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Sewer flow deposits

Sewer flow deposits

Sewer flow deposits

Does anyone know of a simple formula to be used for estimating debris deposits in a gravity sewer?  I'm concerned about a specific location where there is a change in slope from a very steep slope to a mild slope.  I have values for seddimentation in the same line upstream of this point (this line has a different slope).

RE: Sewer flow deposits

I hope zero would be the answer to any formula you try. If you have a change in grade for a gravity system, it's time for a manhole.

RE: Sewer flow deposits

It has been a long time since I was into sewers (just a little pun), but I seem to remember that one of the design considerations was the "self-cleaning velocity".  The flow rate at which the sewer would clean out accumulated sediment.  Is this not done anymore?

Yes, at most slope changes you would expect a manhole, but not always.  The most dramatic case it the "inverted syphon".

RE: Sewer flow deposits

In the US, we use minimum grades for sewers designed to assure a self cleaning velocity.  Usually that is about 2 to 2.5 feet per second ( 0.61 m/sec).  For example, the minimum slope for an 8-inch line is 0.004.  Remember that the velocity at half full is the same as th velocity full for circular pipes.

Russ Faust
Salem, OR USA

RE: Sewer flow deposits

Your minimum velocity should be the scour, or cleansing.  Debris should not settle, or at least be minimal in manholes if they are not properly graded or mortared smooth.  In a storm sewer, there should be basins at each inlet to collect debris (sediments and gravels) prior to entry into the sewer system.  I would also be concerned with excessive energy in the fluid resulting in destructive scour.

KRS Services

RE: Sewer flow deposits

I designed and installed a number of sewer lines. If the inspector doesn't see at least a half moon between manholes, it will be rejected. So how do you get away with changes in grade without a manhole jheidt2543?

RE: Sewer flow deposits

Unless the approved "for construction" drawings include a grade change between manholes, a manhole should be required.  Gtrainor is accurate on this point.

Gtrainor, I didn't think there were any inspectors left who actually look down the pipe after construction?  In most cases the lines are flushed and then camera'd bothe after costruction and at expiration of warranty (if possible).  Even the guys who trained me once upon a time rarely enter into a finished manhole, let alone peer down the pipe.  That's cool!

KRS Services

RE: Sewer flow deposits

I have had it both ways. When I was installing the lines myself (Anchorage, AK), the city inspector would look down the pipe (prior to having any waste in it). Now I am doing construction services as an inspecting engineer, and I look for a full-half moon. Cameras worked great but they are expensive and I don't think its worth it in a new pipe where I can watch some or all the install.

RE: Sewer flow deposits

i haven’t heard of such formula ,but anyway if both of your pipe slopes are above minimum to get self-cleansing you shouldn't worry about debris.

i'v done design and construction for several sewer projects ,we've never changed a slope without a manhole ,but i can't see why not, as long as both of the lines are at a grade steep enough to achieve self-cleansing velocities at the designed proportional depth, which is based on your very near expected flow.

self-cleansing velocities is very important ,but practically you can't always get it ,specially at the very upstream part of a system ,if you insist on flushing upstream you will end up with a massive excavation work downstream.

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