Title page for ETD etd-01222009-042457

Type of Document Dissertation
Author Kobashi, Daijiro
URN etd-01222009-042457
Title Bottom Boundary Layer Physics and Sediment Transport along a Transgressive Sand Body, Ship Shoal, South-Central Louisiana: Implications for Fluvial Sediments and Winter Storms
Degree Doctor of Philosophy (Ph.D.)
Department Oceanography & Coastal Sciences
Advisory Committee
Advisor Name Title
Gregory W. Stone Committee Chair
Chunyan Li Committee Member
Felix Jose Committee Member
Irving Mendelssohn Committee Member
Masamichi Inoue Committee Member
S.A. Hsu Committee Member
Michael A. Dunn Dean's Representative
  • Bottom boundary layer dynamics
  • Sediment heterogeneity
  • Fluid mud
  • Barrier island restoration
  • Sand mining
  • Atchafalaya River
Date of Defense 2008-12-16
Availability unrestricted
Ship Shoal, a shore-parallel sand body, was recently recognized as having a unique physical and biological environment and also as a potential sand resource for coastal restoration in coastal Louisiana. Little is known regarding such dynamics, in concert with fluvial sediments and winter storms, influenced in unique ecosystems, and likely in future potential sand mining. This dissertation addresses such the morphodynamics and sedimentary processes and their implications for the mining from the shoal using field measurements and numerical modeling studies.

During the winter-spring season, fluvial sediment plumes shifted from the prevailing west to southeast during the post-frontal phases, resulted in accumulation of fluid mud on the eastern flank of the shoal and consequent shoal sediment heterogeneity during the spring of 2006; this fluid mud layer strongly interacted with storm waves and currents through the processes of sediment re-suspension, vertical mixing, and hindered settling and redistribution. Studies during winter 2008 represented dynamics dominated by non-cohesive bottom material and hence followed the conventional approaches.

State-of-the-art numerical models for waves, currents and transport provided reasonably well estimation for the study area and showed changes in wave transformation, current variability, and sediment transport for various hypothetical post-dredging scenarios. Sediment re-suspension intensity showed spatial differences along the shoal: high on the western flank of the shoal and a decrease toward the eastern shoal due to the change in shoal bathymetry. The results indicated a favor for the fluid mud accumulation on the eastern flank of the shoal, corroborated by in-situ measurements.

Data suggest that Ship Shoal appears to have recurring sandy and muddy bottoms depending on the amount of storm-induced sediment reworking and fluvially-derived sediments. The fluid mud on the shoal seems to be patchy and does not remain in place as permanently consolidated mud, given the frequency of winter storms and the dispersal shifts. Numerical simulations suggest that targeted small-scale mining would not significantly alter the hydrodynamics and sediment transport over the shoal. Dredging from the eastern flank of the shoal may give rise to lesser impacts than that from the middle and western flank of the shoal. This suggestion is consistent with that from our collaborative biological study.

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