Thursday, January 22, 2009

Theta Implicit Weighting Factor

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

The Theta Implicit Weighting Factor is used in unsteady flow HEC-RAS as a means for providing numerical stability through the imiplicit solution of the St. Venant Equations. Without going into the details, a value of 1 for Theta provides the most stability, but sacrifices some accuracy. A value of 0.6 provides the most accuracy, but is very difficult to stabilize. So...what should we use for our unsteady flow models. There is some disagreement out there among users of HEC-RAS, but here is my take: The manual, and the class that HEC holds suggests working with a Theta value of 1.0, then when your model is stabilized, reduce it as close to 0.6 as possible. In my experience, moderate to complex models never are able to maintain stability with Theta less than around 0.8. At some point in the past, I realized that most of the time, reducing the Theta value did not produce significantly different results. However, many modelers insist that Theta should be reduced. I can't disagree with that. In principle, I believe this is correct. However, in practice, my experience shows that it makes very little difference.

I think this would make a great topic for a paper, and the research would be easy to conduct. Does anyone have any thoughts on this topic?

Thursday, January 8, 2009

Where will my dam breach flood go?

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

One of the first steps (usually in the proposal stage) of a dam breach model is to visualize where the flood wave will travel downstream from the dam. Is there a very obvious main channel entrenched in a canyon (if so, you have a fairly easy task). Many times, dam breach models are not this straight-forward. I have this dam breach project I'm getting ready to start. Just below the dam, two tributaries (fairly canyonized) converge with the primary stream. This spot is wide open and flat. My dilemma is how do I set up my model just below the dam. Water will come out of this breach and go up both tribs and probably swirl around quite a bit just downstream of the dam before eventually heading down the primary stream. Originally I thought about coupling a 2-D model with RAS, but that can be a real pain (not to mention possibly blow the budget). I'm now thinking of setting up the downstream area as a Storage Area in RAS with both secondary tribs included as part of that storage area. Then I'll connect the primary reach to the storage area so that the dam breach flood wave can drain downstream. I've placed inline storage areas further downstream in dam breach models, but never just below the dam. Hope it works. I would love to hear any comments or suggestions...