Science behind Shelly Island says it won’t be around long

By Kip Tabb | Outer Banks Voice on August 14, 2017

Once a treacherous crossing, wading is possible between the Point and the island. Below, how the island got its name. (Pat Morris)

At low tide, Shelly Island is now almost attached to the Point, and even at high tide it is accessible by foot.

What has happened at Cape Point is not rare, according to oceanographer Dr. Jesse McNinch at the Army Corps of Engineers Field Research Facility in Duck.

“It’s pretty common in the summer. The longer period swells we see in the summertime allows an accumulation of sand,” he said.

Yet, witnessing an island rising from the sea is awe-inspiring, even if it’s a sandbar and may be temporary. And if not a rare phenomenon, the size of Shelly Island is exceptional.

At Cape Lookout, Shark Island climbs from the sea periodically. “It can get to be several acres with an elevation of six feet,” McNinch said.

By comparison, if the size estimates of Shelly Island are accurate — one mile in length and one to two football fields across — it would be 50 to 55 acres.

Since it formed several months ago, the island has become a national curiousity. Only a month ago, the strong, deep current of the water between the island and Cape Point made crossing treacherous. It was accessible only by kayak or some other watercraft.

Now, visitors can wade across at some points. The island has created a protected cove, where on a recent Sunday, families were snorkeling, surfing, kayaking, fishing, swimming, wading and collecting shells. Parking at the point was at a premium, and the area looked like a small resort of its own.

If the proportions of Shelly Island are surprising, the processes that created it are known, although there is considerable ongoing research.

As a landform, Cape Point at Cape Hatteras has a scientific name: cuspate foreland. Think of the cusp of something, the point where lines come together, and that’s where the term originated. North Carolina has a number of them.

“The capes of North Carolina are sort of the poster child of the world (for cuspate forelands),” McNinch said.

Whether its Cape Hatteras, Cape Lookout or Cape Fear, they all have similar characteristics. An east-facing beach to the north, extensive shoals to the east with the beach south of the cape curving to the west then back to facing east.

The long east-facing beach of the barrier island is important — that is where the sand originates. The direction, or littoral, of drift of the sand is usually from the north to the south, especially in the winter when north and northeast winds dominate weather patterns.

Those same weather patterns create larger waves that take sand from the Outer Banks beaches and moves it to the nearshore water column.

Filled with sand, the littoral drift heads south until it encounters the cuspate foreland at Cape Hatteras.
When it encounters the Point, its southward drift is interrupted, and it is interrupted by a number of factors.

Diamond Shoals extends far to east. “It sticks out almost 16 kilometers seaward from Cape Point,” McNinch said.
Coupled with the land mass that is Cape Point, the southward drift is stopped.

There are other factors as well, though.

The view toward Cape Point from Shelly Island, which loops around on the left. (Pat Morris)

The Hatteras Bight is south of Cape Point, as the Outer Banks curve west and form a south-facing beach. Within the Bight, circular currents are created, some of them spinning off from the Gulf Stream, which is as close as 12 miles from Cape Hatteras at times.

Especially as the weather warms and the northeast winds subside, the southward littoral drift, unable to overcome the land mass, shoals and new currents, slows and sand begins to accumulate.

“It’s really a beautiful balance of factors,” McNinch said.

Those are the factors that can create a Shelly Island, although as Reide Corbett, co-Program Head of Coastal Processes at the Coastal Studies Institute, notes, there is no way to know exactly what happened.

“Nobody had instruments on this,” he said.

Without instruments to provide the data that fills in the details, there is some guesswork, especially about the size of Shelly Island. It is possible a warmer than normal winter followed by a spring with relatively few major storms played a role.

“My guess is it was set up by the dominant weather patterns of the last few months. It’s been a pretty quiet summer and that would allow material to accumulate,” Corbett said.

Yet despite its size and notoriety, it’s survival as an island or even as a sandbar peaking out from the incessant waves and currents of Cape Hatteras is in doubt.

“I would be surprised if it lasts ’til November,” Corbett said. “It’s definitely an ephemeral feature.”

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