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Barrier Islands

Oregon Inlet, North Carolina

The North Carolina coast is home to some of the world’s most extensive barrier islands. At about 190 miles in length, these narrow strips of sand along the NC coast protect the delicate estuarine shoreline and inland areas from the high energy waves and storms (Pilkey et al, 1998), govern the input of marine sediments into back-barrier wetland systems and support a distinct community of flora and fauna highly evolved to withstand the harsh environment. The formation and migration of inlets along the barrier island chain allows the mixing of freshwater from the rivers and saltwater from the sea, creating estuarine habitats of varying degrees of salinity (Riggs & Ames, 2003). These thin islands form over many centuries through the vertical accretion of marine supplied sediments transported by waves, and are dependent upon the rise and fall of sea level, forming in times of slow sea level rise and dissipating into continuous land surfaces in times of decreasing sea level (Davis & Fitzgerald, 2004). In summation, barrier island ecosystems are governed by continuously changing, high energy dynamics involving both wave and geologic processes.

Hatteras Island, Cape Point

After favorable conditions allow the formation of a barrier island other processes take over to keep the island above sea level. Barrier islands migrate landward in response to rising sea level and migrate seaward in times of decreasing sea level; in NC, for every foot of sea level rise barrier island shorelines should retreat landward about 2,000 feet (Pilkey et al, 1998). Normally, as the front, or oceanside, of the barrier island looses sand retreats landward the soundside gains sand via dune overwash (slowly during sea level rise and much more quickly in storm events) and the whole island migrates (Donnelly et al, 2005). This is how the island maintains its’ width and height; as sea level rises waves that overtop the dunes on the shoreface become more frequent, resulting in shoreface erosion, back-barrier accretion and vertical dune accretion as sand is deposited during overwash.

Overwash caused by Hurricane Isabel, September 2003

These natural processes that maintain barrier islands are compromised as the rate of sea level rise and development increases along the NC coast. Developments that prevent the occurrence of overwash restricts the supply of sediment to the backside of the island and to the top of the dunes, preventing both back-barrier and vertical dune accretion of sediments (Dolan, 1972). This loss of height and width is exacerbated by shoreline erosion resultant from sea level rise, causing the overall erosion and narrowing of barrier islands. As the rate of sea level rise and human developments (including dune stabilization, road construction, soft and hard stabilization along shorelines and the construction of any buildings that restrict overwash sand deposits in low elevation areas) restricting migration occurs the delicate ecosystems along all the coasts, especially those with barrier islands, are increasingly threatened with complete inundation.