• Dean, 1999, Against the Tide
• Godfrey, 1976, Barrier beaches of the east coast
• Hayes, 1979, Barrier island morphology as a function of tidal and wave regime

Barrier islands are detached linear islands of sand and/or gravel(rare) that run parallel to the shore and are back by a bay, lagoon, marsh, or tidal flat. Barrier islands are wave and wind built landforms and form in wave dominated or mixed energy environments, typically microtidal or mesotidal environments.

• Most barrier islands are along trailing edge and marginal sea coasts where continental shelves are wide and gently sloping. Barriers originating on steep shelves generally would not survive rising sea level.
  
• Most barrier island in North America are along the Atlantic and Gulf Coast.

• Barrier islands are elongate islands of sand which parallel the shore and are separated from the mainland by a
  • Bay (microtidal/lmix energy wave dominated)
  • Lagoon
  • Marsh and tidal creek system (mesotidal/mix energy tide dominated)
    
• Barrier islands typically occur in chains; each island is separated from its neighboring islands by inlets. The number of inlets per length along an island chain is reflective of the combined influence of waves and tides. High wave energy tends to close inlets while strong tidal flux keeps them open. Barriers on glaciated coasts, such as the New England coast, may be locally anchored to an isolated rock headland or drumlin.
  
• Barrier islands range in length from 3 to 100 km in length and <1 to 3 km in width
  • As alluded to above, barrier island length reflects hydrographic regime, which controls the size and number of inlets. Storms produce inlets, tides keep them open, and waves transport sediment that fills them in. The balance of these processes determines the length of the barrier islands and number of inlets along a coast. Long barrier islands typically form in regions of low tidal range and moderate to high wave energy.
  • Barrier island width primarily reflects sediment supply and relative sea level fluctuations. Rising eustatic sea level is causing erosion and landward retreat (transgression) of most barrier islands. However, barrier islands receiving large amounts of sediment can build seaward (prograde) despite the rise in sea level. Width can also vary with prevailing wind pattern. Winds blowing across the island promote the formation of large transverse dunes and wide barriers. Narrow barriers are associated with prevailing winds that blow along the barrier axis.
    
• The morphology of a typical East coast barrier is shown in figures 1 and 2.
  
  

  Figure 1. Morphology of a typical east coast barrier. Redrawn from Godfrey, 1971.




Figure 2. Plum Island, MA. Photo taken and low tide. Takes some time to locate the following features then place the cursor over the image to check your interpretation: Foreshore with onshore migrating bar, backshore (between the berm and foredune ridge), dunes, vegetated overwash, forest, high marsh and low marsh.

• Barrier Islands may consist of one or more beach ridges, depending on whether the system is prograding or transgressive. (Discussed below.)
  • Beach ridges, the highest portion composed of dune sediments, represent successive shoreline positions formed during progradation.
  • Transgressive barrier islands commonly lack multiple beach ridges unless they had an earlier history of progradation.
  • Beach ridges, which are transverse dunes, run parallel to the shore at the time of their formation and may be 1 to 30 meters high
  • Adjacent beach ridges may be separated by low areas called swales. 
    
• Back barrier region
  • The lagoon, bay, marsh, or tidal flats backing the barrier islands range form 2 to 20 km in width.
  • Transfer of sediment into the back barrier region is important to the survival of a transgressive barrier system. Sediment is transported into the back barrier region by:
    • Overwash during storms. Dominant process in microtidal systems
    • Flood tidal flow and inlet migration. Dominant process in mesotidal systems
    • Wind. Blows sand from the beach face to the back shore. Once dunes are formed they tend to migrate landward.
    • spit formation
  • As will be discussed later, human intervention typically interrupts the natural flow of sediment, thereby preventing a barrier island system from migrating and keeping up with rising sea level.
    
• Sediment

  Barrier islands are normally composed of sand size sediment but in some instances a substantial amount of gravel may be present. (e.g., some New England Barriers). Barriers with substantial gravel most likely formed by spit growth from a till or rocky headland.
  

Attached barriers are sand and/or gravel ridges that are attached to the shore and are backed by a bay, lagoon, marsh, or tidal flat.

• Varieties
  • A baymouth barrier is a barrier beach that is attached at both ends to headlands and backed by a bay or lagoon.
  • A spit is a protrusion or linear beach extending from a headland. Spits are attached at one end. However, continued growth may extend a spit to a downdrift headland where it can become attached thereby evolving into a barrier beach.
  • A sand or gravel beach ridge is a relatively small beach ridge backed by a marsh or tidal flat.
• Characteristics:
  • Attached barrier beaches are commonly smaller in size and more variable in composition than barrier islands
  • These beaches are not separated from the mainland or other barrier beaches by inlets

• Any theory of barrier island formation must account for the following characteristics:
  • Morphology: elongate parallel to shore and narrow in width
  • Composed predominantly of sand size material
  • Islands form chains

    ---

• Involves the building of offshore bars
• Waves approaching the coast would stir-up bottom sediments
• Where the waves break they would lose their energy and the sediment would be deposited
• This sediment would accumulate and an offshore bar would be formed
• Once the bar became subaerial eolian processes would build it up vertically
• Problems with De Beaumont theory
  • Studies conducted by McKee and Steward (1961) in wave tanks showed that offshore bars could be built no higher than sea level
  • When ½ the bar reaches a certain height the wave action would carry sediment form the top of the bar landward.
  • If barriers formed in this manner why don't be see them in the various stages of formation along the coast today?
  • Absence nearshore deposits landward of the barriers.
    • If a barrier formed from an offshore bar the open ocean conditions should have prevailed prior to barrier island formation
    • This means that landward of the barrier nearshore fauna and sediment should be found.
    • Extensive coring along the GA and TEX coast has shown this not to be true

    ---

Gilbert (1885): Spit Progradation Theory

• Gilbert believed that material which built the barriers came not from offshore sources but rather from along the shore by the process of longshore transport.
• Spits will form which will then be breach to form barrier islands.

