Barrier Islands

Individual barrier islands do not stand alone; instead, systems of islands develop along favorable coastlines. The formation of an island allows other landforms to develop, each characterized by its dominant sediment type and by the water that shapes them. For example, each barrier island has a shoreline that faces the sea and receives the full force of waves, tides, and currents. This shoreline is often called the beach. The beach zone extends from slightly offshore to the high water line. Coarse sand and gravel are deposited along the beach, with finer sand and silts carried farther offshore.

Behind the beach are sand dunes. Wind and plants such as sea oats form dunes, but occasionally dunes are inundated by high water and may be reworked by storm surges and waves. On wide barrier islands, the landscape behind the foredunes gently rolls as dunes alternate with low-lying swales. If the dunes and swales are well developed, distinct parallel lines of dune ridges and swales can be seen from above. These differences in topography allow some soil to develop and nutrients to accumulate despite the porous sandy base. Consequently, many medium to large barrier islands are host to trees (which are often stunted), bushes, and herbaceous plants. Smaller or younger barrier islands may be little more than loose sand with few plants.

The back-barrier lies on the shoreward side of the island. Unlike the beach, this zone does not bear the full force of waves. Instead, the back-barrier region Diagram of the barrier island system. Photo Researchers, Inc. Reproduced by permission. consists of a protected shoreline and lagoon, which is influenced by tides more than waves. Water may occasionally rush over the island during storms, carrying beach and dune sand and depositing it in the lagoon. This process, called rolling over, is vital to the existence of barrier islands and is the method by which a barrier island migrates landward. Sand washover fans in the lagoons are evidence of rolling over. Salt marsh, sea grass, and mudflat communities develop along the sheltered back-barrier. These communities teem with plant and animal life and their sediments are rich in organic matter.

Finally, barrier islands are associated with tidal inlets and tidal deltas. Tidal inlets allow water to move into and out of bays and lagoons with rising and falling tides. Tidal inlets also provide a path for high water during storms and hurricanes. As water moves through an inlet, sand is deposited at both ends of the inlet, forming tidal deltas. Longshore currents may also deposit sand at the delta. Eventually, the growing deltas close the inlet and a new inlet appears elsewhere on the island, usually at a low-lying spot. The size and shape of the inlet are determined by various factors, including the size of the associated lagoon and the tidal range (the vertical height between high and low tide for the area). A large tidal range promotes the formation of many inlets, thereby creating shorter and wider barrier islands referred to as drumsticks. In addition, the larger the lagoon and the greater the tidal range, the deeper and wider the inlet due to the large quantity of water moving from ocean to lagoon and back. Deep, wide inlets occur where the main source of energy shaping the coastal area is tides, or tides in conjunction with waves. In contrast, wave-dominated areas form long barrier islands with narrow bays and narrow, shallow inlets.