Retreating

Charles Fergus
June 01, 1984

"During those long ages of geologic time, the sea has ebbed and flowed over the great Atlantic coastal plain. It has crept toward the distant Appalachians, paused for a time, then slowly receded, sometimes far into its basin; and on each such advance it has rained down its sediments and left the fossils of its creatures over that vast and level plain. And so the particular place of its stand today is of little moment in the history of the earth or in the nature of the beach—a hundred feet higher, or a hundred feet lower, the seas would still rise and fall unhurried over shining flats of sand, as they do today."
—From The Edge of the Sea, by Rachel Carson

On the beach at Assateague, sometimes the surf will wash away sand to expose a black, clotted mass several inches deep, spreading in a 20-foot band or standing in irregular, yard-wide patches. The mass is peat, mummified marsh grass. The surf may also churn up shells of scallops and clams, creatures of the bay; rare is the beachcomber who knows that the shells are thousands of years old. On some barrier beaches, tree stumps jut above the swashface. Like the peat and shells, the stumps tell a story: The island is rolling over itself, heading toward shore.

Barrier islands erode and retreat in the face of a rising ocean. Sea level rise, on the order of a sixteenth of an inch a year, is caused by a global warming trend that, over the last 18,000 years, has been freeing water by melting the polar ice caps. Some barrier islands retreat faster than others; recent rates of sea level rise, say three Penn State geologists, are not enough to have caused the extremely rapid movement of Assateague Island and several neighboring islands, which have in some spots retreated up to 250 feet in the last 25 years.

Al Guber, Rudy Slingerland, and Paul Gayes believe that compaction is an important factor in the accelerated retreat. They have shown that sand, washed over the islands and deposited on the soft sediments of the marsh, compacts the sediments and causes the marsh surfaces to sink. The resulting depressions make further washovers likely. The cycle repeats itself again and again.

"Geologists have never considered that compaction plays a role in barrier island migration," Slingerland says. "They've formed a false conception of how a classic barrier island evolves."

Al Guber sits in the front of the bus. The vehicle, belonging to the Marine Sciences Consortium, is carrying two dozen college students to Wallops Island, where Guber will instruct them in the geology of barrier beaches. The bus trundles up the causeway that rises over the marsh. Guber, who wears glasses and has a dark beard surrounding his lips, turns toward me.

"See those little depressions out in the marsh?" he says, raising his voice above the bus's rumble. "When they dug foundation holes for the causeway piers, they dumped the mud on the surface of the marsh. At first it formed a mound, after a while it settled, and then it started to sink. Now it's shaped like a pan.

"I started wondering about those depressions. I knew that parts of the marsh won't support much weight—sometimes you sink in to your knees. So I had my students do some digging. We checked out spots along the intracoastal waterway, where the Army Corps of Engineers had piled silt they'd dredged out of the channel. Those piles were sinking, too.

"I thought the same thing could be happening behind barrier islands. When there's a storm, especially one during high spring tide, waves can wash across an island. They drive through any weak spots in the dune, carry sand through the breach, and dump it on the marsh.

"On Assateague, there are two big washovers near the northern end, broad flat areas where everything opens up—no dunes, or only low dunes that shift back and forth with the wind. They're really apparent on aerial photos. You can see it happening—the islands creeping in toward the shore."

"A> feel like putting a dollar on the plate every time Al gets going on washovers," Paul Gayes said earlier. "He persuaded Rudy Slingerland that his idea was valid, and then Rudy expanded the theory and worked on the math and physics."

Gayes sat in a cramped office in the geology building. He was leaving the next day for Halifax, Nova Scotia, to begin work on a Ph.D.

"I was the one who did most of the field work," Gayes said. "The title of my thesis is ‘Primary Consolidation and Subsidence in Transgressive Barrier Island Systems.'

"In 1982 we took 33 cores from four different transects across three islands: two on Assawoman, one on Wallops, one on Metomkin. We cored from the low tide zone across the island and 20 to 30 feet out in the bay." (When Guber and Slingerland had mentioned fieldwork, they'd smiled. "You wouldn't want to do it," they said. "Mosquitoes, poison ivy, heat, and 'The Corer.'")

