Barrier Systems Common along passive margins (sediment supply) with moderately wide, gentle sloping continental margin System consists of Barrier islands/spits, back barrier (tidal flats, bays and lagoons) May be small or up to 100s of km long Barrier Island Beach Interior (often dunes) Back Barrier-intertidal sand/mud flat or salt marsh Landward migration by overwash deposits Wave energy is important for barrier formation Tide dominated coasts don’t have them-sand reworked Types Barrier spits Recurved spits, cuspate spits, tombolos Welded barriers Formation Typically form during times of relatively low sea level change. Most modern barriers are between 5000 and 7000 years old. Offshore bar theory Waves move offshore bars into the region where they grow vertically Spit accretion theory Spits detach from the mainland to form islands Submergence theory Drowned coastal dune ridges All three mechanisms seem to occur at different locations Prograding and Retrograding Barriers Prograding-higher sediment supply-beach migrates seaward and island grows wider. Tends to form a series of beach ridges as the barrier grows. Retrograding-lower sediment supply-barrier thins and moves landward Wave Dominated Coasts Long linear barrier island systems (North Carolina) Tide Dominated Coasts-funnel shaped embayments and lack barrier systems Mixed Coasts (tidal influence) (further south in South Carolina (wider shelf = larger tides Barriers are short with many more tidal channels (more tide influence) “drumstick” shaped barrier islands Examples from the East Friesian Islands-Netherlands/Germany Poldering (trapping sediment to fill in for agriculture) has reduced the tidal prism behind the barriers and therefore reduced the size of the channels. These islands have grown in size as there is more sediment available to stay in the system.