Virginia’s barrier islands are moving toward the mainland, study shows

Virginia’s barrier islands on the Atlantic side of the Eastern Shore are moving — and are about to move faster, providing a clue to how shorelines worldwide will be affected by rising sea levels, a new study finds.

For people living on the coast, even if they are not on the barrier islands themselves, the faster movement has a big impact. The retreating islands don’t do as well protecting coastal land from storm surge, waves and flooding.

The study, by Virginia Institute of Marine Science professor Christopher Hein and Louisiana State University’s Giulio Mariotti, started by asking why barrier-island retreat has lagged behind ever-quickening rise in sea levels over the past half century.

The study is based on data from the Virginia islands, including hints from Eastern Shore geology to track 5,000 years of sea-level changes. It focuses on them because they’re not inhabited or developed. That means factors such as beach replenishment and hardening of shorelines with floodwalls and other structures don’t affect their retreat.

The Virginia islands are moving landward at a rate of about five meters (16-1/2 feet) a year, and the study predicts that’s about to accelerate to seven meters (23 feet) a year by 2100.

The reason has a lot to do with the 5,000-year average 1.5 millimeter (six-one-hundredths of an inch) a year rise in sea level off the Eastern Shore. In past 50 years, that pace as increased to 4.5 millimeters (one-fifth of an inch) a year.

The effect of the long term, if modest, rise in sea level “was to build up a vast reservoir of sand and mud across the barrier-island system,” Hein said.

Storms and tides have carved away at that sand and mud, making it easy for those forces to ease the islands toward the mainland.

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But now, with enough sand and mud gone, the rise in sea level can make the islands retreat at a pace that’s 50% faster — and that’s even assuming no acceleration in the pace of sea level rise, which most climate scientists say is unlikely.

“Because the Virginia Barrier Islands are almost entirely undeveloped, they are among the most dynamic in the world,” Hein said. “This study shows that what we are seeing out there today is only a hint of what’s to come given increasing rates of sea-level rise.”

One reason other studies have missed the more complicated link between sea level rise and barrier island retreat ““was because they assumed the geometry of the whole coastal tract — from the lower continental shelf to the upland boundary, including subaerial barriers, inlets and tidal channels — was frozen,” Mariotti said.

“However, the geometry can change.”

The study was published in “Nature Geoscience.”

Dave Ress, 757-247-4535, dress@dailypress.com