intro to coasts / tectonic classification / hydrographic regime / waves /tides / beaches / barrier islands / inlets / deltas
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.
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
LengthBarrier 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.
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.|
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.
Human intervention typically interrupts the natural flow of sediment, thereby preventing a barrier island system from migrating and keeping up with rising sea level.
Barrier islands are typically 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.
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. Phillips and Goldthwaite/Devereau beaches are beach ridges.
Any theory of barrier island formation must account for the following characteristics:
De Beaumont (1845): Offshore bar theory
This hypothesis involves the building of offshore bars. Waves approaching the coast would stir-up bottom sediments. Where the waves break they lose their energy and the sediment is deposited forming an offshore bar. If 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 the bar reaches a certain height continued wave action would carry sediment at 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.
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.
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.
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)
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.
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:
- Slope of the shelf area over which it is migrating: The steeper the slope the slower the migration
- 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
- Sediment supply
- 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.
- Louisiana's Barrier Islands:A Vanishing Resource-USGS fact sheet
- LA Coast Louisiana Coastal Restoration Web Site (USGS)
- Louisiana's Barrier Islands: A Vanishing Resource, Jeffrey H. List, U.S.G.S. Fact Sheet
- West-Central Florida coastal Studies Project
- **Carolina Coastal Science
- Carolina Coastal Journal
- Geomorphology from Space
- ** 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
- 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