Science News - August 28, 2010

University in Houston, isn’t so sure all that sand from Galveston Bay is lost. Because the researchers didn’t determine the size distribution of grains in the sand layer, it’s impossible to tell what proportion of those grains might be small enough to be bumped back to shore by normal wave action, even from waters up to 10 meters deep, he adds.

After looking at historical data, Goff and his colleagues contend that the sand is indeed gone for good. An extensive layer of sand dragged offshore by Hurricane Carla, which struck the Texas coast in 1961, stayed where the storm dropped it for at least a quarter of a century, they note.

Sometimes those layers of storm-shifted sand are buried by quickly accumulating sediment and thus preserved, leaving in the geological record a layer known as a “tempestite,” Goff says. Studying and understanding the modern-day processes that generate such layers can aid researchers trying to interpret the geological record. The presence of a tempestite would indicate that an area was susceptible to major storms, for example, and the size and frequency of such layers within rock formations could yield clues about ancient climates.

Images taken four days before Hurricane Ike struck (left) and two days after
show erosion along the coast near Galveston, Texas. Arrows denote the same spot.

Diving deeper

Because hurricanes strike any given stretch of coast rather infrequently, data on the storms’ effects are hard to come by. On the broader scale, satellite observations give scientists a look at the speed and direction of currents that hurricanes generate at the ocean’s surface, says oceanographer William J. Teague of the Naval Research Laboratory at NASA’s Stennis Space Center in Mississippi. “But once you get below the surface, there’s a void of data,” he notes.

While buoys far off the Texas coast were blown or dragged southwest as far as 35 kilometers during Ike, researchers don’t know how strong the storm-induced currents were, when the buoys

Hidden destruction ike’s winds forced a 5-meter storm surge into galveston bay. much of that water rushed back to sea through the bolivar roads inlet (below, arrow). the flow’s force took the tops off shell-and-gravel ridges and dumped new deposits on the inlet floor (sonar images on right show area in red box on map).

galveston bay bolivar peninsula

moved or what path they took — it’s clear only where the buoys ended up. But new analyses of data collected by seafloor instruments during another hurricane, 2004’s Ivan, could let researchers make some reasonable guesses about water’s scouring power beneath the surface.

Just before Ivan slammed into the Gulf Coast, it passed directly over an array of seafloor sensors deployed in mid-depth waters on the continental shelf south of Mobile, Ala. At that time, Ivan was a category 4 hurricane creeping northward at about 20 km/hr, Teague says. Previously, his team’s analysis of data gathered during the event revealed the largest waves ever measured by instruments (SN: 6/11/05, p. 382). Now,

houstonship
channel
south jetty
south jetty
top: usgs; bottom left: map: usgs, adapted by t. dubÉ; map inset: atlas/map resources; bottom right: j. goff et al/geology 2010