Friday, January 11, 2019

Preview of the new Finite-Volume Approach for 1D Reaches

One of the most anticipated new features soon to come in the next major version of HEC-RAS (Version 5.1) is the option of running unsteady 1D reaches with a finite volume solution scheme.  This will be a fantastic addition to HEC-RAS.  Gary Brunner recently gave me a brief overview of the new finite volume feature we can expect.  But before you ask, there is no set release date for Version 5.1 yet.  But I'm hoping we'll see it within the next year or two.

1D Finite Volume Solution Algorithm

By Gary W. Brunner, P.E., D.WRE
Senior Technical Hydraulic Engineer
Hydrologic Engineering Center

A brand new solution algorithm has been developed for 1D modeling.  A Finite-Volume solution approach, similar to what was added for 2D modeling will be available for 1D modeling in HEC-RAS version 5.1.

The current 1D Finite Difference solution scheme has the following deficiencies:
  1. Cannot handle starting or going dry in a cross section
  2. Low flow model stability issues with irregular cross section data
  3. Extremely rapidly rising hydrographs can be difficult to get stable
  4. Mixed flow regime (i.e. flow transitions) approach is approximate
  5. Stream junctions do not transfer momentum

The new 1D Finite Volume approach has the following positive attributes:
  1. Can start with cross sections completely dry, or they can go dry during a simulation (wetting/drying)
  2. Very stable for low flow modeling
  3. Can handle extremely rapidly rising hydrographs without going unstable
  4. Handles subcritical to supercritical flow, and hydraulic jumps better.
  5. Junction analysis is performed as a single 2D cell when connecting 1D reaches (continuity and momentum is conserved through the junction).

Additionally, the new 1D Finite Volume approached is solved in the same matrix as the 2D equations.  Solving in the same matrix allows for faster 1D/2D model solutions and more accurate flow transfers between 1D and 2D elements.  The equations are solved together and all hydraulic connections are updated together on an iteration by iteration approach, rather than separately, as in previous versions of HEC-RAS.


  1. Hi Chris. 5.1 really does sound like a giant step forward. What kills me though is your comment on 1 to 2 years! I had been building up an expectation of around July this year.

    1. It's possible Mark. HEC does not give out anticipated release dates anymore probably because it is so hard to forecast how long testing takes. My 1 to 2 years estimate is purely a guess on my part. Let's cross our fingers that I'm wrong and it comes out even earlier!

  2. I'm excited to see this addition! I'm hoping the junction method will solve some of the model flexibility issues I'm trying to work around using a storage area junction approach. In particular if this junction method will allow for the mainstem reach to remain continuous and that you will be able to connect and disconnect tributary reaches without having to rename reaches and update flow files when new junctions are added?

    1. I don't think that's the direction they're going with this, but I could be wrong...