Barrier islands are elongate, shore-parallel accumulations of unconsolidated sediment, some parts of which are situated above the high-tide line (supratidal) most of the time, except during major storms (see example in Fig. 1). They are separated from the mainland by bays, lagoons, estuaries, or wetland complexes and are typically intersected by deep tidal channels called tidal inlets. A large percentage of the major barrier islands of the world occur along the coastlines of the trailing edges of continental plates and of epicontinental seas and lakes (e.g., Caspian and Black Seas). Because they are composed of unconsolidated sediments (primarily sand, with gravel being present in some Arctic regions), they most commonly occur on coastal plain and deltaic shorelines (depositional coasts), where the sediment that makes up the islands was ultimately brought to the shore by rivers and streams. Some barrier islands do occur, primarily as spit forms, on leading edges of continental plates and on some glaciated coasts, but they are a minority coastline type in those areas.
The largest single chain of barrier islands in the world occurs along the East and Gulf Coasts of North America. Many of the largest of these North American barrier islands are highly developed with human habitation, some with moderate-sized cities, such as Galveston, Texas, and Atlantic City, New Jersey. Because of the dynamic nature of the coastal zone, beach erosion is a serious problem for some of the more developed barrier islands, invoking major engineering efforts to stem the erosion. One commonly applied technique is the addition of massive volumes of sand derived from elsewhere along the eroding shoreline, a process called beach nourishment. The largest of these beach nourishment projects commonly cost many millions of dollars. Some of the islands have been preserved as national and state parks, to which vacationers flock in season. Consequently, barrier islands have a high socioeconomic profile in North America, especially for the large percentage of the population that lives in coastal areas. Barrier islands also occur on the shorelines of northwestern Europe, the Mediterranean and Caspian Seas, West Africa, and elsewhere.
Depositional coasts have characteristic morphology and sediment distribution patterns controlled by the interaction of waves and tides, with the magnitude of the tides being of particular importance. Accordingly, depositional coasts are commonly classified as microtidal (tidal range, or T. R. = 0–2 m), mesotidal (T.R. = 2–4 m), and mac-rotidal (T.R. = > 4 m). As a generalization, depositional features on microtidal coasts are highly influenced by waves (wave-dominated coasts), whereas those on macrotidal coats are highly influenced by tides (tide-dominated coasts) and those on mesotidal coasts respond to the effects of both waves and tides (mixed-energy coasts). For example, barrier islands do not occur on open-ocean, coastal-plain shorelines with tidal ranges greater than about 4 meters (macrotidal coasts). This is because their primary mechanism of formation, wave action, is not focused long enough at a single level during the tidal cycle to form the island. Furthermore, the strong tidal currents associated with such large tides transport the available sediments to the offshore regions.
Barrier islands exposed to open-ocean waves and tides that are in a progradational mode (i.e., consistently building in an offshore direction) show major differences depending on whether the tides are microtidal or mesotidal. Prograding barrier islands along microtidal shorelines are long and linear, commonly over 25–50 km in length, with a predominance of storm washover features. Those on mesotidal shorelines are stunted, usually less than 16 km in length, with an abundance of large tidal inlets. More tidal inlets are required on mesotidal coasts, because of the large amount of water that moves into and out of the backbarrier regions during a single tidal cycle. During major storms with significant storm surges, microtidal barrier islands are usually washed over and permanent washover fans are formed (e.g., those on the Texas coast), whereas on mesotidal barrier islands, permanent washover fans are not so common, because the system is already adjusted to major influx and outflow of ocean waters during normal spring tides (e.g., those on the Georgia and South Carolina coasts).
The long-term patterns of morphology and sedimentation on most coastal plain and deltaic shorelines have been significantly impacted by the major changes in sea level that occurred during the glacial episodes of the Pleistocene Epoch. During each major glaciation, sea level was lowered significantly, over 100 m during the last glaciation (Wisconsin). When the sea level was lower, major valleys, called lowstand valleys, were carved across the coastal plains and continental shelves. As sea level rose, the valleys were flooded to become major estuaries.
A major consideration with regard to the morphology and stratigraphy of barrier islands is whether they consistently migrate landward (transgressive) or build in an offshore direction (prograding). The general patterns of prograding barrier islands with respect to the effect of tidal range were discussed in the previous section of this article. However, both types of barrier islands, transgres-sive and prograding, may be present on either microtidal or mesotidal coastlines, depending upon the rate of sea-level change relative to sediment (usually sand) supply at that location (not the tidal range). Diminished, or low, sand supply and rapid sea-level rise both promote the development of landward-migrating islands, and vice versa for those that build in an offshore direction. Both prograding and transgressive barrier islands clearly tend to change to some extent over time, with the rates of landward migration and offshore growth being different from place to place. In some parts of the South Carolina coast, for example, the landward-migrating barrier islands may move 3 m or more per year (example in Fig. 2). However, those that build seaward usually grow more slowly, at rates of < 3 m per decade (example in Fig. 1).
