Barrier Islands
Barrier island. Image courtesy National Ocean Service Photo Gallery.
JB: This is Earth and Sky with a story about barrier islands.
DB: Barrier islands are massive dunes. Their bases lie below the tideline, and their peaks barely reach 3 meters – or about 10 feet – above low tide. These ribbons of sand were formed by huge storms, and they help buffer the coast from wave erosion.
JB: The islands stay above water by migrating up the gentle slope of the coastal plain. These low-lying islands struggle against rising sea levels that started 10,000 years ago when the climate warmed dramatically after the last ice age.
DB: A common type of barrier island is called an “overwash” island. When severe storms strike a beach, the waves wash over dunes and the sands fan out toward the back side of the island. As the front side erodes, the back side gains sand. The island gradually rolls over on itself.
JB: This natural system is disrupted when people build artificial dunes to protect beach homes and roads. This prevents waves from depositing sands to the back side of the island. For example, the barrier islands off the Pamlico Sound lagoon in North Carolina are losing their ability to move and are eroding on both sides, getting skinnier and skinnier. And on a global scale, there are no new barrier islands forming.
DB: Thanks today to the National Fish and Wildlife Foundation and to the U.S. Forest Service – supporting the conservation of native fish, wildlife, plants and their habitats. We’re Block and Byrd for Earth and Sky.
The following individuals were interviewed for today’s show. Our thanks to:
Dr. Stan Riggs
Distinguished Professor of Geology
East Carolina University
Greenville, North Carolina
Steve Harrison
Chief of Resources Management
National Park Service Outer Banks Office
More Resources:
Frankenberg, Dirk, The Nature of the Outer Banks, Environmental Processes, Field Sites, and Developmenta l Issues, Corolla to Ocracoke. University of North Carolina Press, 2001.
Leatherman, Steven P., Barrier Island Handbook. University of Maryland Press, 1988. See sample chapter at www.nando.net/ncd/week1/mason3.html.
Schoenbaum, Thomas J., Islands, Capes, and Sounds, The North Carolina Coast. John F. Blair, publisher, Winston-Salem, North Carolina, 1982.
Sackett, Russell, Planet Earth: Edge of the Sea, Time-Life Books, Virginia, 1983.
How Barrier Islands Work (HowStuffWorks.com)
Interview with Dr. Stan Riggs, East Carolina University:
Dr. Riggs is one of the foremost experts on barrier islands and has studied their formation and movement for more than 30 years.
Q: Can you explain how and why barrier islands move?
Riggs: There are several factors that are absolutely crucial. One is rising sea level. The shoreline or the beach is the energy absorbing sponge for the ocean. That beach moves with rising sea level.The second factor is it takes high-energy storms to do it. The third factor is that not all barrier islands are equal. Some have lots of sand. Some don’t.The ones with lots of sand build dune ridges and won’t overwash. These are called complex barrier islands. The barrier islands that overwash are the ones that have little sand. These are simple carriers and the whole barrier becomes the beach.That barrier will march over the top of itself.
Question: Scientists estimate that the barrier islands off the Carolinas may well have moved 40-50 miles from where they originally formed. Why?
Riggs: During the ice age, 14,000-20,000 years ago , the shoreline was 400 feet below present sea level. You can see an old bathtub ring around continental margins. We know where that shoreline was. Rivers flowed across the Continental Shelf. We even find Indian sites there. The shoreline was off the edge of the Continental Shelf. You have to go 60 miles to get to that depth of 400 feet below present sea level. We had beaches at that time and see evidence on the Continental slope.
Sea levels rose dramatically about 10,000 years ago as glaciers melted. If you move the shoreline, you move the beach, which migrated upward and landward to where it is today. Depending on how much sand is available, you might lose a beach or barrier island. The islands may not be exactly the same ones that were 60 miles out. You can replicate this effect in a puddle with a hose, moving sand around. It’s simply on bigger scale.
Sea level rise was not continuous. We are working on these jiggles in to get the detailed history of how climate and sea level changed so we can better understand the processes and dynamics . When climate changes, the sea level changes and so does th nature of vegetation. We are definitely looking at the effects of global warming as part of our research.
Q. What is happening to Cape Hatteras, North Carolina, where the Cape Hatteras Lighthouse was recently moved back from the ocean?
