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.
A new vertical layering approach, called the Sigma-Zed approach (SGZ), has been developed and applied to the EFDC+ model, reducing the horizontal pressure gradient error. 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 in that cell remains constant over time. This is computationally efficient and is now recommended as the standard approach, ensuring greatly improved accuracy.
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.
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 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 using the SIG method.
You can compare the layering options for sigma coordinate and Sigma-Zed in the figures 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. Although there is no advantage to using GVC with EFDC+, EE continues to support the GVC capability for users of EFDC_EPA.