Science News - August 28, 2010

In recent years,
several hurricanes
have reshaped Gulf
of Mexico shores,
numbered here
to correspond
with images
shown.

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.