Hundreds Of Barrier Islands Newly Identified In Global Survey

Crucial shore-protecting structures had been overlooked by misunderstanding.

NOAA-_Outer_Banks.jpg
An aerial image of North Carolina's Outer Banks barrier islands. Credit: NOAA

Earth
has 657 more barrier islands than previously thought, according to a new global
survey by researchers from Duke University and Meredith College.

The researchers
identified a total of 2,149 barrier islands worldwide using satellite images,
topographical maps and navigational charts. The new total is significantly higher
than the 1,492 islands identified in a 2001 survey conducted without the aid of
publicly available satellite imagery.

All
told, the 2,149 barrier islands measure 20,783 kilometers in length, are found
along all continents except Antarctica and in all oceans, and make up roughly
10 percent of the Earth's continental shorelines. Seventy-four percent of the
islands are found in the northern hemisphere.

Barrier
islands help protect low-lying mainland coasts against erosion and storm
damage, and can be important wildlife habitats. The nation with the most barrier
islands is the United States, with 405, including those along the Alaskan Arctic shoreline.

The
survey results appear in the current issue of the peer-reviewed Journal of
Coastal Research.

"This
provides proof that barrier islands exist in every climate and in every
tide-wave combination," says Orrin H. Pilkey, James B. Duke Professor
Emeritus of Geology at Duke's Nicholas School of the Environment. "We
found that everywhere there is a flat piece of land next to the coast, a
reasonable supply of sand, enough waves to move sand or sediment about, and a
recent sea-level rise that caused a crooked shoreline, barrier islands exist."

Barrier
islands often form as chains of long, low, narrow offshore deposits of sand and
sediment, running parallel to a coast but separated from it by bays, estuaries
or lagoons. Unlike stationary landforms, barrier islands build up, erode,
migrate and rebuild over time in response to waves, tides, currents and other
physical processes in the open ocean environment.

The
657 newly identified barrier islands didn't miraculously appear in the last
decade, explains Matthew L. Stutz, assistant professor of geosciences at
Meredith, located in Raleigh. They've long existed but were overlooked or misclassified
in past surveys.

Previously,
for instance, scientists believed barrier islands couldn't exist in locations
with seasonal tides of more than four meters. Yet Stutz and Pilkey's survey identifies
the world's longest chain of barrier islands along a stretch of the equatorial
coast of Brazil, where spring tides reach seven meters.

The
54-island chain extends 571 kilometers along the fringe of a mangrove forest
south of the mouth of the Amazon River. Past surveys didn't recognize it as a
barrier island coast partly because older, low-resolution satellite images
didn't show a clear separation between the islands and mangrove, Stutz says,
but also because the chain didn't match the wave-tide criteria used to classify
barrier islands in the United States, where most studies have been conducted. Scientists
failed to consider that supplies of replenishing sand are so plentiful along
the equatorial Brazilian coast that they can compensate for the erosion caused
by higher spring tides.

Stutz
and Pilkey say the survey's findings--which formed part of Stutz's
dissertation when he was a doctoral student at Duke--illustrate the need for a
new way to classify and study barrier islands, one that takes into account the
complex interplay of local, regional and global variables that shape where the
islands form and how they evolve.

"Are
there clues there to predict which of today's islands might be in danger of disappearing
in the near future?" Stutz asks.

The
potential for significant climate and sea level change this century "underscores
the need to improve our understanding of the fundamental roles these factors
have played historically in island evolution, in order to help us better
predict future impacts," Pilkey says.

"Barrier
islands, especially in the temperate zone, are under tremendous development
pressure, a rush to the oceanfront that ironically is timed to a period of
rising sea levels and shoreline retreat," he says.

A
developed barrier island, held in place by seawalls, jetties or groins, can't
migrate. "It essentially becomes a sitting duck unable to respond to the
changes occurring around it."

CITATION: "Open-Ocean Barrier Islands: Global Influence of Climatic, Oceanographic,
and Depositional Settings," Matthew L. Stutz and Orrin H. Pilkey. Journal
of Coastal Research: Volume 27, Issue 2: pp. 207-222. (2011) doi: 10.2112/09-1190.1

Online: http://www.jcronline.org/doi/full/10.2112/09-1190.1