Because barrier islands are built by waves, they do not develop in tide-dominated environments. Waves are responsible for the longshore transport of sediment, and it is this transport that drives the deposition of sediment to create elongated features consisting of sandy barrier islands. There are, however, numerous examples of barrier islands within mixed-energy as well as wave-dominated environments. In fact, the two fundamental types of barrier islands are recognized as wave-dominated and mixed-energy barriers.

Wave-dominated barrier islands are long, narrow barrier islands with typically widely spaced tidal inlets. Because of the wave dominance, the capability for longshore transport is high, and tidal inlets in this type of system are relatively narrow because longshore transported sediment acts to fill in the inlets and restrict their widths. The tidal deltas on the seaward side of the inlets, ebb-tidal deltas, also tend to be small compared to mixed energy barrier systems because waves tend to limit the distance that ebb-tidal deltas can migrate seaward.

Close up view of the wave-dominated barrier islands of the North Carolina Outer Banks along the mid-Atlantic eastern coast of the U.S.A. Note the long, thin morphologies of the islands and the sparseness of tidal inlets that connect the backbarrier environments to the nearshore and beyond.

Credit: NASA (Public Domain)

Mixed-energy barrier island systems are typically short and wider at one end than the other end. Historically, this type of morphology has been referred to as a drumstick barrier island because of its approximate similarity in shape to the drumstick of a chicken leg. The tidal inlets between these barriers are large because of the relatively higher tidal energy. Compared to wave-dominated barriers, they also have large ebb-tidal deltas because the strength of the tidal currents is able to transport sediment seaward in a regime of relatively low wave energy. The relatively wider end of the island is the result of the accretion of sediment as waves refract around the edge of the ebb-tidal delta, causing a localized reversal in the longshore transport pattern and leading to sand accumulation.

Several hundred kilometers farther south of the North Carolina coast along the eastern U.S.A. is the coast of the state of Georgia with barrier islands that are different from the barrier systems of North Carolina. Because the Georgia barrier islands here are in a mixed-energy environment, waves are not the dominant forcing mechanism of coastal change, and the barriers are shorter with more tidal features such as closely spaced tidal inlets that allow the exchange of large volumes of seawater between the backbarrier and open ocean.

Credit: NASA (Public Domain)

In recent years, there have been numerous studies investigating how barrier island systems respond to change in sea level and impacts created by storms as they erode sediment and destroy coastal property. Because barrier islands form a true barrier along many inhabited coastal zones, they represent a line of defense for inland communities from the destructive power of storm surges and waves that are driven by large storms. Unfortunately, however, there are global trends in barrier size reduction because of reduced sediment input caused by damming rivers, human modifications to coastal systems, storm-driven erosion, and relative sea level rise.

In future modules, we will explore the dilemmas facing communities located on barrier islands and the question of whether these communities can expect to survive.

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