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.