Alternatives to Small Orifice

[mes11]

Seems like most projects I design end up with a small orifice to meter the water quality volume. I need to find some alternatives for these small orifices. 95% of our projects are designed with an extended-dry detention basin for stormwater management. Therefore, a reverse flow pipe isn't an option with those projects. It also seems like no matter how detailed of a pipe configuration that is in the outlet structure there is still a potential for clogging. Thank you for any suggestions you may have.

 

[cvg]

provide a clog free overflow above your normal high water level and you should be ok

 

[beej67]

What region of the country are you in mes11? The answer will vary quite a bit by your requirements and the sorts of land you're working with.

In flat areas like Florida, v notches are the way to go, because their cross sectional conveyance size approaches zero as the head drops, but widens as the head raises, providing a clean out function.

If you're in South Carolina, they like to have floating orifices that skim just below the surface, to provide full settling time for solids in the pond. There are standard details for them, and also some proprietary devices that perform the same function. This allows you to have a larger orifice under smaller constant head.

If you're stuck in the quagmire of the Georgia Blue Book, then hopefully your regulators have a minimum orifice size they'll let you apply to your project. There are also other anti-clogging options, such as that "waterman" chicken foot looking thing they use in Gwinnett. I've also done some projects where we specified a double perforated half round in front of the orifice, with washed 57 stone between the half rounds, and a hinged top, so the water has to move through the half rounds to get in, and the stone can be cleaned out and replaced by maintenance crews down the line. With good maintenance those can work.

When I'm doing underground vaults, I've had success with a turn-down pipe in a localized sump and an orifice drilled in a threaded cap, down in the sump. Make sure to spec OSHA confined space entry rules in your maintenance plan if the thing ever does get clogged, though.

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

http://www.campbellcivil.com

 

[TerryScan]

The skimmer from SC that beej67 mentioned came to my mind. Here is an implementation as adopted by Delaware DOT:

http://www.deldot.gov/information/pubs_forms/const_details/2012/pdf/sd_e22.pdf