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Bed roughness - input directly from grid file (*.grd)

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Hi all

I have done a search on whether it is possible to use the information directly from a grid file (*.grd) as the bed roughness input for TUFLOW. We have been provided a roughness raster grid provided in TAB format which contains 1/n values. The TUFLOW manual recommends however, that the surface roughness be set using the the standard method of creating regional polygons with a set material code and referenced to a TMF file.

From the few topics I've seen, it appears that the command "READ MI CNM ==" is the way to go. I have exported the grid file using Vertical Mapper into a TAB file and then exported it as MIF. Can someone advise on the following:

- are there additional commands required in the TGC and TCF files in addition to the "READ MI CNM"?

- What is the table structure of the CNM tab file?

- I suppose the other material commands lines such as READ MI MAT and the reference to the TMF file (as per the standard method) should be commented out?

Thanks for any assistance, in particular the steps required.

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Hi,

TUFLOW does not directly read in the vertical mapper .grd files.

As you suggest you can do this by specify polygons of CnM values (this could be done by contouring your grid). It could also be done by using the Read RowCol CnM == command (previously Read MID CnM), this file will need to contain a list of the cell and the value in .mid file format. From the TUFLOW manual:

The first three columns in the file must be "n, m, <value>", where n and m are the 2D grid row, column and <value> is the value bed resistance value (CnM). To create this I would take a zpt_check file, select the cell centres and save these into a separate file. This will allow you to define a roughness for each cell, the zpt check file already contains the row (n) and column (m) as the first two attributes. You can then do a point inspection, to assign a value. Lastly you will need to update the table structure so the your columns are n,m and value as described earlier. This can then be read into TUFLOW. You could go through a similar process with a grid check file to average the value across a cell rather than inspecting at centres. This approach is similar to the process used to define elevations with 2d_zpts.

You will need to set the Bed Resistance Model used to Manning's M (tcf command), if you are going to use the 1/n data.

Bed Resistance Values == MANNING M

You may (depending on the version of TUFLOW you are running) need to change the Bed Resistance Cell Sides setting (.tcf). By default TUFLOW interrogates the material values (polygons) at the cell sides (where flow is calculated), by defining the roughness at each cell, TUFLOW will need to interpolate the value to the cell sides (options for interpolation are Average n, average M and Maximum n). See the manual for more info on these commands.

You could also create a gridded Manning's n dataset by calculating 1/M, if you wanted to keep your model in using the Bed Resistance Values == MANNING N.

Hope that helps.

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