How far off to set a baffle plate?

[beej67]

Existing open topped cast in place box serves as an overflow structure for an outlet control. I want to add a baffle to catch oils and floatables. The design stage is going to stage up 2.5 feet above the box crest, so I need to set the baffle box far back enough so the gap between the baffle and the weir box doesn't become the flow control instead of the head over the weir box.

Any rules of thumb on how far to set back a baffle?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[jgailla]

I've always set them at 1 foot off because that's what I was told when I started doing civil design.
However, I've always had them on the inlet side of a staging pond.
What kind of flow are you getting and how wide is the weir?
Could you use pipe friction to calculate a head loss assuming the weir and baffle is a rectangular pipe?

 

[beej67]

The baffle is going to be very large. Probably 2.5 ft in height, and tracing the outside of a 6 ft by 12 ft overflow box on 3 sides, and held on with bolts or rebar.

My initial inclination is to set it about 2 ft off the edge of the box, because that's approximately the dimension that FDOT uses in their standard skimmer detail.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com