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An Algorithm on Vertical Layering in EFDC with Sigma-Zed

Author(s): P. M. Craig; D. H. Chung; N. T. Lam; P. H. Son; N. X. Tinh

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Keywords: EFDC; Sigma-zed; Vertical layering; Horizontal gradients

Abstract: This paper presents a new algorithm on vertical layering to improve the accuracy for horizontal gradient terms for Navier-Stokes as well as advection-diffusion equations governed in EFDC (Environmental Fluid Dynamics Code) developed by Hamrick (1992). The traditional sigma coordinates system uses the same number of layers for all cells in the computational domain. Therefore, it introduces a well-known error in the horizontal gradient terms known as the pressure gradient error (Mellor, et. al. 1994). Especially, these errors are most pronounced in regions with steeply varying bathymetry. A new vertical layering approach has been developed and efficiently applied to the EFDC model, in which the vertical layering scheme has been modified to allow 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. In addition, in case of sediment transport and bed level change due to erosion/deposition the number of layers at each cell is re-determined for each time step to avoid the horizontal gradient error as mentioned above. This makes a basic difference between sigma and sigmazed transformations. The new approach has been tested with several hypothetical test cases. The model has been applied to Lake Washington (Seattle, WA, USA) as an interesting illustration for the success of sigma-zed in solving a practical problem through some comparisons with the measurement.

DOI:

Year: 2014

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