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.