TUFLOW Message 1991

TUFLOW Message
WARNING 1991 - ",a,": Negative depth at Node ",a,": "," y =",f7.2," Bed =",f7.2," Iter =

Alternate Message
WARNING 1991 - ",a,": Negative depth at Node ",a,": "," y =",f7.2," Bed =",f7.2," dh =

Message Type
WARNING

Description
Numerical solution has resulted in a negative depth at 1D node. This indicates that the model is unstable or unhealthy in this area. Note that a separate warning is issued for convergence issues in the 2D domain (see Message 2991).

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:

• Incorrect bed levels upstream or downstream;
  
• A large change in cross-sectional area;
  
• Very steep channel entering a gently or adverse grade channel;
  
• Sudden change in flow area (for example a culvert width entered in mm not m, 1200mm rather than 1.2m);
  
• Issues with 1D/2D linking (incorrect use of Z flag on culvert inlet / outlet);
• Too high a timestep; and
  
• Too small a nodal area.
  

For a more detailed description on identify the start of instability please see Section 8.1.3 Tips for an Unhealthy 1D Domain and Section 8.1.4 Tips for Unhealthy 1D/2D Links of the TUFLOW manual (reference for 2010 version).

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