The Barrier Islands: Islands in Motion

SciWorks Radio is a production of 88.5 WFDD and SciWorks, the Science Center and Environmental Park of Forsyth County, located in Winston-Salem.

It's summertime and maybe you're planning to head for the coast. Or maybe you've already been. After all, North Carolina is famous for its beautiful set of coastal barrier islands. But did you know that those islands are mobile? Around 14 thousand years ago, as the last ice age was coming to a close, sea level began to rise as water was released from the glaciers. 

The barrier islands formed up and down the east coast, either as sand spits that were cut off from land, or sand dunes along the coast, where the water flooded in behind. Back then, the coast and the newly formed barrier islands were about 40 miles further east than they are now which, at the time, was dry land. How did they get to where they are? To explain this, I spoke with Dr. Marlene McCauley, professor of Geology, Earth Science and Environmental Studies at Guilford College in Greensboro.

Instead of being static, they basically wash over themselves. They roll over themselves, and creep further and further. And they do this by a process of overwash and inlet formation, where, during storms, the waves themselves are higher and they wash over that first line of dunes and carry sand to the area behind the dunes. As sea-level continues to rise, those dunes just get washed back into the sound side, and that moves the island closer and closer to shore.

Until recently, sea level has remained steady for a good four thousand years, and so the islands have remained fixed. Without continued overwash, some grasses began to grow in the sand. They died and created a thin layer of soil that supported small plants and shrubs. As those died, they added to the soil until it was thick enough to support the trees of the maritime forests. That can't happen on an island that's migrating.

If you look at something like the Outer Banks, particularly the Bogue Banks, those are tall islands. They're thick islands; big maritime forests. Same with a lot of the Outer Banks. But the further south on the Outer Banks you get, they start getting narrower and then you hit the Core Banks. They're narrower, they aren't forested. They look more like what the islands look like when they're moving. When sea-level is rising. So now we're in a situation where sea-level is rising, the barrier islands are shrinking and getting ready to move. As sea-level rises, if we weren't inserting ourselves into the system, the sand would just be washed from the front side of the island to the back side of the island. As we come south on Highway 12 and go over the Bonner Bridge, you get to an area on Pea Island National Wildlife Refuge where it overwashes all the time. A little inlet formed a few years ago that they've been patching. This is an island that's trying to move, that's just too narrow now. Overwash happens, and then we bulldoze it right off and put it back where it was before. And then it happens again and we bulldoze it off and… Well, obviously you can't keep doing that forever.

The islands are starting to move. So what about the homes and businesses? What about the beaches?

What we do is say, “Well, this beach is really narrow. Let's do something about it. Let's pump some sand onto shore, and that will temporarily build up the beach.” So, a beach replenishment project might last for some years, or if you're really unlucky and a big nor'easter comes, it can get carried off to a depth where it's lost. So these beach replenishment projects aren't permanent. We have to redo, we have to redo, we have to redo at the cost of millions per project.

Regardless of our efforts, the islands are thinning. What will be their fate?

These islands are dynamic systems that have survived for thousands and thousands of years of sea-level rise, and if we don't muck around with them too much, they can handle sea-level rise. If we do muck too much around with them, they can't handle sea-level rise. They'll be underwater. The inlets will get bigger and bigger and there will be less and less of the islands above sea-level. We aren't going to preserve the islands in the place they are. We aren't going to even preserve the ecosystems, because with overwash you can't have a maritime forest. Can the ecosystems migrate fast enough to survive? Maybe. Maybe not. There are a lot of unknowns there.

This Time Round, the theme music for SciWorks Radio, appears as a generous contribution by the band Storyman and courtesy of UFOmusic.com.