Outlet Control Structures (square vs. circular)

[mbherlihy]

I have an outlet control structure in a standard detention pond that I specified as a square concrete shape. I was told that most engineers use ciruclar due to cost, also how well it holds on to an earth pond embankment... is this correct?, and is this a better shape when placing the outlet in an earthen pond embankment?

 

[MrBachelor]

A couple things need to be clarified first:
does your outfall structure contain multiple stage weirs or orifices?
does the design call for the structure to be cast-in-place or pre-cast?
how large of a structure are we talking about here (height and width/diameter)?

I have been working with storm water design for several years now and have never specified a circular outfall structure. I have seen ponds overdesigned and their discharge limited by one outfall pipe, without a structure.
The structures I typical specify have to control the release of multiple storm events and therefore typically have multiple weirs and/or orifices. They are also specified as cast-in-place due to the complexity of the weir/orifice structures and the size of the units. I recently specified a polygonal outfall structure with the largest face containing the primary orifice and the multiple orifice system. It was approximately 30' wide, 12' deep, and 12' tall made of 8" thick reinforced concrete.

If your system requires only a single orifice (and possibly an open top of structure) I see no problem specifying a circular structure. I would count this as the exception rather than the rule.

Nate

 

[cvg]

a CMP or HDPE riser with holes or slots cut in the sides as orifices is quite inexpensive and circular. However, for anything tall or large, you will need to use concrete and it is difficult to construct forming for a circular concrete structure.

I'm not sure I know what you mean by "how well it holds on to an earth pond embankment..."

 

[mbherlihy]

Thank you for asking me to clarify. I have a multiple orifice structure, to handle different frequency storms (ie: FYI this is Georgia, water quality 1 year-24 hour release, channel protection 2 year-24 hour, emergency overflow 100 year). Is there any hydraulic, structural or cost advantage to circular vs. square? And what i meant by 'hold on' to the embankment was,... preventing the structure from sliding or tipping over due to buoyancy...even though it would be constructed into the side of the pond embankment. My intention is for the structure to remain 'rigid' in the earth embankment and not move during water ponding inside the pond.

Most of our structures, i believe are precast... say 4' x 4' inside diameter, no more than 12-14 feet tall. Mine is 16 in this case.

 

[cvg]

circular pipe is available and makes an excellent riser at relatively low cost. However, for larger structures, cast in place might be better and then it should be square if possible. I'm not aware that the shape of the pipe has any influence on it's buoyancy.

 

[bltseattle]

The standard around here is round flow control structures.

See

http://www.wsdot.wa.gov/Design/Standards/Plans.htm#SectionB

B-10.40 and B-10.60

Large projects often require custom structures, which could be cast-in-place or precast vaults with internal weirs, baffles, orifices added.

 

[sammyk]

It doesnt matter what you use as long as your shape area is substantiated by your flow calculations. It sounds like you are using an external weir with multiple control points. If they Cast in Place, they can form any shape that you want.

Just remember the poor guys that have to construct the forms. And also be nice to yourself and make you orifice or weir calculations easier on you with the simplest shape you can get away with (ie circles or squares).