Very fine 2D Modeling

Written by Mike Hross and Chris Goodell | Kleinschmidt Associates

Copyright © The RAS Solution.  All rights reserved. 

I'm a big fan of testing HEC-RAS to the limits.  What can it do?  What can't it do?  These are questions we should all ask at the beginning of a project.  Can HEC-RAS actually prove to be a useful tool in answering hydraulic-related questions for our specific problem?  Shortly after coming to work at Kleinschmidt I was posed with the question, "Can we use HEC-RAS to model a nature-like fishway?  And more specifically, can we extract data from HEC-RAS to inform us of velocities and shear stresses between the many boulders and in the many pools that make up such a complicated feature?"  The answer was YES!  Check this out:

Here's an example of making HEC-RAS work at a very small cell level, in order to get detailed results for design considerations.

And a zoomed in view:

The fishway is constructed using 11 concrete weirs and is approximately 300 feet wide by 400 feet long. The boulders are approximately 5 feet in diameter, with 2.25 feet projecting above the crest of the weirs. The gaps between the boulders range from 3 to 5 feet. In total, there are over 700 boulders in the fishway. 

For the simulation shown, the drop from normal pond to the tailwater downstream of the last weir is approximately 6.4 feet or 0.58 foot per pool. The fishway is passing 890 cfs, 237 cfs of which is spilling over the top most weir from the headpond and the remainder is being supplied by 5 overflow gates. The 5 gates have varying levels of submergence on their tailwater sides, depending upon each one’s location, and they are each passing from 108 to 137 cfs. The velocity varies throughout the fishway, but is generally less than 6 feet/second. The nominal cell size in the mesh is 3 feet but decreases to 0.75 foot on the tops of the weirs to capture hydraulics between the boulder gaps. The largest cells have sizes of 96 feet and are located primarily in the headpond away from the fishway. 

The model is run using the full momentum equation, with an eddy viscosity mixing coefficient of 0.44. The computational timestep is 0.1 second and the Courant Numbers max out at around 0.6, with the highest values being in the cells on the downstream sides of the gates where water is flowing into the fishway. The timestep is very small, but the model does not need to simulate a very long time since it is a quasi-steady model, meaning there are constant boundary conditions (a constant inflow hydrograph at the upstream end of the model and a stage hydrograph at the downstream end of the model).  The model achieves steady state in less than 1 hour of simulated time.  Model run-times averaged about 3 hours.  

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!