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Barrier Systems: Types, Morphology, and Overwash Processes

What’s a Barrier and Why are They Important? Definition: Composition Sink of a Littoral Cell – Sediment arrives as a result of LST Shelter an Ecologically Productive Area Emergent Depositional Feature Built by Overwash During High Wave Events

“Older Barriers” Preserved Inland of the Modern Coast? Cape Orlando

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Barrier Types and Morphology

“No Free Ends” Barriers

Barriers with no free ends include tombolos, and a variety of barriers built from one shore to the other across bays and estuaries Tombolos are barriers that link islands to each other or to the mainland coast and form because of wave refraction and diffraction around the island and development of a littoral transport gradient leading to deposition in the lee of the island. A detached, offshore breakwater can lead to the progressive development of, first, a beach salient then ultimately a mini-tombolo.

Bay-mouth barrier

Mid-bay barrier

Tombolo with intervening marshlands

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“One Free End” Barriers

This class includes a range of spit forms where there is a pronounced littoral drift from one direction, and cuspate forelands that form where there is a convergence of littoral drift from both directions.

Spit Growth: Continuation Spits vs. Flying Spits

Continuation Spits: direction of spit progradation is parallel to the updrift shoreline. Flying Spits: "leave" the coast at an acute angle.

http://www.youtube.com/watch?v=N_LBeJPWqFM

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“Two Free Ends” Barriers (Barrier Islands)

Swash-Aligned vs. Drift-Aligned Barriers

Swash-aligned coasts are oriented parallel to the crests of the prevailing incident waves. They are closed littoral cells in terms of longshore sediment transport, and net littoral drift rates are zero. Drift-aligned coasts are oriented obliquely to the crest of the prevailing waves and are open littoral cells in terms of longshore sediment transport because sediment is in transit through them, and they are therefore sensitive to changes in drift rates. A coastal section may exhibit combinations of swash- and drift-aligned reaches depending on coastal geology and local sediment supply.

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Examples of Swash-Aligned Coasts

Embayed Beaches Log-Spiral Planform

Components of Barrier Systems Mainland Coast - protected by barrier; includes estuaries extending inland Lagoon, Bay, or Marsh - separates barrier from mainland Subaerial Barrier - including the beach, dune, and back-barrier deposits Subaqueous Platform - sediments or bedrock, upon which the barrier is built Shoreface - offshore extension of the beach, area of wave shoaling/breaking Inlet and Tidal Deltas - typical of barrier islands, but also found on all types of barriers

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Sea Level Rise and Barrier Response

Marine cores reveal that relict estuarine deposits (representing environments “behind” the barrier island) underlie beach deposits. This implies barrier migration landward during the most recent interval of the Holocene rise.

Barrier Translation appears to be the most common response to sea level rise.

Shoreline retreat rates, however, are sensitive to factors beyond SL rise, including: gradient of underlying substrate, LST, and back-barrier depositional processes.

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Barrier islands – Theories of Formation

DeBeaumont (1845) presented a theory based on cross shore sediment transport, whereby continued deposition of a submarine bar (originating from wave breaking) eventually accreted to an elevation above sea level, creating a barrier island. In the famous USGS Monograph 1 by G.K. Gilbert (1890) on Lake Bonneville, a theory is presented which offers an explanation that barrier islands were the result spit growth from longshore sediment transport. McGee (1890) argued that the drowned river valleys along the east coast of North America testified to the submergence of the shoreline, which he claimed explained the formation of barrier islands unconnected to the mainland. Hoyt (1967) also supported this theory.