Overview
The Environmental Fluid Dynamics Code (EFDC) is a widely applied three-dimensional hydrodynamic model. In the vertical, EFDC uses a sigma coordinate transformation with the same number of layers for all cells in the domain. While layer thickness varies cell to cell to accommodate varying depths, the constant number of layers introduces a well-known error in the density gradient terms — the pressure gradient error — which is most pronounced in regions with steeply varying bathymetry.
Approach
A new, computationally efficient vertical layering approach was developed and applied to EFDC. The vertical layering scheme was modified to allow the number of layers to vary over the model domain: each cell can use a different number of layers, constant in time, while layer thickness varies in time to accommodate changing depths. The z coordinate system varies for each cell face, matching the number of active layers to adjacent cells. The new version is more computationally efficient than a similarly configured sigma stretch grid, making models with 20 to 50 layers or more practical for typical projects.
Key Findings
The approach was tested with several hypothetical test cases and applied to Lake Washington in Seattle, which has steep bottom gradients and sharp thermoclines. The results indicate that the vertical variation of temperature and the thermal stratification are more accurately reproduced, providing a significant improvement over the earlier sigma coordinate transformation method.