6.1.2 Computational Grid Domain
The model was also applied to the a more extended area, including the Pontchartrain Basin, the Mississippi and Alabama Coasts, including Bay Saint Louis, Biloxi and Mobile Bays. The computational domain consists of 22882 elements and 12281 nodes (Figure 6.1). There are 11 sigma levels placed in a constant distribution. The OB is composed by 57 nodes at the east edge of the domain.
An unstructured grid was used for the Pontchartrain Estuary. The grid has areas with coarse resolution of approximately 15000 m (e.g., open boundary), and fine resolution of approximately 200 m (e.g., Inner Harbor Navigation Canal). Barataria Bay was represented by a single inlet with an equivalent storage volume.
Figure 6.1: Model computational domain composed by 22882 cells and 12281 nodes. 5.8.2 Model Inputs
5.8.2.1 Initial Conditions
The bathymetry for the model was obtained from the National Oceanic and Atmospheric Administration (NOAA) hydrographic and topographic surveys and from the ADCIRC grid version SL15_V3. Figure 6.2 shows the model bathymetry.
Figure 6.2: Model Bathymetry relative to MSL
The initial condition for salinity was also generated using datasets collected by Haralampides (2000), Georgiou (2002), and Georgiou et al., (2007). These datasets were collected from 1997 to 2002. Interpolation methods were utilized to fill the gaps of the areas with little or no information. Some of the isohalines were interpolated to the Open Boundary values (Gulf of Mexico). Thus, the model was run for 120 days with tidal forcing to achieve dynamic equilibrium near the freshwater discharges, e.g., Mississippi River, Mobile Bay, among others; tidal passes and open boundary. Daily discharges were forced to simulate the tributary flows of a year. The salinity distribution for the final hour of the 120-day run was used as the initial condition. Salinity in some areas were modified in a discrete form to maintain a realistic gradient of the system The initial condition for
surface salinity is shown in Figures 6.3.
Despite the model provided at the end of the 120th day a stratified distribution of salinity, no stratification was forced as the initial condition to allow the model to bring the respective stratification. The bottom salinity, at the 120th day, showed that system reached high values at the shorefront of the barrier islands of the three states, while the surface salinity showed that it did not reach high salinity values yet. This result justifies the effect of the baroclinic radiative boundary; thus the OB is capable radiating freshwater in the top layers and force into the system prescribed saltwater values.
The initial elevation and velocity vector field were initialized from the 24th hour
of the 120 day run. The datum is relative to mean sea level (MSL) and the ramp-up period for the boundary conditions was 8 hours.
5.8.2.2 Boundary Conditions
The open boundary was set up with the radiative baroclinic boundary. The tidal range was based on the tidal constituents of the locations of the nodes along the open boundary. The main tidal constituents for the nodes representing the OB are K1 and O1,
each with an amplitude of approximately 15 cm, and phases of 17 and 20 hours, respectively. Tidal constituents were taken from the ADCIRC Tidal Database version 2001 (ADCIRC, 2008). A constant temperature of 25°C was defined for the initial and boundary conditions. The inflow prescribed salinity at the OB was 35 ppt.
