HEC-RAS 3.0

Dear HEC Users

I have successfully used HEC-RAS3.0 for unsteady flow modelling on six projects recently. But I am stuck with the seventh similar project. A error message turned out during post processing and read as "a change (or known value) in WSEL or EG at a node in river station xxx caused the WSEL below the bottom of the cross section".

This error message is the same as in the example I sent to the HEC User mailing list in about March this year, soon after the release of HEC-RAS version 3.0. An American HEC user created a simple two-cross section project (I did the same) and got the same error message. When a relatively small initial flow is given and the downstream Boundary condition is set as normal depth, this cross section would be the second last (interpolated) cross section. If a larger initial flow is given, the cross section would be another one away from the downstream end.

Checking the computation log file found that initial water surface level (shown in ft though Metric unit system is selected) at many cross sections are below the channel bottom when a relatively small initial flow is given . This did not happen for the other six projects I mentioned above.

I tried increasing the number of points in "HTab Param", tried raising "starting EL", tried different time steps, tried different initial flows, tried different distances of interpolated cross sections. All did not help.

I also tried to run with a restart file as suggested in the User manual. Not knowing the format of the required restart file, I used the default file (xxx.O01) appear in the restart file box. The error message indicated that this is a invalid file (wrong format). The box in the HEC-RAS 3.0 user manual shows a file name intl.con but what is the format? A run seems do not generate any file with a extension of .con.

Any help will be very much appreciated.
Thanks and regards!
Qi Xie Ph.D
q.xie@geo-eng.com.au

Hello Qi;

I also tried your project files. It seems that the problem is localized to, as Mike said, the upper most region of the reach and more specifically, to somewhere between RS 8715-7950 (for Q~100 cms) and 8715-6660 (for Q~500 cms). The flow regime there tends to be at or very close to the 'critical' and that could be a source of the problem since the unsteady flow component of the RAS 3.0 " was developed primarily for subcritical flow regime " (User's Manual,Jan 2001,page 1-3).

Regards,
Mostafa Bozorg-Zadeh
Independent Consulting Engineer