Riggs: Rising sea level is the main factor. Buxton Woods, site of the lighthouse, is not overwashing, but all that shoreline is eroding because of human alteration. North of the lighthouse, there is no sand left in the system. That’s why Highway 12 goes to sea all the time. There’s a five mile stretch of overwash barrier that is disappearing partly because of human alteration.
Q. What is the function of a barrier island?
Riggs: A barrier island has a sole function of absorbing the wave energy. If you have a low enough flow and if you have sand, you will build a beach that functions to absorb wave energy.
Q. Why are barrier islands limited to coastlines like the Atlantic seaboard as opposed to the Pacific?
Riggs: If you have a steep vertical cliff and a rocky shoreline, you will not build beaches or barrier islands. Barrier islands form where there are gentle coastlines with lots of sand available but not equally available. For example, where rivers come in, a lot of sand comes in.
Q. How do the mechanics of barrier island migration work?
Riggs: Erosion is happening on a daily basis. When water intersects land it moves sand around. Waves are an efficient sand mover. There are many cycles and scales on which a barrier island moves. On calm days, waves slowly move sand back in. Storms pull sand off again.
The real work is done at a bigger scale. When a storm hits, a beach may erode 200 feet or so, unless prevented by stationary dunes and bulkheads. If you prevent overwash, you prevent the nourishment of the backside of the barrier, which is what the barrier will roll over on in response to rising sea levels. Fo r example, barrier islands off Pamlico Sound (North Carolina, Nagshead area) are eroding on the sound side, because there is no overwash distributing sand to that side of the island. The barrier island erodes on both sides and gets skinnier and skinnier.
Q. What happens when people develop barrier islands?
Riggs: Overwash barrier islands survive by rolling over on themselves. On many islands, we have build dune ridges that have changed the whole dynamics. We get overwash, but onto the dunes, instead of across the island to the sound side. In the southern part of North Carolina, we have some barrier islands rolling over on themselves, but to the north we will lose those barrier islands. We will form them again inland, but they will backstep or make a jump. The process is no longer a continuous tank tread. If we get a big storm, the artificial dune ridges won’t stop anything.
Q. Can you tell me more about your barrier island research?
Riggs: We have recently started a large cooperative research program looking at barrier island processes, involving East Carolina University, North Carolina geological survey, US Geologic Survey(lead agency)?and individuals from other institutions.
We have been out in the field since the middle of June with five or six programs going. For instance, we are running seismic equipment back in the estuaries and the islands. These work like an xray of your body. We’re seeing pictures of the subsurface using sound. This tells us where the old riverbeds and old shorelines were. This fall we’re coming back to drill for samples. We age date the samples to know when we had an estuary and a barrier island. Pamlico Bay was sometimes an open bay. Barrier islands weren’t always there. They come and go.
We are also studying the old Roanoke River channels. We drill and find out how many times sea level had gone up and down. We study the organism types found there to determine salinity. Eventually, we will reconstruct decade by decade the changing climatic and geographic conditions that led to what we have today. Our research ties together the Continental Shelf, barrier islands, and estuaries.
Additional Teacher Resources
U.S. National Oceanic and Atmospheric Administration, Beach Nourishment, Costal Geology: Barrier Islands: Formation and Evolution
This site provides great introductory material for students learning about barrier islands. It includes an introduction, origin of barrier islands, determining age of barrier islands, barrier island environments, barrier island morphology and dynamics, barrier island types, and a concluding summary. The report also contains an assortment of diagrams, maps, and photo images of various barrier islands.
U.S. National Park Service, Gulf Islands National Seashore?Mississippi District, Nature: Islands
Few people realize barrier islands are creations not just of the sea, but also of the land. This site explains that the barrier islands on the northern gulf coast are made of quartz sand eroded and washed down rivers from the eastern U.S. mountain ranges. This means in a sense, a sand dune could have once been, thousands of years ago, on top of a rugged southern Appalachian mountain.
U.S. Geological Survey, Marine and Coastal Geology Program: Louisiana’s Barrier Islands: A Vanishing Resource
This site discusses the erosion of the barrier islands of Louisiana, which is extreme in some cases. In places up to 100 feet of shoreline are disappearing every year. It is predicted that these islands could disappear by the end of this century.
Answers.com: Barrier Islands
This site provides a myriad definitions and information on barrier islands. There is also a variety of photo image examples from all over the world, as well as several links to more information.
