Archive of Spotlight Feature Articles

Fighting the Tide: Cape Hatteras
Lighthouse Retreats from Rising Sea

By Andrew Shepard,
Associate Director, National Undersea Research Center,
University of North Carolina at Wilmington

What is 200 feet high and travels 100 feet per day? If you live on the Outer Banks of North Carolina, you probably know the answer. Cape Hatteras lighthouse has been a beacon for seamen for 130 years from its perch on the north end of Hatteras Island. Sea level rise and erosion have forced the lighthouse off its roost and it is being moved a half mile inland. (See details at http://www.nps.gov/caha/lrp.htm).

In order to protect billions of dollars of property and lives, few options remain for keeping a beach in place.

Hatteras Island is a barrier island. Barrier islands are massive dunes with bases below the tideline and peaks ten feet above low tide. As sea level rises and falls over time, the islands migrate both along and perpendicular to the beach. Now, they are migrating inland as sea level rises. According to a National Research Council study on the lighthouse's predicament (http://www.nps.gov/caha/lighthousereports.htm), if present trends continue, sea-level will rise up to 0.5 feet and the shoreline in front of the lighthouse will retreat 400 feet by the year 2018. By the year 2088, the retreat may reach over a half a mile inland.

Evidence of past barrier islands can be found hundred of miles inland and well offshore of the Carolinas. Before humans developed the coast, it was like a tree falling in the woods-- nature did its thing and no one noticed the quiet movement of the beaches. Things are different now. By 2018, well over half the nation's population will live within five miles of a beach. Houses and hotels now extend from end to end on most of North Carolina's barrier islands.

In order to protect billions of dollars of property and lives, few options remain for keeping a beach in place. First is "hardening" using seawalls, groins and a variety of man-made structures in front of the weak points. Unfortunately, these devices tend to transfer the vulnerable points to either side, creating a problem for someone else. The lighthouse has lots of rip-rap and debris around it from past hardening attempts. The resulting trash does not make for a very desirable beach front. Finally, the sea will not be stopped this way for long. This option is outlawed in North Carolina, unless special variance is granted.

The preferred option to fight erosion is replenishment: placing new sand on the beach from another location. The sand may come from offshore or behind the barrier island. Projects usually involve hundreds of thousands of cubic yards of sand, at a cost of one-to-two dollars per yard. Carolina's beaches are among the most replenished in the world.

At six o'clock in the morning, vessels start to stir from Wanchese, NC. Most are hardy fishermen heading out to hunt for tuna and other species that populate the Gulf Stream edges and eddies. Captain Dan pilots the University of North Carolina at Wilmington's R/V Cape Fear (http://www.uncwil.edu/cmsr/capefear) alongside them, to a spot just a few hundred yards off Hatteras Island.

Dr. Stan Riggs, recently retired professor from East Carolina University, sips coffee and looks at underwater maps. "The barrier islands are in real trouble" notes Dr. Riggs. "Dare County is going to have to spend sixty million dollars to replenish its beaches, plus ten million every two years to protect the urbanized sections of its islands. The next category three hurricane (sustained winds of greater than 110 mph) will likely break through one of the islands." Dr. Riggs, like many other scientists, knows that well-planned replenishment projects can save dollars and lives. Dr. Riggs and Dr. Bill Cleary, University of North Carolina at Wilmington, are being supported by the National Undersea Research Center at UNCW (http://www.uncwil.edu/nurc) to provide data for this purpose. As Associate Director of the funding agency, I am there for the day to follow progress on the project and get an update from Dr. Riggs on the problems we all face on the coast.

Our objective this day, and in the weeks to come, is to describe the deep end of the barrier island that lies below water and try to answer several questions. What lies underneath the sand offshore? What kind of sand is there and where did it come from? Do outcrops poke through the sand and affect how sand moves? Are there good sources of sand that could be pumped up on the beach?

On previous trips, acoustic samplers were used to do fast surveys of a broad area, but these samplers cannot provide enough detail to answer our questions. Divers equipped with cameras and cores will do the work, diving on targets picked out from the acoustic maps. Although our day was cut short by weather and high seas after a few dives, the line of samples off the north end of Hatteras Island revealed no exposed reefs where the acoustic record suggested possible outcrops. There is no better way to know what is down there than by going there.

Dr. Cleary and Dr. Riggs, and several colleagues, have been instrumental in helping the US Army Corps of Engineers plan their replenishment efforts—they often turn to the scientists for advice on potential sand sources and where erosion is likely to be worse. Our goal is not to abandon the beaches we all love, but to protect them as best we can. The lighthouse is not the last sizable object that will likely be moving in the coming decades.

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