Coastal Wetland Disturbance and Recovery From Hurricanes in Coastal Louisiana

With climate change and sea level rise, coastal wetlands will experience increased flooding, saltwater intrusion, edge erosion, and frequency and intensity of tropical cyclones. In coastal Louisiana, wetlands are particularly vulnerable due to decreasing riverine sediment supply and increasing subsidence, which control the resilience and sustainability of marshes. The Atchafalaya (AB) and Terrebonne (TB) basins represent distinct coastal landscapes at the opposite ends of the delta cycle. The AB maintains a strong river connection, which supplies sediment to wetlands, leading to aggradation and progradation. Conversely, the TB basin is disconnected from the river and sediment starved, leading to land loss. These two systems are the experimental regions of Delta-X, a 5-year NASA Earth Venture Suborbital program that studies the interactions between vegetation, hydrology, and geomorphology in Louisiana's coastal wetlands.

As part of this program, we used optical (Sentinel-2) and C-Band radar (Sentinel-1) satellites to classify the AB and TB into vegetation functional groups (e.g., forest, freshwater herbaceous marsh, salt marsh) that are controlled by salinity gradients, flooding frequency and duration, and sediment delivery. Our land cover classification focuses on plant phenology by incorporating winter, spring and fall images to capture plant seasonalities that differ in terms of emergence, die-off, and peak biomass among species. With a 5-6 day revisit time and moderate spatial resolution (~10m), Sentinel-1 & 2 data allowed us to assess seasonal changes in wetland vegetation and hurricane impacts from 2017-2021. For example, after Hurricane Barry, we detected 60-80% decrease in Normalized Difference Vegetation Index (NDVI) within the subtidal wetlands of the AB, but less than 20% decrease in NDVI in the TB. However, NDVI in the AB increased within the same growing season, suggesting quick re-greening and little long-term impact. With very active hurricane seasons in 2020 and 2021, we also compared spatial impacts and the potential for recovery from short-term chemical disturbances (such as salt-burn) and long-term physical disturbances (such as land erosion). These results highlight the power of optical and radar satellite remote sensing for monitoring wetland vegetation dynamics.