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Submerged weir

Submerged weir

(OP)
I'm not sure how to model this situation and any help would be greatly appreciated:

There is an old mill site having a weir, which is about 1.5 ft. high.  Only about 10 ft. downstream of the weir is an 88" x 66" elliptical pipe that crosses under the road.  I have figured out how much storage is behind the weir and also the storage between the culvert and the weir (which is relatively minimal).

I modeled the weir as a pond with a weir outlet and I also modelled the culvert as a pond with a weir (top of roadway) outlet.

Here are some elevations:

Top of weir = el 56.4
Bottom of weir/invert of culvert = 54.90

What I would expect to happen is that once the inflow into the culvert exceeds the culvert capacity, the weir would become submerged, and the pond elevation behind the weir would equal that of the pond behind the culvert.  It's showing the pond behind the weir to be 59' +/- in a 100-yr event, and the pond behind the culvert to be 63' +/-.  How can I get the two surfaces to even out?

Thank you!
Replies continue below

RE: Submerged weir

First off I assume you're using the Storage-indication routing method.

Now if you are using that method it is the incorrect one for this situation.  To make this work properly you need to create a secondary outlet on the pond that is the culvert that is identical to the primary outlet(the weir) on the first pond.  Then connect that outlet to the first pond.

Then under calculation setting select the Sim-Route method. The sim-route(simultanious routing)method can account for the flow-loop and reverse flow.  This should make the two surfaces equal.

This method is inherently less stabil then the other methods so check for any warning messages that you may get.

RE: Submerged weir

Just use two ponds, with one routed directly to the next.  If you have tailwater conditions, there will be a warning message suggesting the used of a tailwater-sensitive routing procedure, such as the Dynamic Storage-Indication method (DSI).  Click the message for details or see www.hydrocad.net/dsi.htm

Sim-Route is NOT required for your situation.  bpattengale is describing a "reverse outlet" which is needing ONLY for reversing flows: when the flow initially goes from A to B, and then changes direction and flows from B to A.  For details see www.hydrocad.net/simroute.htm

Your situation DOES appear to involve tailwater at the weir (since you are expecting it to be sumberged), but NOT reverse flow, so DSI will handle the routing, and a reverse outlet is NOT needed.  Keep it simple, and check the warning messages, which will generally detect the need for more complex methods.

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