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Hurricanes are one of the most devastating earth surface processes. In 2020 and 2021, Hurricanes Zeta and Ida pounded the Mississippi River Delta in two consecutive years, devastated South Louisiana, and raised tremendous concerns for scientists and stakeholders around the world. This study presents a high-resolution spatial-temporal analysis incorporating planialtimetric data acquired via LIDAR, drone, and satellite to investigate the shoreline dynamics near Port Fourchon, Louisiana, the eye of Ida at landfall, before and after the beach nourishment project and recent hurricane landfalls. The remote sensing analysis shows that the volume of the ~2 km studied beachfront was reduced by 240,858 m3 after consecutive landfalls of Hurricanes Zeta and Ida in 2020 and 2021, while 82,915 m3 of overwash fans were transported to the backbarrier areas. Overall, the studied beach front lost almost 40% of its volume in 2019, while the average dune crest height was reduced by over 1 m and the shoreline retreated ~60 m after the two hurricane strikes. Our spatial-temporal dataset suggests that the Louisiana Coastal Protection and Restoration Authority's (CPRA's) beach nourishment effort successfully stabilized the beach barrier at Port Fourchon during the hurricane-quiescent years but was not adequate to protect the shoreline at the Mississippi River Delta from intense hurricane landfalls. Our study supports the conclusion that, in the absence of further human intervention, Bay Champagne will likely disappear completely into the Gulf of Mexico within the next 40 years. [ABSTRACT FROM AUTHOR]

Relative sea-level (RSL) rise associated with decreased fluvial sediment discharge and increased hurricane activity have contributed to the high rate of shoreline retreat and threatened coastal ecosystems in Port Fourchon, Louisiana, USA. This study, based on QuickBird/drone images (2004–2019) and LIDAR data (1998–2013), analyzed the impacts of shoreline dynamics on mangroves (Avicennia germinans) and marshes before and after the initiation of a beach nourishment project in 2013. The coastal barrier and dune crest migrated landward between 1998 and 2013. Meanwhile, the dune crest height increased between 1998 and 2001, then decreased in 2013, probably due to hurricane impacts. The total sediment volume along this sandy coastal barrier presented an overall trend of decline in the 1998–2013 period, resulting in a wetlands loss of ~15.6 ha along 4 km of coastline. This has led to a landward sand migration onto muddy tidal flats occupied by Avicennia germinans (1.08 ha) and Spartina (14.52 ha). However, the beach nourishment project resulted in the advancement of the beach barrier from Nov/2012 to Jan/2015, followed by a relatively stable period between Jan/2015 and Mar/2019. Additionally, both the dune crest height and sediment volume increased between 2013 and 2019. This set of factors favored the establishment and expansion of mangroves (3.2 ha) and saltmarshes (25.4 ha) along the backbarrier environments after 2013, allowing the tidal flats to keep pace with the RSL rise. However, waves and currents caused shoreline erosion following the beach nourishment project between Oct/2017 and Nov/2019, threatening wetlands by resuming the long-term process of shoreline retreat. [ABSTRACT FROM AUTHOR]

Global warming has led to a tropicalization of ecosystems, especially in the northern hemisphere, where freeze-sensitive plants, such as Avicennia germinans , are expanding northward, exposing tropical species to greater annual climate extremes. The effects of this driver on the boreal limits of mangroves need to be investigated for projecting the fate of mangroves in the face of global warming. Based on QuickBird, aerial/drone images, and historic air/seawater temperatures, this work analyzed the 2017–2018 winter freeze impacts on mangroves of Bay Champagne, Louisiana, USA. This winter freeze, marked by 14 days of subfreezing daily minimum temperatures, degraded ~90% (111 ha) of the studied mangroves. This macroclimatic setting caused defoliation and dry branches on Avicennia trees. Such degradation occurred mainly in mangroves established after 2004, where trees of low heights (1–1.5 m) and density between 1000 and 10,000 trees/ha were present mainly on topographically lower tidal flats (13–26 cm above mean sea-level). By contrast, healthy Avicennia trees were typically taller (1.5–2.2 m), growing at intermediate-density (4000–8000 trees/ha) stands, and occupying higher grounds (20–46 cm) in the inner part of mangrove areas. Canopy height and tree density modulated the winter freeze damages on Avicennia trees, as these factors attenuated the wind impacts along a microclimatic gradient. The microtopography of tidal flats may have also influenced the Avicennia degradation. The recovery of mangroves was rapid (~1 year) and regulated by a microclimatic and microtopographic gradient. Winter freeze impacts on mangroves will probably be attenuated as the increase in the stature and density of mangrove trees. These processes could contribute to the continued northward expansion of mangroves. However, recurring winter freezes affect mangrove productivity that is a critical factor for mangroves to keep pace with relative sea-level rise (SLR). • Winter freeze impacts on mangroves of Louisiana were studied by satellite/drone images. • 2017/18 winter freeze degraded ~90% of mangroves mainly among the youngest Avicennia. • Damages on Avicennia were controlled by tree height, density and microtopography. • Microclimatic and microtopographic gradient regulated the mangrove recovery (~1 year). • These processes contribute to the continued northward expansion of mangroves. [ABSTRACT FROM AUTHOR]