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Barrier Islands

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Description

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.

Distribution

Characteristic morphology

Barrier islands are elongate islands of sand which parallel the shore and are separated from the mainland by a

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

Length
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.

Width

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.

Beach Ridges

Beach ridges and the dune ridges that run parallel to shore, and at the time of their formation and may be 1 to 30 meters high. Adjacent beach ridges are separated by low areas called swales. Beach ridges are the highest portion of a barrier. The represent successive shoreline positions formed during progradation, when the beach was building seaward. Barrier Islands may consist of one or more beach ridges, depending on whether the system is prograding (building seaward) or transgressive (retreating). Only barriers that receive enough sediment to keep up with rising sea level can prograde, all others are transgressive.

Transgressive barrier islands are migrating shoreward and typically lack multiple beach ridges unless they had an earlier history of progradation.

Back barrier region
Sediment

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.

Varieties
Characteristics:

Theories of Barrier Island Formation

Any theory of barrier island formation must account for the following characteristics:

De Beaumont (1845): Offshore bar theory
Gilbert (1885): Spit Progradation Theory

Gilbert believed that material which built the barriers did not originate from offshore sources but from sources along shore, such as an eroding bluff, delta or glacial deposit. Spits extending from these regions would be breach to form barrier islands. A modern day example might be Monomoy Island formed by erosion of glacial fluvial deposits on Cap Cod.

Monomoy Island
 


 Hoyt (1967): Beach Ridge Submergence Theory

On the coastal plain where the ocean meets land sandy beaches typically form backed by high dune ridges. Hoyt believed that barrier islands evolved from beach ridges created during a time of lower sea level. Rising sea level breached the ridge and flooded the back creating islands. Wind and wave action enabled the barrier to migrate shorewared keeping pace with rising sea level.

Examples of beaches backed by high dune ridges.

 

 

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.

Regresssive and Transgressive barrier islands

Transgressive Barriers

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.

The resultant form, composition, and stability of a barrier island ultimately depends on the relative influence of each of the following:

 

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. Shoreward migration is accomplished by the transfer of sediment to the back barrier environment by overwash, inlet migration and eolian transport. 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 is called overwash. 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.

Inlet Migration

Along mesotidal barriers, sand that is transported offshore eventually migrates along shore and is carried into the back barrier region by waves and flood tidal 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.

The rate of shoreward migration is determined by:

  1. Slope of the shelf area over which it is migrating: The steeper the slope the slower the migration
  2. Rate of sea level rise: The faster the rise in sea level the faster the rate of migration. sea level is presently rising a little over 1 foot/century
  3. Sediment supply
  4. intervention by man

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 systems

Regressive (prograding) barrier islands are seaward-building barrier island systems. They occur where sediment is supplied to the barrier can outpace the effects of sea level rise.

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