Design of Storm Water Structure

[Reddivari]

Hello,

I have a question about choosing the right type of storm water structure for the below situation:

We installed a 12" diameter sewer about 400' long (with manholes at every 150')to drain storm water into the municipal combined sewer system. The last manhole (which is located in a concrete drive with a steep grade downhill where it meets the public street 100' wide) is approximately 130' from the 36" municipal combined sewer pipe situated in the public way. The municipal sewer runs east-west. The new 12" storm sewer runs perpendicular to the municipal sewer pipe.

During excavation, we find out that there is a 30" high pressure gas main running east west (and 3 ft below the pavement) of the public way. If we run the new 12" sewer under the gas pipe we can't provide the required slope to drain the water into the city sewer. In order to go above the gas main, we need to install a storm water structure ( such as a drop manhole) with the inflow-pipe at a lower elevation and a the outflow pipe at a higher elevation. We would like to have the outflow pipe at an elevation just to allow the required slopes as well as to clear the gas main in the street.

I am wondering whether a drop manhole with an inflow pipe (connected at the bottom) with a back-flow restrictor and an outflow pipe at a higher elevation will work for this situation.

I really, really appreciate your comments.

Thanks,
SR

 

[dicksewerrat]

Resize the 12 inch to something larger and go under?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[Reddivari]

Mr. Cornelius,

The municipal sewer and the gas main are about 35' apart and are at the same elevation. As such, going under the gas main will not provide the required pitch.

Thanks

 

[froude]

It's hard to fully visualize the situation without a plan & profile, but here's a few thoughts...

Regarding going over the gas main: I don't know what your municipalities clearance requirements are, but if the gas main is only 3' below the pavement, your proposed 12-inch storm drain line may not be that far below the pavement when it's all said and done. Also, is the need for outflow at a higher elevation than inflow a design requirement?

Assuming clearance can be worked out, would it be possible to construct the drop manhole between the gas main and sewer main, such that the inflow to the D.M. would clear above the gas line, drop into the D.M., and flow out with positive drainage to the city sewer?

Just my thoughts. Good luck!

 

[Reddivari]

There is no requirement to install outlet pipe at a higher elevation than the inlet pipe as such. But to clear the gas main and stay below the 1-foot thick concrete pavement (assuming the use of a ductile iron pipe at the crossing), we need to raise the elevation of the outflow pipe at the manhole ( which is situated approximately 30' from the municipal sewer) to be able to provide the required slope as well as clear the gas main.

The constraints here are the top of the gas main and the bottom of the street pavement. If we provide the outflow pipe higher enough (than the inflow pipe) we could achieve a positive pitch and also be able to clear the gas main. The dilemma is how we could do this without resulting a backflow into inflow pipe. Can we use some type of restrictor at the inflow pipe to prevent a backup?

 

[Reddivari]

I mean "( which is situated approximately [highlight 130-feet from the municipal sewer) to be able to provide the required slope as well as clear the gas main."

 

[beej67]

A profile would really help.

Would running two parallel 8" sewers for the last stick instead of the one 12" help you gain clearance?



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