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Marine Conservation Science and Policy Service learning Program

Barrier islands are long, narrow, offshore deposits of sand or sediments that parallel the coast line. Some barrier islands can extend for 100 miles (160 km) or more. The islands are separated from the main land by a shallow sound, bay or lagoon. Barrier islands are often found in chains along the coast line and are separated from each other by narrow tidal inlets. Barrier islands serve two main functions. First, they protect the coastlines from severe storm damage. Second, they harbor several habitats that are refuges for wildlife.

Module 1: Ocean and Coastal Habitats

Sunshine State Standards

SC.912.E.6.6, SC.912.L.16.10, SC.912.L.17.17, SC.912.E.7.4

Objectives Understand barrier islands

Locate barrier islands on a map

Describe the processes of erosion and accretion

Describe the formation of sand dunes

Explain the importance of sand dunes

Identify which geographical areas of a barrier island are affected by stronger wind and water energies by characterizing individual sand samples from different localities across the island transect.

Students should hypothesize which areas experience higher wind or water energies by looking for trends in their analyzed sand samples.

Section 8: Barrier Islands

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Vocabulary Accretion - building up of land by physical forces Barrier Island - long, narrow island lying parallel to the mainland and separated from it by bay, lagoon, or marsh Continental Shelf - remaining submerged portion of the coastal plain Erosion- process of being gradually worn away

Island or isle - is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot or holm. A grouping of geographically or geologically related islands is called an archipelago.

Longshore Current - current that runs parallel to the shore within the surf zones Marsh wrack - windrows of dead cordgrass from the marsh left behind on the beach by the wave wash. Sand dunes - a hill of sand piled up by the wind Sediment transport - is the movement of solid particles (sediment), typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Tidal range - is the vertical difference between the high tide and the succeeding low tide. It is the difference in height between high and low water and will vary throughout the tidal cycle.

Wave power - is the transport of energy by ocean surface waves, and the capture of that energy to do useful work — for example for electricity generation, water desalination, or the pumping of water.

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Background 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.

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.

General Information and Theories

Barrier islands, a coastal landform and a type of barrier system, are relatively narrow strips of sand that parallel the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen. Excepting the tidal inlets that separate the islands, a barrier chain may extend uninterrupted for over a hundred kilometers. The length and width of barriers and overall morphology of barrier coasts

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are related to parameters including tidal range, wave energy, sediment supply, sea-level trends and basement controls.

Chains of barrier islands can be found along the world's coastlines of different settings, suggesting that they can form and be maintained in a variety of environmental settings. Numerous theories have been given to explain their formations.

Formation Explanations for the development of barrier islands have been proposed by numerous scientists over more than 150 years. They can be grouped into three major theories: offshore bar theory, spit accretion theory and submergence theory. No single theory can explain the development of all barriers distributed extensively along the world's coastlines. Scientists accept the idea that barriers, including other barrier types, can form by a number of different mechanisms.

Offshore bar theory

One of the earliest ideas to explain barrier island formation was published in 1845 by the Frenchman Elie de Beaumont. He believed that waves moving into shallow water churned up sand, which was deposited in the form of a submarine bar when the waves broke and lost much of their energy. As the bars accreted vertically, they gradually built above sea level, forming barrier islands.

Spit accretion theory American geologist Grove Karl Gilbert first argued in 1885 the barrier sediments came from alongshore sources. He proposed that sediment moving in the breaker zone through agitation by waves in longshore drift would construct spits extending from headlands parallel to the coast. The subsequent breaching of spits by storm waves would form barrier islands Submergence theory

William John McGee reasoned in 1890 that the East and Gulf coasts of the United States were undergoing submergence, as evidenced by the many drowned river valleys that occur along these coasts, including Raritan, Delaware and Chesapeake Bays. He believed that during submergence coastal ridges were separated from the mainland, forming lagoons behind the ridges. He used the Mississippi-Alabama barrier islands (consists of Cat, Ship, Horn, Petit Bois and Dauphin Islands) as an example where coastal submergence formed barrier islands, but his interpretation was later shown to be incorrect as the coastal stratigraphy and sediment ages were more accurately determined.

Along the coast of Louisiana former lobes of the Mississippi River delta have been reworked by wave action, forming beach ridge complexes. Prolonged sinking of the

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marshes behind the barriers has converted these former vegetated wetlands to open-water areas. In a period of 125 years, from 1853 to 1978, two small semi-protected bays behind the barrier had been transforming to the large water body of Lake Pelto, leading to Isles Dernieres's detachment from the mainland.