As illustrated in Figs. 2 and 3, transgressive (landward-migrating) barrier islands are composed of coalescing washover fans, or a washover terrace, that is overtopped at high tides, usually several times a year. In the process of migration, the entire washover terrace complex moves landward, leaving an eroded nearshore zone in its wake. As a result of this type of migration, in three dimensions the entire complex consists of a relatively thin (< 1–3 m) wedge of sand and shell of the washover terrace, which overlies muddy sediment originally deposited in the lagoons or wetlands landward of the islands (see cross section in Fig. 3). Because of their continual landward migration, these types of islands are, needless to say, impractical sites for human development. The trans-gressive (landward-migrating) barrier islands in South Carolina are relatively short, 2–8 km on the average, because new inlets are created where the migrating islands intersect tidal channels (see Fig. 2).
Prograding (seaward-building) barrier islands (Figs. 1 and 3) are typically composed of multiple, relatively parallel linear ridges of sand topped by vegetated sand dunes that originally formed as front-line dunes on the back-beach (called foredunes). The most notable changes on these types of islands occur where adjacent tidal inlets migrate into them or when the inlets expand dramatically during hurricane storm surges. As a result of their offshore growth, in three dimensions these types of barrier islands typically consist of a wedge of sand 7–9 m thick that has built over offshore muds (see cross section in Fig. 3). Most of the major developed barrier islands along the east coast of the United States, which typically are greater than 16 km long, are of this type (e.g., Kiawah Island and Hilton Head Island, South Carolina). When human development occurs on these types of islands, buildings are usually secure from all but the most extreme hurricanes if they have been set back an adequate distance from the front-line dunes and tidal inlets. That security will vanish, however, if a major rise of sea level occurs in the near future as a result of global warming.
The morphology of the prograding barrier islands longer than about 11 km takes on a characteristic drumstick appearance, as shown in Fig. 4. This pattern is most common on mesotidal shorelines. Two factors that enhance the development of the drumstick shape are:
The occurrence of significant masses of sand in the form of large, wave-built intertidal sand bars (swash bars) that develop along the outer margin of a large lobe of sand deposited on the seaward side of the tidal inlet by ebb-tidal currents (called the ebb-tidal delta). These huge swash bars eventually move toward shore and weld to the beach (Fig. 4). This welding process builds out the end of the island that faces the direction from which the sediments come, accentuating its drumstick shape (see photographs in Figs. 1 and 5).
Refraction of the dominant waves around the ebbtidal delta, a process that enhances deposition on the same end of the island where the huge swash bars come ashore. The refracted waves create currents that transport sediment in the opposite direction from that on the open coast. This is a relatively minor reversal from the normal longshore transport direction (see model in Fig. 4), but it allows sand to remain in the inlet area and aids in its accumulation on that end of the island.
Climate plays a significant role in the nature of barrier islands, not so much regarding their morphology and stratigraphy, as was demonstrated for tides and waves, but in the production of sediment types, occurrence of storms, and the types of vegetation present. This is especially true in extreme climates, such as polar regions. For example, the barrier islands on the North Slope of Alaska are eroding away at alarming rates. This erosion continues despite the fact the Arctic Ocean is frozen for many months of the year, producing limited fetch for the waves, hence relatively small waves, and a short season for waves to occur. Even in August, blocks of ice sometimes occur near shore. Composed mostly of gravel, these islands are short (average length < 3 km) and low, with numerous wide inlets. Warming of the Arctic Ocean may be playing a role in the islands' demise, with melting of the permafrost possibly being a major factor.
In the other extreme, barrier islands in hot, arid regions are more prone to have barren sand dunes and extensive sand transport across them. In shallow seas with warm water and high salinities, the sediments of the islands beaches may be cemented with beachrock. As the satellite image of the shoreline of Abu Dhabi in Fig. 6 shows, although probably the hottest and driest barrier island chain in the world, these islands have many of the characteristics of mesotidal barrier islands (the tidal range in Abu Dhabi is 2.5 m). Features such as stunted islands, large tidal inlets with huge ebb-tidal deltas, and complex backbarrier regions stand out. However, typical inlet migration and beach erosion patterns are inhibited by the beachrock, producing some jagged shorelines cemented in place. Also, the ebb-tidal delta sediments are composed of carbonate oolite sand, another signature of arid tropic regions.
