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# Outlet structure for Detention Pond

## Outlet structure for Detention Pond

(OP)
I am reviewing an engineer's drainage report, showing both current and proposed conditions for a housing addition. In the proposed design, a detention pond is designed, with an outlet structure. The details are shown in the accompanying file. To figure the discharge at the elevations, would I use the contracted rectangular weir equation for the 12' wide portion from elevation of 669.50 to 671.50, then use the suppressed rectangular weir equation from 671.50 upwards? If so, what length would I use for the suppressed rectangular weir equation - 10 or 22 feet?

Denise

### RE: Outlet structure for Detention Pond

There are a couple of ways to solve this, but the easiest it to use 12' for the lower weir and 10' feet for the upper. This avoids any overlap (double-counting) of the flow area.

You could allow for contraction in the upper weir (depending on the geometry), but the lower weir is more complex, because it will only contract up to 671.5, where the upper weir starts. The easiest solution is to omit the contractions.

Peter Smart

### RE: Outlet structure for Detention Pond

(OP)

Much appreciated,
Denise

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