The meteorological forcing consisted on a constant precipitation and evaporation of 4.24 mm/day and 3.28 mm/day respectively. Moderate uniform wind was forced to overcome the fictitious effect of solid body rotation observed in Lake Pontchartrain. A prescribed 3 m/s constant wind was used with varying hourly wind direction. The wind direction rate of change was set to 1 degree per hour, counterclockwise. Tributary discharges represent the runoff and major rivers flowing into the Pontchartrain Basin including the Amite, Blind, Tickfaw in Lake Maurepas; the Tangipahoa and Tchefuncte Rivers, the leakage from the Bonnet Carré Spillway in Lake Pontchartrain; the Pearl River near the Louisiana and Mississippi State border; the Wolf and Jourdan Rivers in Bay Saint Louis, the Biloxi and Pascogoula Rivers in Mississippi State; the Mobile River
in Alabama; the MR freshwater diversions Davis Pond in Barataria Bay, Violet in Lake Borgne and Caernarvon in Breton Sound; and the Mississippi River. Figure 6.4 shows the tributary flows. 1 10 100 1000 10000 0 50 100 150 200 250 300 350 Time, days R ive r f lo w s, m 3 /s 10 100 1000 10000 100000 M is s is si ppi r iv e r fl ow
Amite Tangipahoa Tchefuncte Tickfaw Pearl River South LM SW LP SC LP JP SE LP OP NE LP BLaC NE LP BBf LB (Violet) Wolf 2 Jourdan Biloxi Pascagoula Mobile Caernarvon Davis Pond Missippi River
Figure 6.4 Tributary flows used in the model (flows represent a 17 year average of the mean daily flow for each day for rivers in the Pontchartrain estuary, and a 2-5 year daily average for the remaining rivers).
The model reproduced reasonably spring tidal ranges for the system. For example, the stations shown in Table 6.1 indicate that the ranges fall within the margin of
variability of the measured data from the monitoring stations. The results from the west end of the The Rigolets showed a degree of under-prediction; however, the tidal prism propagated properly into the head of the estuary since the tidal ranges at Lake
Table 6.1 Observed and simulated tidal ranges for the spring tide. Lake Maurepas (1) Lake Pontchartrain (2) The Rigolets (1) Waveland
(3) Biloxi Bay (1) SW Pass (4) Mobile Bay (1) Range, m 0.16 0.16 0.25 0.53 0.85 0.68 0.54 Observed Variability, m 0.01 0.01 0.03 0.07 0.11 0.09 0.07 Simulated Range, m 0.17 0.17 0.18 0.66 0.79 0.56 0.60 (1) USGS station at Pass Manchac, The Rigolets, and East Biloxi Bay; (2) DEQ Midlake station in Lake Pontchartrain; (3) NOAA station at the Yacht Club and Mobile Bay; (4) LUMCON station at SW Pass.
Figure 6.5 shows the predicted salinities at seven stations in the study area. The results indicate that FVCOM with tide, wind and hydrologic forcing captures the seasonal variations in salinity in all areas except Lake Pontchartrain where the interaction of wind shear and the density flows in the region of the passes tends to reduce the salt fluxes into the Lake.
0 5 10 15 20 25 0 30 60 90 120 150 180 210 240 270 300 330 360 Jullian day Sali ni ty p p t
Salinity_LP Salinity_Rigolets Salinity_SW_Pass
Salinity_Biloxi Salinity_MS_Sound Salinity_LMaurepas
Salinity_Chef
Figure 6.5: Annual variation in salinity (FVCOM) at selected stations
Table 6.2 show the average observed and predicted salinities at selected stations. This table also indicated the variability in the field data. Figure 6.5 shows the variability in the model results. The salinities are generally within the range of variability; however, the average salinity in the middle of Lake Pontchartrain is under-estimated by more than 1 ppt under wind forcing.
Table 6.2: Observed and simulated average salinities over a one year period
Pass Manchac
(1)
Lake
Pontchartrain(2) The Rigolets(1) Chef Menteur Pass(2)
Biloxi
Bay(1) Mississipi Sound(1) Observed Average, m 0.9 3.3 5.5 5.5 14.6 17.4 Variability, m 0.9 2.0 6.0 5.0 7.0 12.0 Simulated Average, m 1.0 1.8 4.1 2.6 14.6 13.2
Figure 6.6 represents the predicted surface salinities at the time the average system salinity near its minimum (April). The corresponding maximum average system salinities are illustrated in Figure 6.7.
Figure 6.7: Surface Salinity Distribution at maximum system salinity.
Figure 6.8 and 6.9 show the top and bottom salinities at the end of one year of simulation. These figures indicate that there is significant stratification, especially offshore of the Barrier Islands.