Doctor of Oceanography and Coastal Sciences (POCS)
Department of Oceanography and Coastal Science
In-situ observations and a Finite Volume Community Ocean Model (FVCOM) are used to investigate the cold front induced sub-tidal hydrodynamics of Lake Pontchartrain, a semi-enclosed low-salinity estuary with multiple inlets connecting to the open ocean. Observations show that the sub-tidal hydrodynamic responses are highly correlated with the meteorological parameters during cold front events. Model results indicate that, under barotropic conditions, the remote wind effect has the greatest contribution to the overall water level variation, while the local wind stress during cold front events determines the slope for the water level inside the estuary. An examination of a quasi-steady state force balance shows that the water level slope in the north-south direction inside the estuary is determined by the north-south wind stress, explaining ~ 83% of the variability but less so in the east-west direction (~ 43%), a lower value mainly caused by the eastern open boundary at the Rigolets.
Furthermore, under baroclinic conditions, circulation patterns of the lake are examined by numerical experiments. In general, circulation in the low-salinity estuary with restricted openings to the ocean is mainly driven by wind and to a lesser extent by water level fluctuations at the open boundaries. Local wind effect tends to produce downwind flows in coastal, shallow water regions and on the surface, but upwind flows near the bottom, a result consistent with barotropic wind-driven circulations; while the remote wind effect is important mostly near the open boundary. Remote wind effect decreases into the interior due to bottom friction. The quasi-steady state balance is more accurate in the cross-estuary direction than that in the along-estuary direction (R2 ~ 0.94 vs. 0.60) under baroclinic conditions. This difference in the accuracy of the quasi-steady state balance between the cross- and along-estuary directions is caused by the open boundary - a tidally-induced mean slope exists. Furthermore, even if the tidal effect is removed, the accuracy still decreases toward the open end for slopes in both directions. Remote wind effect and residual flow through the eastern open boundary tends to introduce a departure from the quasi-steady state balance in both along- and cross-estuary directions. In addition, there is another reason for the quasi-steady state balance being more (less) accurate in the along- (cross) estuary direction before cold front passages - the relatively higher (lower) occurrence of the wind in that direction.
Results from numerical modeling by FVCOM show that northerly and southerly winds tend to stretch the plume in east-west directions, while easterly and westerly winds constrain the plume to expand in north-south directions. Sensitivity experiments to wind magnitude are conducted. Increasing wind magnitude tends to increase the salt content of the lake except under the westerly wind during which salt content decreases if the wind speed is less than 6 m/s. Increasing wind magnitude can enhance both surface downwind flow and bottom upwind flow and lower the no-motion layer between the two opposite flows. Leaked water before the opening of the spillway has significant influence on the vertical structure of flows and salinity: mixing is facilitated by the large amount of freshwater leaked into the lake; gyres are diminished; the Average Potential Energy Demand (APED) is reduced to very low values; quasi-steady state balance tends to be affected; about 1,500,000 kg of salt content is reduced. The Lake Pontchartrain estuary is completely dominated by the freshwater from the river diversion within 25 days, replacing a total water volume of 9.77×109 m3, indicating that salinity drops to minimum value after 25 days.
Huang, Wei, "Sub-Tidal Hydrodynamics of the Multi-Inlet Lake Pontchartrain Estuary Influenced by Mississippi River Diversion and Wind Associated with Atmospheric Fronts" (2019). LSU Doctoral Dissertations. 5038.
Available for download on Wednesday, August 26, 2020