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-

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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.

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Age of Barrier Islands

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. 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 above, 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

Sea level curves over the past 35,000 years (upper).

Three sea level curves for the late Holocene Age (past

8,000 years) (lower).

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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.

The Boulder Bank

An unusual natural structure in New Zealand may give clues to the formation processes involved in barrier islands. The Boulder Bank, at the entrance to Nelson Haven at the northern end of the South Island, is a unique 13 kilometer-long stretch of rocky substrate a few meters in width. It is not strictly a barrier island itself, as it is linked to the mainland

at one end. The Boulder Bank is composed of granodiorite from Mackay Bluff, which lies close to the point where the bank joins the mainland. It is still debated what process or processes have resulted in this odd structure, though longshore drift is the most accepted hypothesis. Studies have been conducted since 1892 to determine speed of boulder movement. Rates of the top course gravel movement have been estimated at 7.5 meters a year.

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Barrier System Elements

The major sedimentary environments of a barrier island are the beach, dunes, salt marshes, washover fans, subaerial spits and tidal flats. Barrier Island systems consist in six interactive sedimentary environments:

the mainland, a backbarrier lagoon, barrier island, inlets and inlet deltas, barrier platform, and the shoreface

Each element has distinct morphology and sediment patterns within the barrier island system which in turn can be wave or tide dominated. Evolution of each element can affect adjacent environments, as well as the entire system. The mainland part of the lagoon system, and usually has a very different character from a coast facing an open ocean.

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The expansion of barrier islands is related to sand supply, sea level history, and the wave and current regime. If a continued sediment supply, stable sea level, and low to moderate subsidence continues, barriers prograde seaward. A reduction in sediment supply, rise in sea level or a high rate of subsidence will result in landward migration of barriers and reduction in size. If the beach sand is fine, storm waves carry the excess into the lagoon to form overwash fans . Over time, the barrier island may increase in horizontal extent from these processes. As barriers are built forward, dune ridges are built inside the beach. These ridges may become covered with vegetation and stabilized at levels above the beach. If the beach continues to build seaward, a new dune ridge may form closer to the new shoreline, and this in turn become stabilized; thus a series of ridges may develop. Barrier elongation may be a result of accretion by longshore transport. Elongation may also result from migration of a tidal inlet causing one spit to lengthen and another to shorten as the inlet migrates.

The lagoon is an inland body of water, oriented parallel to the coast, separated from the ocean by a barrier and connected to the ocean by one or more restricted inlets that allow communication with the ocean. This enclosure by a barrier and restricted communication with the sea distinguishes lagoons from estuaries. The direct connection of an estuary with the ocean at one end and river flow at the other is quite different from the pattern of water exchange for a lagoon. The variables governing lagoon development are: wave energy impacting on the barrier, tidal range on the open coast and in the lagoon, tidal prism of the lagoon, type and amount of sediment supply. Lagoons are generally shallow with depths that seldom exceed a couple of meters and the sedimentation processes are dominated by stratified flow and tidal motion. The marshes and tidal flats in lagoons are sufficiently shallow that wind-generated wave energy and circulation determine the patterns of sediment accumulation. Rivers (and estuaries) may lie behind lagoons and they discharge into them.

Temperate lagoons and marshes behind barrier islands are sediment sinks for the terrigenous sediments coming from inland streams and the open sea. They border almost half of the Atlantic and Gulf coastline of the United States with 121 barrier and sea islands fronting 8,510 sq. km. lagoons and 5,810 sq. km. marsh only in the 2000 km distance between Long Island, New York and Miami, Florida. Large areas in these lagoons are floored by sand with some admixture of silt and clay. Sedimentary laminations are generally absent in the lagoon sediments because the intertidal sediments are thoroughly mixed by burrowing organisms. Lagoons have variable biotas depending on bottom sediments and salinity, which are distinct from those of the open sea. Tidal flows may move beach sediments into the lagoons. In general, the lagoonal sediments are influenced by climate and local sediment sources.

