December 2014 Newsletter: The Outer Banks - Formation, Erosion & Nourishment

One of North Carolina’s most beautiful and dynamic coastal destinations is The Outer Banks. Many of you are likely aware of the ongoing issues associated with erosion and loss of beachfront along the Outer Banks. We wanted to provide you with a little more background on how these barrier islands formed, why they continue to exhibit such dynamic behavior, and the steps that people have taken to preserve the beaches and islands that make up this fragile ecosystem.

NC Outer Banks — Image obtained from www.outerbanks.org

Barrier Islands – How Do They Form?

Barrier island systems like the Outer Banks are elongated accumulations of sand that have formed offshore and are separated from the mainland by some type of lagoon, estuary, or bay. There are several theories on how these islands formed in the geologic past. A summary of these theories is presented below:

Offshore sand bar development – This theory suggests that barrier islands gradually developed from offshore sand bars. Sand is deposited parallel to the shoreline away from the mainland. Over time, wave action allows for increased accumulation of sand along the bars, and eventually their crest breaches the water surface. Continued sand accumulation leads to the formation of a barrier island, which can then further be stabilized by vegetation growing on the island helping to prevent erosion.

Formation from sand spits – This theory hypothesizes that longshore (parallel to the shoreline) sediment transport along a coastline resulted in the formation of elongated sand spits. Over time, strong storms such as hurricanes created breaches in the spits, forming barrier islands.

Formation from sea level rise – A third theory suggests that barrier islands formed from what were once coastal dunes. During times of sea level rise (or some say coastal submergence), these dunes became isolated from the mainland, forming barrier islands.

Outer Banks barrier island formation — Image obtained from http://www.geo.hunter.cuny.edu/bight/beach.html

If you are at all familiar with the Outer Banks, then you know the beaches are constantly in a state of flux, and significant issues with beach erosion are ongoing. This is because it is in a barrier island’s nature to move towards the mainland. The following processes contribute to the erosion and movement of a barrier island:

Longshore currents and wave action erode sand from one location along a barrier island coast and move it to another, often from north to south along the east coast of the U.S.
Strong waves and storms carry sand over the crest/dune of the barrier island and deposit it on the landward side in the lagoon/bay. This is called overwash.
Hurricanes can cause breaches in the barrier island itself, creating permanent morphological changes and allowing more sand to erode and move landward.

These processes result in the barrier island generally moving towards the mainland over time. For a location like the Outer Banks, this means that the beaches where houses have been constructed are constantly eroding and losing their width, moving closer to the homes. Humans have now developed ways to help stop, or at least postpone, this movement.

Beach Nourishment – Maintaining Coastlines and Preserving Barrier Islands

Beach nourishment NC coastline — Image obtained from www.coastalcare.org

The process of anthropogenic beach nourishment is when humans attempt to combat coastal erosion by creating structures along the coastline and adding sand to beaches in order to maintain their current morphology. Beach nourishment occurs in a wide variety of coastal environments, not all of which are barrier islands. The Outer Banks has numerous beach nourishment projects that have been implemented in an effort to maintain the ecosystem and save buildings.

Beach nourishment often involves finding an offshore source of sand that is similar in grain size, color, and quality to that of the natural sand along the coast. Sand from this offshore deposit is then collected by dredging techniques and re-deposited along the areas of coastline that are experiencing erosion. A variety of techniques are used to located these offshore deposits, including aerial photography, bathymetric profiling, remote sensing, marine geophysical surveys, and physical sampling of the offshore environment. Additionally, sand for nourishment is sometimes collected from near-shore environments, in the bays or lagoons between the barrier island and the mainland where sediment accumulates over time.

A significant beach nourishment project was implemented at the town of Nags Head in the Outer Banks in 2011 following a 10-year period of extensive beach erosion. A company called Coastal Science and Engineering, Inc. (CSE) worked with the town to dredge sand from offshore borrow areas and deposit it along the eroded sections of coastline. To the left is a photo from CSE’s website showing the effects of the nourishment project.

The Outer Banks are a precious resource for our state, and the combination of natural erosion and human impacts has resulted in a continuous struggle to maintain both the barrier islands and the infrastructure that is important for tourism. Please visit the links included throughout this article to learn more about barrier islands, the Outer Banks, and beach nourishment.