TUFLOW Message 2991

TUFLOW Message
WARNING 2991 - Negative U depth at ",a,".

Alternate Message
WARNING 2991 - Negative V depth at ",a,".

Message Type
WARNING

Description
A 2D negative depth has occurred at the cell side. This indicates the solution failed to converge at this point in the 2D domain and is an indicator that the model may become unstable. Note that a separate warning is issued for convergence issues in the 1D domain (see Message 1991).

Suggestions
Import the messages layer in to your GIS package. Check for the number and time at which these warnings occur. A few warnings at a single location or timestep in the simulation are not normally an issue. Persistent warnings however warrant further investigation. Check the following:

• Look at the _zpt_check file to see if there is a steep change in topography. Where appropriate, smoothing out the topography or increasing the roughness may help stabilise the model.
• Look at the uvpt_check file which shows the Manning's n value at each cell side. Check this roughness is suitable.
  
• Repeated warnings may occur due to badly behaving boundary conditions nearby. This can be 1D/2D, a external 2d (such as a HQ boundary) or a 2D/2D boundary.
  
• View the velocity vectors prior to the message , verifying that the flow patterns are realistic.
  
• Check that the timestep is not too large, and the Courant number is appropriate. Rather than simply reducing the timestep, first check the suggestions above!
  

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