanent if the tidal prism (volumn and force of tidal flow) is large enough. Older barrier islands that have accumulated dunes are less subject to washovers and opening of inlets. Wave-dominated islands require an abundant supply of sediment to grow and develop dunes. If a barrier island does not receive enough sediment to grow, repeated washovers from storms will migrate the island towards the mainland.
=== Mixed-energy ===
Wave-dominated barrier islands may eventually develop into mixed-energy barrier islands. Mixed-energy barrier islands are molded by both wave energy and tidal flux. The flow of a tidal prism moves sand. Sand accumulates at both the inshore and off shore sides of an inlet, forming a flood delta or shoal on the bay or lagoon side of the inlet (from sand carried in on a flood tide), and an ebb delta or shoal on the open water side (from sand carried out by an ebb tide). Large tidal prisms tend to produce large ebb shoals, which may rise enough to be exposed at low tide. Ebb shoals refract waves approaching the inlet, locally reversing the longshore current moving sand along the coast. This can modify the ebb shoal into swash bars, which migrate into the end of the island up current from the inlet, adding to the barrier's width near the inlet (creating a "drumstick" barrier island). This process captures sand that is carried by the longshore current, preventing it from reaching the downcurrent side of the inlet, starving that island. Many of the Sea Islands in the U.S. state of Georgia are relatively wide compared to their shore-parallel length. Siesta Key, Florida has a characteristic drumstick shape, with a wide portion at the northern end near the mouth of Phillipi Creek.
=== Fetch-Limited Barrier Islands === Fetch-Limited barrier islands are defined by the difference in waves they are exposed to. These barrier islands are shaped by small, local waves that are on a smaller and lower-energy scale than barrier islands formed by open-ocean waves. These barrier islands are still formed in the same way as ocean-facing islands, but with much lower-energy. Fetch-limited barrier islands can be found all over the world, with over 63% coming from the Northern Hemisphere. North America makes up the largest percentage, with just over 30% of all fetch-limited barrier islands. During fair-weather conditions, fetch-limited islands experience locally generated waves under one meter high. Because wave energy is low under these conditions, most significant geomorphic change occurs during storms. As a result, fetch-limited barrier islands are generally shorter and narrower than ocean-facing barriers.
== Ecological importance == Barrier islands are critically important in mitigating ocean swells and other storm events for the water systems on the mainland side of the barrier island, as well as protecting the coastline. This effectively creates a unique environment of relatively low energy, brackish water. Multiple wetland systems such as lagoons, estuaries, and/or marshes can result from such conditions depending on the surroundings. They are typically rich habitats for a variety of flora and fauna. Without barrier islands, these wetlands could not exist; they would be destroyed by daily ocean waves and tides as well as ocean storm events. One of the most prominent examples is the Louisiana barrier islands.
== See also ==
North Frisian Barrier Island Outer Banks Virginia Barrier Islands New York Barrier Islands Texas barrier islands Sea Islands Long Beach Island Bald Head Island
== Notes ==
== References ==
== Sources == Davis, Richard A. Jr.; FitzGerald, Duncan M. (2004), Beaches and Coasts, United Kingdom: Blackwell Publishing, ISBN 978-0-632-04308-8 Davis, Richard A. Jr. (2016). Barrier Islands of the Florida Gulf Coast Peninsula. Sarasota, Florida: Pineapple Press. ISBN 978-1-56164-8085. Morton, Robert A. (2007), "Historical Changes in the Mississippi-Alabama Barrier Islands and the Roles of Extreme Storms, Sea level, and human activities" (PDF), USGS Report, Open-File Report, U. S. Geological Survey: 43, Bibcode:2007usgs.rept...43M, doi:10.3133/ofr20071161
== External links ==