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# Box Riser with two different size circular orifice openings 3

## Box Riser with two different size circular orifice openings

(OP)
Hello,
I am reviewing drainage which has a detention pond with the outlet structure as shown on the accompanying file.

The outlet structure is a 4' x 4' Riser Box. The top elevation of this box is 1300, while the bottom elevation is 1295.30. There are two circular openings on one side of the riser. The first one, a 9" circle, is at elevation 1295.30 and the second one, a 24" circle, is at elevation 1296.50. Above the 24" circular opening there are rectangular openings, 4' wide by 6" tall, on three sides of the box riser; they are at elevation 1299.00.

How would I figure the discharge through the riser? The following is what I was thinking:
1.) Use the orifice equation to figure the flow through the 9" hole (with the effective head being from the center of the opening).
2.) Add the discharge coming from the 24" opening (also figured by using the orifice equation with the effective head from the center of this opening).
3.) Figure the discharge through the three rectangular openings (add them together collectively). Then add this onto the first two discharges.

Is this correct?
I have a question concerning the effective head of the two hole openings. When the holes are free flowing, I would use the head from the center of the opening. But when the tailwater elevation becomes higher than the center of the opening, I would use the difference in height between the headwater and tailwater elevations. My question is this: how would I know what the tailwater elevation is or do I assume the water free flows through the openings at all times?

I'm sorry to ask dumb questions, but I'm fairly new at analyzing detention ponds and their outlet structures.

### RE: Box Riser with two different size circular orifice openings

The procedure you outline is correct.

You can determine the tailwater by calculating the headwater of the culvert, which is going to be the same as the tailwater in the structure.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

### RE: Box Riser with two different size circular orifice openings

I would model your 24" CGMP as the primary outlet (elev. 1295.25) of the structure, then the other openings you define their 'Routing' through the outlet (rather than defining them as 'Primary' outlets they are defined as Orifices & their routing runs through Device 1 (or whatever row of the table the 24" CGMP occupies).

### RE: Box Riser with two different size circular orifice openings

using the orifice equation will overestimate the flow through your structure. for the bottom 9 inch hole, probably ok because it is so small. but, for the 24" round and the rectangular ones, they will all flow as weirs until they are completely submerged.

you will need to adjust your flow rate through the openings for tailwater submergence if your outlet pipe cannot handle the inflow. This can get very complicated. Best to design your outlet pipe large enough so tailwater is not an issue.

It doesn't appear that you have any freeboard for the 100-year flow. Typically I would suggest at least a foot of freeboard or more

### RE: Box Riser with two different size circular orifice openings

(OP)
I appreciate all who answered my questions.

Thank you!

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