Impact of Hurricane Irma on Coral Reef Sediment Redistribution at Looe Key Reef, Florida, USA

Abstract. Understanding event-driven sediment transport in coral reef environments is essential to assessing impacts to reef species, habitats, restoration, and mitigation, yet there remains a global knowledge gap due to limited quantitative studies. Hurricane Irma made landfall in the Lower Florida Keys with sustained 209 km h^-1 winds and greater than 8 m waves on 10 September 2017, directly impacting the Florida Reef Tract (FRT), and providing an opportunity to perform a unique comprehensive, quantitative assessment of its impact on coral reef structure and sediment redistribution. We used lidar and multibeam derived digital elevation models (DEMs) collected before and after the passing of Hurricane Irma over a 15.98 km² area along the Lower FRT including Looe Key Reef to quantify changes in seafloor elevation, volume, and structure due to storm impacts. Elevation change was calculated at over 4-million point-locations across 10 habitat types within this study area for two time periods using data collected from 1) approximately one year before the passing of Irma and three to six months following the storm’s impact, and 2) from three to six months after, and up to 16.5 months after, the storm. Elevation-change data were then used to generate Triangulated Irregular Network (TIN) models in ArcMap to calculate changes in seafloor volume during each time-period. Our results indicate that Hurricane Irma was primarily a depositional event that increased mean seafloor elevation and volume at this study site by 0.34 m and up to 5.4 Mm³, respectively. Sediment was transported primarily west-southwest (WSW) and downslope modifying geomorphic seafloor features including the migration of sand waves and rubble fields, formation of scour marks in shallow seagrass habitat, and burial of seagrass and coral-dominated habitat. Approximately 16.5 months after Hurricane Irma (during a 13-month period between 2017 and 2019), net erosion was observed across all habitats with mean elevation-change of -0.15 m and net volume change up to -2.46 Mm³. Rates of elevation change during this post-storm period were one to two orders of magnitude greater than decadal and multi-decadal rates of change in the same location, and changes showed erosion of approximately 50 % of sediment deposited during the storm event as seafloor sediment distribution began to re-equilibrate to non-storm sea state conditions. Our results suggest higher resolution elevation-change data collected over seasonal and annual time periods could enhance characterization and understanding of short-term and long-term rates and processes of seafloor change and help guide post-storm recovery and restoration of benthic habitats in topographically complex coral reef systems.