Potential issues with Large Diameter Sewer, Low Design Flows?

[CivDevil]

I'm in the process of redesigning a sewer system for a small commercial project due to a utility conflict. Original intention was to extend a 10" public sewer main along street frontage and tap 6" service off of it for project; this has been scrubbed due to utility conflict blocking 10" main alignment.

New plan is to provide a site service using a pre-existing sewer stub (on other street frontage). Contractor would like to use the 10" pipe that they already have. Est. peak flows for the line are 150 gpm max...with average flows more likely in the 20 gpm range.

I'm concerned that running such low sewer flows through a large diameter line (10" at 1/8"/ft) will lead to clogging of the service line. If I were designing such a system from scratch, I'd use a 4" or 6" dia. line.

Running it through Mannings for 6" I get:
1.9 fps @ 20 gpm (18.5% full; marginal but close to 2 fps min)
2.3 fps @ 40 gpm (26% full)

for 10":
1.7 fps @ 20 gpm (10% full; slow...)
2.1 fps @ 40 gpm (13% full)

Anyone have any thoughts or input? Thanks.

 

[bimr]

There is usually a minimum requirement that service sewers (building sewer to public sewer) have 1 to 2% slope.

 

[coloeng]

Require the contractor to use 4"/6" sewer pipe and get a credit from the supplier for the 10". If the contractor can't get a credit, then the owner pays for the 10" pipe. There should be a credit change order for the contractor not having to install the sewer in the public right-of-way.

 

[Lan123]

How long does the peak flow last. Check the Velocity at peak flow, and, if this is more than 3.0 fps, should be adequate to resuspend the deposited material.

 

[cvg]

just because a contractor wants to save money doesn't mean you should agree. the long term O&M costs to clean out the clogged sewer every 6 months will be paid by the owner, not the contractor and they will blame you for it.

 

[CivDevil]

In the end I've required a switch out to 6", sticking with the known vs. the unknown. Project owner would definitely not thank us for saving a couple thousand bucks (potentially) if the sewer is clogging down the road.

For this building, peak flows would have only occurred one day per week, so interim accumulation could have posed a problem.

Thanks all for your timely and informative comments.