Navigating Vertical Layering in EFDC+: A Comprehensive Guide
Introduction
EFDC+ is a versatile hydrodynamic model capable of operating in 1-, 2-, or 3-dimensional (3D) modes. For 3D simulations, EFDC+ allows users to choose from two vertical layering schemes, ensuring a precise representation of the system’s vertical structure. In this blog, we will guide you through configuring vertical layering and demonstrate how to visualize the results in EEMS.
Configuring Vertical Layering
The Layers sub-menu serves as the focal point for summarizing the vertical layering within the grid. Right-clicking on this menu item unveils a set of options for configuring the vertical layering system, as illustrated in Figure 1. Users can choose between three fundamental options: Standard Sigma grid (SIG), Sigma-Zed (SGZ) Variable layers, or SGZ Uniform layers. In the Water Layers form, users can specify the number of layers and select the type of layering for their model.
Exploring Layering Options
Let us delve into the details of these layering options:
- Standard Sigma Grid:
- This option represents the original EFDC approach using sigma coordinates.
- It employs the same number of layers for all cells in the computational domain.
- The relative thicknesses of these layers must sum up to approximately 1.
- Sigma-Zed Vertical Layering:
- The Sigma-Zed approach allows the number of layers to vary across the model domain.
- Each cell can feature a different number of layers, although this remains constant over time.
- This approach enhances computational efficiency and is recommended for accurate vertical stratification simulation.
- Zonation can be applied using this option, making it versatile for various system types, including stratified systems and those with rapidly changing bed elevations.
- Uniform layer thickness in a cell matches that of neighboring cells, with the bottom layer thickness ranging from 20% to 120% of the overlying layer.
- The model calculates the maximum thickness throughout the domain.
Visualizing Results with Different Layering Options
To compare or display simulation results with different water layer options, follow these steps:
- Create a centerline in 2DH View or import it from an external source to visualize results in a longitudinal section.
- Access 2DV View either from the main menu by selecting “New 2DV View” or directly by clicking the button on the main toolbar.
In 2DV View, you can view the results along the I, J direction or the longitudinal section represented by a red line, as depicted in Figure 3.
Comparative Analysis
Figure 4 provides a vertical profile plot comparing model-generated data (blue) with observed data (red) for Lake Washington under three different water layer cases.
Notice that the Standard Sigma option produces poor vertical temperature stratification (Figure 3). Conversely, Figure 4 demonstrates that the temperature distribution under the SGZ option offers more accurate results. Additionally, using SGZ significantly reduces the total number of computational cells in the water column, reducing model running time.
Conclusion
Configuring vertical layering in EFDC+ is a critical step in achieving precise simulations. The choice between Standard Sigma and Sigma-Zed options dramatically impacts the accuracy of your model’s vertical stratification. By following these steps and selecting the appropriate layering scheme, you can ensure your simulations align with observed data, saving time and computational resources.
