The main findings from this study were that there has been an extensive increase in relatively deep-water coverage (and corresponding losses of land coverage) since 2019 over most of the marshlands and shorelines of the Barataria and Breton Sound Basins in southeastern Louisiana. Deep scouring and widespread erosion of brackish and fresh marshlands (Morton and Barros, 2011) was detected after Hurricane Ida in 2021 NAIP imagery. In the Barataria Basin, the combined impacts of the DWH Oil Spill in 2010 (McClenachan and Turner, 2023), Hurricane Zeta in 2020, and Hurricane Ida in 2021, whose eye traveled just west of Bayou Lafourche on the western margins of the Basin, eroded the estuary into progressively larger patches of deeper water cover. Longer shorelines and marshland edge boundaries were developed, particularly in brackish marsh cover areas. In the Breton Sound Basin, the storm surge of Hurricane Zeta in 2020 resulted in widespread drowning of both freshwater and brackish marsh vegetation cover and generated more compact, less irregular patch shapes of both wetland and water cover. Many square kilometers of sparsely vegetated marshland patches dominated by Spartina sp. that were mapped in 2019 were replaced by open water cover following the storm surge of Hurricane Ida in August, 2021.
Spatial variability in the patch metrics for area coverage and irregularity of shapes increased in deeper-water areas mapped by NAIP imagery between 2019 and 2021 in the Barataria and Breton Sound Basin study areas. Variability in the patch attributes of sparsely wetland covered areas increased markedly between 2021 (post-Ida) and 2023, suggesting that there is presently a more fragmented landscape of regrowing marshland patches dominated by Spartina sp. than was present in 2019.
Hurricanes have been documented to damage marsh plants and reduce wetland biomass production that is essential for organic accretion in coastal zones (Morton and Barras, 2011). Tropical storm surges may decrease marsh elevation by removing substrates near the shoreline and channel edges through scouring and erosion (Barras, 2007), or conversely, intense storms can increase marsh elevation by moving sediment onto tidal flats (Turner et al., 2006; Tweel and Turner, 2014).
In the survey report by Sharp (2021), photographic evidence revealed uprooting and nearly complete drowning of marsh plant cover at damaged Barataria Basin CRMS stations following Hurricane Ida. Some sites in Breton Sound appeared to be “salt burned”, meaning that saline water introduced into freshwater marsh zones from the storm surge killed much of the vegetation cover. Sations in the Mississippi River birds-foot Delta facing Chandaleur Sound commonly saw washed-up wrack (dead reeds and leaf debris mixed with trash) and sediment deposits. This photographic evidence of wetland loss was consistent with the mapping of NDWI class dynamics from NAIP imagery collected in 2019 (pre-Ida) and 2021 (post-Ida).
As an overview of wetland disturbance processes, interior marsh pond formation and enlargement, either elongated or irregular in shape, can result from erosion on pond perimeters, flooding of wetlands within a pond, and coalescing of adjacent ponds (Nyman et al., 1994). Expansion of open water area is often not uniform around an interior pond perimeter, and wetland loss often occurs on a margin that is impacted most by the direction of the wind-driven waves. Coastal Louisiana is a micro-tidal system and the waves interacting with the marsh edges are generated locally within a shallow bay with minimal river current influence (Sapkota and White, 2019).
In storm-impacted coastal areas, “plucked” marshlands (Morton and Barros, 2011) show numerous small, closely spaced, circular or irregular scours or ponds that are distributed over a wide area, resulting in a pockmarked appearance (Mariotti, 2016). Laterally displaced marsh mats and marsh balls can be formed from deep scouring and erosion of brackish and fresh marshlands that commonly form elongated strips or disaggregated clumps of organic matter (Morton and Barros, 2011). High-resolution (< 1 m pixel size) aerial imaging such as that from 2021 NAIP data readily resolved and identified such clumps and mats of organic debris in plucked marshlands at CRMS station numbers 4103 and 387.
Marsh collapse and drowning can occur if ponds on the wetlands zone enter a runaway expansion phase (Nyman et al., 1994). Runaway expansion may occur when ponds are greater than a critical width, because edge erosion by waves increases the fetch (pond width), which can feed back to elevate the wave size and further increase marsh edge erosion (Mariotti, 2016; Ortiz et al, 2017). However, Mariotti (2016) reported that connected ponds in Terrebonne Bay (just west of the Barataria Basin) were expanding even if their diameter was only 10–50 m, suggesting that a threshold width for runaway wave erosion is not well understood. Mariotti (2016) went on to predict that connected ponds of any size would expand if critical inorganic deposition rates are smaller than the rate of sea level rise, a condition common in marshlands of the Mississippi Delta region. High-resolution aerial imaging should readily identify and measure the changes in the areas of small marshland ponds, down to a few square meters coverage.
Marsh edge erosion studies by Sapkota and White (2019) in the Barataria Basin, near the sites measured by Deis et al. (2019) as well, implied that variability in erosion rates, measured at between 0.5 and 3.5 m yr− 1, was attributed to wind duration and soil bulk density and organic matter content. Marsh edges adjacent to the open bay experienced greater erosion rates than the protected sites located adjacent to the shallow water bodies in the northern part of the basin. This difference was attributed to the long fetch of unprotected sites, in contrast to shallow bathymetry in more protected sites and small fetches that produce waves with relatively low power. These edge erosion rates from 2017–2018 reported by Sapkota and White (2019) were somewhat lower than the range of (DWH oiled) shoreline erosion rates (at between 2 and 5 m yr− 1) measured by Deis et al. (2019) following Hurricane Katrina and the DWH oil spill and mapped at 1-m resolution.
To summarize, aerial remote sensing of hurricane impacts on coastal wetlands can be used to support decision-making processes, such as prioritizing restoration efforts, monitoring the effectiveness of management actions, and assessing the long-term resilience of coastal wetlands to climate change. When individual marshland plant species can be identified and mapped every year over the Mississippi Delta plain for changes in abundance, better informed decisions can be formulated about how to remediate areas that are heavily disturbed by intensive hurricane storm surges.