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# Designing Overflow Weir

## Designing Overflow Weir

(OP)
Hey everyone,
I'm designing a ~27,000 sq-ft. wet pond which holds approx. 650,000 gal. and serves a drainage area of approximately 30 acres.

The desired pond water elevation & corresponding weir crest elevation are 465'
I'm trying to determine ideal dimensions for my sharp-crested contracted rectangular weir.
There will be a separate emergency overflow riser at a higher elevation.

I'm a bit confused on determining the Head acting on the weir. I understand H is essentially the distance between the crest height and the EGL.
However the Pressure is atmospheric, the crest elevation is the same as the surface elevation in the pond, and my reference says that for pond spillways the approach velocity is often assumed to be zero.

I'm following local design guidelines for determining my Q25 and I obtained approximately 23 cfs entering the pond over a ~5 minute storm duration.
My rationale is that 23 cfs over 5 minutes is approximately 7000 cubic feet. 7000 cf over the 27,000 pond area gives an elevation rise of ~0.26' across the pond. Is this my design Head acting on the weir?

Once I come up with some weir dimensions and results in HydroCAD, what do I compare against to see if my velocities and flows are acceptable? Or is the allowed velocity more determined by the receiving channel downstream, to prevent scouring?

Thanks in advance, I've tried to do my due diligence as this is my first project of this sort and wanted to check-in with you fine folks.

### RE: Designing Overflow Weir

That's a very rough way to determine the weir head, but the head will be changing constantly with time, so you need a hydrograph routing to get an accurate answer. Since you're already using HydroCAD just adjust the weir dimensions until you achieve the desired behavior.

As for flow velocity, if you look at the device analysis (at the bottom of the pond summary) this will show the velocity for each of the outlets at the time of peak flow. If you're concerned about the velocity in the downstream channel you can model this as a reach.

Peter Smart

### RE: Designing Overflow Weir

(OP)
Great - thank you psmart! Always very helpful.

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