On the Atlantic coast of North America, the geomorphic variation of lagoons range from wide marsh-filled lagoons to open water lagoons with fringing marsh. The open lagoons have a relatively constant water surface area regardless of tidal stage. Expandable lagoons increase their surface area by more than 15 percent between low and high water because of partial emergence of the seabed during low water stages of the tide. Intertidal lagoonal features such as marshes, oyster reefs, mangroves, tidal flats, and

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flood deltas can bring much of the lagoon floor into intertidal water depths. Filling of a lagoon is controlled by the expansion of fringing marshes, transport of sand from the barrier by washover, and tidal delta expansion. Development of these features changes an open lagoon to the expandable state as filling progresses. On the Pacific coast, the barriers are all a form of spit elongation with only a single tidal channel. 1994

In arid and humid regions, silt is the dominant sediment size in the lagoon, and sand is present only along the margins and inlets. If the lagoon is formed behind coral reefs, the deposits may be fine-grained biogenic calcium carbonates, precipitated as aragonite needles by algae and various organisms and by bioerosion of the carbonates. Penicillus is a major contributor of fine aragonite mud. It is one of several lightly calcified green and red algae that disintegrate post-mortem and produce fine crystals of aragonite and calcite. The present rate of production by Penicillus sp. in the Florida reef tract could account for all the fine aragonite mud deposited since the areas were flooded by rising sea level. Mechanical breakdown of Halimeda, mollusks, algae, corals, etc. are another source of fine lime mud. Both sources are abundant in carbonate quiet water environments and the excess probably contributes to large mud banks.

Barrier inlets are channels that separate one barrier island from another. The inlets allow water and sediment transport between the lagoon and open ocean. Tidal deltas form seaward (ebb tidal delta) and landward (flood tidal delta) at tidal inlets where the tidal current spreads out and slows after passing through the narrow channel. The tidal delta sands are largely derived from the ocean, and usually grow inward across the lagoon toward the mainland more rapidly than outward.

The inlets are different in tide-dominated, wave-dominated and transitional barrier systems. Tide-dominated inlets have a deep, ebb-dominant main channel flanked by long, linear channel-margin bars. Flood tidal deltas are poorly developed or non-existent. Most of the sand carried into the lagoons from the seaward side are formed into flood tidal deltas with the head at the inlet, and growth directed laterally into the lagoon. Wave-dominated inlets have large, lobate flood-tide deltas building into wide, open lagoons. The ebb tide delta is small and extends only a short distance from the beach, and tidal channels are generally shallow (less than 6m). Longshore transport of material by wave-generated currents in the surf and swash zones is the major sediment source. Sediments may be transported past an inlet by: 1) bar bypassing in which sand moves along the seaward portion of the ebb tide delta onto the downdrift shore, or 2) tidal bypassing in which the sediment enters the inlet on flood tide and exits on ebb tide downdrift of the inlet. In transitional inlets, major sand bodies are typically concentrated in the inlet throat.

Microtidal areas, such as the Texas coast, have poorly developed ebb-tidal deltas and relatively large flood-tidal deltas because of the dominance of wave-energy flux over a small tidal prism. Ebb-tidal deltas of the mesotidal South Carolina coast are more elongate and extend further seaward than those of the mesotidal New England coast because of decreased wave energy in South Carolina. An increased tidal prism caused by freshwater discharge also may contribute to the increased size of the South Carolina

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ebb-tidal deltas. The diminished size of the flood-tidal deltas in this area, as compared to New England, may be caused by the ratio of open water to marsh in the estuaries and lagoons which creates an ebb-dominant inlet flow and does not allow large flood-tidal deltas to develop.

The main channels and tidal deltas are primarily composed of medium to fine sand. Ripples occur in a wide assortment of sizes and shapes. Bedforms can be classified on the basis of wavelength into ripple marks, megaripples and sand waves as three basic scales of bedforms developed in the tidal flow areas of lagoons. Ripples range up to 60 cm spacing, megaripples from 60 cm to 10 m; and sand waves, greater than 6 m spacing. There is no distinct height difference between two forms, particularly megaripples and intertidal sand waves.

The barrier platform is the stratigraphic substructure of a barrier island. Subsurface materials that support the barrier island are primarily related to the origin and evolution of the barrier island system. Pre-Holocene topographic highs on submerged mainland surfaces provide platforms for 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

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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.

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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.

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.

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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.

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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.

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Barrier Island Types

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.

Figure: Diagram showing the environments of a

barrier island system. (from Blatt, et al, 1980,

Petrology of Sedimentary Rocks)

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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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Activity: Georgia Barrier Islands

Duration: 30-40 Minutes

Description In this lesson students will examine the dynamics of barrier islands

and locate the barrier islands. Causes of erosion will be discussed through examination of the current state of Jekyll Island.

