Detecting wetland loss or gain using Landsat imagery for the Gulf Coast of Louisiana

Extensive wetland areas south of New Orleans were severely impacted by both Hurricane Katrina in 2005 and Hurricane Gustav in 2008. Recent Landsat imagery was used to map wetland cover change in 5-year intervals (from 1985 to 2019) using the Normalized Difference Water Index (NDWI). Substantial recovery of these marshlands was detected by the NDWI difference method since 2009, despite decades of region-wide damage and land loss from gas and oil well drilling and canal construction prior to the 1980s. Areas of marshland gain were validated with 1-meter resolution aircraft imagery and by boat surveys in 2019. These continuing studies are analyzing the past 15-year record of Landsat satellite imagery to better understand the patterns and processes of net land loss in wetlands of Jefferson, Lafourche, and Terrebonne Parishes on the Gulf Coast of Louisiana. Widespread agreement across these three Parishes was discerned for patterns of progressive wetland loss (since 2005, post-Katrina) from two methods: (1) the United States Geologic Survey algorithms of Couvillion et al. (2017, USGS Investigations Map 3381) and (2) the Landsat NDWI annual difference method of Potter and Amer (2020, Journal of Coastal Research). However, several large areas of recent marshland creation that were detected using the Landsat NDWI difference method, and validated with extensive boat surveys in the summer of 2020, were not mapped as wetland gains in the USGS Map 3381. The implications of these findings will be discussed for accuracy in mapping of wetland area transitions in the largest and most dynamically changing coastal marshland region in the United States.