the group’s work gives an unprecedented
look at the effects of huge waves at
depths normally undisturbed by events
at the ocean’s surface. “These data make
it possible to study the physics underneath a hurricane,” Teague says.
In normal conditions, currents crawl
along the ocean bottom at about 1 millimeter per second. But as Hurricane Ivan
passed over, seafloor currents rushed
about a thousand times faster, Teague and
colleagues report in the June 16
Geophysical Research Letters. And while steady
currents are the norm for deep water, the
researchers estimate that shear stresses
induced in seafloor sediments by the
waves were about four times higher than
those triggered by everyday currents.
The strong back-and-forth action of
Ivan’s waves was particularly effective
at stirring up sediment; upward-looking
sonar detected sediment suspended in
the water as much as 25 meters above
the ocean floor.
The researchers did have one surprise,
says Teague: Currents set in motion by the
hurricane were strong enough to shuffle
sediment across the ocean floor for nearly
a week after the storm had passed. During
that week, currents scoured more than
30 centimeters of material from beneath
one of the sensors, he notes.
Results of this study will not only help
scientists improve and calibrate models
of subsurface currents, says Teague, but
they’ll also help engineers make seafloor pipelines more resilient in storms.
This is no small problem: According
to a report released by the Minerals
Management Service in February, at
least 17 pipelines that crisscross the
Gulf seafloor were damaged by wind- or
wave-induced currents during hurricanes Gustav and Ike in 2008.
Scientists’ next question is how much
erosion will result from the powerful movement of water during a major
storm in any given location. Though two
stretches of coast may appear similar,
their erosion rates can differ substantially. Along the Atlantic Coast, the shore
erodes inland, on average, between
60 centimeters and a meter each year;
annual rates of erosion along the Gulf
Coast are double that ( SN: 7/8/00, p. 20).
And while all Gulf shores are at risk of
being struck by major storms, some areas
are more vulnerable to erosion than
others. In southern Louisiana, and particularly in the Mississippi Delta, long-term
sinking, or subsidence — a phenomenon
resulting from reduced deliveries of river
sediment and the ongoing withdrawal
of oil and gas from underground reservoirs — makes barrier islands unusually
susceptible to storms. Rising sea levels
only aggravate the problem.
Even along relatively stable shores
such as Texas’ Gulf Coast, long-term
rates of erosion can differ dramatically
from one swath of land to the next, says
Rice’s Anderson. Analyses of aerial photos suggest that Galveston Island and
the Bolivar Peninsula each lost an average of 1. 3 to 1. 4 meters of beach each
year between the 1930s and the 1980s,
Anderson’s team notes in the June 8 Eos.
Arrows show erosion over time
on Alabama’s Dauphin Island: before
Hurricane Lili (top), just after Ivan and
two days after Katrina (bottom).
The shore of Matagorda Island, southwest of Galveston Island, actually beefed
up by about a meter a year over the same
period, thanks to the convergence of sed-iment-laden currents there.
But long-term average rates of erosion
don’t tell the full story, says geophysicist
Neil Frazer of the University of Hawaii at
Manoa. His team’s studies indicate that
models that include only the average
rates and ignore the temporary effects
of storms don’t do a good job of estimating what portions of the coastline are safe
for development. That’s been true in his
research on Assateague Island along the
Maryland and Virginia coast between
1995 and 2003, and in studies along the
Delaware coast, where data have been
collected since the 1920s.
With coastal populations growing at
the fastest rates in the country, better
estimates of future erosion are becoming more important than ever.
“If I were a shoreline manager, I could
take long-term data and make a best bet
about where the shoreline would possibly be in 2040,” Frazer says. “But then
I could consider the effects of a single
major storm and argue that maybe I’d
better not plan on the shoreline being
there exactly, because that storm could
come along and blow you away…. Things
can look the same for 50 years, then
‘Bang!’ ” he notes.
That happened on the Bolivar
Peninsula with Hurricane Ike, where
erosion averages 1. 3 meters per year, but
Ike sluiced away at least 30 times that
in one fell swoop. Using predictions of
erosion based on average rates but also
including the temporary impacts of a
major storm might have made prudent
developers think twice about building
structures on the low-lying land — or at
least let the developers know that their
buildings needed taller stilts. s
s K.S. Doran et al. “Hurricane Ike:
Observations and analysis of coastal
change.” U.S. Geological Survey
Open-File Report 2009-1061.
Report available at pubs.usgs.gov/