warning messages.

[kjmartin]

I am trying to rid my model of all warning messages.

Does anyone know how to get rid of the "energy equation can not be balanced" or the "parabolic search meathod failed to converge on critical depth" errors? Any hints on common model mistakes that would cause these?

Any help would be much appreciated.

Kim

[Chris Maeder]

Kim:

The most common correction for the warning messages that you are getting is to add additional cross-sections to your model. It sounds like your cross-sections are spaced too far apart.

Also, you might have a mixed flow regime (i.e., both subcritical and supercritical flow). You might want to run your analysis as a mixed flow model.

Finally, HEC-RAS has an option to find multiple critical depths. By default, HEC-RAS uses the "Parabolic Method", which is faster than the "Multiple Critical Depth Search" method, but only finds one critical depth. To change this, from the Steady Flow Analysis dialog box, select Options | Critical Depth Computation Method. This helps for difficult cross-sections.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com

The answer to how to get rid of the warning message "energy equation can not be balanced" is not an easy one. There are many potential solutions to this one. One way I usually try is to decrease the distance between cross sections by using the interpolation method if the u.s. and d.s. cross sections are similar or by generating new cross sections from the mapping when they are not similar. The potential causes of the message are abrupt changes in flow, cross section shape, Manning's n, channel
slope, etc.

I don't know that you need to worry about getting rid of the "parabolic search method failed to converge on critical depth" warning message. That is usually caused by the shape of the cross section not being conducive to determining the critical depth of cross section using the parabolic search method. The shape/area/roughness of the cross section changes abruptly at the elevation near the computed critical depth. You probably only need to be concerned about this message if your computed flow depth is near critical depth or is transitioning between subcritical and supercritical flow.

John G. Morgan
john.morgan@cte-eng.com

To Kim,

HEC RAS likes the cross-sections NOT to have flat bottoms, also consider the ratios of convergence (4 to 1) and expansion of (1 to 1 ). Flows outside these ranges are just storage.

DD