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Sanitary Sewer Capacity Equation

Sanitary Sewer Capacity Equation

Sanitary Sewer Capacity Equation

I'm reviewing new developments for sanitary sewers. Usually I've seen sanitary sewer analyzed by capacity, using mannings (pretty simple). But I have a developer providing computation for an existing over capacity pipe section. They are providing inlet water surface and max water surface elevation and stating "while under capacity per mannings equation, HGL computation shows that HGL stays below the crown of the pipe, there fore the section of existing pipe is adequate. How can it be adequate if the pipe section is under capacity per mannings? I'm new to pipe design review. Any help is appreciated

RE: Sanitary Sewer Capacity Equation

The developer is developing an alternative reality to advocate a less expensive (to the developer) solution. Recommend that you stand firm and reject.

RE: Sanitary Sewer Capacity Equation

What is your role in this?
If you are in the design approval role then reject the design. The mannings equation approach is well tried and adopted.

"Any water can be made potable if you filter it through enough money"

RE: Sanitary Sewer Capacity Equation

By the time that pipe is overloaded, the developer will be long gone.
Is this work done to a municipality's standards? They're going to own the system when the developer sells off the last unit. It seems like they would not deviate from manning's.

RE: Sanitary Sewer Capacity Equation

I’m working under a P.e. And we are the approving authority for the municipality. I usually meet with our P.e. To discuss the developments but it is not due for a while. I just did not understand how you can argue the surface outlet water elevation if the mannings show the pipe section can not handle the flow from the new development. I wasn’t sure how the water elevation could even be mathematically related to capacity. Unfortunately our municipality is behind and we do not have a set design standard that developers can use. They usually follow other nearby justifications design standard. Thanks for all the help.

RE: Sanitary Sewer Capacity Equation

generally accepted practice for sanitary sewer design in the US is to use the Mannings equation. Normal depth provides a basic estimate of the capacity and Mannings can be used with appropriate manhole and junction losses to generate a water surface and hydraulic grade line which would more accurately show the flow condition in the pipe. It sounds like your developer engineer is either
1) trying to pull a fast one or
b) does not understand basic hydraulics

RE: Sanitary Sewer Capacity Equation

It's been a while, but I think 10 States, which is adopted in most US states, mandates Manning's.

RE: Sanitary Sewer Capacity Equation

I will take a crack at this…

It sounds to me like the developer's engineer is claiming that simply maintaining open channel flow in the sewer is sufficient justification to call the pipe "under capacity". Let's say his(?) calculations show a depth ratio of 90% at the estimated future peak flow. Yes, the pipe would be flowing as an open channel under these conditions, but it would have precious little reserve capacity before entering the undesirable surcharge flow regime. This is important because the estimated future peak flow is just that, an estimate. Actual future peak flows might exceed that estimate, which is why having reserve open channel flow capacity is so important for sewers and storm drains.

Let's also say that your agency has a requirement for this size pipe to not exceed a depth ratio of 50%. Obviously still open channel flow, but with a much greater reserve capacity. So, while his "design" maintains open channel flow at the estimated peak, his "design" violates your agency's requirements and provides far too little reserve capacity. Thus, his design is rejected and he is forced to eat liver and onions for a month.

As an aside, back in the mid-1980s, I modeled about four miles of a City of Los Angeles trunk sewer and its watershed as part of a study evaluating a proposed service area swap with an adjacent special district. I noticed that the City's sewer design criteria was not nearly as conservative as I was used to seeing. I don't remember the exact numbers, but it went something like this: instead of requiring a max depth ratio of 50% up to 18", then 67% up to 24", and 75% above that, the City was using something like 50% up to 12", then 67% up to 18", and 75% above that. I asked one of the City's engineers about it and he told me that because the area I was studying was gradually transitioning from single-family to multi-family, and they had a pretty good handle on what the actual peak flows would be, the City decided (probably with pressure from developers) to liberalize the design criteria in this watershed rather than require upsizing miles of pipes and tearing up miles of roads. In this case, the City determined that cutting into their reserve capacity made more engineering and economic sense.


