SEDIMENTATION FROM THE 29 AUGUST 2005 LANDFALL OF HURRICANE KATRINA ALONG THE GULF COAST

Hurricane Katrina flooded coastal Louisiana, Mississippi, and Alabama to a depth of over 5 m, deposited a discontinuous sheet of sand up to 80 cm thick, and flooded inland locally more than 5 km. Where observed the sand sheet contains complex internal stratigraphy that records an initial rapid sedimentation event followed by deposition of an inland-advancing dune.

Most of the deposition along the Mississippi and Alabama coasts occurs within 200-300 m of the shore. Sand sheets are locally up to 80 cm thick, but thin rapidly landward, and appear to be halted by even low topographic relief. We did not notice the sand selectively infilling low spots. The deposits are composed of moderately well to well sorted medium to coarse sand, mostly subrounded quartz grains, with local concentrations of magnetite. The sand often has an erosive base, and generally fines upward and landward, although the limited range of grain sizes available on the beach makes this trend difficult to discern.

At East Ship Island (30.224°N, 88.907°W), we measured an 80 cm thick section of storm deposit that records the changing hydraulic conditions during passage of the storm. The base of the deposit is erosive, and is marked by a strong organic layer. Above this marker the sand is plane laminated or massive for ~5 cm, followed by 15 cm of climbing ripples. The remaining 60 cm are dominated by a single planar cross-bedded unit with preserved ridge crests.

The East Ship section appears to have been deposited rapidly, and was dominated by bedload, except at the beginning of deposition. We envision the waves to have initially eroded the beach, and only during the waning phase of the storm did velocities again drop to the point that deposition could occur. The initial depositional unit reflects deposition from suspension, followed by climbing ripples as the flow became unable to suspend medium sand. Finally, a low dune, still at high sedimentation rates, progressed over the area, but was unable to travel more than about 200 m inland before deposition ceased.