Sand and mud deposited by Hurricane Katrina on Deer Island, Biloxi Bay, Mississippi

....................................................................................................................... 2 AUTHOR’S STATEMENT................................................................................................ 3 INTRODUCTION .............................................................................................................. 3 DEER ISLAND .................................................................................................................. 4 METHODS ......................................................................................................................... 5 STORM WATER LEVELS................................................................................................ 6 DEER ISLAND OVERWASH DEPOSITS ....................................................................... 6 Gulfside deposits............................................................................................................. 7 Bayside deposits.............................................................................................................. 7 Flow direction indicators ................................................................................................ 8 INTERPRETATION........................................................................................................... 8 Katrina vs. Rita ............................................................................................................... 8 Overwash structure ......................................................................................................... 8 Early deposition .............................................................................................................. 9 Peak storm deposition ..................................................................................................... 9 Late-stage deposition .................................................................................................... 10 Grain size trends ........................................................................................................... 11 DISCUSSION................................................................................................................... 12 Implications for distinguishing tsunami and storm deposits......................................... 12 Geologic record of previous storms on Deer Island ..................................................... 13 CONCLUSIONS............................................................................................................... 13 ACKNOWLEDGMENTS ................................................................................................ 14 REFERENCES ................................................................................................................. 14 ABSTRACT Hurricane Katrina overwash berms on both sides of Deer Island, Mississippi, include sub-horizontally layered, landward dipping cross-bedded, and visually structureless units. Thickness of Katrina sand varies along two parallel profiles from 30-90 cm landward of Gulf of Mexico beach dunes, 0-10 cm for 90-140 m across the island, and 30-60 cm on the Biloxi Bay back beach. In Gulfside deposits, vertical samples of sub-horizontally layered and visually structureless lower units vary more in skewness than mean grain size, whereas cross-dipping and structureless upper units vary more in grain size than in skewness. This shift in grain size properties probably represents the addition of a new sediment source that occurred with changing wind directions at landfall. In Bayside deposits, sub-units vary widely in both skewness and mean grain size. In the thickest Gulfside Katrina deposit, a normally graded mud layer marks a prominent structural transition between underlying sub-horizontal layering and overlying steeply dipping cross-beds. Unlike the bedload deposition of the sandy units, the mud layer settled from suspension during low bottom velocity conditions that occurred while the island was still overtopped by storm surge. Wave heights near Deer Island reached their maxima while storm surge was still low, 10 hours before Katrina’s landfall. At peakHurricane Katrina overwash berms on both sides of Deer Island, Mississippi, include sub-horizontally layered, landward dipping cross-bedded, and visually structureless units. Thickness of Katrina sand varies along two parallel profiles from 30-90 cm landward of Gulf of Mexico beach dunes, 0-10 cm for 90-140 m across the island, and 30-60 cm on the Biloxi Bay back beach. In Gulfside deposits, vertical samples of sub-horizontally layered and visually structureless lower units vary more in skewness than mean grain size, whereas cross-dipping and structureless upper units vary more in grain size than in skewness. This shift in grain size properties probably represents the addition of a new sediment source that occurred with changing wind directions at landfall. In Bayside deposits, sub-units vary widely in both skewness and mean grain size. In the thickest Gulfside Katrina deposit, a normally graded mud layer marks a prominent structural transition between underlying sub-horizontal layering and overlying steeply dipping cross-beds. Unlike the bedload deposition of the sandy units, the mud layer settled from suspension during low bottom velocity conditions that occurred while the island was still overtopped by storm surge. Wave heights near Deer Island reached their maxima while storm surge was still low, 10 hours before Katrina’s landfall. At peak