Simple (?) Sanitary Sewer design problem

[drhenne]

We want to create 3 lots for the construction of single family residences. A town sewer and manhole is adjacent to the one of the lots. The other two lots will require a sewer extension about 400 feet long. Local and State ordinances require 2 fps velocity for an 8 inch pvc pipe flowing full. Minimum grade is .005 ft/ft. Existing grade and sewer inlet conditions would allow the extension grade to be as steep as .01 ft/ft. My pipe-full calculations using mannings (N=.013)indicate a velocity of 2.45 f/s, which would be satisfactory per ordinance. My concern is this - will 3 units provide enough flow for the sewer to operate properly, considering scouring, etc ? The required daily design flow per unit is 270 gpd. This is probably a routine problem for an engineer, but I am stuck. Could someone please show me the proper way to approach this design question?

 

[dicksewerrat]

For pvc, N= .01. Three units is a bit low. Should try to get the lots on the other side and build 6 units. Try to keep the flow velocity close to 2.5 fps. Faster and the solids will settle out.

 

[gibfrog]

Can you use a smaller pipe size? Like a 6" or maybe even a 4"? Per the Ten States Standards, (

http://www.dec.state.ny.us/website/dow/10states.pdf)

you want to use the smallest pipe that will carry your flow due to flushing concerns. You also will want to place clean outs every 70' or so on a 4" line.
However, many municipalities require 8" sanitary pipe in a right-of-way at 0.4% and 6" minimum at 0.7% for a lateral line with multiple connections. Remember, once the pipe is in the ROW, it is the municipality's headache.

Clifford H Laubstein
FL PE 58662

 

[KRSServices]

At .5%, 8" I get a velocity of about 3.5 f/s.

gbfrog is correct, the municipality or regulator will normally specify the minimum criteria for the sewer line. This is done to ensure adequate sizing for the future of the line, and yes, initially, the flows will be excessively low for that size of line. The future connections onto that line (beyond your subdivision) are the main considerations, unless of course, there is absolutely no possibility of further expansion of that line. Then a sound arguement could be made to reduce the diameter to better match the flows.

KRS Services

http://www.krs-services.com

 

[TVE]

I've recently dealt with a similar situation so...

1) Manning's 'n' is whatever the local authority says it is. Some places use 0.013 for ALL pipe materials while others allow different values for different pipe materials. (Apparently there are sufficient conflicting studies so that there is no clear answer on this point.)

2) Most hydraulics texts have a chart showing the relationship between %Q, %V and Flow Depth. If you can locate one of those, that should help you determine a more representative flow velocity. (In most cases they have the chart to show how there is actually a "zone" where the pipe can theoretically flow at a higher capacity than at "full flow".)

3) Have you asked the local authority "is there planned expansion past your project?" If there is, they may not want a pipe at a higher slope as it will limit how far the sewer can be extended. By installing YOUR length of sewer, you may open the door to further development in the near future that could add flows and eliminate your low flow concerns. (ie, your calcs would be something like "3 houses now and 10 houses later = pipe flowing reasonably full&quot

David Dietrich
Thames Valley Engineering

 

[Chris73]

I wouldn't go smaller than 6" in size. Definitely you want to determine the more relevant average daily flow through the pipe and check the velocity at x% full status. This velocity should be min. 2'/sec if you want to avoid sedimentation and potential odour problems.