USGS Scientists Monitor Coastal Damage from Hurricane Lili
Lafayette, LA-U.S. Geological Survey scientists are continuing to monitor the effects of Hurricane Lili, which slammed into the Louisiana coast Oct. 3, with winds in excess of 110 miles per hour.
Hydrologists from the USGS Baton Rouge Water District Office measured water levels in rivers and bays before and during the hurricane on a 24-7 basis and continue to keep an eye on water levels throughout the state. Geologists from the USGS Center for Coastal Geology, St. Petersburg, Fla., are using sophisticated technology to compare pre- and post-hurricane information on Louisiana barrier islands to determine erosion and accretion caused by Lili. Additionally, biologists from the USGS National Wetlands Research Center (NWRC) are flying the coast to assess coastal wetland damage.
According to Charles Demas, Chief of the USGS Water Resources Office in Louisiana, USGS stream gauges supported by the U.S. Army Corps of Engineers reported storm surges as much as three feet above normal at 40 miles upstream from Atchafalaya Bay. Gauges 20 miles upstream from the bay recorded water flowing upstream as fast as four feet per second. Other gauges along the coast registered storm surges up to five and a half feet and upstream flows of up to six and a half feet per second. For real time-time water data from the USGS in cooperation with the Louisiana Department of Wildlife and Fisheries, go to http://wwwdlabrg.er.usgs.gov/hydrowatch.htm.
USGS geologists used LIDAR, an airplane-borne scanning laser that measures elevations of the earth's surface, to survey central Louisiana barrier islands affected by the hurricane. They are comparing these surveys with ones acquired before the hurricane to measure change, according to Abby Sallenger. LIDAR stands for light detecting and ranging, similar to radar, which stands for radio detecting and ranging. This work is done cooperatively with the Louisiana Department of Natural Resources and the National Aeronautics and Space Administration.
Biologists and geographers are using aerial video to document substantial marsh and barrier island damage. USGS NWRC Director Bob Stewart said, "These wetlands are the state's first line of defense against winds and flooding from storm surge. By buffering winds and absorbing floodwaters, they protect people and property. They are also valuable as wildlife habitat and nursery areas for fish and shellfish supporting the seafood industry and related businesses."
Stewart added, "These wetlands are also needed for protecting coastal oil and gas infrastructure. Port Fourchon was hard hit during Lili. Almost 20 percent of U.S. oil and gas go through the port; and it's a critical link to the Louisiana Offshore Oil Port, which handles 13 percent of the nation's foreign oil and connects by pipeline to 35 percent of the U.S. refining capability."
Hurricane damage is especially of great concern to the state, Stewart said, because it has already lost an average of 25 to 35 square miles of land a year to open water over the past 50 years. In addition to hurricanes and storms, causes of land loss are natural subsidence and lack of marsh replenishment because of levees, he said.
Immediately after the hurricane, Stewart dispatched a rapid response team to survey storm damage. Veteran hurricane observers Tommy Michot, USGS wildlife research biologist, and Chris Wells, USGS geographer, have flown over the coast several times since Hurricane Lili and used aerial video and automated mapping technology to document damage. In addition, they had flown the coast just a week earlier to observe effects of Tropical Storm Isidore.
Coastal Wetlands, Erupting Gas Wells, Fish Kills, and Muskrat Eatouts
Michot and Wells reported that Hurricane Lili greatly affected barrier islands as well as fresh and intermediate-salinity marshes and some mainland tree and shrub damage. Salt marshes, they said, were largely unaffected.
In addition to finding barrier island and coastal marsh damage, Michot and Wells observed a gas well erupting 300 feet into the air near White Lake. They also saw a large fish kill, extending from Freshwater Bayou to Marsh Island, near where the eye of the storm had made landfall. For a stretch of 30 miles there were long bands of tens of thousands of tiny menhaden floating in nearshore waters or washed ashore. They died, apparently, from lack of oxygen in the water. Demas reported fish kills in the Atchafalaya Swamp.
At Marsh Island, Michot and Wells saw storm effects on marsh that was already stressed by muskrat "eatouts" (overgrazing). They had first spotted these sites last spring. Now, after Lili, they noted resuspended sediments in these areas, resulting in a "cafe au lait" colored water. They speculate that these areas may not recover because of decreased elevation.
