Storm and Sanitary sewer grade conflict - options needed

[DanTVE]

Hi everyone,

I am working on project, in which we have a grade issue with storm and sanitary sewers. At the crossing of the two sewers, the 12" sanitary will be going exactly through the center of the 42" storm sewer. There is no room to adjust the grades of either sewer. I need to come up with a creative solution. Has anyone ran into a similar problem, and what was your solution?

So far I have come up with the following solutions:

1. Install sanitary sewer through a storm manhole structure to where the two cross. The sanitary sewer pipe could be installed through a pipe casing that is fixed the the MH structure. I would have to prove the sanitary pipe going through the MH does not impact the flow capacity of the storm. This method is definitely not ideal, but would be cost effective.

2. Inverted siphon of the sanitary under the storm. Due to low sewer flows and maintenance issues, this is my least preferred option.

3. Install a pump station and forcemain to pump the santitary flows above (or below) the existing storm. Continue by gravity after that point.

It will be a challenge to get options 1 and 2 approved by the city.

Attached is a very crude sketch of my 3 options.

Thanks,
Dan

 

[gbam]

Nice home theater layout. I dont think that is the correct sketch. Have you considered a siphon for the stormdrain?

 

[cvg]

method 1 which is sometimes called a conflict structure is recommended over either of your other two options. The structure needs to be large enough so that is has minimal impact on the hydraulics of the system. Expect that it will collect debris and that the sewer department may object to this until you explain the very high installation and maintenance costs of the alternatives. A 4th option is to relocate the sewerline or re-align the storm drain.

 

[jgailla]

A conflict manhole as you describe is the most common way to address this issue in my region.
cvg, as always, has some good comments concerning system hydraulics and debris issues from the conflict manhole. It is, however, probably the best way to do it from a maintenance point of view.
If you go to the website of your nearest large utility, they will probably have a design standard drawing for such a manhole. If your local doesn't you can find one by searching jea.com. I'd attach a copy but my computer is broken and I can't get to my library.

 

[DanTVE]

Thanks cvg and jgailla. I didn't know they were called "conflict manholes" - learned something new. I was able to find a couple detail drawings by doing a quick search on google. I will check with our local utility commission to see if they have any standards on this.