weir coefficient

[alberto.bonomi]

Hey

I'm modelling a serie of weirs....

I 've read about the weir coefficient and I'm using the value of 1.44 (I have meter as unit), but I've seen that this value can be different.

If I change the value of a weir, where the water depth increases more than I think is resonable, the system can't model the river.

Have you an advice about how to change this value?

My weirs are all broad crest weir and have the same width of the channel bed, so I can say there aren't narrowing on the weir, right?

Should I use the value of 1.44 or should I change it? How?

I hope you can help me, I'm new in this list but only one has answered to me until now....
thanks

[MattZ]

I would refer to any hydraulics textbook for tables of weir coefficients. Then you can check to see if 1.44 is appropriate for your model.

Does that help answer your question? I assume you know how to change the coefficient within the HEC-RAS program.

[alberto.bonomi]

I thank you for the answer.

I asked here because I haven't found in internet an appropriate table to define the value for my weir coefficient.

I'm a new user so I have some problems to rde the instabilities of the unsteady solution, and I'm doing my best to resolve it.

I thought the problem could be caused by these weirs, and their coefficient...

My weir are a sort of jump, and I modelled them using the same shape of thge upstream section.

So I used the vale of 1.4, but I haven't found support to this value.


If you say it can be ok, then I use it.

Thanks.


And a question more, if you can answer to me.

The mixed solution is to use only if I have supercritical and subcritical....

but in my example with many weirs I have results only if I use this mixed solution.

Can I us it or is absolutely wrong?

For me mixed solutuion sounds as:

I use the right solution undepending if I' m subcritical or supercritical.

Thanks

[MattZ]

You will have to run your simulation using mixed flow if you suspect that there is a flow regime change in your model. For example, if the weirs cause a hydraulic drop (subcritical flow to super critical flow), then you have to run in mixed flow.

I suggest going to a library and finding an actual hydraulics textbook for your weir coefficient question. Sometimes the internet does not have much information on such a specific topic.

[alberto.bonomi]

thanks for the help.

bye

[Hydraulics Tool]

Regarding your question about mixed flow. The mixed flow option can provide stabitlity to the model even if you are 100% subcritical. If, for example, you have flow approaching critical depth (in the subcritical regime), the inertial terms in the Cons. of Momentum equation become very sensitive to small changes in hydraulic parameters. So much so that they have historically caused instabilties in unsteady flow modeling near critical depth. The mixed flow option in RAS unsteady flow uses a technique called Local Partial Inertia that reduces the inertial terms in areas of your model that get close to a Froude number = 1. The rate of reduction can be controlled by you, but the default usually works well. The reason this technique works is because as you get close to critical depth, the inertial terms, although extrememly sensitive, are actually a very small contribution to the solution.

So, as a result, I always have mixed flow checked when I run unsteady flow models, regardless of my assumption of the flow regime.

[MattZ]

Hydraulics Tool:

Thanks for the detailed insight! I would have to say that most of my knowledge comes from practical use and I am not as familiar as you are with the solution schemes of the model.

Where did you get your experience with the model? It is always good to have another experienced modeler to bounce ideas off of.

[Hydraulics Tool]

MattZ-

I agree, it's good to be able to bounce ideas here in this forum. With regard to my experience with RAS, I used to work on its development and I currently teach RAS classes (Basic, Advanced Unsteady, and Dam Breach). I have to admit, I'm not great at checking back here often. Let me know if you want to swap contact info...

[MattZ]

Sounds like a great idea! Email me at:

htownholdem@yahoo.com

I work in private consulting, but I also teaching basic HMS and RAS courses. I work mostly in Texas, but I have taught in CA and I am going to NJ next week to teach an unsteady flow class.

[hi_nook]

Hi,
Regarding your question about weir coefficient.
i think weir coefficient can not greater than 1.
Try to use about 0.8

i think i can help you
NOOK

[Matthias Fritz]

that depends on the definition...

In germany i.e. it must be below 1 (common 0,5-0,7)
In USA the coefficient includes the term 2/3 sqrt(2g) - so their coefficient is normally above 1...