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Open channel with zero slope

Open channel with zero slope

Open channel with zero slope

(OP)
Any suggestions on how to model open channel flow in a channel with zero slope and a drain located about halfway along its length?  It is a unifomrly shaped asphalt channel.

Thanks
dpajr

RE: Open channel with zero slope

Start with the critical point at the drain, that will give you a starting hgl for the flow rate you are evaulating
then I would use Manning's but each calculation will only be good for a short distance as the 'normal' level will increase to create at false slope the farther you get from the drain.  eg there will be a slope in the hgl and egl but it changes as you get away from the drain.

Hydrae

RE: Open channel with zero slope

Backwater effect.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Open channel with zero slope

The use of a mannings equation will not help you in this scenario.

As msquared48 has said a backwater calculation is probably the way to go, however, you will have to consider how the flow is actually getting to the drain.

Looking at the smaller picture 0% slope= 0 conveyance....

You will probably have to consider a more broader approach considering the flow paths u/s of your area of interest, is there enough momentum u/s to actually carry the flow to your area of interest???? A RAS model may be the way to go...

Hope this helps.  

RE: Open channel with zero slope

(OP)
Thanks to everyone for the quick response.

RE: Open channel with zero slope

Mannings equation is for uniform flow.  The situation you describe is classic "gradually varied" flow.   

RE: Open channel with zero slope

It can be modeled like a pond.  Your flow would be a weir calc or an orifice depending on the head on the inv.  Model it like a riser only scenario in your modeling software.

RE: Open channel with zero slope

I would model it as a very large diameter tank (not necessarily cylindrical) with an orifice and let the surface area of the pond and flow thru the orifice vary with time.

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