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.

Location of barrier islands
Characteristics and morphology of barrier islands


Attached barrier beaches

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.


De Beaumont (1845): Offshore bar theory

Gilbert (1885): Spit Progradation Theory

 Leontyev and Nikiforov (1966): Higher Still Stand Theory


 Hoyt (1967): Beach Ridge Submergence Theory
 

 

Stage 1: Approximate 15, 000, when sea level was 85 meters below present beach ridges developed along the late Pleistocene shoreline which was much farther out along the continental shelf.

Stage 2. Rising sea level at the end of the Pleistocene results in breaching of the beach ridge and flooding of the region behind it . The beach ridge becomes a barrier island backed by a bay or lagoon.

Stage 3. The barrier system migrates landward across the shelf and sea level continues to rise.


Figure 2. Stages of barrier island formation according to Hoyt. Modified from Pilkey and others 1978. Note that on the gentle slope of the continental shelf a meter rise in sea level results in almost a kilometer of landward migration.


Landward retreating barriers are called transgressive (figure 2) and prograding barriers that are building out are termed regressive. Most modern Holocene barriers, especially along the East coast, are transgressive in response to rising sea level. However some barriers that receive a large supply of sediment are regressive despite the present rise in sea level.
 

Figure 3. Cross-section through a typical transgressive barrier island. Redrawn from Godfrey 1976. Note the shoreface deposits are rolling back over grassland and back barrier marsh deposits.

  • 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)


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Lindley Hanson/email /Gls214
Department of Geological Sciences, Salem State College, Salem, MA