Keywords: one-way nested model, hydrodynamics, flooding and drying, DIVAST.

A one-way nested version of the hydrobiological model, depth integrated velocity and solute transport (DIVAST) is introduced. DIVAST is a depth integrated, time-variant model capable of simulating two-dimensional distributions of currents, water surface elevations and water quality constituents. The nested model allows flooding and drying of intertidal areas, a process often present in areas where nesting is required but usually excluded from nested models due to the added complexity and instabilities. The model allows flooding and drying of both the nested grid interiors and the nested open boundaries (in which case the nested boundaries effectively behave as dynamic boundaries).

A nested simulation involves one outer grid which contains one or more inner nested grids. Each nested region (or child) is entirely contained within a single coarser grid (the parent). The model allows multiple levels of nesting, in which case children are also parents. The fine grids may be telescoped to any depth and several fine grids may share the same parent at the same level of nesting. The nested grids allow any integer spatial and temporal refinements of the parent grid (the spatial and temporal refinements are usually, but not necessarily the same). Nested grid open boundary data is interpolated (both in time and space) from parent grid data; a linear technique is used for temporal interpolation while an inverse distance weighted technique is used for spatial interpolation.

The model was tested in Cork Harbour due to its extensive areas of sand- and mudflats. Model performance was assessed by comparing results from the nested model with those from coarse and fine models of the Harbour. Extensive testing has shown that the model is stable and is capable of computing hydrodynamic activity to a high degree of accuracy. The model performs equally well in areas of flooding and drying and in deeper waters. It was found that the accuracy of the nested model is dependant on the location of the open boundaries of the nested domains. The nested boundaries of each child are driven by data obtained from its parent which are inherently erroneous. These errors are passed from parent to child across the open boundary and propagate into the child domain. However, errors were found to decrease with distance from the open boundaries and nested boundaries should therefore be located sufficiently distant from the area of interest so as to minimise their influence. Boundaries should also be located in those areas of the parent domain where accuracy is high, in order to minimise the error being passed from parent to child. Careful consideration should therefore be given to the location of nested boundaries. Significant reduction in computation time was achieved by using the nested model. Nesting is a very cost-effective method of improving model accuracy by increasing model resolution in the region of interest without incurring the cost of fine resolution over the full model domain.

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