Monday, February 8, 2016

Is it normal with a 2D full momentum model to run without any WSEL errors? Or should I expect a few?

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants 
Copyright © The RAS Solution 2016.  All rights reserved.   

You can have a 2D full momentum model without errors if:

  • Your cell size and computation interval is appropriate
  • Everything is moving very slowly
  • Stages are high
  • Slopes are shallow
  • N values are high
  • Discharge doesn’t change too suddenly
  • It’s a 2D only model (i.e. no connections to 1D)
  • You are a skilled HEC-RAS modeler
  • A combination of any or all of these (and some other factors I’m not thinking of), sure, you can have no errors. 
Is it normal?  I certainly wouldn’t call it abnormal.  Actually, you’d be surprised at how well the 2D solution works compared with the 1D solution.  I’ve found I get a lot less errors and instabilities in 2D areas than I do with 1D reaches.  Most of the time, my big errors happen at and around 1D/2D connections.

If you are lucky to get a simulation with no errors, double check your solution-make sure it looks reasonable.  Your instincts are correct and you should question the results.  But if everything looks good and reasonable, chalk it up to your skill and claim your bragging rights! 

1 comment:

  1. Hi Chris,

    I am modeling a 1D/2D dam break (sunny day) with v5.0.1. The reservoir is modeled as a 1D SA element. Downstream is modeled 2D with 100ft grids (grids around break lines are smaller). The dam (weir) and breach is modeled with the SA/2D Area Connector. I'm using a 0.5 second time step with the full momentum equation.

    In the simulation I received the following message about 2 minutes after the end of the breach formation time:

    In green text it read...
    1D/2D Flow Area -31 Laf Dam

    The error lasted for about 23 minutes in the simulation (with the number becoming less negative with each time step); the simulation did not crash and has kept running (not finished running yet).

    Do you have any thoughts as to what caused this message?

    Thank you.

    Eric

    ReplyDelete