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katebradbrook

Flow over HX lines is grid size dependent

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I have just built my first ISIS-TUFLOW model (but concerns would apply for ESTRY too) and am concerned how the actual flow entering the 2D domain is very dependent on grid size - there are 2 factors here - one is that the ZC levels along the HX cells in my smaller grid (30m) pick up some low points missed by the larger grid (50m) - this results in greater flow to 2D domain from 30m grid along these spills. In another location where spill is over just 2 cells in both grids, this represents a flow width of 100m in larger grid and 60m in smaller grid and consequently more flow goes in here for larger grid. These factors overwhelm any attempt at traditional 2d grid dependence assessments. If I went to 10m cells, flow entering would be different again....maybe there needs to be some sort of porosity applied along HX lines that can take account of 3D zln points at a closer spacing than the grid size? Anyway, I'm now thinking of putting the spills back into ISIS and having SX links to 2d domain - not what is normally recommended, but I feel it would allow a better representation of spills (I could have elevations at a much closer spacing than my 2d grid). Any comments?

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It sounds like there is flow controlling detail in the real world domain that simply can not be expected to be well represented using large 2D grid cells. This is not a fundamental issue with 2D modelling, rather a problem with of cell size compared to the detail required. This detail can readily be put into the 1D domain instead of the 2D using spills and SX as suggested, although this may effect the precise locations of water entering the 2D domain.

If precise location of inflows is less of an issue, or the water level in the 1D is reasonably flat and low points in the spill coincide well with low points in the 2D at the boundary location (ie water will actually flow in at more or less the right place), then SX boundaries will work well.

If the precise location of where flows enter the 2D domain is critical to your project, you may wish to try using a finer grid resolution to model the boundary (one which can pick up the relevent detail). If you have multiple 2D domain capability this could then be joined to the coarser grid for the rest of the flood plain.

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Kate

Sounds to me like it is showing how your linked model is grid size dependent. If its only a few points of inflow then it is practical to refine locally but for a more typical isis-tuflow floodplain model that sounds a fair enough conclusion for the topography you describe.

SX can work as commented already but may still not give you consistent inflows if there isnt a clear path away from the inflow cell at the grid size you use as other cells may affect the downstream water level transferred back to isis. Where the flow path isnt aligned with the grid, we've had a few issues with culverts through banks unless the lower flow path is imposed on the topography which again can be dependent on grid size - though you can define porosity and restrict conveyance at cells as used at bridge piers.

Good luck.

Tony

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