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Orifice size...

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SMIAH

Civil/Environmental
Jan 26, 2009
482
I'm trying to make my point versus a consultant to make sure, as a municipal engineer, that a proposed retention/sedimentation pond design will work.

For this particular design (it is), sediment control will be achieved with (read this) one (1) single orifice (diameter = 31/2'') placed at the bottom of the pond.

Consultant is telling us that the released flow will be over a long period of time...
I'm with them on this though. Even think it will be over forever or until the water evaporates of infiltrates.

What should be the minimum orifice diameter that should be specified for a sediment pond?
I know it might depend on the inflow/sediment load/debris but... from experience vs clogging issues?
 
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well, would you rather have one 3.5-inch orifice or three 2-inch orifices? Either way, they may clog. to avoid clogging, an upstream trash rack can be used. orifice should be vertical and should not be too near the bottom. Personally, if I was writing the regulation, I would specify a minimum size of at least 6 inches.
 
Thanks cvg.
I'm with you on the 6'' minimum.

I know they want to maximize detention time (say 24 hours), but this is fooling people.
 
1" is too small for a sediment pond, but can work fine for a permanent pond. Channel protection requirements in Georgia have driven orifices sizes for small permanent ponds down to an inch to two inches. We just use a lot of anti-clogging measures on them. I've seen orifices that small in Florida projects as well.

When your ponds are all relatively vertical (10+ ft of head, often in north Georgia) then you can't meet 24hr drawdown requirements with a 6", or even a 3" orifice. In the last decade, 1" drawdown orifices aren't uncommon at all in this region.





Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
 
1" orifice - perhaps the regulators mean well, but that's rediculous. We can generally get away with a bleed off rate of 1 cfs without any issues and with retention ponds generally limited to 3 feet deep, 1 inch orifice just wouldnt cut it.
 
The 24hr drawdown...

When I see a pond with 1x orifice of 3.5'' to meet this requirement, then I'd rather try to maximize detention as you can with 1x 6'' diameter orifice.

Then we won't have to perform maintenance after each storm.
 
I just finished up a job for a bank branch here in Atlanta. Required bleed down volume for the 2 acre site was 8,500 cf. Got about half out of infiltration systems, and the remaining 4100 cf was in an underground pipe manifold. Max head on the orifice was 3.9 ft, average head on it was 2 ft, according to the way GA likes to estimate it. You do the math. (answer = 1" orifice)

I did a project down in Florida a few years ago where the discharge limitation was 49 cfs per square mile of project area, at the 25 year storm event, regardless of what your existing site conditions were. SFWMD basically backed their way into that number based on downstream flow restrictions. If you've got any ideas on how to detain a 10+ acre development to 0.08 cfs per acre discharge during a 25 year Florida hurricane without a very small orifice, please share them. We went with a 2" orifice on that job.

Small orifices can work, if you make sure you protect them properly from clogging. I like to use a reverse pipe with a threaded cap in the outlet control structure, and drill an orifice in the cap. That way you can get at it to clean it if necessary, and trash tends to fall away from the orifice, not towards it. Have the reverse pipe pull from the center of the water column through a trash rack.

That said, nobody in Alabama or Mississippi would dream of putting a 1 inch orifice in an outlet control structure. As always, stormwater is very different depending on your state and region.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
 
reverse pipe with a threaded cap in the outlet control structure, and drill an orifice in the cap

This might be alot different than to use a simple 3.5'' orifice at the bottom of a pond.

I am in Canada... Ice and debris.

Thanks for advice.
 
Hard to find two more different regions than Canada and the American southeast, but they do these things where it snows too. Here's a link to a detail from Iowa showing a reverse sloped pipe:


That's similar to how they like to see them here. In that detail, just replace the valve with a threaded cap and a hole in it.

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