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Hydraulic Grade Line below invert of pipe!

Hi,

We have a Storm Drainage network with Soakaways scattered along the ROW and all interconnected with overflow pipes. The overflow lines are achieved with a control structure in the pipe line. However, I have modelled the network but at certain locations, the HGL seems peculiar which is running below the pipe invert and in V shape. The overflow connection is same as any other upstream/Downstream overflow pipes(Refer to screenshots below) where HGLs are running normal. Appreciate if anyone can elaborate this unusual


behavior of HGL.

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  • Thanks Scott and Jesse,

    I have reverified all the Pipe inverts and found O.K.

    My observation is inline with Jesse's point 1 that this HGL instability  is arising due to the overflow conduit being dry or not having any lateral connection. If I add a lateral connection or remove the control start structure for the overflow conduits then the hydraulic grade line seem to behave normal.

    Is it normal and can be ignored?

  • If the dry condition is expected then these initial unstable results could be okay if you see stable results after you progress through the simulation. Is the HGL improving when the conduits are "not" running dry? Are you seeing stable results later in the simulation?

    Is adding a "lateral" providing some inflow there? Also, what are the conduit controls structures set for? What do they accomplish?

    I would also recommend to try the troubleshooting options as suggested by Jesse above. Try with different time steps and calculation options to see if the unstable results are improving.


    Regards,

    Yashodhan Joshi

  • Hi Joshi,

    Yes, the HGL tends to become stable as the run-off begins to flow in the conduit.

    The proposed system is sort of combined Infiltration chambers with Overflow drainage lines(see below screenshot)

    The laterals are connecting the Inlets to the Overflow pipes directly where Manholes are at a distant locations.

    The start control structures allows the flow through it after the upstream infiltration chambers are filled.

    We have at some locations with no laterals to connect to the overflow lines and there HGL tends to be unstable as long the Overflow pipe is dry.