10 Tips for Troubleshooting Unsteady HEC-RAS
Unsteady HEC-RAS can be a useful tool for modeling complex river systems; however, it can also be more difficult to troubleshoot than steady state models. The model sometimes crashes suddenly and displays a red bar across the computation window stating that the model is “unstable.” Below are some tips to avoid the dreaded “red bar” and for developing a stable running unsteady HEC-RAS model.
- Model stability can be sensitive to the computation interval. Lowering the computation interval of your plan file may reduce your error and make the model more stable.
- If a drop structure is present in the model, the best way to represent it is as an inline structure.
- Lateral and inline structure stability factors can improve model stability. In order to improve model stability, try increasing your stability factor from 1.0. The stability increases as you approach 3.0, but keep in mind that this also makes your accuracy decrease. The most accurate answer will come from using a stability factor of 1.0.
- Try adding extra cross-sections in locations where errors are higher than 1.0 or where the model cannot converge on several time steps.
- Check if an appropriate weir coefficient is used for deck and roadway. Check the elevations of the ineffective flow areas at structures. If the structure is overtopped, the ineffective flow area on the downstream side of the structure should overtop simultaneously.
- Adjusting Theta (0.6 to 1.0) can improve model stability or increase the accuracy of the output. As you approach 1.0 your stability will increase, but if you approach 0.6 your accuracy will rise.
- If the model crashes at the very beginning of the simulation, check your initial conditions such as flows, storage area elevations and downstream boundary condition. If it crashes during the simulation, check the water surface profiles to pinpoint the location of the problem.
- The HEC-RAS model cannot go dry at any time during a simulation. Adding an initial flow and keeping it as small as possible may provide the necessary stability. Also make sure that the initial flow matches your minimum flow.
- Adjust the starting elevation of the hydraulic curves for cross-sections so that the elevation starts at the cross-section invert.
- Revise the hydraulic curves for every cross-section and structure. Keep in mind that you need to have smooth conveyance curves. In most cases abrupt changes can be fixed by adding extra horizontal n-value break points.