HEC-RAS: spillway approach height for overflow gate

[MarkusV]

Hello everyone,

I am trying to model a simple (open top) overfall weir in HEC-RAS 4 (e.g. a stop log weir). HEC-RAS can handle sharp crested overfall gates like this with the Rehbock equation. The weir coefficient according to Rehbock needs the weir height ("spillway approach height" in HEC-RAS). So far so good.
Problem: HEC-RAS wants me to enter the spillway approach height as a constant property of the gate in the geometry editor. Which doesn't make sense, since the value obviously changes with the gate position (A quick sketch is attached).

This negates the whole point of having the gate openings be part of the flow data, doesn't it, since I would have to alter my spillway approach height (i.e. my geometry files) for every run with different gate openings.

Am I missing something here? Or is this a flaw in HEC-RAS? Any insight or advice would be appreciated.

Cheers
Markus

 

[sknight701]

Sounds like you might want to try inputting performance curves for your gage. Select "user defined gate curves" from the gate type selector, and input the performance data. See pages 6-71 thru 6-74 of the users manual. Hope this helps.

 

[MarkusV]

Thank you for your suggestion. You are right, of course, in that if all else fails I could use a user-defined curve.

However, the whole point of the Rehbock equation (and the beauty of a sharp-crested weir ) is, that I can dynamically calculate a relatively precise weir coefficient without the need to supply a curve. And HEC-RAS now specifically offers calculating the weir coefficient using the Rehbock equation (is this new in 4.0? I think so...), but it should then automatically derive the spillway approach height using the gate opening position assotiated with the current profile instead of asking for some constant value the way it does.

 

[sknight701]

I see your point. The program should take the gate opening from the flow data, a mean ground surface elevation from the geometric data, and calculate the approach height for the profile.

I must admit, you have me stumped. I'll try looking into this a little more. If you find out anything, please post it.