Eastern Barrier Islands: Terra Un-Firma
Hog Island is currently heavy at the top and thin at the bottom. But 100 years ago, the dimensions were reversed.
Credit: Bruce Hayden
Something must be wrong with the barrier islands.
Every year, homes topple into the ocean, beaches shrink, and people who
think they own terra firma find their property lines underwater.
The property owners are upset, but the government is stymied. How can the Federal
Emergency Management Agency (FEMA) set rates for flood insurance without knowing
how bad the erosion hazard really is?
FEMA and NSF scientists joined forces to find an answer.
Part of the problem is that people simply do not understand the barrier island
system, says ecologist Bruce Hayden. The strips of sand that line the eastern
coast aren't necessarily shrinking, they're moving and changing shape. "The islands
are extremely dynamic," he says. "Due to currents and breaking waves, sand moves
along the coast. On Hog Island, ninety percent of the sand you walk on was put
down since the turn of the century."
But Hayden, of the University of Virginia, admits this isn't an answer that FEMA
can live with. So as part of an NSF Long-Term Ecological Research Program, he
and his colleagues tracked the islands' morphing tendencies over two centuries.
While Hayden stopped short of claiming a pattern, his team did find phenomena
that may help FEMA set policies.
Their investigation started with Virginia's Hog Island, a seven-mile strip of
sand that is currently narrow at the bottom and fat at the top.
That wasn't the way the island looked when it had 250 residents. In 1890, the
town plat shows a bulge at the island's bottom. By 1930, the island was shifting
north, and the residents moved inland, but they left behind their official records
of the island's shores. Since then, the island's movements have been recorded
in the U.S. Coast and Geodetic Survey and in aerial photos.
Hayden and his colleagues used these data and further in situ research to create
a computer model of the island's changes from 1800 to 2050.
The computer image shows the island waxing and waning and allows researchers
to identify the years when the sands' patterns change directions. Within the
sample years, sand reversed direction twice, once in 1870 and again in 1969.
After finding that other barrier chain islands had similar changes in those years,
Hayden started looking for mechanisms that could cause the shift.
The answer, he says, is mostly in the storm patterns. East coast storm frequency
was near a minimum around the turn of the century and reached a maximum in the
late 1960s. While other weather systems, such as the Bermuda High, move sand
northward, the storms and their waves push sand southward, matching the observed
patterns of sand movement.
Hayden acknowledges that his work does not give FEMA a formula to work with,
but he has provided a more realistic picture of the shifting barrier islands,
where the only thing that is stable is the sands' continuous movement.