HEC-RAS Modeling Best Practices

The following best practices should be used when defining, analyzing, and reviewing a HEC-RAS model:

  1. The model results should be reviewed for consistency and accuracy.
  2. The cross section detailed output warning messages should be reviewed and be able to be explained.
    Cross Section Results
  3. The profile plot should be checked, especially at roadway crossings. The energy gradeline should be plotted along with the computed water surface elevation.
  4. Bridge opening results should be closely reviewed to make certain that the low chord clears the computed energy gradeline elevation.
  5. Cross sections should be tall enough to contain the defined discharge values. Otherwise, the software will extend the cross section ends vertically to contain the flow.
  6. Cross sections need to be placed at representative locations describe changes in geometry.
  7. Additional cross sections should be added at locations where changes occur in discharge, slope, velocity, and roughness.
  8. Cross sections need to be added just upstream and downstream of roadway crossings (bridges and culverts), inline structures (dams and spillways), and levees (flood walls).
  9. Steeper channels require more cross sections.
  10. Streams flowing at high velocities may require cross sections every 100 ft (30 m) or less.
  11. Large uniform rivers with flat slopes in rural areas require cross sections every 1000 to 1500 ft (300 to 500 m).
  12. Rivers in urban regions require cross sections every 500 ft (150 m) or less.
  13. As a starting point, cross sections can be spaced at 5 times the active channel width.
  14. Review any cross sections where the computed water surface elevation is equal to critical depth. Likely these cross sections are flowing as supercritical flow and the mixed flow (subcritical and supercritical) HEC-RAS analysis should be performed.
  15. Downstream boundaries cannot have a flat or adverse (negative) slope. If necessary, extend the downstream end of the model to provide positive (downward) slope for the channel bed.
  16. Check the ineffective flow areas top elevations at roadway crossing structures. If a roadway structure is overtopped, the ineffective flow area on the downstream side of the structure should also overtop and not be blocked.
  17. Start out with a simplified model and confirm the model is running successfully. Then add complexity to the model in iterative steps, confirming for each iteration that the model is running successfully.

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