East of hurricane landfall, from Point Au Fer Island to the Chandeleur Islands, the state's chain of barrier islands and barrier shorelines exhibited marked shoreline retreat. Many islands were severely eroded on their southern and eastern portions.
The barrier islands also had abundant deposits (overwash) of sand into the marsh, as much as 150 to 600 feet. Some overwash fans, which are subtidal sand picked up from the Gulf and deposited over the island, extended beyond the northern shores of the island. There also appeared to be subtidal build up (accretion) of sand on the western end of some islands. Overwash sediment buries existent vegetation, but it also raises elevation and provides substrate for new vegetation.
Raccoon Island, closest of the Isles Dernieres chain of barrier islands to the eyewall of the hurricane, suffered extensive beachside erosion. The vegetated part of the island, however, remained intact. Raccoon Island is a major nesting colony for brown pelicans, the state bird. Only in recent years has the brown pelican population begun to recover in Louisiana.
Grand Isle had much shoreline erosion. Areas that formerly supported wax myrtle have degraded to an intertidal zone where the freshwater shrub cannot live. Erosion along the beach varied from a few yards to tens of yards inland. In one area, Gulf waters destroyed the dune system and moved sand and mud across Louisiana Highway 1 into fishing camps.
Fresh, Intermediate-Salinity Marshes
Michot and Wells reported that the freshwater and intermediate-salinity marshes sustained great damage. Lili's damage to the marshes was from the combined effects of storm surge, winds and wave action.
This impact was reminiscent of that caused by the category four Hurricane Andrew in 1992 but was not as extensive.
Resulting damage varied. Some marshes (including their vegetation and the substrate it grows in) were torn. Some were folded like an accordion. Others were displaced with their vegetation actually lifted and moved to another area. Still others were inverted or flipped over. And many were covered with debris (wrack).
Damage was most apparent in the marshes east of Cote Blanche Island and south of the Gulf Intracoastal Waterway. There was an overall browning of vegetation, especially in the fresh and brackish marshes. This area, 50 miles southeast of Lafayette, is where the hurricane's northeast quadrant passed.
Nearshore and Mainland
Similar marsh damage was seen at the Louisiana Wildlife Management Area and Game Preserve, west of the hurricane landfall.
In general, near-shore and mainland shrubs and trees were subject to leaves being stripped (denuding), salt-spray poisoning, limb and trunk snap and uprooting. Unlike Hurricane Andrew, Hurricane Lili only minimally damaged the willows and other vegetation in the Atchafalya Basin. Less than five percent of the trees exhibited snaps and blow down from Lili.
The shoreline from Port Fourchon to Caminada Pass eroded severely. In many places the Gulf of Mexico waters and inland ponds are now continuous, allowing strong tidal currents into formerly placid inner lagoons and thus subjecting the lagoons to erosion. Along the Fourchon Beach, back marsh and bays are now open to the Gulf, which places Louisiana Highway 1 and the LOOP facility at greater risk.
The artificial barriers installed due south of Port Fourchon appeared to have inhibited beach erosion. The barriers are a restoration project under the Coastal Wetlands Planning, Protection and Restoration Act, also known as the Breaux Act.
The combination of breakwaters, fences and dune plantings in some coastal restoration areas along the coast seems to have lessened storm damage in some areas to varying degrees, Michot said. In contrast, rock breakwaters on the Gulf side at Raccoon Island may have actually increased erosion by causing scouring behind them.
Other Hurricane Research
The USGS NWRC studies storms and hurricanes worldwide by using aerial photography, satellite data and on-the ground studies. Examples of such research include extensive studies on the effects of the 1992 Hurricane Andrew, resulting in 19 published articles; the 1998 Hurricane Georges; and the 1998 category five Hurricane Mitch that hit Central America, especially Honduras.
To learn more about the effects of hurricanes, the USGS NWRC staff peers into the past. For example, they study aerial photography from Hurricane Camille in 1969. They can even learn about hurricanes from past centuries by analyzing tree rings from ancient trees.
In addition to hurricanes, the center monitors coastal land loss and studies issues related to nutria damage, subsidence, forestry, coastal plains, amphibians, invasive species and fire ecology.
For additional hurricane information, see www.nwrc.usgs.gov.
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Note to Editors: JPEG Graphic: marsh photo labeled with damage effects available at Web site: www.nwrc.usgs.gov.
Caption: Mainland marsh east of Cote Blanche Island demonstrating various kinds of marsh damage caused by Hurricane Lili.
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