duck bill

[hany]

I want to know the water saface in the upstream of a canal that has a duck bill weir in it ?any body know how i can enter the information of duck bill weir in hecras?

[dhales]

Depending on what you are designing for, you would get a higher WSE just modeling a straight weir.

[hany]

Dear dhales
What do u mean by can get the higher WSE ?could u explain more about that?
If there is a trapezoidal canal and we have a duck bill weir in downstream of it ,and we want to know water surface upstream of the weir such an in 100 meter and 200 meter upstream of it .
Plz tell me more about that .
Best regards

[dhales]

If you have a channel and you want to model a weir that is non-linear in the channel, the highest wse will occur when you assume a linear weir across the length of the channel.

For instance, if you are designing a road alignment, then the finished surface grade of the road usually needs to be at least 1-3' above the upstream WSE.

What I would do is (going downstream before the start of the weir), stop at the station where the weir begins and model a linear weir at this point. This will give you the highest WSE possible. Just make sure you start the weir at the most upstream point.

This is how I would do it, because I do not believe HEC-RAS can efficiently model 2-D flow. However, the methodology I've explained is a good work-around. Let me know if you have any more questions.

[yann.viala]

Hi !

I don't know if you can introduce a value for weir lenght larger than the width of the associated cross section, but here is another way to compute such waterline.

If your duck bill is not submerged, and normally such structure is designed to operate in non-submerged flow, you precisely know the height vs discharge law on the weir. So you can finished your model at the weir station and simply use the downstream boundary condition to describe the weir behavior.

Sincerly,
YV.

[hany]

what do u mean by you precisely know the height vs discharge law on the weir?
how would u enter the weri properties such as wier width and c and .... ?
If u could plz tell some more about that
best regards

[yann.viala]

Hello,
You want to know the WS elevation upstream the weir, so you can limit your model to the part upstream the weir, finishing it at the weir station. Doing that, your downstream boundary condition is the height vs discharge law of the duck bill.

So you can now introduce point by point a law of the following type as a rating curve in the boundary conditions interface :
Q=Cd.L.sqrt(2.g).h^3/2
where Cd is the discharge coefficient (around 0.4), L the crest length, g the acceleration of gravity (9.81 m/s^2 in metric system) and h the water height above the crest. All units are in the International System.

You can of course use another law.

Sincerly,
YV.