Hydrodynamics, Bottom Boundary Layer Processes and Sediment Transport on the South-Central Louisiana Inner Shelf: the Influence of Extratropical Storms and Bathymetric Modification.
Date of Award
Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
Gregory W. Stone
The south-central Louisiana inner shelf has several distinctive characteristics that are not well accounted for by common hydrodynamic and sediment transport models, including a high-frequency wave regime, low fair-weather hydrodynamic energy levels, a coastal orientation parallel to most storm approaches, and a prominent submerged sand body (Ship Shoal). To address these unique considerations, inner-shelf processes were studied using three instrumentation systems deployed on the seaward and landward sides of the shoal between November, 1998 and January, 1999. These instruments were designed to measure hydrodynamic characteristics, suspended sediment concentration, and bed level, which were used to calculate bottom boundary layer parameters and predict sediment transport. Quasi-periodic extratropical storms were the most important forcing mechanism during the deployment, typically causing increases in wave height and frequency, mean and oscillatory current velocity, shear velocity, suspended sediment concentration, and sediment transport, which was predominantly offshore. One energetic event was not initiated by local storm activity but consisted of a group of high waves propagating from offshore. Some landward sediment transport also occurred during typical fair weather conditions. Considerable inter- and intra-storm variability was noted and a storm classification system was established. Type 1 Storms were associated with anticyclonic activity, northeasterly winds, southerly waves, and southwesterly currents and sediment transport. Type 2 Storms were migrating cyclones that generated energetic, rotational, pre- and post-frontal winds and currents, and caused high northerly swell that transformed into southerly sea. Overall, northwesterly winds during these events caused southeasterly currents and sediment transport. Aside from increases in wind speed, Type 3 Storms were similar to fair weather. Ship Shoal influenced hydrodynamics significantly. Mean wave height and period on the landward side were 36% and 9% lower, respectively, than on the seaward side, due to attenuation. Across-shelf currents were offshore on the seaward side and onshore on the landward side, where flow speed was 10% higher. Sediment flux across Ship Shoal appears to have been divergent during fair weather conditions and convergent during extratropical storms. It is clear, therefore, that winter hydrodynamic and sedimentary processes on the "low energy" Louisiana inner shelf are very dynamic, largely due to extratropical storms.
Pepper, David Alton, "Hydrodynamics, Bottom Boundary Layer Processes and Sediment Transport on the South-Central Louisiana Inner Shelf: the Influence of Extratropical Storms and Bathymetric Modification." (2000). LSU Historical Dissertations and Theses. 7383.