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Barrier Islands: Formation and Evolution


Introduction

Barrier islands are elongate accumulations of sand that are separated from the mainland by open water in the form of estuaries, bays, or lagoons. These primarily sandy islands have become in great demand for both residential and recreational development. Beaches on the seaward side of barrier islands are the principal location for beach nourishment. In order to properly manage these important natural resources it is important to understand the origin, dynamics, and probable future of barrier islands and their associated beaches. The following discussion will examine these complex coastal environments and the nature of the coastal processes that affect the evolution of barrier islands in order to assist decision making in better assessing potential measures to stabilize barrier island beaches.

Although barrier islands are quite extensive along the coasts of the United States, they can only be found along 15 percent of the world's existing coastlines. Most of the Atlantic and Gulf Coasts of the United States are comprised of barrier islands, and there are numerous such islands found along both the southeast and northern coasts of Alaska. The Pacific Coast, extending from Washington to California, is characterized by numerous short barrier spits that are elongate, primarily sand accumulations, generally connected to the mainland at a rocky headland.

Origin of Barrier Islands

The origin of barrier islands has been the subject of debate for more than a century. There are three prevailing theories that have been proposed attempting to explain their origins. The earliest theory is based upon waves concentrating on sand along the shallow water adjacent to a mainland shoreline. Waves transport sand landward until a sand bar is formed. As the crest of the sand bar reaches near sea level, the waves then begin to break over the top of a bar. This process continues over time until a fully-developed sand bar ultimately emerges, initially within the intertidal zone, and then eventually rising above the high tide. In the absence of major storms, opportunistic vegetation may develop on such a sand island. This vegetation traps additional sand, and builds small dunes that eventually become much larger formations. Such barrier island development has been observed at numerous locations along the Gulf Coast of Florida where islands of several kilometers or longer have developed in only a couple of decades.

Another theory of the origin of barrier islands is based on the premise that elongate sand spits were once connected to the adjacent mainland. These sand accumulations became isolated as the result of violent storms that breached the narrow and low-lying barriers, thereby forming islands. Such origins may be inferred from a few examples found along the Gulf Coast, where hurricanes routinely drive extremely high energy waves across such islands, resulting in multiple breaches.

A third theory of origin assumes that dunes lying along a low-lying coastline provide the core for the development of a barrier island during the period of rising sea level. As sea levels flooded these coastal areas, it is proposed that dunes rose to form elongate islands, which over time became separated from the mainland by the rising open water. This origin can neither be proven nor discounted, largely because there are simply no documented barrier islands that were formed assuming such a scenario. Nor are there any existing barrier islands that can be interpreted as having been formed as the result of the drowning of a coast.

Age of Barrier Islands

Figure 3
Figure 3. Sea level curves over the past 35,000 years (upper).
Three sea level curves for the late Holocene Age (past 8,000 years) (lower).

Geologically speaking, barrier islands are young features; the vast majority are less than 7,000 years in age, and most are probably less than 3,000 years old. Barrier island formation is dependent upon the complex interaction existing between waves, sea level change, and the availability of sediment.

In order for island barriers to develop, it is important for shoreline and water depth to remain essentially unchanged. This condition provides the time required for wave action to build sand accumulations that eventually become barrier islands. The quantity of available sand and the rate of change in sea level are the principal factors that determine the amount of time required for the complete development of a barrier island. As indicated below, there is strong evidence suggesting that a sea-level increase of one to two centimeters per year that occurred as the result of melting glaciers was simply too rapid for adequate barrier island development.

There have, in fact, been rapid and significant sea level rises that have occurred over the last 20,000 years, since the melting of glaciers that occurred during the latter phases of the last Ice Age. The overall trends in sea level are illustrated by Figure 3, which shows the position of the shoreline as the result of sea level rise and fall over the past 30,000 years.

As the great glaciers of North America and Europe began to melt, the enormous volume of water produced by the melting ice caused sea levels to rise rapidly and dramatically. It is estimated that this rise averaged one to two centimeters annually. That level of increase, for example, is almost ten times the current rate of sea level rise, which is approximately two millimeters per year. This rapid increase in sea level persisted for some 12,000-13,000 years. The total rise in sea level during this entire period is estimated to be at least 100 meters. As this gradual rise in sea level persisted, the shoreline constantly moved inland across the coastal plains of the Atlantic and Gulf Coasts, thereby effectively preventing island barriers from developing.

