8.3Identifying the Start of an Instability
Instabilities usually start with a one or a few computational points “bouncing” as a result of poor convergence of the mathematical equations being solved. To help identify the start of an instability, negative depth warnings are issued if the depth in a 2D cell or a 1D node becomes falls below –0.1m. Negative depth warnings are usually a pre-cursor to an instability. It is not uncommon, particularly in areas of rapid wetting and drying for negative depths to occur before the computational point is made dry (inactive). Hence a buffer of –0.1m is used before reporting a WARNING.
The WARNINGs are sent to the _messages.mif file. Bring these into your GIS and they point directly at the location of the negative depth. If the number of these warnings are substantial (eg. if a model remains stable but with minor instabilities), select some of the first negative depth warnings in the attribute data (Browser Window in MapInfo) and display just those. The warnings are in order of occurrence. By tracing through the negative depth warnings in the vicinity of the instability, the trigger point of the instability can often be located.
Also see Depth Limit Factor (1D domains) and Instability Water Level (2D domains).
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