Decreased Size in Downstream Stormwater Pipe 1

[lakjdsfkljaklfdj]

Hello all and thanks in advance for any guidance you provide.

I'm reviewing a site plan amendment for an industrial site. The site has an existing private drainage system outfalling to a public inlet via a 10-inch PVC pipe. The amendment will add buildings and therefore increases the runoff rate. To accommodate the increased runoff, the designer is proposing a system with a max 16-inch pipe, and, in an apparent effort to save some dollars, the designer is proposing to not upsize the entire run of 10-inch pipe to the inlet. Rather, the system would entail conveying runoff from the new 16-inch pipe to the existing 10-inch pipe. This would save the project from upsizing about 400 feet of existing 10-inch pipe under a parking lot. Applicable design criteria specifically state jurisdiction includes privately owned and maintained systems and specifically disallows decreased pipe size in the downstream direction. However, we have a variance process...which leads to my question: If we allow the variance, can we expect any adverse effects to the public system.

By the way, I realize there is no hydraulic rationale for a 16-inch pipe conveying to a 10-inch pipe. I understand the designer is specifying a larger pipe to convey runoff from the surface and temporarily store the water due to temporary backwater effects from the smaller downstream pipe. Frankly, I think this is smart creative, I just don't want the public system to suffer from some unrecognized effects.

 

[jgailla]

What happens when something gets into the 16 inch pipe which is larger than 10 inches? This is a terrible idea. A 16 inch pipe isn't enough to store water for flow attenuation. I'd like to hear the rationale for variance.

 

[cvg]

this could be a clogging point, however i would seriously hope that you are not allowing large 16 inch objects to go down your drainage system. a manhole at the junction point will allow cleaning

 

[lakjdsfkljaklfdj]

@jgailla I agree. While the probability of objects larger than 10 inches entering the system is low, the consequences may be great, i.e. flooding of the buildings. As with all variances, I'm sure the reason they propose this is to save a buck, but since financial hardship is not a valid basis for a variance, they would come up with a more creative stated reason that would immediately trigger my BS sensor. I also don't understand why they would propose such a compromise to save maybe $10K on a $7M project.

@cvg Good point about the manhole. I looked back at the proposed design and they do have a manhole at the 16-inch to 10-inch junction. For public systems we require minimum 18-inch RCP, but allow other materials and sizes on the private side.

 

[ACtrafficengr]

It doesn't take a large object to clog a pipe. A sufficient quantity of aggregate will work as well.

My glass has a v/c ratio of 0.5

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

http://xkcd.com/319/

 

[thedarkside]

@kornolio Your point about financial hardship not being a valid basis for a variance is a good one.

That being said, is the 16-inch pipe large enough to accommodate the additional backwater from the 10-inch pipe? If the pipe needs to be 16 inches just to carry the design storm, methinks they would need to upsize that if they want to use it for detention. If they want to do large pipe --> smaller pipe, then they should treat it as a detention facility and design it accordingly. (looking @ how does that affect the HGL upstream, providing a maintenance agreement if your jurisdiction requires that for SWM facilities, using an orifice with potentially a trash rack and an overflow feature).

As far as adverse effects to the public system from the size decrease - how close is this to a roadway? If the backwater from the constriction floods the upstream system since this 16-inch pipe doesn't appear to be sized for detention, where will that water go? Into the public roadway?

 

[FacEngrPE]

In my locality the design report - including downstream impact is a requirement of a site drainage plans submittal. As plans examiner it should be in your pervue to examine the report.
The need for the requirement became obvious after several flooding events that all resulted from the impact of development on the downstream storm water collection system.

Fred

 

[GeoEnvGuy]

Was the slope of the 16 and 10 inch storm sewer considered in total flow.