Coastal barriers in the United States

September 20, 2012, 4:21 pm
Source: USFWS

Introduction

Coastal barriers are unique land forms that provide protection for diverse aquatic habitats and serve as the mainland's first line of defense against the impacts of severe coastal storms and erosion. Located at the interface of land and sea, the dominant physical factors responsible for shaping coastal land forms are tidal range, wave energy, and sediment supply from rivers and older, pre-existing coastal sand bodies. Relative changes in local sea level also profoundly affect coastal barrier diversity.

Coastal barriers:

  1. Consist primarily of unconsolidated sediments (sand, gravel, etc.);
  2. are subject to wind, wave, and tidal energies;
  3. are subject to the impacts of coastal storms and sea-level rise;
  4. buffer the mainland from the impact of storms;
  5. include associated landward aquatic habitats that are protected from direct wave attack by the fastland (non-wetland) portion of the coastal barrier; and
  6. protect and maintain productive estuarine systems which support the Nation's fishing and shellfishing industries.

Types of Coastal Barriers

Coastal barriers may be described by their relationships to the mainland as bay barriers, tombolos, barrier spits, and barrier islands. Additional areas which function as coastal barriers include dune and beach barriers, and fringing mangroves. The term "mainland" includes the continental land mass as well as large islands such as Long Island, New York and the Hawaiian Islands. Definitions of the various kinds of coastal barriers follow.

caption Cape Lookout barrier island, North Carolina. (Source: U.S. Fish and Wildlife Service)

Bay barriers are coastal barriers that connect two headlands, and enclose a pond, marsh, or other aquatic habitat. The terms bay mount bar and bay bar are synonymous.

Tombolos are sand or gravel beaches which connect one or more offshore islands to each other or to the mainland. The terms connecting bar, tie bar, and tying bar are synonymous.

Barrier spits are coastal barriers that extend into open water and are attached to the mainland at only one end. They can develop into a bay barrier if they grow completely across a bay or other aquatic habitat. On the other hand, bay barriers can become spits if an inlet is created.

Barrier islands are coastal barriers completely detached from the mainland. Barrier spits may become barrier islands if their connection to the mainland is severed by creation of a permanent inlet. The barrier island represents a broad barrier beach, commonly sufficiently above high tide to have dunes, vegetated zones, and wetland areas.

Dune or beach barriers are broad sandy barrier beaches, with hills or ridges of sand formed by winds, which protect landward aquatic habitats.

Fringing mangroves are bands of mangrove along subtropical or tropical mainland shores in areas of low wave energy. Many of these areas are located behind coral reefs, which together with the mangroves themselves, provide significant protection for the mainland from storm impact.

Location of Coastal Barriers

Coastal barriers occur on all the coastlines of the United States. One of the longest and best defined chains of coastal barriers in the world occurs along the United States shoreline bordering the Atlantic Ocean and the Gulf of Mexico. This chain contains over 400 barriers and totals about 2,700 miles of shoreline. The coastal barriers from Maine to Texas show a high degree of regional diversity, controlled by differences in climate and in the physical processes shaping barrier shorelines. Long, continuous barriers with small ebb-tidal deltas are produced by longshore currents along wave-dominated coasts. These barriers are typified by the coastal barrier islands along the south Texas coast which are long, generally narrow, and cut by widely separated tidal inlets with large sand accumulations in the back-barrier bays, and small or nonexistent seaward shoals. Similar barrier islands are also found in parts of Louisiana, the Florida panhandle, southeast Florida, North Carolina's Outer Banks, the south shore of Long Island, and the Cape Cod segment of the Massachusetts coast. Tide-dominated coastlines support large ebb-tidal deltas. The Georgia coastal barrier islands typify a tide-dominated coastline: they are relatively short and stubby and are separated by stable tidal inlets with an average spacing of 9 miles. Tide-dominated barriers also occur in northeast Florida, most of South Carolina's coast, along the Delmarva Peninsula, Massachusetts, and in some areas of Louisiana and Texas.

