Nourishment, Sea Level Rise and the Big Picture

In March of this year, the town of Southern Shores decided to move forward with beach nourishment, piggybacking on the Kitty Hawk project to replenish a small section of threatened shoreline just north of the Kitty Hawk town line.

With that decision, Southern Shores joined every other oceanfront town in Dare County north of Oregon Inlet who have all moved forward with approving beach nourishment projects.

There are other areas in southern Dare County where beach nourishment is being implemented or is scheduled for 2017. Nourishment is being used to keep NC 12 open at S Curves in Rodanthe until a permanent fix is in place; and the north end of Buxton on Hatteras Island will be nourished this summer.

The United States was the first nation to approve a nourishment project, for Coney Island, NY in 1922. Wrightsville Beach in 1939 was the first North Carolina town to nourish its beach.

Dare County is somewhat of an outlier in the world of beach nourishment—it is one of the last of North Carolina’s oceanfront counties to utilize the process to protect its beaches and property. To the north, Currituck County has not yet opted to nourish its beaches; nor has Ocracoke Island, although most of its beaches are part of Cape Hatteras National Seashore, and the rules for nourishing federal lands are far more stringent than areas under local jurisdiction.

NC12 BridgeThe use of beach nourishment as a shoreline management tool has been steadily increasing for a number of years. In Europe, every nation that has a shoreline has nourished some part of its beaches; the same goes for Asia and Australia.

Dr. Reide Corbett, Program Head of Coastal Processes at the Coastal Studies Institute on Roanoke Island has discussed beach nourishment with the public at a number of forums. The rising sea level is something he notes frequently in his discussions.

“We can stand here and argue about what causes sea level rise until we’re blue in the face. It doesn’t matter, in my opinion. The process is happening,” he said.

He points out, though, that the rate of sea level rise is not constant across all land. “Here on the Outer Banks sea level is rising about 4mm (about .16”) per year. It does vary depending on where you are in North Carolina. The reason it’s different is because the rate the landmass is sinking varies depending on where you are,” he said. The southern part of the state, as an example, is seeing a sea level rise about half of the Outer Banks level.

For Dr. Corbett, beach nourishment is one of the tools in the tool chest to mitigate the effect of a rising sea level on the Outer Banks. He has spoken at a number of forums on the subject, and has completed studies to help understand when and where the most effective places for nourishment are located.

He also points out that nourishment is limited in its scope and there are other factors involved that are important to consider when developing shoreline mitigation plans.

“The first thing we have to do is define our coast,” he said. “For North Carolina, we have about 300 miles of ocean shoreline. I’m not suggesting that’s not important. But we should recognize we also have 12,000 miles of estuarine shoreline. And that estuarine shoreline is as important, if not more important, than the ocean shoreline when it comes to the ecology of our system, and when it comes to protection from inundation.”

He adds to that definition, saying, “The coast does not just refer to that shoreline, but to that area that is influenced by that water that is adjacent. So that shoreline could be a few hundred feet or maybe even a mile inland.”

The Effects of Sea Level Rise on the Estuarine System

Sea Level Rise Impact on Estuaries photoThe effects of sea level rise on the estuarine side of the Outer Banks is complex, but there is visible evidence in that area of sea level rise.

Marsh habitats are designed to transition from fresh to brackish to salt, but that transition typically occurs over extended periods of time.

“It is a natural progression,” Christine Pickens, Coastal Restoration and Adaptation Specialist at Nags Head Woods said. What seems to be happening, however, is the transition to a saltwater or brackish environment is so rapid that some of the marshes do not seem to be successfully transitioning.

In the Alligator River National Wildlife Refuge (ARNWR) just west of the Outer Banks, this has resulted in many trees dying, which can be seen driving along US 64 or US 264.

In an interview before he retired in October of last year, outgoing ARNWR supervisor Mike Bryant described what was happening.

“We’ve got this big chunk of land that’s so low in the landscape, and as the sea rises we’re losing land,” he said. “We’re also getting a big change in the plant community from forest to marsh at a very rapid rate. This rate of change is not normal.”

The marsh habitat may be the key to slowing the retreat or erosion along the shores of the Outer Banks sounds.

A marsh exists both horizontally–the distance from the shore that it extends–and vertically, the thickness of the marsh vegetation and sediments above the sediment floor. Possibly more importantly than the actual thickness is the marsh’s ability to continue to grow vertically through the addition of sediment or plant matter from the marsh itself. “Marsh habitats tend to have a slower erosion rate, likely due to the root systems, than other coastal shorelines–sediment bank and swamp forest,” Corbett said.

Erosion rates become important because a rise in sea level, although it effects shorelines, does not affect all shorelines in the same way.

“Depending on where you are, the importance of shoreline change versus sea level rise might be different,” he said. “Wetlands tend to erode at a slower rate because of all the roots that hold the material together. The erosion is not really important. It’s the inundation.”

This is not the first time there has been a sea level rise. In fact, sea levels fluctuate significantly over time based on a number of factors. During the last Ice Age, when vast amounts of water were trapped in ice, the coastline of North America was approximately 200 miles to the east.

What appears to be unique in this cycle is the rate at which sea level is rising, and that rapid rise is having an effect on the marshes and estuaries that typically protect the shoreline.

This is not, however, a gloom and doom scenario for the Outer Banks. Although marsh and wetlands are often the first areas to feel the impacts of inundation, they are also the most resilient and nature’s natural protection. The estuarine shoreline of North Carolina is relatively undeveloped, which is helpful.

A detailed map of the interior coastline of North Carolina was completed for the NC Division of Coastal Management in 2012, and Corbett led the team that created the map. What it shows is surprising.

“If we look at some general statistics… 65% of that shoreline is marsh. That’s a huge amount of really important habitat,” Corbett said. “A key thing to note here is only 5% of our shoreline has been modified. That is also really important. There are other systems, like Mobile Bay, that’s 40 or 50% modified.”

It’s an important characteristic of the shoreline. Over millennia, shorelines have evolved to deal with the effects of changing sea levels. On the ocean side, nourishment and zoning regulations are tools that help to mitigate the effects of a rising sea level.

The 12,000 miles of estuarine shoreline is not a candidate for the mitigation tools used on the ocean side. What appears to be the most effective means of dealing with changes in the environment is to leave the natural processes in place.