Lateral Weir and flow convergence

[elisabethb]

I am new to HEC-RAS – but have read through the users manual and reference manual before getting started…

I am modeling a creek with a semi-abandoned oxbow. The oxbow receives flow from the main channel only under high flow events once the water has reached a high enough elevation that it can flow over a steep left bank. The intention of my modeling is to see how frequently there is water in the oxbow to see if it will be viable fish habitat during high flow.

I set up the main channel model first – using a lateral weir to signify the connection to the oxbow. When I run the model with the tail water connection to “out of reach” and the flow optimization on there is no problem and I get what look to be reasonable results. Then when I added in the oxbow reach and changed the tail water setting of the weir to “cross sections of a river/reach” the model fails to converge. Any insight into what I am doing wrong or better approaches is much appreciated.

Best,
-E

[Scott Miller]

I am reading this as a 1D to 1D model. Is that correct? Open the Unsteady Flow Analysis window, where you set up the plan, and open Options>Calculation Options and Tolerances… On the General tab, you can set the 1D theta, the number of iterations, and the Lateral Structure Flow Stability Factor. Make sure theta is 1.0 to start out with, and try a stability factor of 3.0. They both trade off accuracy for stability. See if that helps.

[elisabethb]

Hi Scott,

Thanks for the response. Yes – I am using a 1D model – but also steady flow analysis. It is a small system and we only have a limited amount of measured flow & stage data so I didn’t think I had enough info for an unsteady analysis.

Do you have any other suggestions?

[cameron]

You can turn on detailed output for lateral structures in the plan|options|Set Log File Output Level menu. Just specify the lateral structure and use level 2 output or greater.

Once it is done running, a log file is created in the project folder which shows the iterations and what it is trying to solve. It makes an assumption based on your initial assumption in the Steady Flow Data|Options|Initial Split Flow Optimization window.

It tries your first assumption and then calculates something, it then takes what was calculated and keeps going until it either converges or stops due to max iterations. You can specify a few things in the tolerance window on how many iterations and a Flow Tolerance Factor.

I usually look to see if it is converging or with the default 20 iterations. I then begin to tweak the weir coefficient up or down to see how sensitive it is. You can then play with it until it converges or sometimes you may have to break it up into multiple lateral structures to get it to work correctly.

One thing to note is that the discharge in the split reach is important as it can cause the model to converge or diverge really easy. What I mean by this is if you have the flow spilling into another reach that is supposed to be dry, HEC-RAS doesn’t allow dry reaches so you have to put a dummy flow in there (10cfs or something). The dummy flow can impact the lateral structure so you may have to tweak that as well.

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