An analysis of land-change data from the U.S. Geological Survey (USGS) found that 217 square miles of Louisiana’s coastal lands were transformed to water after hurricanes Katrina and Rita swept across the coastline in August and September 2005, the USGS announced. The agency now will use satellite imagery and aerial photography to determine how much of this transformation may be permanent.

In November 2005, the USGS’s National Wetlands Research Center reported that about 100 square miles of land were transformed to water in southeastern Louisiana. The new analysis includes additional data and gives for the first time an estimate of 98 square miles of land changed to water in southwestern Louisiana, and updates the land transformed to water for southeastern Louisiana from 100 to 119 square miles.

Land transformed to water along the coast and on barrier islands further reduces Louisiana’s natural protection from future storms, the USGS said. Louisiana already had lost 1,900 square miles of coastal lands, primarily marshes, from 1932 to 2000. The 217 square miles of potential land loss from the 2005 hurricanes represent 42 percent of what scientists had predicted would take place over a 50-year period from 2000 to 2050.

The National Wetlands Research Center calculates land changes by comparing geographic information-system data bases that include vegetation cover to satellite images obtained from the USGS Center for Earth Resources Observation and Science (EROS) in Sioux Falls, S.D. EROS provided images of coastal Louisiana taken between Oct. 16 and Oct. 25, 2005 by the Landsat Thematic Mapper satellite. These images were compared to some collected between Oct. 13 and Nov. 7, 2004. Further analyses were made by comparing the 2004 imagery to that of 2001 to provide an estimate of normal variations in seasonal land- and water-area changes before the 2005 hurricanes.

The new study was done to provide preliminary information on land-to-water area changes in coastal Louisiana that were present shortly after Katrina and Rita. The study also will serve as a regional baseline for monitoring wetland recovery.

Some of the land-to-water transformations are permanent and were caused by direct removal of land by storm surge, the USGS said. Others may be transitory or temporary, including flooded and impounded areas. Only an analysis of land after future growing seasons will determine how permanent the changes are. Temporary land change could be caused by remnant flooding of marshes; removal of floating and submerged aquatic plants; scouring of marsh vegetation; or even water-level variations caused by normal tide and meteorological variation between the satellite images that the analysis was based upon.

There were even some new land gains created by winds depositing wrack; marsh moved by the storm surge; aquatic vegetation that was possibly misidentified or classified; or water-level variations caused by normal tidal and meteorological variations. Those gains also could be temporary, the USGS said.