Weirs in HEC RAS 2D

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Weirs in HEC RAS 2D

Ikenna
Dear All,
I have a few questions to ask - apologies if these have been addressed elsewhere or are otherwise obvious; I'm a first time user.
I've got a 2D model set up in HECRAS with culverts modelled as SA/2D connections using the 'weir and culverts' option. The area being modelled is VERY flat and we are trying to optimise culvert openings to allow for what is essentially overland flow across a road. I believe it is set up correctly because when i look at result profiles i can see the effect of the weir (road centreline), and in the geometry editor i can see HW and TW as well as flow over the weir.
My questions are:
1. HECRAS uses only the 2D solver during the runs - is this correct? Or have i missed something in setting it up?
2. In RAS Mapper I'm expecting to see a water line on either side of the weir, which i have modelled to be 70m wide (broad-crested), but HECRAS seems to ignore the width of the weir in the 2D results... again, have i done something wrong?
Can provide screenshots.
Thanks,
Ikenna
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Re: Weirs in HEC RAS 2D

jarvus
1.  You have a single 2D area with a weir that is inside the 2D area?  The flow through the culvert is computed separately.  It is computed just before each iteration of the 2D solver.

If everything is very flat and the flow over the road looks like "overland flow" as opposed to an elevated road grade that has "weir" type flow, you should check the "Normal 2D Equation Domain" and not the "Use Weir Equation".  This will allow the 2D solver to compute the flow over the road and it will do a better job.  The flow through the culvert is still computed separately.

On the Storage Area Connection Weir Data editor, I do not believe that the "Weir Width" is used as part of the computations.  It just changes how the weir is drawn on the geometry editor.  RASMapper does not prevent water from being displayed across this width.

There can be some confusing issues between what is in the terrain data and what is in the weir elevation data.  For the elevation of the top of the road (this is the elevation part of the user entered station/elevation data), does this elevation match what is in the terrain?  If so you can ignore the next discussion.  If not:

The weir station/elevation is used during the 2D computations.  However, the underlying terrain is not modified by the user entered station/elevation.  So, for instance, if you enter a high station/elevation data there may not be any flow over the road during the computations.  However, when the results are displayed by RASMapper, it is using the original underlying terrain so it may look like the road is overtopped even though it hasn't been.
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Re: Weirs in HEC RAS 2D

Ikenna
Hi Jarvus,
Thanks so much for the very detailed response.
MW
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Re: Weirs in HEC RAS 2D

MW
In reply to this post by jarvus
Hi Jarvus

I'm having issues modelling culvert crossings through a road embankment in which the terrain data elevation and the weir elevation data are the same. Hec-Ras won't let me run the unsteady simulation because my culvert invert sits below the grid elevation (which is of the road centreline so realistically, my culvert should sit below this elevation).  

You mentioned this situation below:
"There can be some confusing issues between what is in the terrain data and what is in the weir elevation data.  For the elevation of the top of the road (this is the elevation part of the user entered station/elevation data), does this elevation match what is in the terrain?  If so you can ignore the next discussion."

How do I get around this issue?
Is there an alternate method to model culvert crossings in a 2D flow area?

Thanks
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Re: Weirs in HEC RAS 2D

jarvus
The inlet and the exit of the culvert can not be underground.  (There is no provision that I am aware of for a storm drain to a completely buried culvert.)

It is ok if the culvert is underground in the middle (such as under the road centerline) but the culvert can not be lower than the lowest point of the two cells on either side of the road centerline.

It may be that all you need to do is make the cells larger.  Say the inlet of the culvert is at an elevation of 100 feet and the top of the road is at 110 feet.  The cell needs to be large enough that some part of the cell is at, or below, 100 feet.  If the cells are tiny, say only 2 feet by 2 feet, then the lowest point in the cell might only by 109 feet.  If the cell is 100 feet by 100 feet, part of the cell may be far enough away from the road embankment that it is below 100 feet and you would no longer get the message.

I hope this makes sense.