Goal Exploration of Georgia’s barrier islands with emphasis on Jekyll Island and the

consequences of beach erosion.

Objectives define barrier islands

locate Georgia’s barrier islands on a map

describe the processes of erosion and accretion

describe the formation of sand dunes

explain the importance of sand dunes

identify sea oats and explain their importance

Materials 2 Trays for sand dunes

Sand

Spray bottle of water

Personal battery operated mini fan

Pictures of dunes and sea oats

Poster of Jekyll Island

Observation worksheets

Preparation Prepare a sand box to demonstrate the effects of wind and water erosion on an

area of the beach.

Procedure Introduction:

1. Begin by asking students; how many of you enjoy the beach? (Wait for responses.) I enjoy the beach too. As a matter of fact, I really like Jekyll Island. Because it is a great place to enjoy the beach.

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2. Did you know there is something very special about Jekyll Island? Can anyone tell me why Jekyll Island is so special? (Allow time for responses.) Jekyll Island is a Barrier Island and that is why it is unique.

3. What is a barrier island? A barrier island is a long, narrow strip of land that lies parallel to the mainland. It is separated from the mainland by a bay, a lagoon or a marsh.

4. Barrier islands, which border the U.S. Atlantic and Gulf coasts, protect the mainland from the damaging effects of wind and waves. In addition to Jekyll, Georgia has 7 other major barrier islands. (Display Georgia map.) Let’s see how many you can name and locate? Since we have already identified Jekyll, we will locate it on our map first. (1. Call on students to name an island and place the name cards on the correct island. 2. Ask for volunteers to come and place the island name cards correctly on the map.) Tybee, Skidaway, Wassaw, Sapelo, Little Saint Simons, Saint Simons, Cumberland. Great job!

5. (Display poster of Jekyll Island) Today, we are going to focus on Jekyll Island because of the amount of beach erosion that is happening there. What is erosion? (Listen to responses and restate the definition of erosion from the vocabulary list.)

6. Erosion is the process of being gradually worn away. Why should we be worried about the beaches eroding? (Listen to responses and reply accordingly.)

7. What factors are causing the erosion process on Jekyll Island? Waves, wind, currents and on Jekyll the shipping channel adds to this situation. Jekyll Island is the smallest island in the eight major island clusters along the coast of Georgia. It is only 7.5 miles long and 2.3 miles across at the widest point. Jekyll Island covers 5700 acres, but 1400 of those acres are marshland. The erosion of the beach from parts of Jekyll Island is a great concern to many residents and environmentalist.

Development of Concepts/Core (Experience-Share) Before we begin learning about the erosion process, you may be wondering where the sand on the beach came from. The sands of barrier islands come from the weathering of the Piedmont and the Appalachian Mountains. Here again, we must remember the definition of erosion. In Georgia, sands coming from mountains are deposited in the slow, meandering streams of the lower Piedmont and Coastal Plain and very little ever reach the coast. Most sand on the beach comes from the continental shelf and is pushed up by the waves. The continental shelf is the remaining submerged or under water portion of the coastal plain. As waves travel across the continental shelf, they pick up grains of sand and bring them to the shore. Although the same forces – wind, waves, and currents – affect all barrier islands, each island is an individual unit. Let’s talk about the different forces that affect Jekyll Island. (Jekyll poster for this information)

