When computing or validating a model, the following error message is generated:
"Elevation (Relative) values within the Irregular shape definition must be non-negative."
This can occur when the relative elevation of an irregular channel cross section type has values that are negative.
"Elevation (Relative)" is basically a reported water level, so the elevation entered in this table is relative to invert elevation for the cross section. Basically, the lowest value in the table should be a zero, which will be at the invert elevation.
Browse to the cross section(s) generating the error message and open the properties. Click the ellipsis (...) button in the cell for irregular Channel Section.
This will open the Irregular Channel dialog.
Make sure that the elevation values entered for "Elevation (Relative)" are set so they are relative to the invert elevation of the cross section. The lowest value will be zero. The highest will generally not go above the ground surface elevation.