SEDIMENTATION FROM 2005 HURRICANE KATRINA ON THE MISSISSIPPI AND ALABAMA GULF COAST BARRIER ISLANDS

Vertical sequences of sedimentary structures in sediments deposited by Hurricane Katrina in the northern Gulf Islands record the intensification of a single, landward directed, surge of water with time. This suggests that these sediments record temporal acceleration of the flow, rather than waning flow, and further suggest that these sediments can be used as recorders of the maximum flow velocity during the storm.

We studied Katrina sedimentation along shore-normal transects ranging from Cat Island (to the east, and closest to the storm track) to Dauphin Island (to the west) along Gulf Islands National Seashore. Along these transects, which cut through washover fans generated by Katrina, approximate sediment thickness ranges from 20 cm to 80 cm. The deposits are composed of moderately well to well sorted medium to coarse sand, mostly subrounded quartz grains, with local concentrations of heavy dark minerals. These minerals made sedimentary structures visible—at each island, we noted that deposition began with climbing ripples, progressed to 2D dunes, and often ended with a transition to 3D dunes or plane lamination.

The storm stripped bark from trees and left debris to a height of at least 8m above the base of East Ship Island, and 4m above base on Horn Island. Additionally, small trees and grasses were bent over by the storm, all directed landward, rather than in the direction of wind flow. The storm eroded both the east and west tips of all the barrier islands, rather than the typical hurricane pattern of preferential erosion at the eastern end.

The combination of grain size data, flow depth data, and the vertical sequence of sedimentary structures will enable detailed estimates of flow velocity for each area with time. Initial estimates of flow velocity suggest that, unlike wind velocity, maximum water velocities do not simply decrease with distance away from the storm center, but may vary with factors such as barrier height and overall barrier width.