Evidence from a variety of sources further indicates that the rate of sea level rise decreased significantly about 7,000 years ago. Since that time, it is estimated that the rate of increase has approximated the present rate of approximately two millimeters per year. Barrier islands in parts of both the Atlantic and Gulf Coasts began to form at that time. Examples of such formations can be found along the coasts of Texas, Georgia and South Carolina. In much of the United States, sea level rose about 10 meters.

About 4,000 years ago another change in the sea level rate took place. Three theories have been put forward seeking to explain the sea level events over the last 3,000 years. The first theory suggests that sea level reached present levels at that time. A second theory suggests that sea level reached points at or near present levels, but that these levels have risen or fallen as much as a meter or more over the last 3,000 years. A third hypothesis proposes that sea levels have simply risen gradually (about 0.1 mm/yr) over the last 3,000 years. At present, none of these theories can be proven or discounted as being the most probable scenario in describing the formation of barrier islands.

Barrier Island Environments

There are numerous distinct environments found within any particular barrier island. Although one or more of these environments may be present for any given barrier island, the overall scheme appears both consistent and predictable. Taken in sequential order, and beginning from open ocean to the outermost reaches of the back-barrier, these environments are as follows: nearshore, beach, dune, washover fan, marsh, tidal flat, and the adjacent estuary/lagoon.

Nearshore Environment
The narrow zone immediately seaward of the shoreline is typically called the nearshore environment. This area generally extends from the shoreline across the zone of longshore sand bars and troughs. Usually longshore sand bars and troughs are present, but this is not always the case. The nearshore environment includes the normal surf zone where breaking waves occur. The number of longshore bars present depends upon the gradient, or slope, of the nearshore environment. Generally speaking, the more gentle the gradient, the more bars will be present. Thereby, steep nearshore gradients tend to have few or no longshore bars. Longshore bars, when present, tend to persist throughout the entire year, but can move or change shape as seasons change or conditions vary.

Beach
The beach, or visible portion of the profile, is the most familiar of the barrier island environments, and in many respects is the most important because it affords protection from wave attack to the landward upland environments (where development is typically located). The beach extends from the shoreline landward, and often includes numerous changes in topography such as sand dunes, sea walls, or other man made structures. A beach is typically divided into the seaward sloping foreshore and the nearly horizontal backshore.

The foreshore is considered to be that area where the last vestiges of waves rush up and back. This constitutes what is referred to as the swash zone, although such wave action can often cover an entire foreshore. This area is also the zone of the intertidal portion of the beach and may range in width depending upon slope or gradient.

The backshore is generally dry except during the occurrence of severe storms and their associated storm tides. Under normal conditions, the backshore is subjected only to wind action that blows the dry sand landward, creating dunes. Opportunistic dune or beach vegetation may occupy this portion of the beach.

Storms can cause a beach to erode, and can result in a uniformly seaward-sloping beach. This typically occurs during the winter months. During the calmer summer months, the beach gradually accumulates sediment as the result of currents, produced by low waves that return sand landward to the foreshore, or emergent, portion of the profile. If there is long-term erosion of a beach, it may be due to a variety of phenomena, including storms, high rates of sea level rise, interruptions in the longshore transport system along the beach, or inappropriate construction practices along the shoreline area that function to interrupt the longshore movement of sand.

Dunes
The landward transport of sand causes the backshore environment to accumulate sand as dunes, where the growth of opportunistic vegetation on the backshore area traps blowing sand as it moves above the beach surface. As mounds of sand accumulate, vegetation continues to grow upward, forming substantial dunes. The principal factors involved in sand dune development are the presence and width of dry beach, and the abundance of sediment being supplied to the backshore areas. Dunes can provide excellent protection for the landward portion of a barrier island, and, when possible, their continued growth should be encouraged.

Figure 5
Figure 4. Aerial photograph showing a washover apron on Dauphin Island, Alabama. These washovers were produced by Hurricane Frederic in 1979.

