Unsteady Flow HEC-RAS Model
Troubleshooting
© 2025 CivilGEO, Inc. All Rights Reserved. Except as otherwise permitted by CivilGEO, Inc., this publication, or parts thereof, may not
be reproduced in any form, by any method, for any purpose. Certain materials included in this publication are reprinted with the
permission of the copyright holder.
Trademarks
GeoHECRAS, GeoHECRAS 2D, GeoHECHMS, and GeoSTORM are trademarks of CivilGEO, Inc., in the USA and other countries.
All other brandnames, product names or trademarks belong to their respective holders.
Disclaimer
THIS PUBLICATION AND THE INFORMATION CONTAINED HEREIN IS MADE AVAILABLE BY CIVILGEO, INC. “AS IS.” CIVILGEO, INC.
DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE REGARDING THESE MATERIALS.
Civil 3D Integrated
Stormwater Software
GeoSTORM
Get Free Training
800-488-4110
www.civilgeo.com
Table of Contents
1
2
3
4
5
6
6
6
Unsteady Flow HEC-RAS Model Troubleshooting – Introduction...................
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Time Step Issues.................................................................................................
Cross Section Issues..........................................................................................
Boundary Conditions Issues..............................................................................
Computational Options Issues..........................................................................
Additional Issues and Tips.................................................................................
Other Issues….............................…...…………………........................……………
Other Tips.....................................……………...........................….................……
Chapter 1 Unsteady Flow HEC-RAS Model Troubleshooting – Introduction
Unsteady Flow HEC-RAS Model Troubleshooting - Introduction
completion, even when the user has entered poor data. Therefore, the fact that the program
executes a complete run does not necessarily mean that the results are good. The user must
carefully review the results to ensure that they adequately represent the study reach and that they
are reasonable and consistent. The software is by no means a substitute for sound engineering.
The software contains several features to assist the user in model development, problem
resolution and results review. These features include: built-in data checking, an errors, warnings
graphical and tabular output to review the results and check data consistency.
The following sections describe various issues that should be considered when troubleshooting
1. Model stability can be very sensitive to the computational time step. Lowering the
computation time step may r educe computational instabilities and make the model more
stable.
2.
3.
some of the cross sections, causing the model to become unstable.
4. Too small of a computational time step can cause the model to become unstable. For
unstable. In addition, the computational run times can get overly long.
5.
6. However, for dam failure models a much shorter time step is required to account for the steep
change in discharge.
7. A trial and error method can be used to test various computational time steps to see which of
the largest time steps works while providing accurate results and minimal convergence errors.
Time Step Issues
Chapter 2 Time Step Issues
Chapter 3 Cross Section Issues
Cross Section Issues
1. Place additional cross-sections at locations where the model cannot converge on several time
steps. Keep in mind that if the cross sections are placed too close together, the numerical
solution will cause wave steepening and the model will go unstable on the rising limb of the
2.
following situations:
•
versa.
•
versa.
•
3. It is better to use real terrain geometry for constructing additional cross sections rather than
just interpolating cross sections.
4.
structure should also overtop and not be blocked.
5.
Ineffective flow areas should be marked as “permanent” so that during the routing, the
large increase in conveyance area and cause the model solution to oscillate by having an
the next iteration or time step.
6.
conditions.
7.
cross-section invert.
8. Revise the hydraulic parameters for every cross-section and structure to provide additional
conveyance curves can be minimized by adding additional horizontal Manning’s roughness
locations.
Boundary Conditions Issues
Chapter 4 Boundary Conditions Issues
1.
the model.
2.
hydrograph value at start up.
3.
4.
numerical shock.
cause the model to fail. Take the rising limb hydrograph data and manually stretch it out for a
longer time period.
5. Inconsistent initial conditions can cause the model to become unstable immediately upon
model start up.
6.
7.
8.
Chapter 5 Computational Options Issues
Computational Options Issues
1.
convergence accuracy of the model—especially when the model has lateral weirs and storage
areas.
2.
3. Do not use the “Convert Bridges to Lids” option. This option often causes model instability
4.
models, slightly increasing these values might help in getting the model to stabilize. The
•
•
•
Chapter 6 Additional Issues and Tips
Additional Issues and Tips
6.1 Other Issues
1. If a drop structure is present in the HEC-RAS model, the best way to represent this is with an
inline structure.
2. Lateral and inline structure stability factors can improve model stability. To improve model
3.
4. If the model crashes at the beginning of a simulation, check initial conditions such as
discharge values, storage area elevations, and downstream boundary conditions. If the model
model stability locations.
5.
elevation to match initial model conditions.
6.2 Other Tips
1.
the model to fail in its computations.
2.
model. Perhaps one of the structures is causing the model to fail. If the model suddenly runs
after removing the structures, then place the structures back in, one at a time, to see which
3.
could be occurring, which often causes model instability.
4. Use the Modified Puls Routing
5. Use the Variable Time Step option where the HEC-RAS computational engine dynamically
recomputes the required time step during the simulation based upon the Courant number
Integrate HEC-RAS
directly with Civil 3D
Get Free Training
800-488-4110
www.civilgeo.com
www.civilgeo.com
© 2025 CivilGEO, Inc. All rights reserved.
The CivilGEO logo, “GeoSTORM”, “GeoHECRAS”, “GeoHECHMS” and “Ready to Engineer” are registered trademarks of CivilGEO, Inc.
Civil 3D and AutoCAD are registered trademarks of Autodesk, Inc.
ArcGIS is a registered trademark of Esri, Inc.
Call us 24x7 toll free 800-488-4110 (US and Canada)
CivilGEO, Inc.
708 Heartland Trail, 3rd Floor
Madison, WI 53717, USA
800-488-4110
608-729-5100
Toll Free:
info@civilgeo.com
sales@civilgeo.com
support@civilgeo.com
General:
Sales:
Support:
®
CivilGEO G2 Reviews
4.7/5.0 Rating, Over 400 Reviews