HEC-RAS Overtopping Problems

[jeffd64]

Hi All,

I am currently running into a problem with HEC-RAS. I currently and modeling a bridge where the immediate upstream cross section is approximately 26-ft away. When I run the model, the water surface elevation at the immediate upstream x-sec is 2-3ft above the bridge x-sec. The model currently has all the flow being conveyed through the structure. I know this is not physically possible.

I am using the standard step method for both high and low flows.

I used the momentum equation but that did not work.

I also used the pressure/weir flow module too. But that did not work either.


Any tips?

 

[beej67]

It *may* be physically possible depending on the geometry. That's not to say your model is right, but rather that it *could* be right, particularly if your bridge deck is close to the crown of the culvert. Rapid drops in HGL before the opening of a culvert typically happen when the culvert is in inlet control, but the slope of the culvert is steep, so there's a free surface in the pipe while maintaining a high headwater just outside it. Go fish through the FHWA nomagraphs and you'll see some diagrams of the case.

If I saw this behavior, and I didn't expect it, I would probably start by checking the culvert slopes and inlet loss coefficients for data entry error. HEC-RAS is quirky and there could be any number of things going on, but those two are the first I'd check based on the actual science, to see if my model was actually giving me a correct answer for the wrong input.





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