Three theories exist as to how barrier landforms formed. The first suggests that shallow water waves churned up sand, creating underwater sandbars. As the bars accumulated sand, they broke through the surface of the water, forming a barrier island. The second theory proposes that barrier islands are the result of breached sand spits: long coastal landforms connected to the mainland at one end. The theory suggests that high-energy waves produced during violent storms eroded the narrow connecting barrier, separating the spit from the mainland. The third theory suggests that barrier islands were formed as the result of rising sea levels and underwater dunes. Researchers at the National Oceanic and Atmospheric Administration (NOAA) say no evidence supports this theory; however, NOAA does not discredit it as a possibility.
Both tides and waves shape barrier landforms. Tidal inlets allow water to flow into and out of lagoons and bays with the changing tides, and provide a path for rising water during storms and hurricanes. Tidal deltas are formed as a result of sand deposits on either side of tidal inlets. Tidal deltas will eventually fill in with sand and close the inlet. At some point, a new tidal inlet will open at another low-lying area of the island.
The two types of barrier landforms are wave dominated and mixed energy -- identified by their shape and size. Wave-dominated barrier landforms are typically long and narrow and are the result of the interaction between approaching waves and the longshore currents they produce. Tidal inlets on wave-dominated barrier landforms are widely spaced and have small cross-sectional areas. Mixed-energy barrier landforms are generally shorter landforms that are wide on one end and narrow on the other. The interaction between waves and large tidal ranges result in the formation of numerous, deep tidal inlets, giving mixed-energy barrier landforms a drumstick appearance.
Barrier landforms have numerous environments. The near-shore environment, immediately seaward of the shoreline, is the area where breaking waves occur. It is also called the surf zone. The beach is the visible portion of a barrier landform and extends landward from the shoreline. Dunes are the result of a backshore accumulation of sand. Growing vegetation traps sand as it moves from the beach surface and accumulates into mounds.
Washover fans are found on the landward portion of a barrier landform -- usually the result of strong, storm-induced waves and currents washing over the landform and depositing sediment on the landward side. An increase in water level can also contribute to the appearance of washover fans. Wetlands occur on the landward fringe of a barrier landform and typically form along the intertidal portion of a washover fan. Wetlands are typically areas of high vegetation. Tidal flats also occur on the landward side of a barrier landform, but are covered in fine sediment and have no vegetation.
