## Sanitary Sewer Capacity Equation

## Sanitary Sewer Capacity Equation

(OP)

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

## RE: Sanitary Sewer Capacity Equation

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

Regards

Ashtree

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

## RE: Sanitary Sewer Capacity Equation

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

## RE: Sanitary Sewer Capacity Equation

1) trying to pull a fast one or

b) does not understand basic hydraulics

## RE: Sanitary Sewer Capacity Equation

## RE: Sanitary Sewer Capacity Equation

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.

--Fred

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"Is it the only lesson of history that mankind is unteachable?"

--Winston S. Churchill

## RE: Sanitary Sewer Capacity Equation

## 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

"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."

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.

arodgers1986…

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.

Fred

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"Is it the only lesson of history that mankind is unteachable?"

--Winston S. Churchill