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jiwaszko

Water Ignores Altered DEM

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Hi,

I'm in a bit of a odd situation; I have created a levee in my 2d domain using Read GIS Z Shape (Z=2, Shape Opti= ADD). I can see that the DEM has successfully been amended to include the desired levee by checking the "DEM_Z.flt" check file . However, the modelled flood appears to ignore the altered DEM (see attached, red is DEM_Z.flt, blue is modelled water level).

Furthermore, looking at the depth results I can see the depth is -1.65 m over the levee, however checking the messages.mif file, no warnings of negative depths are recorded for this location.

The model incorperates three (3) 2D domains, but I don't think this would be presenting the issue.

Does anybody have any ideas about how I might be able to remedy this? I'd love to hear your input.

Cheers,

Josh

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Hi Joshua,

The 2d_zsh polyline in your model had been set as thin line, which will raise the elevation only in the cell sides and cell corners. The water level output is based on cell centres and it gets interpolated into the cell corners for the map output. When a thin 2d_zsh line is raising the elevation in the cell sides and water is present at both sides of the raised elevation, the map output would interpolate the water level from the cell centres on both sides of the thin zsh line to the cell sides and cell corners. The water in not going through the levee, but the map output might be misleading in this case by displaying interpolated water level at the location of the levee. You can examine the velocity vectors around the levee to confirm if the water is going through or around the levee.

The negative depth in the map output is caused because it is calculated from the interpolated water level and the cell corner elevation. in this particular case, the interpolated water level is lower than the cell corner elevation, so the water depth became negative. This negative depth is normally prevented by the default “Zero negative Depths == ON” command, but it was turned off for some reason in this model. Setting to OFF should only be effective if the original cell interpolation method (Map Output Corner Interpolation == METHOD A) applies, but your model uses Method C.  Using the OFF option is then not recommended.

The Zero negative Depths command is a legacy command provided for backward compatibility. Default setting to ON zeroes depths at cell corners for map output if negative. The negative depth could arise in old builds of TUFLOW when interpolating the water level at the cell corners from the surrounding cell centres, due to the ZH Zpt being higher than the interpolated water level. 

Please change the Zero Negative Depths to ON. If you wish, you may consider changing the thin 2d_zsh line to thick by changing the Shape_Width attribute from null value to 3 to overcome the issue with misleading map output. 3m is the cell size, so the 2d_zsh will raise the whole cell instead of the cell sides only.

 

Kind Regards,

Pavlina

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Is it possible that the water is ending up behind the embankment via another route? If the embankment is only a thin z-line/shape and cells on either side end up wet then in the results it will look like the water surface goes straight through the embankment, but isn't actually doing so. The step change at the embankment amid the otherwise very flat levels might indicate this..? Perhaps check our your velocity vectors and see if any are going through the embankment, or if all velocities by the embankment are actually running parallel to it (in which case the embankment is holding firm). If you haven't already, it'd also be a good idea to step through your model results and not just examine the maximum water surface, as this will be more informative for the flood development and progression, so would hopefully give you a clear pointer to the source of your flooding on the 'wrong' side of your embankment.

Hope that helps!

...Is what I wrote on the day that the original question was posted! Just found it when tidying up some old tabs in my browser. Oops. Mostly it agrees with Pavlina, but I think there are a couple of extra bits that are worth having out there.  Sorry if it's too late to be of use on this project...

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