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The first is the Longshore Current.: Sand on the beach is constantly shifting and migrating due to the physical forces of wind, waves, and currents. Winds, waves, and currents determine the direction and rate of sand transport. The longshore current is a current in the water that runs parallel to the shore within the surf zones. During most of the year, winds from the northeast generate waves which hit the beach at an angle. These small, low-energy waves generate the longshore current which transports sand along the beaches in a north to south direction. The transported sand is usually deposited on the south end of the island in the process of accretion. Who can tell me what I mean by accretion? Accretion is the building up of land by physical forces. (Use the map to illustrate this point. Show the difference in the size of the north end of the island compared to the south end.) The longshore current is a strong flowing current that transports large amounts of sand in shallow waters. Who can tell me what other force plays a part on changing the shape of beaches? (Allow time for discussion.) Waves play a big part. Most of the sand-moving work is done by intense storms that come a few times a year. When a wave breaks on the shore, most of the energy is spread out along the shoreline and the wave drops its load of particles. Waves constantly moving sand onto the beach results in re-distribution of the sand. From late spring through early fall, Georgia gets soft southeasterly and easterly winds, the seas are calm, waves are gentle, and storms infrequent. The transport of sand is toward the shore and sand is deposited high on the beach making for a broader and flatter beach. During the winter when northeast storms bring higher winds and steeper waves, higher wave energy removes sand from the beach resulting in a narrower, steeper beach. Now that we know something about the causes of erosion of beaches, let’s discover how the island protects its self naturally. What element is located on the beach to protect the island? (Listen for answers. Give clues to encourage discussion.) We all know that a sand dune is a hill of sand piled up by the wind. (Display picture of dunes.) Sand dunes protect the island from wind and waves, provide natural beach stability, and a supply of sand for the changing beach, as well as habitat for plants and animals. Sand dunes begin with marsh wrack. Have you ever been to the beach and there were dark dirty looking lines in the sand? That was marsh wrack. Marsh wrack is simply dead cordgrass from the marsh that is left behind on the beach by the waves. The marsh wrack becomes a mesh that traps wind-blown sand and seeds. Because of this, the marsh wrack plays a vital role in forming new sand dunes. Plants stabilize dunes as sand continues to accumulate. Support for the dune comes from the root network and shoots of sea oats and other dune plants. Sea oats are a tall grass (Uniola panicolata) that grows on the coast of the southern United States and helps hold sand dunes together. Sea oats are highly adapted to the dune environment. Their long curly leaves and tall oat heads trap windblown sand which quickly causes them to become buried. (This could be sketched on the board to assist the visual learner.) By

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growing vertical runners, which produce daughter plants on the surface of the growing dune, sea oats stay ahead of the accumulating sand while most other plants become buried and die. Sea oats are master dune builders. Because of their vital role in building and stabilizing dunes, sea oats are protected by law; there is a very stiff fine for picking or damaging the plants. Erosion is a major force that affects the barrier islands. This is a process which typically occurs at the northern end of the island. On Jekyll, that process has increased due to the shipping channel which blocks any supply of new sand from coming in. The beach represents one of the most dynamic environments on earth. As we have learned the condition of the beach system depends on three elements: wave energy, wind, and sand supply. The complex interaction and balance among these elements determines the shape of the beach and its position. (Provide students the observation worksheets.) I have two different beach environments. (Display trays.) One beach has sand dunes formed and the other does not. We are going to use this spray bottle of water and my mini fan to represent wind and water. This will emulate the erosion process. I want you to record you observations. (Simultaneously, have one student spray the water and another hold the fan for 60 seconds on each tray of sand. After the experiment, have students share their findings from their observations sheet.) The experiment we just conducted happens everyday on our beaches and barrier islands in America. Wrap Up/Review/Reflection :( Process-Generalize-Apply) Now that we have studied the changing shape of the Jekyll coastline, what forces can change the shape of other barrier islands? Who is affected by erosion? Look at the two illustrations on the handout. (A Changing Coastline.) Can you explain the difference in the two pictures? Is this an example of erosion or accretion? What could be added to the coastline to prevent more erosion from happening? Questions:

1. Describe how dunes form. 2. Why are dunes important? 3. What holds dunes together? 4. Which end of barrier islands tend to erode? 5. Which end of barrier islands tend to accrete? 6. Which direction does a longshore current run? 7. Does the long shore current move sand from the beach? 8. Define erosion. 9. Define accretion.

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Name: ____________________

Date: ____________________ Barrier Island Observation Sheet Record your observations. Possible effects to look for:

1. What effects does the wind have on the beach without the sand dunes?

2. What effects does the water have on the beach without the sand dunes? On the beach with the sand dunes?

3. How do the sea oats affect the beach environment?

4. What other observations can you see?

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Activity: Sand Transport Analysis

Duration: 2 hours

Objectives

Students will learn to collect and analyze data in a scientific manner. The goal of this project is to identify which geographical areas of a barrier island are affected by stronger wind and water energies by characterizing individual sand samples from different localities across the island transect. Students should hypothesize which areas experience higher wind or water energies by looking for trends in their analyzed sand samples.

Major Understandings

Barrier Islands are formed and constantly reshaped by the transport of sand by water and wind.

The type and intensity of these forces vary in different sections of a barrier island. For example, there is more activity on top of s dune than directly behind it.

