HEC RAS Dam Break Analysis

[damitall]

I am having difficulties with my dam break model:

The model seems to work fine for a say 100 year event with an unsteady flow of about 100 cfs peak flow.

I have problems when I try the dam break flow hydrograph which as a peak flow of 11,000 cfs which rises and falls over a 1 hour time interval. I believe my problem lies within the steep change in flow over even the smallest time interval. I've tried reducing the time interval to 1 second and put cross sections a up to 5 feet distance but nothing has helped.

Any suggestions.
Thanks
Dave

 

[SMIAH]

Instability can be the result of, let's say, a couple of factor.

You could check the unsteady defaults flow parameters.

I believe this is case by case...

 

[DMcGrath]

Been there, done that...After a while you get used to seeing the red status line appear during the run.

Four things I have done that seem to help at times:

1. For your hydrograph, select the check box for "time step adjustment options" and put in a reasonable flow change in the box, say 10 or 20 cfs. This will help the program swallow the rapid, large changes in flow more easily.

2. Also in the hydrograph, establish a minimum flow of 50 or 100 cfs. HEC-RAS hates to run dry.

3. Since you are concerned with dam breach analysis and potentially only the max high water, you may be able to ignore the initial instability of the run as the model settles down for the "real" flows.

4. Under the "Options" for unsteady flow analysis, put in some warm-up time steps. Sometimes this helps too.