Existing Flat Sewer

[GoldDredger]

I have a new project for a school that will build a 2,000 seat arena at an existing campus. The arena will generate 310 Fixture Units, or about 0.25 cfs during peak use.

The sanitary tie in will be a private 4" lateral, currenty serving a maintenance building and baseball concession stand (low use). After having the lateral exposed and shot for location and elevation, I calculated it is 900-feet at 0.1% slope to the tie in public manhole.

The mannings capacity for the pipe is a quarter of expected peak flow, at about 0.06 cfs.

The question for me is do I just design 900 feet of new bipass line down an existing road, or propose a connection to the undersized (and old) line.

For the second option, I would propose a new sanitary manhole. Flows above existing downstream capacity could surcharge the manhole temporarily until catching up.

Normally I wouldn't consider anything but a new bypass line, however the arena's peak use will be limited to a few times a year at best (basketball games).

Any opinions?

 

[Maury]

The existing 4" is to small even for a service. It should be at least 6" diameter. The slope of 0.10% is much to flat. The usual design is a flow velocity of 2 fps for the pipe flowing full. I don't understand how this would have been permitted

I would suggest replacing all of the sewer with a properly sized and sloped gravity line or an ejector system and force main.

 

[bimr]

Estimating the flow from such an installation is more art than science.

One rule of thumb is to assume that 25% of the patrons will use the restroom during an intermission. That would calculate to an estimated sanitary flow of around 50 gpm. That is about 1/2 of what you estimated.

Sure you may save a few dollars to skip the installation of a new lateral, but how much of an embarassment would it be to have a sewer overflow on a brand new project?

The existing sewer line is inadequate and should be increased in capacity.

 

[cvg]

surcharge the manhole temporarily until catching up

- you had better do some calcs on this one. Unless you have a really big manhole or a lift station with a large wet well, you won't get much storage in a manhole. Plus it will be a big mess, hope you never need to do any maintenance in that manhole...

 

[nackra]

I recommend checking PEAK FLOW capacity for your system, i.e. think of lines at the bathrooms at halftime. Potential for numerous flushes and all at once.

I had a project where we had to add a second grinder pump station to handle PEAK FLOWS for the concession/snack bar/bathroom out building at a high school sport field.

 

[GoldDredger]

Thanks for the responses guys.

I just don't think it is worth the risk, so I am proposing a bipass sanitary line.

If the owner needs to start cutting costs after bids come in, I could propose that to them as an option. At least then it would be easier for me to explain the risks involved and let it be their decision.

 

[dicksewerrat]

Why not combine to two lines and then the owner will only have one line to maintain.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[GoldDredger]

Sewerrat, that is what I plan to do. So it isn't so much a bi-pass line as simply a replacement line.

 

[Timmyutah]

Grand Canyon university?