The Grid tab provides the options by which the user may set the vertical layering system, set masks, and also set the model position at its center as shown in Figure 1. The Sigma Zed (SGZ) vertical layering option is implemented with EFDC+ alongside the previous option of Standard Sigma grid (SIG). A drop-down option for the Water Layers is provided and the options described below.

Figure 1  Domain Tab: Grid

Standard Sigma Stretch Grid

The Standard Sigma option for Water Layers is the previous option provided in all versions of EFDCPlus. When this is selected the SGZ options are not displayed.

The table in the Water Layers frame shows the primary water layer settings (KC = number of water layers) and the distribution of the relative layer thicknesses.

The relative thicknesses must add to 1 (or very close). EFDC_Explorer checks this for the user. EFDC_Explorer will automatically divide the water layers into equal fractions based on the number of layers specified by the user. Note that layer 1 is the bottom most layer and that the highest layer number is the surface layer (i.e. KC).

A feature here that is very useful during preliminary testing of a model application is the rapid adjustment of layering simply by changing the number of layers. EFDC_Explorer simply reallocates the layering based on the new KC and then adjusts all the boundary conditions to reflect this change. Use this feature with care, but it has been used on many applications and found to work well.

The I J Map: Maximums frame display the maximum number of rows and columns in the I J map.

Sigma Zed Vertical Layering

EFDC_Explorer implements DSI's improved version of the EFDC code, the EFDC_DSI_SGZ. DSI has developed this version to deal with pressure gradient errors that occur in models that have steep changes in bed elevation. This new version of the code is called the Sigma Zed code, or EFDC_SGZ. This version contrasts with the conventional sigma stretch version (SIG) of the EFDC code in terms of vertical layering in the model. SIG uses a sigma coordinate transformation in the vertical direction and uses the same number of layers for all cells in the domain. To accommodate the varying depths over the model domain in the SIG approach, the thickness of the layers vary from cell to cell but the number of layers and fraction of depth for each layer are constant. SIG approach introduces a well-known error in the density gradient terms, otherwise known as the pressure gradient error (Mellor, et. al. 1994). These errors are most pronounced in regions with steeply varying bathymetry.

The new vertical layering approach (SGZ approach) developed and applied to the EFDC model is computationally efficient and significantly reduces pressure gradient errors. In the EFDC_SGZ model, the vertical layering scheme has been modified to allow for the number of layers to vary over the model domain. Each cell can use a different number of layers, though the number of layers for each cell is constant in time. As with the Sigma Stretch approach, the thickness of each layer varies in time to accommodate the time varying depths. The z coordinate system varies for each cell face, matching the number of active layers to the adjacent cells. The new version is computationally more efficient than a similarly configured Sigma Stretch grid, thus making models with 20 to 50 layers or more practical for typical projects. 

A comparison of SIG and SGZ grids is shown in Figure 2.

Figure 2  Vertical grids for Sigma stretch (left) and Sigma Z (right).

The model grid specification as SIG or SGZ can be made in the main form of EE from the Domain tab | Grid sub-tab as shown in Figure 3. The first option Standard Sigma is the conventional gridding scheme of EFDC model.

The two options for implementing SGZ option in EE are a) Specified Bottom Layer and b) Uniform Layer Thickness. The second two options i.e., SGZ Specified Bottom Layer and SGZ Uniform Layer Thickness, are both SGZ model options with the following attributes.

Specified Bottom Layer

In Specified bottom layer, the layer thickness of the adjacent cells in the vertical do not align in the same plane. Maximum depth in the model is first calculated and the maximum layer thicknesses are determined based on maximum depth. Zonation can be applied using this option. This is the robust option and can be applied on any type of systems such as stratified systems, systems with rapidly changing bed elevation.

Uniform Layer Thickness

Layer thickness of the cell is mostly of the same size as the neighboring cell. The bottom layer thickness can be from 20-120% of the overlying layer. The maximum thickness is first calculated throughout the domain. Thickness at each layer for the cell with maximum depth is then calculated by multiplying DZCK with maximum thickness. Zonation cannot be applied in Uniform Layer Thickness option. Uniform Layer Thickness is suitable is quiescent systems where there are very little external flow influence such as lakes without steep sloping sides.

When computing buoyancy integrals in the model, there are differences between layer thickness and difference between elevation. Using the uniform layer thickness, the differences between layer thicknesses is very small except for the regions with minimum sigma layers.
The SIGMA ZED Layering Options form provides the user with the option of specifying maximum and minimum layers to be used in the model. After selecting Set SGZ Layering and OK, the layers are all automatically updated to the Sigma Stretch layering.

Figure 3  Domain Tab: Layering Options (SIG & SGZ)

Masks

In the Masks frame the Use check box enables the user to enable masks (zero thickness flow barriers) in EFDC. The MASK.INP file will then be required by EFDC. EFDC_Explorer can generate and modify masks using the Modify button as shown in Figure 4. The Create Masks utility does not automatically turn on the mask computation, it just generates the masks and writes the MASK.INP file. The user must manually click the mouse cursor inside the Use Masks check box to use the masks generated.

Figure 4  Mask Editing Tool.

Masks can be viewed, added and deleted in the ViewPlan function. When viewing the Bottom Elevations, RMC on a cell (Enable Editing must be checked first) and the cell properties are displayed, along with the mask options.

The Channel Modifiers frame allows the user to select the use of EFDC channel modifiers. These are like pipes that connect two cells. The Channel Modifier Flag option box is used to enable the use of the channel modifiers capability of EFDC. The normal "on" option is when this flag is set to 2. If set, EFDC requires the MODCHAN.INP file to be input.

The most flexible approach to create and edit channel modifiers is to:

1. turn on this option (i.e. ISCHAN=2),
2. enter the ViewPlan,
3. select Modchannel,
4. enable edit, and
5. use the right mouse click to add, delete or modify channel modifiers (i.e. "pipes").

Care should be exercised in using channel modifiers as model instability is sometimes increased and mass balance errors can occur.

Model Position at Center

Latitude and longitude settings the center of model are accessed in Domain | Grid | Model Position at Centre as shown in Figure 1 above. These settings are required for EFDC+ to calculate solar radiation as well as mapping features in EE.  The user should select the Modify button to update these values.  If the user has not entered a UTM zone for the model the default will be UTM zone 10 and the Northing and Easting will be set to the model centroid.  EE reads the UTM zone from C91 (netCDF settings) in the EFDC.INP and updates to the latitude and longitude are stored in C46C (atmospheric location and wind function coefficients).  Northing and Easting values are calculated based on the UTM and lat/long values.