Analysis and Modeling of Hurricane Impacts on a Coastal Louisiana Lake Bottom

• Main Author: Freeman, Angelina
• Other Authors: Stone, Gregory W.
• Format: Others
• Language: en
• Published: LSU 2010
• Subjects: Oceanography & Coastal Sciences
• Online Access: http://etd.lsu.edu/docs/available/etd-01242010-185628/

Tropical cyclone impacts on wetland, terrestrial, and shelf systems have been previously studied and reasonably delineated, but little is known about the response of coastal lakes to storm events. For the first time, tropical cyclone impacts on a shallow coastal lake in the Louisiana coastal plain have been studied using direct lines of evidence and numerical modeling. Using side-scan sonar, CHIRP subbottom and echo sounder bathymetric profiles, the lake bottom and shallow subsurface of Sister Lake was imaged pre- and post-Hurricanes Katrina and Rita to provide a geologic framework for assessing the effects of these storms. Box cores were collected to provide site-specific ground truth data to further evaluate the accretion or erosion of sediment over the short storm period between synoptic geophysical surveys. Coupled hydrodynamic models MIKE 21 and MIKE 3 were used to hindcast Hurricane Rita conditions, and clarified sediment transport and deposition patterns in the geologically complex Sister Lake region. X-ray radiographs of box cores showed clear increments of recent event sedimentation (1 - 10 cm in thickness), corroborated with radionuclide dating as being products of the storm period. High percentages of approximately 40% fine sand in the storm layer and its thickness relative to an average long-term sedimentation rate of 2.0 mm/yr suggest that storm-related deposition is a large factor in Sister Lake sedimentation. Modeling results from Hurricane Rita forcing conditions hindcast maximum water elevations of approximately 1 m and wave heights of 1 m in Sister Lake. Bed shear stresses across almost the entire model domain prior to Hurricane Ritas landfall were above the critical value causing erosion of fine bottom sediments, and quickly decreased in the western portion during Ritas landfall, indicating significant deposition in this western portion of the lake. The ideal event sedimentation unit that would result from the storm conditions hindcast from the numerical model was corroborated with stratigraphy identified in box cores; units with an erosional base overlain by recently deposited silty material topped by clays. This study provides a framework and fundamental understanding of lake bottom characteristics and impacts of storm-related physical processes on erosion and deposition.