Sand is made of many different minerals that vary in density.

Sand grains vary in size.

The distribution of minerals and average grain size varies along a beach transect.

More energy is required to move heavier sand grains.

By observing the grain size and mineralogy variations in sand across a barrier island, it is possible to determine which areas are affected by higher energies.

Materials

Binocular microscope

Metric scale (preferably digital)

A sieve set containing at least five sieve sizes (Sieve sets available through science catalogues.) Make sure that sets include a wide range of sieve sizes appropriate for the spectrum of grain sizes in your area of study. (I.e., silty sand needs finer sieves.)

Locking sandwich bags

Strong magnets

Glass slides-2 for each group

Plastic cups (small)

Rubber Cement

Tissue paper (thin)

Calculator

Permanent Markers

Tape measure (metric)

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Bug repellent/ sunscreen

Screening

Procedure

Have students read the introduction section and follow up with a pre-lab discussion on wind and water transport of sand. Students should have a basic grasp of the Major Understandings before beginning this project.

Choose a transect to cover. An ideal transect will be unaltered by human activities, yet, not be covered in poison ivy or other potential hazards. The transect should also contain the major zone of a barrier island; Beachface, Berm, Foredune, Backdune and Bay beach.

Look at the local tide tables. Times of low tide will be better for sampling variety.

Divide class into enough groups to cover all areas to be sampled.

Collect samples

Assist students in graphing and analyzing data

Pre-Lab Questions/Discussion guide

1.) In which areas of a barrier island would you expect exposed to larger amounts of energy?

2.) What would you expect to find in the areas of higher energy? Average grain size? Percentage of heavy minerals? These include magnetite, garnet, staurolite, and olivine.

3.) What kind of relationship would you expect to find between average grain size and mineral content?

Students should discuss as a class which areas they would like to sample. Be sure they include areas on the beach face, berm and dune (both front and back)

Introduction

Barrier Islands

Barrier Islands occur along the length of shorelines that have a lot of sediment. Barrier islands are not stationary structures. They migrate landward or seaward as the sea level changes. There are several different sub-environments across barrier islands. The beachface, also called the intertidal zone, is the area on a beach that extends from the water level of low tide to the berm, a terrace like ridge built by storm waves. The next section is called the backshore, and includes the area from the berm to the dunes on the other side of the barrier island.

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Sand

Sand comes from rocks. As rock is exposed to the weathering conditions and geological processes, it gradually breaks down into sediments. This idea is a keystone in the geological cycle. What type of minerals make up a certain sand is therefore directly related to the type of rocks from which it originated. (It also depends on the average energies of a particular beach.) Glaciers created Long Island, therefore the sediment that forms the sand is derived from glacial sediment. Most beaches are dominated by quartz, a light colored mineral that is more resistant to weathering than most other minerals. This is why most beaches appear to be white. If you take a closer look at the sand, you will notice that quartz is one of many minerals. On Long Island beaches, among the quartz grains you can see lesser amounts of magnetite, garnet, staurolite, and sometimes olivine.

Sand Transport

A barrier island is affected by both wind and water processes. The effect of these energies varies with different kinds of sand grains. The equation for kinetic energy is

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E=1/2 mv2 . According to this equation, if the wind and water are moving at the same speed, the water, which has a greater mass, will have more kinetic energy. Therefore water has a greater capacity to transport larger and heavier grains. The areas affected by water energy are the beachface, the bay beach and sometimes the berm (during storm weather). The other parts of a barrier island are all shaped by aeolian, or wind processes, which change in intensity over the island zones. Areas subject to stronger forces will have a greater percentage of large and/or heavy mineral grains.

Procedure

Data collection

1.) Each group will choose one zone along the beach. 2.) Each student should create their own data sheet that will include Date: Time: Location: Distance from reference point: Sample #: Comments: 3.) With the meter stick, measure the distance from the ocean or the reference point to your site and scoop sand (about the size of a half-cup) to al least 5 cm. depth. Put the sand in a plastic bag and label the bag with site area, distance from ocean or reference point, and group number using the permanent marker. 4.) Back in the lab, put the samples on a piece of paper away from any drafts. Leave them overnight under a lamp to dry.

Sieving

1.) Once the samples are dry, decide which sieves to use and in what order they should be stacked. Separate the samples by grain size.

