Vertical Layering Options

EFDC+ can serve as a 1-, 2-, or 3-dimensional (D) hydrodynamic model.  For 3D simulations, two different vertical layering schemes are supported, providing you the ability to appropriately represent the vertical structure of your system.

Layering options for 10 layers: (0) Sigma Coordinate, (1) SGZ Variable layers, (2) SGZ Uniform layers.

Sigma Coordinate

The sigma coordinate (SIG) approach uses the same number of vertical layers everywhere in the model domain, regardless of the water depths.  This is the conventional EFDC approach and is common in most modern hydrodynamic modeling codes, but it is subject to horizontal pressure gradient errors when the system has steeply sloping beds relative to the horizontal grid size. This can be seen in Figure 2a, which demonstrates how thermocline is not as well reproduced when using the SIG method.

Figure 2a: Vertical profile for temperature of model (blue) and data (red) for Lake Washington with the SIG approach.

Sigma-Zed Options

To overcome the pressure gradient errors in the SIG approach, the SGZ approach was developed and applied in the EFDC+ model (Craig et al., 2014). This approach allows the number of layers to vary over the model domain based on the water depth. As a result, each cell can have a different number of layers, though the number of layers in that cell remains constant over time. The SGZ approach is computationally efficient and is now recommended as the standard approach, ensuring greatly improved accuracy. Three SGZ options are now available with release of EFDC+ 12.1:

  • SGZ: Specified Bottom Layer (GRIDV = 1)
  • SGZ: Uniform Layers (GRIDV = 2)
  • SGZ -Specified Thickness from Top (GRIDV = 3)

Figure 2b demonstrates how thermocline is well captured in the simulation by using the SGZ approach. Learn more about Sigma-Zed and see examples of how it has been applied to water bodies in North America such as Lake Washington and Lake Mead here. 

Figure 2b: Vertical profile for temperature of model (blue) and data (red) for Lake Washington with the SGZ approach.

Generalized Vertical Coordinates

EFDC+ no longer supports the General Vertical Coordinate (GVC), which was available in the EPA version of EFDC. The GVC and the Sigma-Zed approaches are similar, though the Sigma-Zed approach produces more accurate results and is significantly faster than a similarly configured GVC model. Although there is no advantage to using GVC with EFDC+, EE continues to support the GVC capability for users of EFDC_EPA.