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Refine grid using xb_grid_ygrid

TD
Thomas Dix, modified 5 Years ago.

Refine grid using xb_grid_ygrid

Capillary Posts: 6 Join Date: 6/5/15 Recent Posts
Hi,

I am new to x-beach and am looking to refine a 2D grid which I have created using the matlab toolbox. I want a finer resolution in the nearshore and in the centre of the model to capture processes around a groyne system and a coarse resolution toward the boundaries of the grid.

I am using the function xb_grid_ygrid to refine the alongshore profile so that it is focused on the centre of the grid using the following settings:

ygr = xb_grid_ygrid(yy1,'dymin',2,'dymax',20,'area_type','center','area_size',0.5);

Where yy1 is my initial alongshore grid spacing, dymin is the minimum cell size, dymax is the maximum cell size, area_type 'centre' refines the mesh in the centre of the grid to an extent of 50% (0.5) of the total area size.

I am happy with the resolution output for the grid in the nearshore but this process creates a fine mesh to the seaward boundary extent (see attached picture) which is not required. I am looking for a way to create a grid which becomes coarser going offshore, even in the region defined using xb_grid_ygrid (the area highlighted in the attached image).

Is there a way to achieve this output in xbeach?

Thanks,

Tom
BH
Bas Hoonhout, modified 5 Years ago.

RE: Refine grid using xb_grid_ygrid (Answer)

Swell Posts: 226 Join Date: 5/20/11 Recent Posts
Hi Tom,

I assume you ask this question in context of computational efficiency. In that case the answer to your question is: no, you can't.

XBeach works with curvilinear grids. The Matlab toolbox only supports rectilinear grids (which is a special case of a curvilinear grid). Both grid types have in common that the number of cross-shore and alongshore grid cells is constant throughout the grid. Therefore specifying X alongshore grid cells near the waterline will automatically result in X alongshore grid cells near the offshore border.

Using rectilinear grids the alongshore resolution is also constant. Using curvilinear grids the alongshore resolution may vary over the domain so the grid near the waterline may cover a small beach stretch quite detailed whereas the grid near the offshore border may cover a much larger distance. Your bathy seems to be reasonably alongshore uniform, so stretching the offshore border wouldn't help you much, I suppose. Moreover, curvilinear grids are not supported by the Matlab toolbox. You can use the RFGrid tool provided with Delft3D for this.

Did you use the xb_grid_xgrid function for the cross-shore discretization? This function will make deeper areas more coarse, so you might win some computational time in the cross-shore resolution of the offshore area.

What you really are looking for is an unstructured grid. The XBeach core is being implemented in D-FLOW FM which supports that. But this is an ongoing development and I have no idea on the expected release date.

Bas
TD
Tom Dix, modified 5 Years ago.

RE: Refine grid using xb_grid_ygrid

Capillary Posts: 6 Join Date: 6/5/15 Recent Posts
Bas,

Thanks for the advice, makes sense. Yes I was looking at this to improve processing efficiency, although I haven't tested the model yet to assess run times (fingers crossed!!!). I have used xb_grid_xgrid and it does make the offshore cells much coarser which, as you said, should help to some extent.

Thanks again,

Tom
SK
Samuel Keogh, modified 4 Months ago.

RE: Refine grid using xb_grid_ygrid

Capillary Posts: 2 Join Date: 3/9/20 Recent Posts
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