Washover Fans
An increase in water level or large waves associated with storms can sometimes cause barrier islands to occasionally be washed over, forming what are referred to as washover fans. Low-lying islands generally permit widespread washover, whereas islands having dunes permit water to channel only in the lower spaces between dunes. Strong storm-induced currents over-washing a barrier island can carry abundant sand from the nearshore and beach inland. On a natural, undeveloped barrier island, this phenomenon is the primary method of naturally removing sand from a beach. Overtopping of barrier islands during storm events causes sand to accumulate into a fan-shaped feature on low barrier islands, where the washover or overtopping process can be quite significant, these fans coalesce to form washover aprons, as shown in Figure 4.

Individual fans may extend over hundreds to thousands of acres but are generally only 10-15 centimeters thick. It is not uncommon for washover fan deposits to accumulate in several layers, each representing a single storm. The washover deposits comprise the landward portion of a barrier island. Scientists have demonstrated that through the process of overwash, many barrier islands can actually conserve mass and will lose very little sediment during major storm events. Santa Rosa Island, located along the Florida panhandle, is a well documented example of this occurrence. In 1995, although struck almost directly by Hurricane Opal, very little sediment eroded from that barrier system.

Wetlands
The landward fringe of a barrier island is typically a wetland, generally a highly vegetated environment. In the lower latitudes this fringe can typically be populated by mangroves, while in the mid-to-higher latitudes this area is generally salt marsh. Wetlands develop along the intertidal portion of washover fans. They can provide excellent stabilization and protection from erosion for the landward shoreline of a barrier island.

Tidal Flats
The unvegetated intertidal zone on the protected landward side of the island represents the environment referred to as the tidal flat. These gently sloping island margins are typically covered with fine sediment and can be occupied by numerous bottom dwelling invertebrates. Such tidal flats are typically the distant portions of the washover fans. The width of the tidal flats is generally proportional to the tidal range of a given location.

Barrier Island Morphology and Dynamics

Barrier islands are acted upon by both wave and tide generated processes. Waves can range in both size and direction, and can cause beach erosion due to the fact that they can produce longshore currents flowing parallel to the shoreline. As tides rise and fall, waves interact differently with the nearshore profile, or the seaward portion of the barrier island. Tidal currents at inlets, separating barrier islands, typically produce sand bodies at the seaward mouth of the inlet or tidal channels. These sediment bodies may influence those processes that affect adjacent beaches. As the various processes of coastline interact with the barrier islands, extensive and yet relatively predictable changes occur. Such inlet shoals contain large reservoirs of sand, acting as sediment sinks, where large volumes of sand accumulate. These shoals are used as sand sources or borrow sites for beach nourishment.

Barrier Island Types

Figure 4
Figure 5. Diagram showing the environments of a barrier island system. (from Blatt, et al, 1980, Petrology of Sedimentary Rocks)

The combination of wave- and tide-dominated processes results in two distinct types of barrier islands determined largely by their shape: wave-dominated barriers and mixed-energy barriers.

Wave-Dominated Barriers
The interaction of approaching waves, and the longshore currents produced by those waves, cause the formation of long, narrow barrier islands. This type of barrier island is typically characterized by widespread washover fans. Due to their length, which at Santa Rosa Island, Florida and Padre Island, Texas exceeds 75 kilometers, the associated inlets are widely spaced with relatively small cross-sectional areas, the latter due to the influence of predominating longshore currents.

Mixed-Energy Barriers
Many coastal areas experience a combination of both tidal and wave influence, giving a unique size and shape to the barrier islands that are formed. Such barrier islands generally tend to be short, wide at one end (typically drumstick shaped) and narrow at the other, and are separated by large, stable, tidal inlets with large sand shoals at their mouth. The typically wide barrier island-inlet interface is the result of abundant sediment that accumulates as beach ridges. This sand accumulation can be caused by the bending of the waves around the ebb delta, at the mouth of the inlet, thus causing a local reversal in longshore current and net movement of sand. These reversals in the direction of sand movement interrupt the net direction of sand movement to the barrier island. This lack of sediment leads to erosion of that particular portion of the barrier island.

Summary

Barrier islands are valuable natural resources that provide recreational areas, sensitive coastal habitats and ecosystems, and protect the marshes and coastal estuaries during storms. These islands characterize most of the Atlantic and Gulf Coasts and over the past 30 years have become densely developed, especially in New Jersey and Florida. As a result, erosion of these barrier island beaches can adversely affect the local, state, and regional interests that reside, recreate, or economically depend on the beaches. Thus, preservation and enhancement of barrier island beaches protects these interests and is the major impetus for beach nourishment.

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