Land Area Changes in Coastal Louisiana after the 2005 Hurricanes: Marsh Communities

The U.S. Geological Survey (USGS) assessed changes in land and water area in coastal Louisiana within two months of Hurricanes Katrina and Rita (Aug. 29 and Sept. 24, 2005, respectively) by using Landsat Thematic Mapper (TM) satellite imagery. Land-water conditions before the storms were represented using imagery acquired in the fall of 2004. A series of Landsat TM scenes acquired in the fall of 2005 represented conditions after the storms. Comparison of the 2004 and 2005 imagery showed a total increase in water area of 777 km², but this measurement includes 215 km² of flooded lands consisting of flooded burned marsh and flooded agricultural and developed areas occurring after the hurricanes. Adjusting for these flooded lands, the estimated increase in water area (and decrease in land) is 562 km².

The 2004 and 2005 land-water data sets were combined with a data set containing 2001 marsh vegetation community types in order to identify new land-water changes within marsh communities. The greatest water area increases were in the fresh marsh community (316 km²) and the intermediate marsh community (233 km²). Lesser water area increases occurred in the brackish marsh community (85 km²) and the saline marsh community(73 km²). Remnant surge flooding caused some of the increased water area in the fresh and intermediate marsh communities. However, fresh and intermediate marsh communities located near to or east of the hurricane landfalls experienced detectable surge shearing at Landsat TM spatial resolutions. Brackish and saline marsh communities appear to be more resilient to shearing than fresh and intermediate marsh communities.

The purpose of this analysis was to provide preliminary information on land area changes shortly after Hurricanes Katrina and Rita. Estimation of permanent losses cannot be made until several growing seasons have passed and the transitory impacts of the hurricanes are minimized.