"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: Sanitary Sewer Capacity Equation

fel3, I think you've nailed it. Talking with some other engineers at other municipalities, the consultant decided to show that the depth of flow is still under capacity for their proposed upstream development. I was told as long as the HGL is contained in the pipe, then theoretically you can claim the pipe has enough capacity even though mannings equations shows "Q" being over capacity. I still have a few days to make a decision, but honestly it still does not make sense of how mannings can show over capacity, but the HGL is still "contained". Is it because of friction loss that is assumed in the HGL computations?

RE: Sanitary Sewer Capacity Equation

if a mannings normal depth shows it being over capacity (say over 90%), then the HGL calculation will likely show it to be slightly higher since it takes into consideration the manhole and junction losses downstream which are additional head losses not considered in a normal depth calculation for a pipe. again, it does not sound to me like they have a good handle on the hydraulics. since you are approving the design, than you should be able to decide if "contained in the pipe" is acceptable.

RE: Sanitary Sewer Capacity Equation

Sewers are usually designed for full flow at the peak hour. Most municipalities do not require reserve capacity. The following statement is typical:

"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."

Quote (arodgers1986 (Civil/Environmental) )

But I have a developer providing computation for an existing over capacity pipe section.

Is this over capacity statement based on witnessing of the sewer flows or is it a deign calculation?

RE: Sanitary Sewer Capacity Equation


Around here (my experience is in southern and central California), every agency I have dealt with requires handling the design peak flow at well less than full, starting with a max depth ratio of 50% for smaller pipes (the cut-off varies), then allowing a bit more depth for larger pipes, and a bit more for even larger pipes. BTW, These requirements are for dedicated sanitary sewers, not combined sewers, which we have very little of here.


The HGL can be maintained within the pipe and the pipe can still be over capacity. This occurs when "over capacity" is defined by regulation (e.g. my 50%, 67%, etc examples) and not by just the raw hydraulics.


"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: Sanitary Sewer Capacity Equation

A little late for this but my guess is that the designer is using a software like SewerCAD that calculates the HGL based on Gradually Varied Flow conditions. I've run under a similar problem while designing a storm sewer as discussed here:


RE: Sanitary Sewer Capacity Equation


Are you Tony? This is Fred, late of QK.

"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

RE: Sanitary Sewer Capacity Equation


I was told as long as the HGL is contained in the pipe, then theoretically you can claim the pipe has enough capacity even though mannings equations shows "Q" being over capacity.

Well, let's all be very careful with our words here.

If you plug the full diameter (100%) of a pipe into Manning's equation for the depth of flow, and you use that to calculate the capacity of the pipe, you are actually under-calculating the pipe's open channel flow capacity. A pipe flows at maximum open channel flow capacity at a depth of approximately 92% full. (that's from memory, someone check that) The reason for this has to do with the definition of "normal flow." Normal flow depth is derived from determining a flow depth where the energy gained from the slope of the pipe balances out with the energy lost due to friction. That last 8% of pipe depth doesn't gain you much conveyance in terms of cross sectional area, but it gains you a lot of friction. So if you plot conveyance vs flow depth ratio, the relationship actually doubles back on you inside that last few percent full.

This is one of the reasons why the FHWA culvert design nomographs and such get so complicated.

This factoid does not usually matter for sanitary sewer design, because sanitary sewer design standard usually dictate a depth of flow equal to far less than 92%, to account for solids and peaks and such.

Basically what I'm saying, is be careful what you consider to be "pipe capacity" when having this discussion. Pipe capacity as determined from software will usually take this stuff into account, but pipe capacity from your manning's excel spreadsheet sitting on your corporate server might not, and you could get into a situation where the HGL is shown under the pipe crown even though your local spreadsheet is telling you the pipe is at or over capacity by a bit.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Sanitary Sewer Capacity Equation


No that’s not me :)

RE: Sanitary Sewer Capacity Equation


Well darn. I know a Tony Akel who is a civil engineer in my area.

"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

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