Multi-dimensional hydrodynamic and water quality models are widely used to simulate the physical and biogeochemical processes in the surface water systems such as reservoirs and estuaries. Most of the models have adopted the Eulerian grid modeling framework, mainly because it can reasonably simulate...
Multi-dimensional hydrodynamic and water quality models are widely used to simulate the physical and biogeochemical processes in the surface water systems such as reservoirs and estuaries. Most of the models have adopted the Eulerian grid modeling framework, mainly because it can reasonably simulate physical dynamics and chemical species concentrations throughout the entire model domain. Determining the optimum grid cell size is important when using the Eulerian grid-based three-dimensional water quality models because the characteristics of species are assumed uniform in each of the grid cells and chemical species are represented by concentration (mass per volume). The objective of this study was to examine the effect of grid-size of a three dimensional hydrodynamic and water quality model (EFDC) on hydrodynamics, mass transport and water quality simulations in the Saemangeum Reservoir(Korea). Three different horizontal grid resolutions, respectively representing coarse(CG), medium(MG), and fine(FG) grid cell sizes, and three different vertical grid resolutions(3, 5, 10 layers) were used for sensitivity analysis. The EFDC was constructed and calibrated with surveyed bathymetry data and field data including water level, temperature, salinity, sediment and water quality obtained in 2008. The results obtained from the sensitivity analysis can be summarized as follows. For different grid resolutions, the flow directions and vector fields appeared similar in the reservoir, but the magnitude of flow velocities in specific areas were different. In particular, the maximum flow velocity simulated with FG model was two times greater than that with CG model near the outflow gates. The simulation results of numerical tracer showed that the grid resolution affects on the flow path, mass transport, and mixing zone of upstream inflow, and results in a bias of temporal and spatial distribution of the tracer. With the CG, in particular, the model overestimated diffusion in the mixing zone, and fail to identify the gradient of concentrations between the inflow and the ambient water. Temperature and salinity simulation results of FG model showed better agreement with the field data compared to CG model and adequately replicated the spatial and temporal variations of the variables. The transport and deposition of sediments were also affected by the horizontal grid resolution. Sediments were transported and deposited to further downstream along the thalweg with FG model compared to CG model. The water quality simulation results were also sensitive to the grid resolution, specifically near the mixing zone between freshwater and seawater. The relative errors of MG to FG were greater than those of CG. Meanwhile, the effect of vertical grid resolution on the temperature and salinity simulations was not significant except during the summer stratification period. Temperature stratification and salty water intrusion were better represented with high vertical resolution. In conclusions, the grid resolutions have a significant effect on the contaminant transport and mixing zone between freshwater and seawater in the Saemangeum Reservoir. Thus, the FG model is recommended for accurate simulations of spatial distributions of sediments and water quality in the reservoir. Meanwhile, the MG model is also feasible for less complex simulations of water temperature and salinity only as the biases between MG and FG are neglectable.
Multi-dimensional hydrodynamic and water quality models are widely used to simulate the physical and biogeochemical processes in the surface water systems such as reservoirs and estuaries. Most of the models have adopted the Eulerian grid modeling framework, mainly because it can reasonably simulate physical dynamics and chemical species concentrations throughout the entire model domain. Determining the optimum grid cell size is important when using the Eulerian grid-based three-dimensional water quality models because the characteristics of species are assumed uniform in each of the grid cells and chemical species are represented by concentration (mass per volume). The objective of this study was to examine the effect of grid-size of a three dimensional hydrodynamic and water quality model (EFDC) on hydrodynamics, mass transport and water quality simulations in the Saemangeum Reservoir(Korea). Three different horizontal grid resolutions, respectively representing coarse(CG), medium(MG), and fine(FG) grid cell sizes, and three different vertical grid resolutions(3, 5, 10 layers) were used for sensitivity analysis. The EFDC was constructed and calibrated with surveyed bathymetry data and field data including water level, temperature, salinity, sediment and water quality obtained in 2008. The results obtained from the sensitivity analysis can be summarized as follows. For different grid resolutions, the flow directions and vector fields appeared similar in the reservoir, but the magnitude of flow velocities in specific areas were different. In particular, the maximum flow velocity simulated with FG model was two times greater than that with CG model near the outflow gates. The simulation results of numerical tracer showed that the grid resolution affects on the flow path, mass transport, and mixing zone of upstream inflow, and results in a bias of temporal and spatial distribution of the tracer. With the CG, in particular, the model overestimated diffusion in the mixing zone, and fail to identify the gradient of concentrations between the inflow and the ambient water. Temperature and salinity simulation results of FG model showed better agreement with the field data compared to CG model and adequately replicated the spatial and temporal variations of the variables. The transport and deposition of sediments were also affected by the horizontal grid resolution. Sediments were transported and deposited to further downstream along the thalweg with FG model compared to CG model. The water quality simulation results were also sensitive to the grid resolution, specifically near the mixing zone between freshwater and seawater. The relative errors of MG to FG were greater than those of CG. Meanwhile, the effect of vertical grid resolution on the temperature and salinity simulations was not significant except during the summer stratification period. Temperature stratification and salty water intrusion were better represented with high vertical resolution. In conclusions, the grid resolutions have a significant effect on the contaminant transport and mixing zone between freshwater and seawater in the Saemangeum Reservoir. Thus, the FG model is recommended for accurate simulations of spatial distributions of sediments and water quality in the reservoir. Meanwhile, the MG model is also feasible for less complex simulations of water temperature and salinity only as the biases between MG and FG are neglectable.
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