The origin of barrier islands has been a matter of conjecture in the geological literature for well over 100 years. The processes of beach accretion between storms and the formation of a foredune landward of the beach, primary components of barrier islands, are well understood, and their common occurrence on depositional shorelines is not surprising. The aspect of barrier islands that is most difficult to understand is the fact that they occur offshore of the mainland separated by a topographically low area, a lagoon or estuarine complex in most cases. Any theory for the origin of these islands must account for their mysterious offshore location.
Numerous hypotheses for the origin of barrier islands have been proposed, including the following three examples.
Elongation of sand spits away from some kind of headland, with segmentation of the spits as they grow as a result of the formation of permanent tidal inlets through them during storms, creating individual islands separated from the mainland by an open-water lagoon (as illustrated in Fig. 7).
Elevation of an offshore bar or flooding of a line of foredunes along the shore.
The transgressive—regressive interfluve hypothesis, which has been well documented on the coasts of North Carolina and South Carolina.
One of the major proponents of the spit elongation hypothesis, John Fisher, cited the northern part of the Outer Banks of North Carolina as one of his examples of barrier islands that have been formed by that mechanism. This mode of origin has been suggested for other areas in the world, for example those islands on the central Texas coast. This clearly is one way barrier islands can form, but not the only one.
There is no doubt that many barrier islands have formed without the aid of spit elongation. Two major ideas have been proposed to account for the elevation and permanence of barrier islands independent of spit elongation. Some observations along the Gulf Coast of the United States illustrate that during the elevated tides and unusually high water levels that accompany hurricanes, a major offshore bar may be formed by the large waves. When the water level recedes, the bar emerges and, under the right circumstances, may survive to become a barrier island. A second idea is that a line of typical foredunes forms along the beach during a stillstand or slowly rising sea level. A relatively abrupt rise in sea level floods the line of foredunes. During this abrupt rise, the foredunes are not completely eroded away by the waves. The remnant of the original dune line becomes an island. Both of these processes seem reasonable, but their documentation is still somewhat in question.
The mode of formation for most of the larger prograding barrier islands on the South Carolina coast, such as Kiawah Island, is clear and well documented in studies by Tom Moslow and D. J. Colquhoun. Four major steps take place in this mode of formation:
A narrow, landward-migrating barrier island moved rapidly across what is now the inner continental shelf, leaving behind a thin lag of coarse material on top of an erosion surface across the continental shelf, called the transgressive surface of erosion.
The topography over which the shoreline advanced was irregular, and estuarine waters flooded the numerous river valleys formed when the shoreline was further offshore. Isolated, primary transgressive barrier islands, consisting of washover terraces composed of coarse-grained sand and shell, continued to migrate landward on the exposed interfluves between the drowned lowstand valleys.
When sea level stopped rising and a relative stillstand occurred about 4500 years ago, shoals developed at the entrances of the estuaries created by the drowning of the valleys, and a longshore sediment transport system was initiated along the face of the stranded barrier islands. Over time, beach ridges began to develop, eventually impinging upon the adjacent estuary entrances. As a well defined inlet throat evolved, a shoal off the entrance (ebb-tidal delta) formed, around which sediment was bypassed, augmenting beach-ridge growth on the adjacent barrier island.
As the barrier island matured, and minor fluctuations of sea level occurred, parts of some of the originally prograding beach ridges were eroded as a result of tidal-creek and tidal-inlet migration.
The end result of all this was a prograding, drumstick-shaped barrier island, such as the ones illustrated in Figs. I, 4, and 5. However, this hypothesis does not account for how the transgressive element, the original barrier island, formed.
Beaches / Hurricanes and Typhoons / Sea-Level Change / Tides
- Transgressions and regressions, in Papers in marine geology: Shepard commemorative volume. , ed. New York: MacMillian and Co, 175-203. 1964.
- Geology of the Holocene barrier island systems. Berlin: Springer-Verlag. , ed. 1994.
- Origin of barrier island chain shoreline, Middle Atlantic States. Geological Society of America Special Paper 115: 115-67. 1967.
- Barrier island morphology as a function of tidal and wave regime, in Barrier islands, from the Gulf of St. Lawrence to the Gulf of Mexico. S. Leatherman, ed. New York: Academic Press, 1–27. 1979.
- Barrier island formation. Geological Society of America Bulletin 78: 78-1136. 1967.
- A celebration ofthe world's barrier islands. New York: Columbia University Press. , and . 2003.
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