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Tidal inlets are defined as major tidal channels separating individual barrier islands or barrier spits and adjacent headlands. Two types of barrier islands are recognized: (1) those that consistently migrate landward (transgressive), and (2) those that build seaward (prograding or regressive). The most common types include those that formed from (1) elongation of sand spits from major headlands; (2) drowning of coastal sand ridges; and (3) landward migrating transgressive barriers that stabilized and then prograded seaward during the mid-Holocene on the interfluves between the major lowstand valleys. Large tidal inlets developed in these former river valleys. The influence of tidal range vs. average wave height plays an important role in determining the morphology of the barrier islands and the character of the tidal inlets. As an example, the outer margins of the Georgia Bight, where the tidal range is microtidal and wave heights are greatest, most barrier islands are long (28-30 km), wave-dominated, and transgressive. Toward the mixed-energy head of the Bight, where the tidal range is mesotidal and waves are relatively small, barrier islands tend to be short (<15 km), regressive, and drumstick-shaped. Tidal inlets having large flood-tidal deltas are mostly restricted to the microtidal margins of the Bight, whereas inlets with large ebb-tidal deltas dominate the mesotidal head of the Bight. Most tidal inlets form by the following mechanisms: (1) storm-generated scour channels; (2) spit growth across the entrances to flooded valleys during the mid-Holocene; (3) intersection of major tidal channels by landward-migrating transgressive barrier islands; and (4) tidal prism-controlled, evenly spaced inlets that evolve in regions lacking major former river valleys. [ABSTRACT FROM AUTHOR]

There are more than 15,000 barrier islands in fetch-limited nearshore environments around the world. About half that number are actively evolving (eroding, accreting, migrating) in response to oceanographic processes and are the subject of this study. The remaining half consists of inactive islands protected by surrounding salt marsh or mangroves. Despite their global abundance these islands have not been previously systematically studied or even recognized as a major landform type. More than 70% of fetch-limited barrier islands are found on trailing edge coasts because conditions there are favorable for formation of sheltered waters. Fully 50% are found in the coastal zone of Australia, Mexico, and Russia. We identify eight different types of fetch-limited barrier islands based on genesis and mode of occurrence. Most of the active islands form in estuaries or bays (Spencer Gulf Australia), behind open ocean barrier islands (Pamlico Sound, North Carolina), or on flood tidal deltas of open ocean tidal inlets (Tapora Bank, New Zealand). Others occur on river deltas sheltered by offshore islands (Menderes Delta, Turkey), in sheltered bays with thermokarst topography (Yensei Bay, Russia), and on glacial outwash plains in fjords (Golfo Esteban, Chile). Due to a Holocene sea level drop, some southern hemisphere islands have been stranded above mean sea level and are intermittently active (Maputo Bay, Mozambique); they are only surrounded by water during spring tides and storms. Intermittent islands also form under conditions of high tidal amplitude (Kings Bay, Australia). Fetch-limited barrier islands are much smaller than their open ocean counterparts, averaging roughly 1 km long and 50 m wide and 1 to 2 m maximum elevation. They evolve in similar fashion to ocean barriers except that overwash is almost always the dominant island building process and dune formation is much less important. The two biggest distinctions between open-ocean and fetch-limited barrier islands are (1) complete evolutionary dependence on storms and (2) the important role of salt marsh and mangrove vegetation in controlling the shape and location of fetch limited barrier islands. Stabilized by salt marshes and mangroves, vegetative control is responsible for the irregular shape of some fetchlimited barrier islands and often plays a role in creating the foundation upon which the island evolves. Few of these islands are settled or developed at present, but it is likely that in midlatitudes they will soon be under development pressure. [ABSTRACT FROM AUTHOR]