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Sigma Zed Vertical Layering
EFDC_Explorer8.0 now 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.
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The two options for implementing SGZ option in EE8 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
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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
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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.
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