Geography shifts in the wake of the storm
10 October 1998
From New Scientist Print Edition.
HURRICANE Georges has not only left behind a trail of misery, it may also force cartographers to redraw maps of the Mississippi Delta.
Until Georges roared through on 27 September, the Louisiana coast was protected by a low-lying 50-kilometre arc of sand called Chandeleur Island. But when Thomas Michot of the US Geological Survey in Lafayette flew over the area after the storm, he saw "about a hundred" islands, some separated by several kilometres of water.
If Chandeleur Island has been permanently breached, this may have lasting consequences for the region's ecology. The shallow waters surrounding Chandeleur contain one of the largest seagrass beds in the Gulf of Mexico, which supports a wealth of invertebrates. These are in turn eaten by fish and birds.
Louisiana's coastal defences may also suffer. The island breaks the force of storm waves before they reach threatened marshes along the Louisiana coast. It also protects New Orleans, which is vulnerable to flooding despite being 60 kilometres inland.
The damage is the worst since photographic surveys began in the 1930s, says Greg Stone of Louisiana State University in Baton Rouge. Although Georges was not as severe as Hurricane Camille in 1969, it moved much more slowly, so the island took more of a battering from waves and wind.
Geomorphologists consider Chandeleur one of America's most threatened coastal barrier islands and expected it to disappear within a few hundred years. The damage wrought by Georges will hasten its demise.
From issue 2155 of New Scientist magazine, 10 October 1998, page 16
http://www.newscientist.com/article/mg16021552.900.html
Study: Katrina, Rita may have KO’d islands
Scientists fail to find Chandeleur’s sandy start to rebuilding
By MIKE DUNNE -- Advocate staff writer
Published: Jul 31, 2006
Shea Penland, professor and chairman of the Department of Earth and Environmental Sciences at University of New Orleans, points at a map book that shows the erosion of barrier islands over time along the Louisiana coast.
Researchers mapping the Louisiana coast say they can’t find the sand and sediment that normally rebuilt the Chandeleur Islands east of New Orleans after past big storms.
Hurricanes like Camille in 1969 and Ivan in 2004 pushed sand to the rear of the islands in what is known as “overwash fans,” or splays of sand usually in the shape of an open folding fan. After each monster storm, the island chain slowly rebuilt itself on those fans, always a little smaller than before the storms.
Underwater mapping around the islands and in the Gulf of Mexico shows no such fans or any other sands and sediments that might help rebuild the islands, according to University of New Orleans geology professors Shea Penland and Mark Kulp. The mapping also shows evidence of landslides on the seafloor slopes on the Gulf side, which they say has not been seen before. The now-deeper water might increase the size of some of the waves hitting — and eroding — the remnants, they said. Another research vessel mapping another part of the chain apparently found the same landslide scars, Kulp said.
Dawn Lavoie of the U.S. Geological Survey, working with Penland and Kulp, even wonders if Katrina so severely damaged the islands one has to ask: “Is there a threshold (that has been reached) where the island won’t rebuild itself?”
Penland said the islands act as a “speed bump,” or barrier for storms approaching southeastern Louisiana and coastal Mississippi. Computer models designed to predict hurricane flooding show the islands help reduce the storm surge in the marshes protecting the New Orleans area, he said.
Penland is leading the Barrier Island Comprehensive Monitoring effort to map the Louisiana coast, barrier islands and offshore zones. It is being funded by the Louisiana Department of Natural Resources, U.S. Geological Survey, U.S. Army Corps of Engineers and UNO.
“This is the biggest assessment of the coastline” in more than 20 years, said Penland, who is also director of the geology department at UNO. “We are looking at every map that has ever been made since 1850” in trying to decide how Louisiana’s coastline and offshore waters and barrier islands have changed.
Kulp is overseeing the underwater mapping, which will provide new data to complete a 100-year history of seafloor change along the coast.
All of the data will eventually be available for the state, Corps of Engineers and other agencies that will be designing future coastal restoration projects.
Kulp’s underwater surveys found what appear to be seafloor landslides on the Gulf of Mexico side of the northern tip of the island chain. Another ship doing similar work along the central Chandeleurs found similar seafloor collapses, Kulp said. The two groups have not had a chance yet to share data, Kulp said.
The wind, waves and tides have always moved sand around the islands but they always found an equilibrium, Kulp said. But Katrina was “a major event that moved volumes of sediment” that “tampered with the equilibrium.”
Penland said a 1980s survey found the islands pretty stable “and we thought they would still be around in 300 years. Now, we are saying, maybe a decade.”
Historically, the fronts of the islands lost 20-30 feet of land each year, usually being rolled to the back of the island in those “overwash fans.”
Between 1996-2004, that loss grew to 300 feet per year following hurricanes like Georges, Lili and Ivan and Tropical Storm Isidore. Hurricanes Katrina and Rita practically blew the islands away, leaving only remnants.
“Katrina events tell us a lot,” Penland said. With forecasts for more powerful storms in the future, lessons from Katrina may become very valuable for decision-makers of the future, Penland said.
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