by Assign BC will open the CMS-Flow Boundary Conditions window (Figure 8). A time series of salinity can be assigned along the WSE-Forcing boundary by clicking the Curve undefined under Salinity on the left hand side of the dialog.
The time series is specified either by clicking the Import button to read a salinity boundary input file in xys format (Figure 9) (Aquaveo 2010) or by manually entering time and salinity values in two separate data columns or by importing salinity data from an opened Excel file by Copy/Paste.
Because the WSE-forcing boundary of the Humboldt Bay grid is located outside the bay in open coast, salinity measurements at Trinidad Bay, approximately 18 km north of the CMS domain by CeNCOOS at Humboldt State University (2010), were assigned to the CMS-Flow open boundary. A 30-day time series of the salinity data is shown for the WSE-forcing boundary in Figure 8. Salinity at this location varies between 30.5 and 33.5 ppt that show apparent influence of the ocean on salinity variations during the period.
ii) Flow rate-forcing boundary: Following the same steps as specifying WSE-forcing boundary, salinity values at freshwater inflow boundaries can be assigned together with flow specifications.
Only a few rivers and creeks flow into Humboldt Bay and no stream measurements are available. Although small, there are seasonal variations in freshwater inflows to the bay. Relatively large amount of land runoffs flows into the bay during the rainy season in northern California from October to April. For demonstration purpose in this CMS application, 10 ppt was assigned for a constant river flow in the Elk River (Figure 7). A non-zero salinity value is given here considering that the location of the Flow rate-forcing cellstring is within the intertidal zone.
SIMULATION RESULTS: For the Humboldt Bay demonstration, the CMS-Flow simulation was conducted for a 30 day winter period (December 2007). Depth-averaged current and salinity fields near the bay entrance channel were retrieved from two snapshots of the CMS results at Day 19 of the simulation, corresponding to the flood and ebb currents, respectively (Figure 10).
The current speed is between 1 and 2 m/sec at the Humboldt Bay entrance during the flood and ebb tides. The bay is dominated by relatively high salinity water and low salinity water fills the downstream reach of the Elk River, consistent with the salinity specifications along the WSE-forcing and the Flow rate-forcing boundaries. Figure 10 shows that high salinity ocean water is brought into the bay by the flood current. A salt front near the mouth of the Elk River indicates that intrusion of freshwater plume into the bay. The plume is pushed upstream of the river by strong flood current passing by to the Arcata Bay and extends further into Humboldt Bay during the ebb tidal cycle.
SALINITY TRANSPORT MODELING AT WHITE DITCH AREA, LA: Located southeast of New Orleans on the coast of Louisiana, the White Ditch area has been experiencing the shortage of fresh water, sediments and nutrients from the Mississippi River, and saltwater intrusion due to sea level rise and storms. As a result, extensive wetland loss and degradation of tidal marshes have occurred in the area through the years. A hydrodynamic and salinity modeling analysis was conducted by URS (2010) to evaluate alternative designs for a fresh water diversion from the Mississippi River to the project area.
CMS-Flow was developed for the White Ditch project. The model domain extends about 60 km along the NW-SE directions and about 30 km along the SW-NE direction, and includes the Mississippi River levees, the Mississippi River-Gulf Outlet (MRGO) channel, and other features (Figure 11). The size of the CMS grid cells ranges from 24 to 600 m with the fine grid along the White Ditch (Mississippi River) and the coarse grid in the Gulf of Mexico.
The CMS was driven by the tide data downloaded from U.S. Geological Survey (USGS) station (7374527) (Figure 11), Northeast Bay Gardene near Point-A-LA-Hache, LA (http://waterdata.usgs.gov/nwis). Rainfall data were obtained from the NOAA Port Sulfer Station (167471) and from a Belle Chasse station. Daily evaporation data were not available at the project area. Evaporation rates specified in the CMS were based on a study conducted by Cooke et al. (2008). Freshwater flows were measured from the Caernarvon Diversion and the Mississippi River. The New Orleans District, USACE, manages the Caernarvon Freshwater Diversion Project and provided daily flow information for the study.
The salinity data for the CMS were collected on a field survey conducted from 20 July through 23 July, 2009 by URS. The sampling stations are shown in Figure 12. Flow velocity, temperature, salinity and turbidity were collected periodically between 21 and 23 July, 2009 at the primary stations (N1, N2, N3, S1, S2, and S3). Less frequent flow velocity, temperature and salinity measurements were collected at the secondary locations (Oak River Channel, N4, N5, N6, S4, S5, S6, S7, S8, S9, S10, and S11).
Statistical analysis of the salinity data was performed to compute the median, maximum and minimum salinity at each station. The time-averaged salinity values over the last four days of the CMS simulation corresponding to the time period of the measurements by URS (2010), were compared with the measurements (Figure 13).
The time- and depth-averaged salinity in this wetland area ranges from about 3 to 10 ppt. The variability of salinity is less than 1 ppt at the landward stations (Points 33-35) and can reach as high as more than 3 ppt at the seaward stations (Point 36) over the 4-day survey period. The salinity data show a general trend in the survey area. Salinity is relatively high at the southeast corner of the CMS domain in nearshore waters of the Gulf of Mexico, and decreases towards the northwest. High salinity also occurs along the east bank of the Mississippi River and decreases to the northeast of the CMS domain. The general trend indicates that significant sources of freshwater in the White Ditch area are from the Caernarvon Diversion. CMS-Flow tends to underestimate the salinity at some stations, but overall the CMS results show good agreement between measurements and calculated salinity, and well represent the spatial pattern and temporally averaged salinity levels.
CONCLUSIONS: The depth-averaged salinity calculation procedure by the CMS was demonstrated in this technical note. The Humboldt Bay example shows that the procedures to set up the CMS for salinity calculation are straightforward and user-friendly. The White Ditch example provides an estuarine application of salinity calculations and verification for the CMS results, which well reproduce the spatial salinity distributions in the wetland area. Due to lack of measurements such as rainfall and evaporation, detailed bathymetry and salinity data, some assumptions were made for the study. Further improvement can be made as more data are collected in the future.
ADDITIONAL INFORMATION: This CHETN was prepared and funded under the Coastal Inlet Research Program (CIRP) and was written by Dr. Honghai Li (Honghai.Li@usace.army.mil, voice: 601-634-2840, fax: 601-634-3080) of the U.S. Army Engineer Research and Development Center (ERDC), Coastal and Hydraulics Laboratory (CHL), Dr. Christopher W. Reed (Chris_Reed@URSCorp.com) of URS Corporation, and Mitchell E. Brown (Mitchell.E.Brown@usace.army.mil) of ERDC, CHL. Alejandro Sanchez of CIRP provided hydrodynamic model information for the Humboldt Bay example. The CIRP Program Manager, Dr. Julie D. Rosati (Julie.D.Rosati@usace.army.mil), the assistant Program Manager, Dr. Nicholas C. Kraus, the Chief of the Coastal Engineering Branch at CHL, Dr. Jeffrey P. Waters, and Dr. Lihwa Lin, the Coastal Engineering Branch, reviewed this CHETN. Files for the example may be obtained by contacting the author. This CHETN should be cited as follows:
Li, H., C. W. Reed, and M. E. Brown. 2010. Salinity Calculations in the Coastal Modeling System. Coastal and Hydraulics Engineering Technical Note ERDC/CHL CHETN-IV-XX. Vicksburg, MS: U.S. Army Engineer Research and Development Center. An electronic copy of this CHETN is available from http://chl.erdc.usace.army.mil/chetn or http://cirp.usace.army.mil/pubs/chetns.html.
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