Capacity of sewer pipes on steep slopes

[hogart26]

Some engineers have indicated that for open channel flow applications at a certain steep pipe slope the Manning Equation is no longer applicable and another method must be used to determine the carrying capacity of the pipe. I'm looking for input on: (1) whether others agree with this concept, (2) if so, what is the limiting pipe slope for say an 8-inch pipe (assuming that the limitation varies with pipe size), and (3) what method is used to calculate the pipe capacity when the pipe you are designing exceeds this limiting pipe slope. Thanks.

 

[RWF7437]

As far as I know there is no theoretical limit to the application of the Manning Equation. The slope (S) in the equation is the slope of the hydraulic grade line; NOT the slope of the pipe. For this reason the Manning Eq. may be used for both open channel flow and for closed pipe flows

There are practical limits, however, to the maximum velocity in a sanitary sewer or storm drain. These limits are chosen to reduce erosion of pipe walls or linings, to prevent water hammer, to reduce energy losses, and to prevent off gassing of H2S and other gasses. Minimum velocities are also chosen; mainly to prevent deposition of solids.

For buried pipes there are limits to slope dictated by soil stability and the need to prevent pipe separation at joints.

Drain pipes in buildings, however, are often vertical but they rarely, if ever, flow full. Their capacity is usually limited by inlet conditions which are governed by the size and shape of the inlet acting as an orifice.

Ask your "some engineers" what they think the "limiting pipe slope" is and how they would calculate it.

good luck

 

[bimr]

Not sure exactly what you are asking, but I would offer the following.

Sanitary sewers are typically limited by a maximum mean velocity of 8 to 10 ft/sec. This maximum velocity is used to prevent erosion of a sewer, which is possible at higher velocities.

At higher velocities and low depth of flow, solids may settle on the inverts, from where the solids will be lodged so firmly that the solids will not detach.

An 8% slope will give you the 10 ft/sec velocity in an 8” diameter sewer. The manning equation is still valid if you are operating within the maximum recommended sewer velocity.

 

[civilperson]

Drop Manholes are used to lose elevation in steep terrain vs steep pipe slopes.

 

[Unotec]

Are you usinbg this sewer pipe as a drain (gravity flow) or pumping through it?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[davefitz]

Based on a long ago discussion with my late grandfather- a NYC plumber from 1916-1954:

Long runs of 2-phase sewer pipes are usually sloped at not greater than 1/8" / ft to avoid separation of solids from liquid.

On a steep slope, a typical plumbing technique is to run short horizontal runs at 1/4" to 1/8" / ft and once the pipe nears grade, drop vertically by ( 3 ft) and again proceed horizontally at 1/4 -1/8" /ft. Complicating factors may be code or spec requirements for rod-outs etc.

But with single phase rainwater or fine floc, a steep slope might be raionalized.