Understanding the 2 ft/s minimum sanitary "self-cleaning" velocity 3

[khu]

I need some help understanding the minimum 2 ft/s "self-cleaning" velocity referenced by the 10-state and many other standard documents. I've read the other posts on this subject, but my specific question was never quite answered that I could find.

"All sewers shall be designed and constructed to give mean
velocities, when flowing full, of not less than 2.0 feet per second
(0.6 m/s), based on Manning's formula using an "n" value of 0.013."

I've always taken this to mean that at my design flow, I need to have a minimum velocity of 2 ft/s in the pipe. However, the line "when flowing full (half full included in some documents)" has me a little confused. My issue is that on a development that I'm involved with, my design flows for the first phase are producing less than 2 ft/s. When the second phase comes online, I'm meeting the 2 ft/s requirements for my actual calculated flows. For the first phase though, I would need some of these pipes over 3% to meet the 2 ft/s at my design flows, which is going to cause a very deep pipe at my wet well 1000' away.

I'm just confused why the 10-state indicates that the 2 ft/s requirement is when the pipe is flowing full (or half by some standards). My pipes do meet that criterion, but they will never actually flow half full. Even with Phase 2 flows coming in, my max d/D is only 0.3 in an 8" pipe. Do I design my slopes to meet 2 ft/s at my design flow or 2 ft/s flowing full/half full?

 

[HTURKAK]

- flowing full or half full literally the same.. make a simple calculation and see ..

- All sewers shall be designed and constructed to give mean velocities, when flowing full, of not less than 2.0 feet per second
(0.6 m/s), based on Manning's formula using an "n" value of 0.013.

IMO, the requirement is more than clear.. ( when flowing full, of not less than 2.0 feet per second ) It is not specific design flow..Some standards directly provide seperate minimum slope for each dia.

 

[1503-44]

The most efficient gravity flow occurs at somewhat less than flowing at a full depth, hence the Minimum velocity requirements specifications may try to address both cases.

What seems clear is that 2ft/sec should be the Minimum flow for any situation, especially at the design depth flow rate and if you expect not to meet that for any length of time, you run the risk of debrie accumulation and possible unexpected maintenance costs. In that case you should so advise your client that Minimum velocities can not be meet in certain circumstances and resolve any potential issues before proceeding to final design.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

Ah the joys of designing for future developments.

The main issue for me is that you don't really know if the second phase will ever be built.

So maybe, although it costs a bit more, why not design one pipe for phase 1 and a second pipe for phase 2 and then connect them every now and then and run them into the same wet well.

Either that or have some sort of included regular flushing in the first phase.

But ~300m of two drainage pipes run side by side? Still cheaper than a very deep hole / trench for one. Maybe.

An design for the design flow. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TerryScan]

I design in a coastal plain. There is rarely the luxury of slopes steeper than near minimum. While it is desirable that the pipe be "self cleaning" by having 2fps at the actual design flow, this is often impracticable when the design flow is low.

Designing a 2fps pipe slope when flowing full or half in these situations at least allows for a "cleaning" flow if the pipe is flushed using a water truck.

 

[khu]

Designing a 2fps pipe slope when flowing full or half in these situations at least allows for a "cleaning" flow if the pipe is flushed using a water truck.

This makes sense. Our local regulations list the 2fps as a guideline with a requirement for the system to have self-cleaning velocities when flushed. This all makes a bit more sense now. One thing that was confusing me is that some jurisdictions do not have the language for "when flowing full or half-full", which made me think the 2fps was a hard requirement for your specific design flows. Not necessarily the case for my area.

So maybe, although it costs a bit more, why not design one pipe for phase 1 and a second pipe for phase 2 and then connect them every now and then and run them into the same wet well.

The issue is that I'm already at the minimum diameter allowed of 8", so either way I do it, increasing the slope is the only thing that would get me higher velocity. After some of the other comments here and more research on my end, it appears the language is seeking to have a pipe that can "self clean" when flushed in a maintenance scenario and does not necessarily need to have 2fps at all flow scenarios.

 

[bimr]

Why sewers are always designed for partial full flow conditions?

To maintain an open channel flow condition and to provide space above the surface of the sewage, the sewer is designed to be partially full flow. To control depositions in the sewer, it's necessary to maintain the flow rate.

All sewers shall be designed and constructed to give MEAN velocities, when flowing full, of not less than 2.0 feet per second.

Ten States

The purpose of the standard is to minimize the need for sewer cleaning. It is a common that in new developments, that the initial flows will not meet the flow requirement.

 

[Waxwing]

Thanks for the insight TerryScan. Sizing sewer pipes is an interesting thought process for engineers starting out, since increasing the pipe diameter can result in decreased velocities in certain instances.

 

[kckcei]

It also refers to fore mains, not just gravity lines.
The difference is, the velocity at the operating point in the forcemain should be at the self cleansing velocity.

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