"We used a Vibracorer," Gayes said. "It's a contractor's vibrator—they use them to compact poured cement—modified to remove cores of soil from the ground. The whole thing weighed about 100 pounds. It was bulky, awkward as hell. About the only place you could carry it easily was on the swashface. I had two undergraduate assistants, and fortunately one of them was on the football team. We had the corer, pipe, and a tripod and winch to pull the pipe out of the ground. We took to leaving the corer on the dunes at night."

A Vibracorer, explained Gayes, has a 5-horsepower motor that vibrates 10,000 times a minute, shimmying a 3-inch aluminum pipe into the ground. After drilling a core, Gayes would lug the pipe home and snip it down each side with a circular saw, exposing a cylinder of gray mud 15 to 30 feet long.

"The first thing we measured," Gayes said, "was void ratio: total void space to total volume. In other words, how much water existed in a sample of mud, sand, and clay.

"Then in the lab we conducted one-dimensional consolidation tests. We'd cut a series of plugs out of a core, each from a different horizon in the column. I'd take a plug and add increasing amounts of weight, to compact the sediments and drive the water out. We used standard tests developed by civil engineers back in the ‘20s to predict subsidence under bridge supports, buildings, roads, whatever."

"A clay sample," Rudy Slingerland had told me, "will have many clay platelets arranged randomly—some pointing one way, some another, with lots of water in between. When you add weight, the sample compacts: The platelets line up perpendicular to the force being applied, like pennies in a pile. The water gets squeezed out."

In the lab, Gayes found that a coarse-grained sediment with a low void ratio cannot be compacted as easily as a fine-grained sediment with a higher void ratio. This finding squared with field results indicating that islands made of fine-grained sediments were sinking—and retreating—more rapidly than islands grounded on coarser stuff.

The coring transect that showed the most rapid compaction knifed across Assawoman Island, the next barrier south of Wallops. Assawoman, says Gayes, is privately owned. Developers want to build on the island; the Nature Conservancy wants to buy and preserve it. The owner hasn't decided which group to sell to.

"If you put a high-rise behind the dune on Assawoman," said Gayes, "you'd have your first two stories in the basement before long.

"An engineer can tell you not to build on compactible sediments," he added. "If much of Assawoman is compacting, the island wouldn't be suitable for development. If the Nature Conservancy asks me about Assawoman, I'll tell them."

The beach at Wallops: low and broad, hard and wet. Windblown sand streams across it like powder snow over ice.

A dozen students are digging a test trench at the back of the beach near the dune. Al Guber, crouching with his back to the wind, scratches in the sand with a driftwood stick. He draws ocean waves, the hump of a dune, sand pushed through the dune and dumped behind it.

"You get washovers. The rate of compaction depends on how much sediment is loaded, how fast it's dumped on the marsh, and physical properties of the marsh, such as sediment size. The island moves toward shore, the rate of retreat affected by the amount of sand being resupplied to the beach

"Wallops" (he moves over a foot and draws a long, narrow line) "is supplied with sand from the north, from Assateague, through littoral drift—a general southward drifting of sand along the shore. There's a channel, here, between Assateague and Wallops; the larger grains of sand settle out in the deep water, and only the smaller grains come on down. So Wallops is a fine-grained barrier island. It's compacting rapidly, and this subsidence contributes to its rapid retreat toward shore.

"Wallops is retreating, Assawoman is retreating, Metomkin is retreating so fast it's practically breaking apart. Farther south, the islands are in better shape. I'd guess they're getting sand from offshore bars."

Guber stands and buries his hands in his pockets. Tan strings of sand snake across the beach, the color of wet cement.

"Look how mobile the sand is," Guber says. "Sand and a rising sea level. They're the keys to understanding why a barrier island retreats.

"Some day, Wallops is going to smother the salt marsh. It's going to roll back onto the mainland. If sea level continues to rise, this rolling back will be rapid, like it is right now. When sea level stops rising—and if our ideas on compaction are correct—the barrier will still roll back onto the mainland.

"When we enter a new ice age, glaciation will tie up water in ice, and sea level will fall drastically. Offshore, a whole new set of barrier islands will form."

Last Updated June 01, 1984