2.) Once sediments have been separated, take sieves apart, being careful not to lose too much sand. Dump the sand onto a piece of paper and weigh the different sizes. Calculate the weight percentage of each using chart 1.

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Chart 1

Sieve # Mass (g) Weight % = 100* mass/total mass

. . .

. . .

. . .

. . .

. . .

. . .

. . .

Total:

3.) Find the average grain size of your site using the formula:

Sieve grain size (mm)=d

Weight % = p

Average grain size=(d1)(p1)+(d2)(p2)+……(dn)(pn)

For (n) sieve sizes

4.) Data from each group should be publicly posted, so that it can be plotted together. Using a spreadsheet, or by hand on graph paper, students should create a graph plotting average grain size against the location on a beach. Figure 2 is an example of such a plot.

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Mineralogical Analysis

Before classifying the minerals, students should be able to identify the major minerals found in the sand. Minerals will be separated into heavy and light categories. Light minerals include quartz and feldspar. Heavy minerals include magnetite, garnet, staurolite, and olivine, along with other trace minerals. See Chart 1 below

If you see: It might be:

Clear and Colorless, or White Quartz

Black, Dark Grey Magnetite

Pink (Light or Dark) Garnet

Orange, Brown (Light or Dark) Staurolite

Light green Olivine

Opaque and Whitish Feldspar

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2.) Next, figure out the mineralogy of the samples. This can be done in two ways.

Grain Count:

Use binocular microscope.

a.) Students should first choose a relatively small grain size that is represented in all samples. For example sieve sizes .25mm-.17mm. Note: Larger grain sizes tend to contain a less diverse mineralogy and are less interesting to look at for the students, but distribution trends remain similar. However, if the grains are too small, then the minerals are hard to count. See examples below.

Each group is to create a slide by applying rubber cement to the slide and pouring the sample over the slide. Be sure to hold the slide over a cup to catch the excess sand.

Students should glue a gridded slide onto the sand side on the previous slide. Each

student will be responsible for the light and dark grain counts of a particular square slot. Using the supplied chart, each student will count the number of heavy minerals and light minerals.

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Once again, all the class data should be collected and plotted together. This time the plot will be the percent of heavy (dark) minerals against location on the beach. Figure 3 is an example of such a plot.

Chart 2

Student # Light Minerals Heavy Minerals Total Volume % Heavy minerals

Example 156 102 =156+102=258 =100*102/258=39.5%

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

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Physical Separation with Strong Magnets:

This method uses the composition of magnetic minerals such as garnet and magnetite to physically separate the dark and light minerals. If the magnet is not strong enough, it will only pick up magnetite.

a.) Take a part of your sample and pour it on a piece of white paper

b.) Cover the magnet with tissue paper

c.) Use the magnet to remove as much magnetic material as possible by "stamping" the magnet onto and off the sand. Then lift the magnet off the tissue paper, while holding it over a cup or another piece of paper. The magnetic material will fall off the tissue paper.

d.) Weigh both magnetic and non-magnetic sample portions.

e.) Once again, calculate weight percents and post data for the class

f.) Create graph of weight percent of heavy minerals vs. distance from ocean.

Discussion Questions

1.) Look at your first graph. Which areas have the largest size grains on average? Why do you think this is?

2.) Look at distribution of heavy minerals. How is this graph similar and different from the first graph?

3.) Do you see any resemblance in the graphs created by grain counts and magnet separations?

4.) What do these graphs suggest about the relative forces of wind on a barrier island? (Where are they strongest and where are they the weakest?)

_____________________________________________________________________________

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Resources http://www.csc.noaa.gov/beachnourishment/html/geo/barrier.htm http://en.wikipedia.org/wiki/Barrier_island http://science.howstuffworks.com/environmental/conservation/issues/barrier-island.htm http://www.ugaextension.com/camden/4h/documents/BarrierIslandsLesson2.doc http://www.tpwd.state.tx.us/learning/resources/activities/coastal/essentialislandsactivity.phtml http://www.chipr.sunysb.edu/eserc/SummerEducationalInterns/Katie/lessonplan.html 3dparks.wr.usgs.gov/nyc/shoreline/beaches.htm www.geo.hunter.cuny.edu/bight/beach.html www.beg.utexas.edu/UTopia/glossary.html http://uwf.edu/rsnyder/ffnwf/barrier/barrier.html http://geology.uprm.edu/Morelock/barrsys.htm www.epa.gov/region6/water/edu/sourcebookcorr.htm