TUFLOW Message 2999

TUFLOW Message
UNSTABLE 2999 - Below <Instability Water Level> at <Grid Cell>. Time = <Simulation Time>; Water Level = <Water Level>; 2D Domain = <2D Domain>.

Alternate Message
UNSTABLE 2999 - Above <Instability Water Level> at <Grid Cell>. Time = <Simulation Time>; Water Level = <Water Level>; 2D Domain = <2D Domain>.

Message Type
ERROR

Description
Modelled water level went above or below the threshold limits.

Suggestions
Import the messages layer in to your GIS package. Check for the number and time at which these instabilities occur. Check the following:

• Look at the elevations with the _DEM_Z check file or the 2d_zpt check file to see if there are any issues with the 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.
  
• Does the instability 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. These may need to be reviewed / modified.
  

If the instability occurs at the start of a simulation close to a boundary check that the initial conditions match the boundary conditions at the start of the simulation. For example if the IWL is set to 0 and the boundary water level is 10m at the start, this will cause a 10m surge of water into the model and will likely cause an instability.

• View the velocity vectors prior to the instability may help identify the start of the instability. There is little point at viewing the vectors at the final timestep as these are likely to be wild!
  
• 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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