Can I achieve 90secs detention in a stormwater drainage system's pipes

[36zoms25]

Is it possible to achieve 90seconds detention for storm water quality in a storm drainage system's pipes instead of using a detention box? Is this a good practice?

 

[beej67]

What state are you in?

I've never heard of "90 second detention" for water quality before, wondering where that comes from. I usually see 24 to 72 hours, not seconds.

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

http://www.campbellcivil.com

 

[36zoms25]

I am in Florida

 

[cvg]

the word "detention" as used in the context of storm water quality generally means detention in a pond in order to allow sediment to settle out of the water, prior to discharge through a pipe.

 

[msquared48]

It could mean the relative time the flow takes to flow through a bioswale, not a pond.

Mike McCann
MMC Engineering

 

[cvg]

ok, bioswale, pond, lake, sediment basin, whatever, but the point is the same

 

[36zoms25]

cvg, you're absolutely correct. The detention box is used to allow sediment to settle out of the storm water, prior to discharge through a pipe.
I want to know if the pipes in the system can serve that Purpose, and if it is a good practice.

 

[cvg]

no because pipes cannot trap sediment without some sort of structure constructed such as a weir and unless the flow is extremely low, the sediment wouldn't settle out anyway. I have never seen this used as a stormwater BMP.

 

[msquared48]

cvg:

From a time of concentration donsideration, a pond, or any body of water, if it is full and a]has no storage capacity, the transit time would be zero across the pond. However, for a bioswals, the time of concentration would depend on the length of the bioswale, the material in the bioswale, and the slope. It could be designed to be 90 seconds point being, for whatever reason.

From a water quality standpoint though, the pond would have the greater settling or cleaning time over the bioswale. Depends whaqt you need I guess.

Mike McCann
MMC Engineering

 

[cvg]

how did we get onto bioswales? I thought the question was about pipes vs a "detention box" (whatever that is)?

 

[francesca]

Strikes me as a bit of a maintenance nightmare to trap sediment in a pipe.

 

[psmart]

To provide effective settling time, you need to reduce the flow velocity (and ideally bring it to zero) as you would for a traditional detention pond or "detention box". In a conveyance pipe there's no velocity reduction, so very little settling will occur. A pipe also doesn't provide a reservoir for the sediment to collect, or allow for clean-out, as francesca pointed out. I don't see how a pipe would provide the same functionality as your "detention box".


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ACtrafficengr]

If you could get settlement in pipes, how would you clean them out? The best system (on paper) is worthless unless it can be maintained.

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[beej67]

I see.

Some of yall are a little off on this. You absolutely can get sediment to fall out of flow in a pipe or channel where water is moving. (slowly)

The math for sediment pond design is based in its roots on Stokes Velocity. If you want to trap 80% of your sediment, you do a grain size distribution analysis, figure out what size is 80% or larger, then figure out how long it takes that grain size to fall at Stokes Velocity through a viscous fluid (water) to the bottom of your pipe. Then you size your pipe to reduce the velocity enough so the grain has time to hit the bottom of the pipe before your water is discharged out the other side. ADS a while back built some water quality devices on this premise, which were large HDPEs with pre-welded baffles in them, and a couple of inspection/cleanout manholes.

In fact, that same Stokes Law (falling velocity of spheres) analysis is at the heart of the nomagraphs used in the Georgia Erosion and Sedimentation Control Manual for sizing sediment ponds.



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

http://www.campbellcivil.com

 

[beej67]

Now, all that said, I wouldn't design a regular storm system to do that. Do you want to get sued when the pipe cloggs up? I wouldn't.

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

http://www.campbellcivil.com