Secondary Barriers

If a suitable sediment source and sufficient wind, waves, and tidal energy exist, a secondary barrier may occasionally form behind the seaward coastal barrier. Secondary barriers are located in large, well-defined bays or in lagoons on the mainland side of coastal barrier systems. These barriers are maintained primarily by internally generated wind waves rather than open ocean waves. Consequently, secondary barriers are generally smaller and more ephemeral than barriers along the open coast. Nonetheless, these barriers are formed of unconsolidated sediments just like most oceanic barriers and, more importantly, they also protect vital fish and wildlife habitat and provide substantial protection for the mainland during major storms.

Value of Coastal Barriers

Coastal barriers provide important services that are the foundations of a strong economy and healthy environment. They offer habitats that support a variety of fish and wildlife, protect mainland communities from severe storms, serve as popular vacation destinations, and support local economies.

Coastal Barriers and Natural Resources

Coastal barriers protect the aquatic habitats between the barrier and the mainland which contain resources of extraordinary scenic, scientific, recreational, natural, historic, and economic value. Together with their adjacent wetland, marsh, estuarine, inlet, and nearshore water habitats, coastal barriers support a tremendous variety of organisms. Millions of fish, shellfish, birds, mammals, and other wildlife depend on barriers and their associated wetlands for vital feeding, spawning, nesting, nursery, and resting habitat. These habitats are also critically important for many species harvested in the Nation's commercial fish and shellfish industries. The barrier and its associated habitats are one ecological system, and the health and productivity of the entire system depend on the rational use of all the component parts.

Coastal Barriers and Severe Storms

Under normal weather conditions, only aquatic habitats immediately adjacent to coastal barriers are exposed to direct wave attack. However, major coastal storms routinely affect the entire landward aquatic habitat. This habitat survives major storms because coastal barriers receive the brunt of the ocean's energies. Storm waves break on the barrier beach, leaving a diminished wave to travel into the wetland. At the same time, the wetland stores storm flood waters, easing the flood pressure on the mainland. Without extensive sand beaches protecting many bluffs and terraces, damages from violent storms would be much greater. Sand acts as a brake or drag on waves. Where there are barrier beaches fronting embayments, the sand absorbs the energy much as it does at the base of cliffs. The principal danger to beaches and barriers is not intense storms but a steady reduction in the sand supply caused by dams on tributary streams and the diversion or interruption of littoral transport along the seaward edge of beaches and barriers by bulkheads, groins, and jetties. In some situations, mining of beach sand has contributed to the problem. Spits and low-lying barrier beaches survive severe storms with relatively slight effects as long as there is a supply of sand available to restore the beach. A severe storm is a short-term phenomenon, repeating the annual cycle of changing width and slope of the beach within a few hours. Sometimes a spit is eroded back or shortened and the dunes reduced or moved, but the sand begins to build up again towards its equilibrium condition almost as soon as the storm ends. The entrance to a bay and/or river mouth may be relocated or shoaled, but this sometimes also happens without storms. Shoaling of harbor entrances may be dangerous to navigation and require dredging to restore an entrance channel.

Development of Coastal Barriers

Besides bearing the brunt of impacts from storms and erosion, most coastal barriers are made of unconsolidated sediments (sand, gravel, etc.). This geological composition alone makes them highly unstable areas on which to build. Despite their instability, many coastal barriers have been developed. In the past, this development was encouraged by the availability of Federal flood insurance and other types of Federal financial assistance.

Through its unique free-market approach to conservation, the Coastal Barrier Resources Act has been instrumental in ensuring that the Federal government does not encourage the development of these coastal barrier habitats.

 

caption Image by Jef Poskanzer

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the U.S. Fish and Wildlife Service. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the U.S. Fish and Wildlife Service should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

Glossary

Citation

(2012). Coastal barriers in the United States. Retrieved from http://www.eoearth.org/view/article/51cbed4a7896bb431f691174

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