Use of sub-grid approaches in the modelling of estuaries with salt marsh systems

Spearman, J. and Baugh, J. and McCoy, M. (2004) Use of sub-grid approaches in the modelling of estuaries with salt marsh systems. Technical Report. HR Wallingford, Wallingford.

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Abstract

Produced within Defra Project FD1905 (EstProc). Dendritic systems (which for the sake of this study effectively means saltmarsh and mudflats, although the general principles apply to all such systems) have irregular bathymetry which can vary significantly over length-scales of 1m or less. These small length scales are considerably less than the practically viable scales of 2D flow model grids for applied modelling. For (nonresearch) estuary modelling scales are typically of the order of 50-100m in estuaries (and greater for large systems). The normal course of action open to the numerical modeller is to enhance the grid resolution over the (dendritic) mudflat/saltmarsh up to the point where the flow within the system becomes acceptably similar to the observations. However higher grid resolutions lead to larger numbers of grid cells which can lead to impracticably long run times. The subject of this report, produced within Defra Project FD1905 (EstProc), is the development of an approach to include the effect of the small-scale bathymetric variation on flows without increasing the grid resolution. This type of approach is termed “sub-grid” modelling. The motivation behind the research is to develop a means of including the effect of the dendritic/sub-grid bathymetric variation on the current flows whilst keeping run times to satisfactory levels.

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