  ---

 Leontyev and Nikiforov (1966): Higher Still Stand Theory

• Barriers formed from offshore bars which were built during a higher sea level stand
• They also suggested uplift as a secondary mechanism of raising offshore bars above sea level.
• Problems:
  • Theory depends on higher sea level still stands. (This may not be a problem in glaciated high-latitude regions if the bars can be shown to have formed during deglaciation)
  • In addition the absence of open marine sediments landward of the barrier poses the same objection to the offshore bar theory.

    ---

• Where the ocean meet the land in many instances beaches will be formed.
• The wind may create high dune ridges immediately landward of the beach.
• Examples:
  • Crane beach: 12-15 m
  • Outer banks, NC; 15-35 m
  • Barriers along SC; 1-12 m
• Coalescing of these dunes produced beach ridges
• If during the formation of the beach ridges or most likely afterward there is submergence (rise in sea level) the area immediately landward of the barrier dune ridge will be flooded to form a lagoon, bay, etc.

• The resultant form, composition, and stability of a barrier island ultimately depends on the relative influence of each of the following:
  • Sediment supply
  • Sea level fluctuations
  • Slope of the continental shelf (figure 2)

• Retreat results when the rate sea level rise overcomes the rate of sediment supply
• Process of shoreward migration is accomplished by the transfer of sediment to the back barrier environment
• When the barrier island is attacked by waves sediment is transported both offshore by waves and behind the barrier by overwash or inlet formation (Effects of Hurricane Isabel Sept 03)
  •  Overwash: The discontinuous flow or pulse of sediment charged water which occurs in response to the storm wave runup and storm surge over-topping
  • overwash occurs where there is a low or breach in the foredune ridge
  • Storm wave carry sand which is deposited in a tongue- or fan-shaped deposit called an overwash fan
  • If islands are backed by marsh deposits the overwash sediment may bury the marsh
  • The overwash process, which erodes sediment from the front and carries it to the back, is a cannibalistic process which preserved the existence of the barrier.
• In mesotidal regions, sand that is transported offshore eventually migrates along shore and is transported to the back barrier region by waves and flood currents through inlets. Formation if tidal deltas and inlet migration help to fill in the back barrier region and allow the barrier to migrate shoreward.
• Problems with transgressive systems along the east coast (e.g. North Carolina):
  • Due to the rise in sea level the islands are migrating shoreward
  • The rate of shoreward migration is determined by:
    • Slope of the shelf area over which it is migrating
    • Rate of sea level rise
    • Sediment supply
    • intervention by man
      • The steeper the slope the slower the migration
      • The faster the rise in sea level the faster the rate of migration
      • sea level is presently rising a little over 1 foot/century
      • North Carolina: Island-migration rate is 100-1000 times the rate of sea level rise, depending on the slope. (For every foot of sea level rise the island migrates 100 to 1000 feet inland)

• Regressive (prograding) barrier islands: seaward-building barrier island systems
  • Examples: Galveston Island and Padre Island, Texas; East Frisian Islands, West Germany
  • Processes:
    • Sand is supplied to the barrier faster than sea level rise
    • results in a progradation of the shoreline

• Plum Island, MA
• Louisiana's Barrier Islands:A Vanishing Resource-USGS fact sheet
• LA Coast Louisiana Coastal Restoration Web Site (USGS)
• Louisiana Coastal erosion homepage (USGS)
  • Louisiana's Barrier Islands: A Vanishing Resource, Jeffrey H. List, U.S.G.S. Fact Sheet
• Hurricane Impacts on the Coastal Environment
• West-Central Florida coastal Studies Project
• Insider's Guide: History of the Outer Banks
• **Carolina Coastal Science
• Carolina Coastal Journal
• Geomorphology from Space
  • Cape Hatteras
  • Cape Canaveral
• ** East Beach, RI virtual field trip
• The NC Natural Guide to Barrier Island Dynamics
• Greenhouse effect, sea level rise, and barrier islands Case Study of Long Beach Island, New Jersey, By James G. Titus Environmental Protection Agency
• Topographic Mapping of Myrtle Island of the Virginia Coast Reserve, Charles R. Carlson and Raymond D. Dueser, Utah State: Nice air photos
• Shoreline erosion rates along the Texas Coast: Maps with brief discussion
• ** West Central Florida virtual field trip
• Quaternary Stratigraphy and Depositional History of the Central South Carolina Coast and Inner Shelf: Implications to Coastal Change , Jack L. Kindinger and others, USGS Center for Coastal Research

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Lindley Hanson/email /Gls214

Department of Geological Sciences, Salem State College, Salem, MA