Seafloor Change Around the Mississippi Barrier Islands, 1920 to 2016—The Influence of Storm Effects on Inlet and Island Morphodynamics
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Seafloor Change Around the Mississippi Barrier Islands, 1920 to 2016—The Influence of Storm Effects on Inlet and Island Morphodynamics By James G. Flocks, Noreen A. Buster, and Owen T. Brenner Open-File Report 2019–1140 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DAVID BERNHARDT, Secretary U.S. Geological Survey James F. Reilly, Director U.S. Geological Survey, Reston, Virginia: 2020 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov/ or call 1–888–ASK–USGS (1–888–275–8747). For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Flocks, J.G., Buster, N.A., and Brenner, O.T., 2020, Seafloor change around the Mississippi barrier is- lands, 1920 to 2016—The influence of storm effects on inlet and island morphodynamics: U.S. Geological Survey Open-File Report 2019–1140, 23 p., https://doi.org/10.3133/ofr20191140. ISSN 2331-1258 (online) Acknowledgments This study was funded in part by the U.S. Army Corps of Engineers Mississippi Coastal Improvement Project and builds upon the recent U.S Geological Survey reports of DeWitt and others (2018) and Buster and others (2017). The authors would like to thank Karen Morgan, David Thompson, and Stokely Klasovsky for their review of this report. iii iv Contents Abstract ...............................................................................................................................................................1 Introduction ..........................................................................................................................................................1 Setting .................................................................................................................................................................3 Storm History and Navigation Channel Maintenance ......................................................................................5 Methods ...............................................................................................................................................................7 Results and Discussion .......................................................................................................................................9 Long-term Change (1920–2016) ...................................................................................................................11 Recent Change (2009–2016) ........................................................................................................................13 Elevation Profiles ...........................................................................................................................................14 Temporal and Spatial Sedimentation Patterns ...............................................................................................18 Conclusion .........................................................................................................................................................20 References Cited ...............................................................................................................................................22 Figures 1. Maps showing regional locations, the Mississippi Barrier Islands, and the various features discussed in this report ............................................................................................................................................2 2. Satellite images showing the pre-storm and post-storm effects of Hurricane Katrina on the island morphology of West and East Ship islands ............................................................................................3 3. Digital elevation models generated from the 1920s, 2009, and 2016 bathymetric data .........................4 4. Trackline maps showing survey extent from 1916–1920; 2008–2009; and 2016 ..................................8 5. Maps showing elevation change determined by calculating the difference between DEMs for an ap- proximate 96-year period from 1920s to 2016; an approximate 7-year period from 2009 to 2016; and an approximate 89-year period from 1920 to 2009 ..............................................................................10 6. Stacked elevation profiles from the three time periods along the gulf and sound sides of estW Ship and East Ship islands, and through Camille Cut ..................................................................................15 7. Stacked elevation profiles from the three time periods longitudinally across Little Dog Keys and Dog Keys passes, Little Dog Keys Inlet, and Dog Keys Pass .....................................................................16 8. Stacked elevation profiles from the three time periods along Horn Island, longitudinally across Horn Island Pass, and onshore-offshore through the Horn Island Pass .......................................................17 9. Bar graph showing sediment volumetric change per unit area of each reference subsection for the long and short time periods; and rate of change for each reference subsection during the same time periods ..................................................................................................................................................19 Tables 1. Tropical storms passing within 185 kilometers of Ship Island since 1920 ..............................................6 2. Accretion and erosion volumes, net change, and rate of change for the reference subsections that represent key morphologic features .....................................................................................................12 v Conversion Factors International System of Units to U.S. customary units Multiply By To obtain Length centimeter (cm) 0.3937 inch (in.) meter (m) 3.281 foot (ft) millimeter (mm) 0.03937 inch (in.) kilometer (km) 0.6214 mile (mi) kilometer (km) 0.5400 mile, nautical (nmi) meter (m) 1.094 yard (yd) Datum Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88, GEOID12A), and converted to mean lower low water (MLLW) for analysis purposes. Units of all vertical measurements are in meters. Horizontal coordinate information is referenced in the geographic coordinates World Geodetic System of 1984 (WGS 84); however, data were projected into Universal Transverse Mercator (UTM) coordinate system for analysis purposes. Abbreviations DEM digital elevation models EAARL Experimental Advanced Airborne Research Lidar Geoid12B vertical datum NAVD88 GIS geographic information system GMT Generic Mapping Tools GOM Gulf of Mexico GUIS Gulf Islands National Seashore H-sheets hydrographic sheets ka kilo annum, thousand calendar years ago kts knots m3 cubic meters mb millibars MHW mean high water MLLW mean lower low water mm millimeters MS Mississippi Sound MsCIP Mississippi Coastal Improvement Project NAD83 North American 1983 datum NAVD88 North American Vertical Datum of 1988 nmi nautical mile NOAA National Oceanic and Atmospheric Administration NPS National Park Service SPCMSC St. Petersburg Coastal and Marine Science Center T transects USACE U.S. Army Corps of Engineers USGS U.S. Geological Survey UTM Universal Transverse Mercator v.e. vertical exaggeration WGS84 World Geodetic System of 1984 WHCMSC Woods Hole Coastal and Marine Science Center yr year vi Seafloor Change Around the Mississippi Barrier Islands, 1920 to 2016—The Influence of Storm Effects on Inlet and Island Morphodynamics By James G. Flocks, Noreen A. Buster, and Owen T. Brenner Abstract The Mississippi Barrier Islands in the northern Gulf of Mexico experienced high rates of spatial change over recorded history. Wave-induced sediment transport induced island migration, landward retreat, and inlet evolution. These processes can be measured using repeat bathymetric surveys to analyze elevation change over time. This study analyzes digital elevation models created from three time periods where bathymetric data have been collected: the 1920s, 2009, and 2016. The models are compared to assess volumetric change between surveys and characterize morpho- logic responses to natural and human-influenced processes. Although all the islands within the study area experienced a loss of area over the period of study, the nearshore and tidal inlets experience both accretion and erosion that vary spatially and temporally. Major morphologic changes include westward island migration, expanding ebb-tidal deltas, and changes in inlet dimensions. This study is a collaboration between the U.S. Geological Survey, the U.S. Army Corps of Engineers, and the National Park Service to establish baseline physical and pre-restoration morphologic conditions preceding a major restoration of the islands as part of the Mississippi Coastal Improvement Project. Introduction The barrier island platform located offshore of the State of Mississippi in the Gulf of Mexico supports five islands: Cat, West Ship, East Ship, Horn, and Petite Bois (fig. 1), although the island system has a history