If you detect a possible stability problem, and you are unable to find the location using the graphical output discussed above, another option for finding the location of the problem is to turn on the detailed log output for debugging. Detailed log output is turned on by selecting Options and then Output Options from the Unsteady Flow Simulation manager. When this option is selected the following window will appear:

Figure 7 54. Detailed Log Output Control
As shown in Figure 7-54, the tab labeled "Detailed Log Output" allows the user to control this option. Three check boxes are listed. The first box can be used to turn on an echo of the hydrograph input to the model. This can be used to ensure that the model is receiving the correct flow data. The second check box can be used to turn on an echo of the computed hydrographs that will be written to the HEC-DSS. This is a good option for checking what was computed. However, if the user has selected to have hydrographs computed at many locations, this could end up taking a lot of file and disk space. The third check box, labeled "Write detailed log output for debugging", is used to control the detailed output of results from the unsteady flow simulation. Selecting this option will cause the software to write detailed information on a time step by time step basis. This option is useful when the unsteady flow simulation is going unstable or completely blowing up (stopping). Checking this box turns on the detailed output for every time step. The user has the option to limit this output to a specific time window during the unsteady flow simulation. Limiting the log output is accomplished by entering a starting date and time and an ending date and time. Additionally the user can request that detailed log output only be written when the program reaches a certain number of iterations.
Viewing Detailed Log Output. After the user has turned on the detailed log output option, re-run the unsteady flow simulation. The user can then view the detailed log output by selecting View Computational Log File from the Options menu of the Unsteady Flow Simulation window. When this option is selected the detailed log output file will be loaded into the default text file viewer for your machine (normally the NotePad.exe program, unless you have changed this option within HEC-RAS).
The detailed log output file will contain the following output:
DSS Output: Shows all of the hydrograph data that will be used as input to the model, including data read from HEC-DSS.
Unsteady Flow Computations Output: Detailed unsteady flow calculations including:

  • Job control parameters
  • Initial conditions calculations
  • Detailed output for each time step

Table Output: Final computed hydrographs that are written to HEC-DSS.
The program lists the computed initial conditions from a backwater calculation for each of the river/reaches. They are listed in the order they were computed during the backwater analysis, which is downstream to upstream. An example of the initial conditions output is shown in Figure 7-55 below.

Figure 7 55. Example of Initial Conditions Output.
During the unsteady flow computations, the program will output detailed information for cross sections, bridges/culverts, inline weir/spillways, lateral weir/spillways, storage areas, and storage area connections. This information should be reviewed closely when the software is having stability problems. An example of the detailed output for cross sections is shown in Figure 7-56 below.

Figure 7 56. Detailed Output at a Cross Section
When the program has stability problems, it will generally try to solve them by iterating. An example of a stability problem is shown in Figure 7-55. In this example the program did not solve the equations to the specified tolerances, and therefore it was iterating to improve the solution. As shown in Figure 7-57, the program iterated to the maximum number of iterations. At the end of the iterations a warning message states "WARNING: USED COMPUTED CHANGES IN FLOW AND STAGE AT MINIMUM ERROR. MINIMUM ERROR OCCURRED AT ITERATION XX." This message means that the program could not solve the unsteady flow equations to the required tolerance within the specified number of iterations (default number of iterations is 20). Therefore it used the iteration that had the least amount of error in the numerical solution.

Figure 7 57. Example Detailed Time Step Output for Cross Sections
One way to find and locate potential stability problems with the solution is to do a search in the file for the word "WARNING". The user then needs to look at the detailed output closely to try and detect both where and why the solution is going bad.
The variables that are printed out during the iterations are the following:
Iter= Iteration Number
River = The name of the river in which the largest stage error is occurring.
Station = River station with the largest error in the calculated stage.
Elev = Computed water surface elevation at that river station
DZ= The "Numerical Error" in the computed stage at that location
Storage= Name of the storage area.
Zsa= Computed elevation of the storage area
Dzsa= The "Numerical Error" in the computed storage area elevation.
River= The name of the river in which the largest flow error is occurring.
Station= River station with the largest error in the calculation of flow
Q= Computed flow
DQ= The "Numerical Error" in the computed flow at the listed river station
After the iterations output, the program will show the computed stages and flows for all of the cross sections in which the user has selected to have hydrographs computed. This is also useful information for detecting model stability problems. It is not always obvious as to which cross section or modeling component is causing the problem. Sometimes the program may blow up at one cross section, but the real problem is caused by a cross section upstream or downstream from this location. Detecting, finding, and fixing stability problems will require lots of experience to become proficient at it. Good luck, and don't get to discouraged!!!