Vertical Layering Options
EFDC+ can serve as a 1, 2, or 3D hydrodynamic model. For 3D simulations, two different vertical layering schemes are supported providing you with the ability to appropriately represent the vertical structure of your system.
A new vertical layering approach has been developed and applied to the EFDC+ model which reduces the horizontal pressure gradient error. This new scheme has been named the Sigma-Zed approach (SGZ). The Sigma-Zed approach allows 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. This approach is computationally efficient and is now recommended as the standard approach as accuracy can be greatly improved.
Learn more about Sigma-Zed and see examples of how it has been applied to water bodies in North America such Lake Washington and Lake Mead here.
The sigma coordinate (SIG) approach uses the same number of vertical layers everywhere in the model domain, no matter what the water depths are. This is the conventional EFDC approach and is common in most modern hydrodynamic modeling codes. However, this approach is subject to horizontal pressure gradient errors when your system has steeply sloping beds relative to the horizontal grid size. This can be seen in Figure 2, to the right, which demonstrates how the thermocline is not as realistically represented SIG method.
A comparison of the layering options for sigma coordinate and Sigma-Zed is shown in the figure to the right.
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. EE continues to support the GVC capability when using EFDC_EPA instead of EFDC+.