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Sewer manhole drop

What is the impct in the design of sewer network, when i consider a drop of 20 mm between inlet pipe and the outlet pipe in manholes? How I can deide the drop weather 20 mm or 15 mm or etc? Are there any basic calculations involved to decide the drop? Please let me know?

Ravi

 

  • Have you ever been involved in trying to install a manhole with a 15 or 20 mm drop? To a certain extent, our ability to model may exceed the construction crew's ability to place a manhole floor with an accuracy of +/- 1 mm.

    If you think about a run of say 100 m between manholes, the drop is on the order of 1000 mm.  Unless you are in a very flat terrain where every bit of drop is important, it doesn't make a great deal of difference how much the invert drops across a manhole.

     Having said that, I know there are owners that do specify say a 1 in (25 mm) drop in a manhole. The  Ten State Standards essentially recommend you maintain the same slope in the manhole as in the sewer. If a manhole is roughly 1 m across and the slope is 0.004. then you should have a minimum 4 mm drop. Try measuring that during construction when you're placing a big concrete structure.

    Tom

  • Thanks for explanation. Why I am asking this question is our Client want a drop of 20 mm between the inlet and outlet of the manhole. Normaly what i am doing is in the "default design constraints" Under the node I match the crown and put 20 mm offset. In some places when we doing manuall adjustment to accomodate the site condition we may mistakely keep 35 mm or any other small change value, But client is insisting on 20 mm? Are there any design improtents on this?

     Regards

    Ravi

     

  • Generally this manhole drop is to compensate for energy losses in the structure. So if "typical" headloss in the structure is 20mm, you drop the pipe by 20mm to compensate.  

    However there is no such thing as a "typical" headloss in a structure. Sometimes you might get a 2mm loss, sometimes you might get 200mm - it depends on the flows, pipes diameters, the number of incoming pipes, bend angle, etc. etc. and it will change for every different storm event.

    So a 20mm drop might be an ok rule of thumb based on past experience, but my opinion is that variations should also be acceptable. In fact I agree with Tom that the drop across a manhole doesn't make much difference in most systems (i.e. most systems would work fine with 0mm drop), especially in systems with some steep pipes.

    If the client is very worried about sewer overflows, you can use SewerCAD to perform a detailed hydraulic grade line analysis to (hopefully) confirm that everything will be ok.

    Good luck.

     

    Regards,

    Mal

     

          

  • Thankyou Mal.

    In the mean time can you tell how I can run a detailed hydraulic grade line analysis? Please

     

    Regards

    Ravi

  • Ravi:

     The hydraulic grade line is computed in all pipes at all time in every SewerCAD analysis.  So, to do 'detailed hydraulic grade line analysis' you would simply make good use of the Profile tool (go to View > Profiles in the menu) in SewerCAD to have a look at the hydraulic grade line throughout your system in your design event and check storm events.  Make sure that you build profile views of all your trunk lines, at a minimum, and throughout several of your longer "feeder" lines, particularly those with large contributing loads.  Ensure that the hydraulic grade line is contained within the pipe at the peak flow of all your events (there's a little red hash mark shown on the profile that indicates the max HGL for the scenario shown, so it's easy to tell whether that is inside the pipe) by viewing these profiles.

     HTH,

    Kim