In recent years,
have reshaped Gulf
of Mexico shores,
scientists are seeing better than ever
how much land storms chisel from
the edges of continents— not just in
populated areas, where homes and
infrastructure are damaged or demolished, but in the kinds of remote areas
that previously went unsurveyed after
storms. Along with improving damage estimates right after a major storm
strike, researchers hope to do a better
job of predicting future shoreline locations and assessing prospects for shore
recovery after a storm.
K.S. DORAN ET AL/USGS 2009
The degree of earthmoving these
studies are finding is impressive. A
major storm can essentially pick up
and relocate entire barrier islands, such
as those that help protect Atlantic and
Gulf coasts. Yet the analytical models
that coastal planners use to estimate
where coasts will lie in the future rely
mainly on gradual, persistent rates of
erosion in the past, not the effects of
occasional large storms. Those transient effects have, until recently, been
hard to estimate, since any given location is rarely hit by a big storm.
The churning of sediments beneath a
storm has always been hard to see. But
by using shipboard sonar to map the
underwater landscape, scientists got an
in-depth view of massive movements of
seafloor material by Hurricane Ike.
Beneath a storm’s winds, rushing
water can reshape the underwater landscape, especially when a storm surge
blasts onto land and then recedes across
barrier islands or through narrow inlets.
Besides dramatically rearranging features in navigated areas such as shipping
lanes, storms leave clues in the sediments
that can help scientists interpret ancient
sediments long since turned to stone.
When Ike’s eye reached the eastern
end of Galveston Island, just southwest
of Bolivar Peninsula, hurricane-force
winds — those measuring 119 km/hr or
greater — covered a 180-kilometer-long
stretch of coast. West of the eye, winds
blew away from land and water was
pushed offshore. But east of the eye,
water surged inland across the Bolivar
Peninsula and piled up in Galveston Bay
to heights up to 5 meters.
As the storm passed and winds abated,
that water returned to sea, says John A.
Goff, a marine geologist at the University of Texas at Austin. As water levels
in the bay dropped, flow was channeled
through the inlet separating Bolivar
Peninsula and Galveston Island. Buoys
marking the shipping channel through
the inlet, the main route leading to and
from the port of Houston, were swept
seaward as much as 13 kilometers — quite
a feat considering the buoys’ anchors
weighed more than 5 metric tons each.
By comparing sonar data gathered
in the inlet a week and a half after Ike
made landfall with data gathered just
four months earlier, Goff and his colleagues could readily assess the storm’s
erosive effects. Before the hurricane,
several broad, 3-meter-tall ridges of
shells and loose gravel, sculpted by the
normal flow of tides in and out of the
inlet, stretched across the inlet’s floor,
the researchers reported in the April
Geology. Those ridges had been stable
for decades, Goff says.
Flow rushing through the inlet after
the storm pushed the ridges seaward 40
to 50 meters and scoured away as much
as two-thirds of the ridges’ height. Data
gathered in May 2010, more than a year
and a half after the hurricane, show that
the shells and gravel are again being
pushed into heaps, but those nascent
ridges are accumulating where the post-Ike remnants were dropped, not where
the ridges stood before the storm.
Finer material, such as sand and silt,
was probably carried even farther out to
sea, Goff notes. By analyzing sediment
samples taken from the seafloor at varying distances from the Bolivar Peninsula,
the researchers were able to estimate the
amount of material stripped from shore.
The team estimates that fresh deposits of sand and mud stretched up to
15 kilometers offshore. Goff says
that the piles include about 300,000
cubic meters—or about 37,500 large
dump truck loads — of sand from each
kilometer of coastline. And because
previous studies of the seafloor in
this area have suggested that material at water depths greater than
5 meters typically stays put under
normal conditions, much of that sand
probably won’t be coming back to shore.
But scientists are still learning how
to predict shoreline recovery. John B.
Anderson, a marine geologist at Rice
Gulf of Mexico
On Texas’ Bolivar Peninsula, Hurricane
Ike eroded (red areas) or deposited (blue)
a meter or more of sediment.