Calculate Minimum Sewer Slope

[MrPhatz]

Hello All,

I am trying to develop a spread sheet that will help calculate the minimum slope for sewer pipes. I would like to be able to go in either direction with my calculations but am not sure of what version of Manning’s equation to use to solve for the following.

1. Calculate the minimum pipe slope given flow rate (cfs) and velocity (ft/s).

2. Calculate velocity given flow rate (cfs) and slope (ft/ft).

Any help is appreciated.

 

[cieg22]

I believe the requirements are that the slope produce a min velocity of 2ft/s for a pipe flowing full in a gravity flow situation.

 

[Ron]

A flow velocity of at least 3ft/sec is used to reduce deposition (sediment), while a max. of 5ft/sec is needed to prevent scour. Set your limits in there.

 

[hydroponder]

The Great Lakes-Upper Mississippi River Board of State Public Health and Environmental Managers - Recommended Standards for Wastewater Facilities (generally referred to as the Ten State Standards {IL, IN, IA, MI, MN, MO, NY, OH, Ontario, PA & WI}) recommends that all sewers be designed to give mean velocities, when flowing full, of not less than 2.0 feet per second (0.61 m/s), based on Manning's formula using an "n" value of 0.013. Here are the recommended pipe slopes:

8in - 0.40
10in - 0.28
12in - 0.22
14in - 0.17
15in - 0.15
16in - 0.14
18in - 0.12
21in - 0.10
24in - 0.08
27in - 0.067
30in - 0.058
33in - 0.052
36in - 0.046
39in - 0.041
42in - 0.037

 

[Hoagie]

3 fps is preferred min, 2 fps is min.. 10 fps is the upper limit. (Merrit).

 

[dicksewerrat]

3 fps is better and for new PVC use .011 as 'n'.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

If you are working with the US customary units;

The Manning equation:

Q = (0.463/n) * D^8/3 *S^1/2 (US customary units)

Q= cubic feet/second

D = pipe diameter in feet

S = slope of energy grade line (ft/ft)

n= coefficient of roughness

See McGraw Hill Series Wastewater Engineering Collection and Pumping of Wastewater

 

[jartgo]

You will also want to check with your local regulators. As all of the information here appears accurate at first glance, as you can see, opinions will differ as to what velocity you need and what n value you use. Most state regulations are clear in telling you to meet their required minimum velocity using the n value that they tell you to use. They will also tell you what the minimum slopes are to be for each pipe size.

It kind of takes some of the engineering out of the work but the regulators like to regulate.

 

[rconner]

ASCE Manual Number 60 [Water Environment Federation Manual Number FD-5], “Gravity Sanitary Sewer Design and Construction”, includes the following statement about pipe materials and Manning’s n:

“Generally, Manning’s n for a given sewer, after some time in service, will approach a constant which is not a function of the pipe material but represents the grit accumulation and slime build -up on the pipe walls. This n
will be on the order of 0.013. A coefficient which will yield higher friction losses should be selected for sewers where disturbing influences are known or anticipated. Because of the empirical nature of each formula, conservative design is prudent.”

While many contemporary piping materials and linings etc. including pvc have exhibited in clean water flow coefficients very close to the smooth pipe curves, beyond this pvc and pipe thereof is also known to be a rather “oleophilic” material in some long-term service (see e.g. page 63 of the document at

http://www.epa.state.oh.us/dsw/permits/sso solutions asce epa guidance 2004.pdf.),

and I have also noticed that per some research including recently published in the ASCE “Journal of Environmental Engineering” accessible at

http://php.scripts.psu.edu/users/p/c/pcj5004/CE370H/ARTICLES/SewerBiofilm.pdf,

bio-films can also form and reportedly result in Mannings “n” higher than some more optimistic promotions.

 

[civilperson]

binr's Manning's equation is different than I recall:
Q = V A
V = (1.46/n) H<2/3 x S<1/2

 

[bimr]

Substituting for the hydraulic radius, the Manning equation for pipes flowing full becomes:

V = (0.590/n) * D^2/3 *S^1/2 (US customary units)