Case Studies

Lake Pontchartrain Estuarine System
New Orleans, LA - August 2001

Contributed by: Ioannis Georgiou University of New Orleans

Geographic and Bathymetric Features of the Lake Pontchartrain Estuarine System. View AVI movie (3.45 MB)

Ioannis Georgiou works at the University of New Orleans, Department of Civil and Environmental Engineering as a doctoral candidate in Engineering and Applied Science.

Ioannis' work includes 2- and 3-D numerical modeling of the Lake Pontchartrain System – simulating hydrodynamics, transport, and water quality processes. This effort involves the collection of field data, analysis, construction of laboratory scaled physical models, and implementation of numerical hydrodynamic and transport models.

These models serve mainly to investigate the flow exchange and dynamics of impeding salt water currents and the water quality impact these density currents have on the Lake Pontchartrain ecosystem.

Data visualisation plays a big part in applying his numerical models. Viewing animated time steps during the execution of the solution is invaluable. The ability to slice 3-D plots and volumes enable Ioannis to look inside a process and discover hidden details.

Our Image of the Month is geographic and bathymetric (depth) features of the Lake Pontchartrain Estuarine System. The data was collected during the summer of 1998 with acoustic instruments that measure depth by reading reflected signals off the bottom, and from digitised hydrologic maps provided by the National Oceanic and Atmospheric Administration (NOAA).

The plot was generated by loading the depth data file, which included a longitude, latitude and the corresponding depth, and then triangulating the data points to create a surface plot. The contour option was checked to show the depth contours. Value-blanking was used on the depth variable to block out points falling on land. Additional contour levels were used to show more detail in the areas where the depth did not vary significantly. Finally, the map showing the location of Lake Pontchartrain was loaded into a different frame, made transparent and popped to the front.

Bathymetric features shown in this plot are of specific importance in constructing a mesh for the three-dimensional model. The intrusive high density saltwater currents sink to the bottom waters of the lake – thus depth variability would significantly affect the dynamics of the flow.

Ioannis is specifically investigating the impacts of saltwater intrusion from the Inner Harbor Navigation Canal (IHNC) on the south shore of Lake Pontchartrain. The IHNC is part of the Mississippi River Gulf Outlet (MRGO), which connects Lake Pontchartrain with the Gulf of Mexico. The connection between the MRGO and Lake Pontchartrain is achieved via the IHNC.

Map showing the Mississippi River Gulf Outlet (MRGO), the Inner Harbor Navigation Canal (IHNC), and adjacent areas.

"The Mississippi River Gulf Outlet system, built in 1963 as a shortcut for oceangoing ships that were headed to New Orleans to upload and receive cargo, has resulted in tremendous environmental damage such as salt water intrusion and wetland loss, along with weather and flooding threats."

For Lake Pontchartrain this intrusion has led to the endangerment of fresh water marshes that surround the lake. When the high salinity water is pushed into fresh water marshes it eventually destroys the habitat. High saline water causes the fresh and brackish (slightly salty) water dependent plants and animals to die. Plants and invertebrates such as clams are typically affected most. Birds and fish are able to swim out of the low oxygen water, but ultimately driven out of the area due to salinity changes. When the plants are destroyed in a marsh, the area becomes more susceptible to wetland loss. The roots no longer hold soil together, allowing the soil to wash away with the tide and storms.

There are no specified acceptable levels of salinity, but depending on what you are investigating, a certain limit is apparent. Biologists discovered that clam concentrations in Lake Pontchartrain were diminished not by high salt content in the water column, but by low dissolved oxygen resulting from the salt intrusion. In the case of the freshwater marshes, the Cypress trees that dominate the area cannot tolerate salt concentrations exceeding six parts per thousand (ppt for fresh water is sero ppt, and ocean water is variable around 30-32 ppt).

The contour plot above shows salinity concentration contours (top) and the dissolved oxygen concentration contours (bottom). This field data obtained in Lake Pontchartrain is to be used for calibrating the numerical model. The image shows the density current originating from the navigation canal (left) as it moves into Lake Pontchartrain (right). A depth irregularity near the canal, consisting of a scour hole and a dredge hole, normally fills up with higher salinity water from the canal. Once these irregularities fill, this higher salinity water proceeds into Lake Pontchartrain as as a thin layer in the bottom waters as shown in the plot. This thin layer of higher salinity water is often associated with low dissolved oxygen, as seen in the bottom plot. The goal is to investigate the dynamics of this density current and the associated water quality impacts, and try to calibrate numerical models to be used as predictive tools in the future.

After establishing hydrodynamics in Lake Pontchartrain, Ioannis and his research team will attempt to relate circulation patterns, temperature and salinity to dissolved oxygen, by solving additional equations. Once this step is successfully completed, they will have a complete picture, allowing them to address possible solutions such as structural modifications to the canal, or additional structural components to help control the intrusion.

XY plot showing salinity gradients (bottom salinity minus surface salinity). The data is from the field surveys obtained using a digital handheld YSI instrument.

TECPLOT TIP: If you want your contour legend not to look overloaded with floating numbers add more contours from the menu, then use the skip option to show fewer numbers but all the colors. If your legend looks very small, enlarge it by increasing the legend line spacing.

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