Barrier Island - Formation

Formation

Explanations for the development of barrier islands have been proposed by numerous scientists over more than 150 years. They can be grouped into three major theories: offshore bar theory, spit accretion theory, and submergence theory. No single theory can explain the development of all barriers distributed extensively along the world's coastlines. Scientists accept the idea that barriers, including other barrier types, can form by a number of different mechanisms.

However, there are some general requirements for formation. Barrier island systems develop most easily on wave dominated coasts with a small to moderate tidal range. Coasts are classified into three groups based on tidal range: microtidal, 0–2 metre tidal range; mesotidal, 2–4 metre tidal range; and macrotidal, >4 metre tidal range. Barrier islands tend to form most dominantly along microtidal coasts, where they tend to be well developed and nearly continuous. They are less frequently formed in mesotidal coasts, where they are typically short with tidal inlets common. Barrier islands are very rare along macrotidal coasts. Along with a small tidal range and a wave-dominated coast, there must be a relatively low gradient shelf. Otherwise, sand accumulation into a sandbar would not occur and instead be dispersed throughout the shore. An ample sediment supply is also a requirement for barrier island formation. The last major requirement for barrier island formation is a stable sea level. It is especially important for sea level to remain relatively unchanged during barrier island formation and growth. If sea level changes are too drastic, there will not be enough time for wave action to accumulate sand into a dune, which will eventually become a barrier island through aggradation. Barrier islands need the sea level to remain constant so that waves can concentrate the sand into one location.