Session 4: Barrier Island, Dunes, Salt Marshes

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Barrier Island Evolution

A barrier island is made up of the following habitat zones: Salt marsh—Low-lying area on the sound-side of a barrier island that is stabilized by cord grasses and flooded by daily tidal activity. Helps purify runoff from main land streams and rivers. Barrier flat (overwash)—Formed by sediment pushed through the dunes by storms and stabilized by grasses. Often flooded daily during high tide. Dunes—Sand carried and deposited by winds and stabilized naturally by plants and sometimes artificially by fencing. Can be flooded during storms. Beach—Ocean side of the island with sand deposited by wave action. Covered by salt water twice daily.

Beach Diagram

Three main hypotheses for the formation of barrier islands Spit formation and Segmentation Off-shore Sand Bar Theory Dune-Ridge Submergence

Spit Formation and Segmentation Spit starts to grow from point of land Spit is extended along the coast by longshore currents Spit is breached during a storm, forming a tidal inlet and barrier island

Off-shore Sand Bar Theory Wave energy picks up sediment and deposit sediment offshore. (Gulf Coast)

Dune-Ridge Formation Dune ridge along the coast Rise in sea level starts to submerge dune Barrier island and lagoon formed from ridge

Longshore Current Waves typically strike a beach at an angle, causing a current to run parallel to the coast –In NC, the longshore current runs from North to South Longshore current allows sediment to be carried down the beach.

Longshore Current

Movement landward Transgression is the movement of islands landward –Rate of sea level rise overcomes the amount sediment supply Transgression is caused by waves carrying sediment from the beaches and dunes to the back of the barrier island and depositing it in washover fans. The natural movement of a barrier island is to migrate landward as sea level rises. –Front side of the island is eroded by wave action –Back side must grows from the buildup of washover

Evidence of Transgression Studies of sediment on barrier islands show peat, tree stumps, oyster shells and layers of mud on the beach side –Once were part of the marsh, now found on the beach Marsh has a floor of thick, black mud from the ocean side As the island rolls back, the beach moves back over the old marsh. Today’s beach is where the marsh used to be. –Island rolls over on itself like a conveyor belt

Dynamics Prograding barriers – expand seaward. They form when there is abundant sand and sea-level rise is at a minimum. Retrograding barriers – result from an imbalance in the sand supply and sea-level rise. There is no longer enough sand to build the barrier or keep it stationary. Sand may be transported across the barrier called overwash. Aggrading barriers – built vertically over time at the same rate as sea-level has risen, allowing them to stay in the same position.

Beach Profiling